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A black hole is a region of space from which nothing, not even light, can escape. It is the result of the deformation of spacetime caused by a very compact mass. Around a black hole there is an undetectable surface which marks the point of no return, called an event horizon. It is called "black" because it absorbs all the light that hits it, reflecting nothing, just like a perfect black body in thermodynamics.[1] Quantum mechanics predicts that black holes also emit radiation like a black body with a finite temperature. This temperature decreases with the mass of the black hole, making it unlikely to observe this radiation for black holes of stellar mass.

Despite its invisible interior, a black hole can be observed through its interaction with other matter. A black hole can be inferred by tracking the movement of a group of stars that orbit a region in space. Alternatively, when gas falls into a stellar black hole from a companion star, the gas spirals inward, heating to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and Earth-orbiting telescopes.

Astronomers have identified numerous stellar black hole candidates, and have also found evidence of supermassive black holes at the center of galaxies. In 1998, astronomers found compelling evidence that a supermassive black hole of more than 2 million solar masses is located near the Sagittarius A* region in the center of the Milky Way galaxy, and more recent results using additional data find evidence that the supermassive black hole is more than 4 million solar masses.
Contents
[hide]

* 1 History
o 1.1 General relativity
o 1.2 Golden age
* 2 Properties and structure
o 2.1 Physical properties
o 2.2 Event horizon
o 2.3 Singularity
o 2.4 Photon sphere
o 2.5 Ergosphere
* 3 Formation and evolution
o 3.1 Gravitational collapse
+ 3.1.1 Primordial black holes in the Big Bang
o 3.2 High-energy collisions
o 3.3 Growth
o 3.4 Evaporation
* 4 Observational evidence
o 4.1 Accretion of matter
o 4.2 X-ray binaries
+ 4.2.1 Quiescence and advection-dominated accretion flow
+ 4.2.2 Quasi-periodic oscillations
o 4.3 Gamma ray bursts
o 4.4 Galactic nuclei
o 4.5 Gravitational lensing
o 4.6 Alternatives
* 5 Open questions
o 5.1 Entropy and thermodynamics
o 5.2 Black hole unitarity
* 6 See also
* 7 Notes
* 8 References
* 9 Further reading
* 10 External links

History
Schwarzschild black hole
Simulation of gravitational lensing by a black hole which distorts the image of a galaxy in the background (click here for larger animation)

The idea of a body so massive that even light could not escape was first put forward by geologist John Michell in a letter written to Henry Cavendish in 1783 to the Royal Society:

If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae, with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity.
—John Michell[2]

In 1796, mathematician Pierre-Simon Laplace promoted the same idea in the first and second editions of his book Exposition du système du Monde (it was removed from later editions).[3][4] Such "dark stars" were largely ignored in the nineteenth century, since it was not understood how a massless wave such as light could be influenced by gravity.[5]
General relativity

In 1915, Albert Einstein developed his theory of general relativity, having earlier shown that gravity does influence light's motion. A few months later, Karl Schwarzschild gave the solution for the gravitational field of a point mass and a spherical mass.[6] A few months after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution for the point mass and wrote more extensively about its properties.[7] This solution had a peculiar behaviour at what is now called the Schwarzschild radius, where it became singular, meaning that some of the terms in the Einstein equations became infinite. The nature of this surface was not quite understood at the time. In 1924, Arthur Eddington showed that the singularity disappeared after a change of coordinates (see Eddington coordinates), although it took until 1933 for Georges Lemaître to realize that this meant the singularity at the Schwarzschild radius was an unphysical coordinate singularity.[8]

In 1931, Subrahmanyan Chandrasekhar calculated, using general relativity, that a non-rotating body of electron-degenerate matter above 1.44 solar masses (the Chandrasekhar limit) would collapse.[citation needed] His arguments were opposed by many of his contemporaries like Eddington and Lev Landau, who argued that some yet unknown mechanism would stop the collapse.[9] They were partly correct: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star,[10] which is itself stable because of the Pauli exclusion principle. But in 1939, Robert Oppenheimer and others predicted that neutron stars above approximately three solar masses (the Tolman–Oppenheimer–Volkoff limit) would collapse into black holes for the reasons presented by Chandrasekhar, and concluded that no law of physics was likely to intervene and stop at least some stars from collapsing to black holes.[11]

Oppenheimer and his co-authors interpreted the singularity at the boundary of the Schwarzschild radius as indicating that this was the boundary of a bubble in which time stopped. This is a valid point of view for external observers, but not for infalling observers. Because of this property, the collapsed stars were called "frozen stars,"[12] because an outside observer would see the surface of the star frozen in time at the instant where its collapse takes it inside the Schwarzschild radius. This is a known property of modern black holes, but it must be emphasized that the light from the surface of the frozen star becomes redshifted very fast, turning the black hole black very quickly. Many physicists could not accept the idea of time standing still at the Schwarzschild radius, and there was little interest in the subject for over 20 years.
Golden age
See also: Golden age of general relativity

In 1958, David Finkelstein identified the Schwarzschild surface r = 2m [in geometrized units, i.e. 2Gm/c2, where r is the radius of the surface and m is the mass of the black hole] as an event horizon, "a perfect unidirectional membrane: causal influences can cross it in only one direction".[13] This did not strictly contradict Oppenheimer's results, but extended them to include the point of view of infalling observers. Finkelstein's solution extended the Schwarzschild solution for the future of observers falling into the black hole. A complete extension had already been found by Martin Kruskal, who was urged to publish it.[14]

These results came at the beginning of the golden age of general relativity, which is marked by general relativity and black holes becoming mainstream subjects of research. This process was helped by the discovery of pulsars in 1967,[15][16] which were within a few years shown to be rapidly rotating neutron stars. Until that time, neutron stars, like black holes, were regarded as just theoretical curiosities; but the discovery of pulsars showed their physical relevance and spurred a further interest in all types of compact objects that might be formed by gravitational collapse.

In this period more general black hole solutions where found. In 1963, Roy Kerr found the exact solution for a rotating black hole. Two years later Ezra T. Newman found the axisymmetric solution for a black hole which is both rotating and electrically charged.[17] Through the work of Werner Israel,[18] Brandon Carter,[19][20] and D. C. Robinson[21] the no-hair theorem emerged, stating that a stationary black hole solution is completely described by the three parameters of the Kerr–Newman metric; mass, angular momentum, and electric charge.[22]

For a long time, it was suspected that the strange features of the black hole solutions were pathological artefacts from the symmetry conditions imposed, and that the singularities would not appear in generic situations. This view was held in particular by Belinsky, Khalatnikov, and Lifshitz, who tried to prove that no singularities appear in generic solutions. However, in the late sixties Roger Penrose[23] and Stephen Hawking used global techniques to prove that singularities are generic.[24]

Work by James Bardeen, Jacob Bekenstein, Carter, and Hawking in the early 1970s led to the formulation of the laws of black hole mechanics.[25] These laws describe the behaviour of a black hole in close analogy to the laws of thermodynamics by relating mass to energy, area to entropy, and surface gravity to temperature. The analogy was completed when Hawking, in 1974, showed that quantum field theory predicts that black holes should radiate like a black body with a temperature proportional to the surface gravity of the black hole.[26]

The term "black hole" was first publicly used by John Wheeler during a lecture in 1967. Although he is usually credited with coining the phrase, he always insisted that it was suggested to him by somebody else. The first recorded use of the term is in a 1964 letter by Anne Ewing to the American Association for the Advancement of Science.[27] After Wheeler's use of the term, it was quickly adopted in general use.
Properties and structure

The no-hair theorem states that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, charge, and angular momentum.[22] Any two black holes that share the same values for these properties, or parameters, are indistinguishable according to classical (i.e. non-quantum) mechanics.

These properties are special because they are visible from outside the black hole. For example, a charged black hole repels other like charges just like any other charged object. Similarly, the total mass inside a sphere containing a black hole can be found by using the gravitational analog of Gauss's law, the ADM mass, far away from the black hole.[28] Likewise, the angular momentum can be measured from far away using frame dragging by the gravitomagnetic field.

When an object falls into a black hole, any information about the shape of the object or distribution of charge on it is evenly distributed along the horizon of the black hole, and is lost to outside observers. The behavior of the horizon in this situation is closely analogous to that of a conductive stretchy membrane with friction and electrical resistance, a dissipative system (see membrane paradigm).[29] This is different from other field theories like electromagnetism, which does not have any friction or resistivity at the microscopic level, because they are time-reversible. Because the black hole eventually achieves a stable state with only three parameters, there is no way to avoid losing information about the initial conditions: the gravitational and electric fields of the black hole give very little information about what went in. The information that is lost includes every quantity that cannot be measured far away from the black hole horizon, including the total baryon number, lepton number, and all the other nearly conserved pseudo-charges of particle physics. This behavior is so puzzling that it has been called the black hole information loss paradox.[30][31][32]
Physical properties

The simplest black hole has mass but neither electric charge nor angular momentum. These black holes are often referred to as Schwarzschild black holes after the physicist Karl Schwarzschild who discovered this solution in 1915.[6] According to Birkhoff's theorem, it is the only vacuum solution that is spherically symmetric.[33] This means that there is no observable difference between the gravitational field of such a black hole and that of any other spherical object of the same mass. The popular notion of a black hole "sucking in everything" in its surroundings is therefore only correct near the black hole horizon; far away, the external gravitational field is identical to that of any other body of the same mass.[34]

Solutions describing more general black holes also exist. Charged black holes are described by the Reissner-Nordström metric, while the Kerr metric describes a rotating black hole. The most general stationary black hole solution known is the Kerr-Newman metric, which describes a black hole with both charge and angular momentum.

While the mass of a black hole can take any positive value, the charge and angular momentum are constrained by the mass. In Planck units, the total electric charge Q and the total angular momentum J are expected to satisfy

Q^2+\left ( \tfrac{J}{M} \right )^2\le M^2\,

for a black hole of mass M. Black holes saturating this inequality are called extremal. Solutions of Einstein's equations violating the inequality exist, but do not have a horizon. These solutions have naked singularities and are deemed unphysical. The cosmic censorship hypothesis rules out the formation of such singularities through the gravitational collapse of realistic matter.[35] This is supported by numerical simulations.[36]

Due to the relatively large strength of the electromagnetic force, black holes forming from the collapse of stars are expected to retain the nearly neutral charge of the star. Rotation, however, is expected to be a common feature of compact objects, and the black-hole candidate binary X-ray source GRS 1915+105[37] appears to have an angular momentum near the maximum allowed value.
Class Mass Size
Supermassive black hole ~105–109 MSun ~0.001–10 AU
Intermediate-mass black hole ~103 MSun ~103 km = REarth
Stellar black hole ~10 MSun ~30 km
Micro black hole up to ~MMoon up to ~0.1 mm

Black holes are commonly classified according to their mass, independent of angular momentum J or electric charge Q. The size of a black hole, as determined by the radius of the event horizon, or Schwarzschild radius, is roughly proportional to the mass M through

r_\mathrm{sh} =\frac{2GM}{c^2} \approx 2.95\, \frac{M}{M_\mathrm{Sun}}~\mathrm{km,}

where rsh is the Schwarzschild radius and MSun is the mass of the Sun. This relation is exact only for black holes with zero charge and angular momentum, for more general black holes it can differ up to a factor of 2. The table on the right lists the various classes of black hole that are distinguished.
Event horizon
Main article: Event horizon
Image:BH-no-escape-1.svg
Far away from the black hole a particle can move in any direction. It is only restricted by the speed of light.
Image:BH-no-escape-2.svg
Closer to the black hole spacetime starts to deform. There are more paths going towards the black hole than paths moving away.
Image:BH-no-escape-3.svg
Inside of the event horizon all paths bring the particle closer to the center of the black hole. It is no longer possible for the particle to escape.

The defining feature of a black hole is the appearance of an event horizon—a boundary in spacetime through which matter and light can only pass inward towards the mass of the black hole. Nothing, not even light, can escape from inside the event horizon. The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if such an event occurred.[38]

As predicted by general relativity, the presence of a large mass deforms spacetime in such a way that the paths taken by particles bend towards the mass. At the event horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hole.[39]

To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole.[40] Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it.[41] At the same time, all processes on this object slow down causing emitted light to appear redder and dimmer, an effect known as gravitational redshift.[42] Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.

On the other hand, an observer falling into a black hole does not notice any of these effects as he crosses the event horizon. According to his own clock, he crosses the event horizon after a finite time, although he is unable to determine exactly when he crosses it, as it is impossible to determine the location of the event horizon from local observations.[43]

For a non rotating (static) black hole, the Schwarzschild radius delimits a spherical event horizon. The Schwarzschild radius of an object is proportional to the mass.[44] Rotating black holes have distorted, nonspherical event horizons. Since the event horizon is not a material surface but rather merely a mathematically defined demarcation boundary, nothing prevents matter or radiation from entering a black hole, only from exiting one. The description of black holes given by general relativity is known to be an approximation, and some scientists expect that quantum gravity effects will become significant near the vicinity of the event horizon.[45] This would allow observations of matter near a black hole's event horizon to be used to indirectly study general relativity and proposed extensions to it.
Singularity
Main article: Gravitational singularity

At the center of a black hole as described by general relativity lies a gravitational singularity, a region where the spacetime curvature becomes infinite.[46] For a non-rotating black hole this region takes the shape of a single point and for a rotating black hole it is smeared out to form a ring singularity lying in the plane of rotation.[47] In both cases the singular region has zero volume. It can also be shown that the singular region contains all the mass of the black hole solution.[48] The singular region can thus be thought of as having infinite density.

An observer falling into a Schwarzschild black hole (i.e. non-rotating and no charges) cannot avoid the singularity. Any attempt to do so will only shorten the time taken to get there.[49] When they reach the singularity, they are crushed to infinite density and their mass is added to the total of the black hole. Before that happens, they will have been torn apart by the growing tidal forces in a process sometimes referred to as spaghettification or the noodle effect.[50]

In the case of a charged (Reissner–Nordström) or rotating (Kerr) black hole it is possible to avoid the singularity. Extending these solutions as far as possible reveals the hypothetical possibility of exiting the black hole into a different spacetime with the black hole acting as a wormhole.[51] The possibility of traveling to another universe is however only theoretical, since any perturbation will destroy this possibility.[52] It also appears to be possible to follow closed timelike curves (going back to one's own past) around the Kerr singularity, which lead to problems with causality like the grandfather paradox.[53] It is expected that none of these peculiar effects would survive in a proper quantum mechanical treatment of rotating and charged black holes.[54]

The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory.[55] This breakdown, however, is expected; it occurs in a situation where quantum mechanical effects should describe these actions due to the extremely high density and therefore particle interactions. To date it has not been possible to combine quantum and gravitational effects into a single theory. It is generally expected that a theory of quantum gravity will feature black holes without singularities.[56][57]
Photon sphere
Main article: Photon sphere

The photon sphere is a spherical boundary of zero thickness such that photons moving along tangents to the sphere will be trapped in a circular orbit. For non-rotating black holes, the photon sphere has a radius 1.5 times the Schwarzschild radius. The orbits are dynamically unstable, hence any small perturbation (such as a particle of infalling matter) will grow over time, either setting it on an outward trajectory escaping the black hole or on an inward spiral eventually crossing the event horizon.

While light can still escape from inside the photon sphere, any light that crosses the photon sphere on an inbound trajectory will be captured by the black hole. Hence any light reaching an outside observer from inside the photon sphere must have been emitted by objects inside the photon sphere but still outside of the event horizon.

Other compact objects, such as neutron stars, can also have photon spheres.[58] This follows from the fact that the gravitational field of an object does not depend on its actual size, hence any object that is smaller than 1.5 times the Schwarzschild radius corresponding to its mass will indeed have a photon sphere.
Ergosphere
Main article: Ergosphere
The ergosphere is an oblate spheroid region outside of the event horizon, where objects cannot remain stationary.

Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still, called the ergosphere. This is the result of a process known as frame-dragging; general relativity predicts that any rotating mass will tend to slightly "drag" along the spacetime immediately surrounding it. Any object near the rotating mass will tend to start moving in the direction of rotation. For a rotating black hole this effect becomes so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still.[59]

The ergosphere of a black hole is bounded by the (outer) event horizon on the inside and an oblate spheroid, which coincides with the event horizon at the poles and is noticeably wider around the equator. The outer boundary is sometimes called the ergosurface.

Objects and radiation can escape normally from the ergosphere. Through the Penrose process, objects can emerge from the ergosphere with more energy than they entered. This energy is taken from the rotational energy of the black hole causing it to slow down.[60]
Formation and evolution

Considering the exotic nature of black holes, it may be natural to question if such bizarre objects could exist in nature or to suggest that they are merely pathological solutions to Einstein's equations. Einstein himself wrongly thought that black holes would not form, because he held that the angular momentum of collapsing particles would stabilize their motion at some radius.[61] This led the general relativity community to dismiss all results to the contrary for many years. However, a minority of relativists continued to contend that black holes were physical objects,[62] and by the end of the 1960s, they had persuaded the majority of researchers in the field that there is no obstacle to forming an event horizon.

Once an event horizon forms, Roger Penrose proved that a singularity will form somewhere inside it.[23] Shortly afterwards, Stephen Hawking showed that many cosmological solutions describing the Big Bang have singularities without scalar fields or other exotic matter (see Penrose-Hawking singularity theorems). The Kerr solution, the no-hair theorem and the laws of black hole thermodynamics showed that the physical properties of black holes were simple and comprehensible, making them respectable subjects for research.[63] The primary formation process for black holes is expected to be the gravitational collapse of heavy objects such as stars, but there are also more exotic processes that can lead to the production of black holes.
Gravitational collapse
Main article: Gravitational collapse

Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. For stars this usually occurs either because a star has too little "fuel" left to maintain its temperature through stellar nucleosynthesis, or because a star which would have been stable receives extra matter in a way which does not raise its core temperature. In either case the star's temperature is no longer high enough to prevent it from collapsing under its own weight (the ideal gas law explains the connection between pressure, temperature, and volume).[64]

The collapse may be stopped by the degeneracy pressure of the star's constituents, condensing the matter in an exotic denser state. The result is one of the various types of compact star. Which type of compact star is formed depends on the mass of the remnant — the matter left over after changes triggered by the collapse (such as supernova or pulsations leading to a planetary nebula) have blown away the outer layers. Note that this can be substantially less than the original star — remnants exceeding 5 solar masses are produced by stars which were over 20 solar masses before the collapse.[64]

If the mass of the remnant exceeds about 3–4 solar masses (the Tolman–Oppenheimer–Volkoff limit)—either because the original star was very heavy or because the remnant collected additional mass through accretion of matter—even the degeneracy pressure of neutrons is insufficient to stop the collapse. After this, no known mechanism (except possibly quark degeneracy pressure, see quark star) is powerful enough to stop the collapse and the object will inevitably collapse to a black hole.[64]

This gravitational collapse of heavy stars is assumed to be responsible for the formation of stellar mass black holes. Star formation in the young universe may have resulted in very heavy stars, which upon their collapse would have produced black holes of up to 103 solar masses. These heavy black holes could be the seeds of the supermassive black holes found in the centers of most galaxies.[65]

While most of the energy released during gravitational collapse is emitted very quickly, an outside observer does not actually see the end of this process. Even though the collapse takes a finite amount of time from the reference frame of infalling matter, a distant observer sees the infalling material slow and halt just above the event horizon, due to gravitational time dilation. Light from the collapsing material takes longer and longer to reach the observer, with the light emitted just before the event horizon forms delayed an infinite amount of time. Thus the external observer never sees the formation of the event horizon; instead, the collapsing material seems to become dimmer and increasingly red-shifted, eventually fading away.[66]
Primordial black holes in the Big Bang

Gravitational collapse requires great densities. In the current epoch of the universe these high densities are only found in stars, but in the early universe shortly after the big bang densities were much greater, possibly allowing for the creation of black holes. The high density alone is not enough to allow the formation of black holes since a uniform mass distribution will not allow the mass to bunch up. In order for primordial black holes to form in such a dense medium, there must be initial density perturbations which can then grow under their own gravity. Different models for the early universe vary widely in their predictions of the size of these perturbations. Various models predict the creation of black holes, ranging from a Planck mass to hundreds of thousands of solar masses.[67] Primordial black holes could thus account for the creation of any type of black hole.
High-energy collisions
A simulated event in the CMS detector, a collision in which a micro black hole may be created.

Gravitational collapse is not the only process that could create black holes. In principle, black holes could also be created in high-energy collisions that create sufficient density. However, to date, no such events have ever been detected either directly or indirectly as a deficiency of the mass balance in particle accelerator experiments.[68] This suggests that there must be a lower limit for the mass of black holes. Theoretically, this boundary is expected to lie around the Planck mass (mP = √ħc/G ≈ 1.2×1019 GeV/c2 ≈ 2.2×10−8 kg), where quantum effects are expected to make the theory of general relativity break down completely.[69] This would put the creation of black holes firmly out of reach of any high energy process occurring on or near the Earth. Certain developments in quantum gravity however suggest that the Planck mass could be much lower: some braneworld scenarios for example put it much lower, maybe even as low as 1 TeV/c2[70] This would make it possible for micro black holes to be created in the high energy collisions occurring when cosmic rays hit the Earth's atmosphere, or possibly in the new Large Hadron Collider at CERN. These theories are however very speculative, and the creation of black holes in these processes is deemed unlikely by many specialists.[71] Even if such micro black holes should be formed in these collisions, it is expected that they would evaporate in about 10−25 seconds, posing no threat to Earth[72]
Growth

Once a black hole has formed, it can continue to grow by absorbing additional matter. Any black hole will continually absorb gas and interstellar dust from its direct surroundings and omnipresent cosmic background radiation. This is the primary process through which supermassive black holes seem to have grown.[65] A similar process has been suggested for the formation of intermediate-mass black holes in globular clusters.[73]

Another possibility is for a black hole to merge with other objects such as stars or even other black holes. This is thought to have been important especially for the early development of supermassive black holes, which are thought to have formed from the coagulation of many smaller objects.[65] The process has also been proposed as the origin of some intermediate-mass black holes.[74][75]
Evaporation
Main article: Hawking radiation

In 1974, Stephen Hawking showed that black holes are not entirely black but emit small amounts of thermal radiation.[26] He got this result by applying quantum field theory in a static black hole background. The result of his calculations is that a black hole should emit particles in a perfect black body spectrum. This effect has become known as Hawking radiation. Since Hawking's result, many others have verified the effect through various methods.[76] If his theory of black hole radiation is correct, then black holes are expected to emit a thermal spectrum of radiation, and thereby lose mass, because according to the theory of relativity mass is just highly condensed energy (E = mc2).[26] Black holes will shrink and evaporate over time. The temperature of this spectrum (Hawking temperature) is proportional to the surface gravity of the black hole, which for a Schwarzschild black hole is inversely proportional to the mass. Large black holes, therefore, emit less radiation than small black holes.

A stellar black hole of one solar mass has a Hawking temperature of about 100 nanokelvins. This is far less than the 2.7 K temperature of the cosmic microwave background. Stellar mass (and larger) black holes receive more mass from the cosmic microwave background than they emit through Hawking radiation and will thus grow instead of shrink. To have a Hawking temperature larger than 2.7 K (and be able to evaporate), a black hole needs to be lighter than the Moon (and therefore a diameter of less than a tenth of a millimeter).[77]

On the other hand, if a black hole is very small the radiation effects are expected to become very strong. Even a black hole that is heavy compared to a human would evaporate in an instant. A black hole the weight of a car (~10−24 m) would only take a nanosecond to evaporate, during which time it would briefly have a luminosity more than 200 times that of the sun. Lighter black holes are expected to evaporate even faster, for example a black hole of mass 1 TeV/c2 would take less than 10−88 seconds to evaporate completely. Of course, for such a small black hole quantum gravitation effects are expected to play an important role and could even – although current developments in quantum gravity do not indicate so[78] – hypothetically make such a small black hole stable.[79]
Observational evidence

By their very nature, black holes do not directly emit any signals other than the hypothetical Hawking radiation; since the Hawking radiation for an astrophysical black hole is predicted to be very weak, this makes it impossible to directly detect astrophysical black holes from the Earth. A possible exception to the Hawking radiation being weak is the last stage of the evaporation of light (primordial) black holes; searches for such flashes in the past has proven unsuccessful and provides stringent limits on the possibility of existence of light primordial black holes.[80] NASA's Fermi Gamma-ray Space Telescope launched in 2008 will continue the search for these flashes.[81]

Astrophysicists searching for black holes thus have to rely on indirect observations. A black hole's existence can sometimes be inferred by observing its gravitational interactions with its surroundings.
Accretion of matter
See also: Accretion disc
Formation of extragalactic jets from a black hole's accretion disk

Due to conservation of angular momentum, gas falling into the gravitational well created by a massive object will typically form a disc-like structure around the object. Friction within the disc causes angular momentum to be transported outward allowing matter to fall further inward releasing potential energy and increasing the temperature of the gas.[82] In the case of compact objects such as white dwarfs, neutron stars, and black holes, the gas in the inner regions becomes so hot that it will emit vast amounts of radiation (mainly X-rays), which may be detected by telescopes. This process of accretion is one of the most efficient energy producing process known; up to 40% of the rest mass of the accreted material can be emitted in radiation.[82] (In nuclear fusion only about 0.7% of the rest mass will be emitted as energy.) In many cases, accretion discs are accompanied by relativistic jets emitted along the poles, which carry away much of the energy. The mechanism for the creation of these jets is currently not well understood.

As such many of the universe's more energetic phenomena have been attributed to the accretion of matter on black holes. In particular, active galactic nuclei and quasars are thought to be the accretion discs of supermassive black holes.[83] Similarly, X-ray binaries are thought to be binary star systems in which one of the two stars is a compact object accreting matter from its companion.[83] It has also been suggested that some ultraluminous X-ray sources may be the accretion disks of intermediate-mass black holes.[84]
X-ray binaries
See also: X-ray binary

X-ray binaries are binary star systems that are luminous in the X-ray part of the spectrum. These X-ray emissions are generally thought to be caused by one of the component stars being a compact object accreting matter from the other (regular) star. The presence of an ordinary star in such a system provides a unique opportunity for studying the central object and determining if it might be a black hole.
Artist impression of a binary system with an accretion disk around a compact object being fed by material from the companion star.

If such a system emits signals that can be directly traced back to the compact object, it cannot be a black hole. The absence of such a signal does, however, not exclude the possibility that the compact object is a neutron star. By studying the companion star it is often possible to obtain the orbital parameters of the system and obtain an estimate for the mass of the compact object. If this is much larger than the Tolman–Oppenheimer–Volkoff limit (that is, the maximum mass a neutron star can have before collapsing) then the object cannot be a neutron star and is generally expected to be a black hole.[83]

The first strong candidate for a black hole, Cygnus X-1, was discovered in this way by Charles Thomas Bolton[85] and Webster and Murdin[86] in 1972.[87][88] Some doubt, however, remained due to the uncertainties resultant from the companion star being much heavier than the candidate black hole.[83] Currently, better candidates for black holes are found in a class of X-ray binaries called soft X-ray transients.[83] In this class of system the companion star is relatively low mass allowing for more accurate estimates in the black hole mass. Moreover, these systems are only active in X-ray for several months once every 10–50 years. During the period of low X-ray emission (called quiescence), the accretion disc is extremely faint allowing for detailed observation of the companion star during this period. One of the best such candidates is V404 Cyg.
Quiescence and advection-dominated accretion flow

The faintness of the accretion disc during quiescence is thought to be caused by the flow entering a mode called an advection-dominated accretion flow (ADAF). In this mode, almost all the energy generated by friction in the disc is swept along with the flow instead of radiated away. If this model is correct, then it forms strong qualitative evidence for the presence of an event horizon.[89] Because, if the object at the center of the disc had a solid surface, it would emit large amounts of radiation as the highly energetic gas hits the surface, an effect that is observed for neutron stars in a similar state.[82]
Quasi-periodic oscillations
See also: Quasi-periodic oscillations

The X-ray emissions from accretion disks sometimes exhibit a flickering around certain frequencies. These signals are called quasi-periodic oscillations and are thought to be caused by material moving along the inner edge of the accretion disk (the innermost stable circular orbit). As such their frequency is linked to the mass of the compact object. They can thus be used as an alternative way to determine the mass of potential black holes.[90]
Gamma ray bursts

Intense but one-time gamma ray bursts (GRBs) may signal the birth of "new" black holes, because astrophysicists think that GRBs are caused either by the gravitational collapse of giant stars[91] or by collisions between neutron stars,[92] and both types of event involve sufficient mass and pressure to produce black holes. It appears that a collision between a neutron star and a black hole can also cause a GRB,[93] so a GRB is not proof that a "new" black hole has been formed. All known GRBs come from outside our own galaxy, and most come from billions of light years away[94] so the black holes associated with them are billions of years old.
Galactic nuclei
See also: Active galactic nucleus
The jet originating from the center of M87 in this image comes from an active galactic nucleus that may contain a supermassive black hole. Credit: Hubble Space Telescope/NASA/ESA.

It is now widely accepted that the center of every or at least nearly every galaxy contains a supermassive black hole.[95] The close observational correlation between the mass of this hole and the velocity dispersion of the host galaxy's bulge, known as the M-sigma relation, strongly suggests a connection between the formation of the black hole and the galaxy itself. [96]

For decades, astronomers have used the term "active galaxy" to describe galaxies with unusual characteristics, such as unusual spectral line emission and very strong radio emission.[97][98] However, theoretical and observational studies have shown that the active galactic nuclei (AGN) in these galaxies may contain supermassive black holes.[97][98] The models of these AGN consist of a central black hole that may be millions or billions of times more massive than the Sun; a disk of gas and dust called an accretion disk; and two jets that are perpendicular to the accretion disk.[98]

Although supermassive black holes are expected to be found in most AGN, only some galaxies' nuclei have been more carefully studied in attempts to both identify and measure the actual masses of the central supermassive black hole candidates. Some of the most notable galaxies with supermassive black hole candidates include the Andromeda Galaxy, M32, M87, NGC 3115, NGC 3377, NGC 4258, and the Sombrero Galaxy.[99]

Currently, the best evidence for a supermassive black hole comes from studying the proper motion of stars near the center of our own Milky Way.[100] Since 1995 astronomers have tracked the motion of 90 stars in a region called Sagittarius A*. By fitting their motion to Keplerian orbits they were able to infer in 1998 that 2.6 million solar masses must be contained in a volume with a radius of 0.02 lightyears.[101] Since then one of the stars—called S2—has completed a full orbit. From the orbital data they were able to place better constraints on the mass and size of the object causing the orbital motion of stars in the Sagittarius A* region, finding that there is a spherical mass of 4.3 million solar masses contained within a radius of less than 0.002 lightyears.[100] While this is more than 3000 times the Schwarzschild radius corresponding to that mass, it is at least consistent with the central object being a supermassive black hole, and no "realistic cluster [of stars] is physically tenable."[101]
Gravitational lensing
Further information: Gravitational lens

The deformation of spacetime around a massive object causes light rays to be deflected much like light passing through an optic lens. This phenomenon is known as gravitational lensing. Observations have been made of weak gravitational lensing, in which photons are deflected by only a few arcseconds. However, it has never been directly observed for a black hole.[102] One possibility for observing gravitational lensing by a black hole would be to observe stars in orbit around the black hole. There are several candidates for such an observation in orbit around Sagittarius A*.[102]
Alternatives

The evidence for stellar black holes strongly relies on the existence of an upper limit for the mass of a neutron star. The size of this limit heavily depends on the assumptions made about the properties of dense matter. New exotic phases of matter could push up this bound.[83] A phase of free quarks at high density might allow the existence of dense quark stars,[103] and some supersymmetric models predict the existence of Q stars.[104] Some extensions of the standard model posit the existence of preons as fundamental building blocks of quarks and leptons which could hypothetically form preon stars.[105] These hypothetical models could potentially explain a number of observations of stellar black hole candidates. However, it can be shown from general arguments in general relativity that any such object will have a maximum mass.[83]

Since the average density of a black hole inside its Schwarzschild radius is inversely proportional to the square of its mass, supermassive black holes are much less dense than stellar black holes (the average density of a large supermassive black hole is comparable to that of water).[83] Consequently, the physics of matter forming a supermassive black hole is much better understood and the possible alternative explanations for supermassive black hole observations are much more mundane. For example, a supermassive black hole could be modelled by a large cluster of very dark objects. However, typically such alternatives are not stable enough to explain the supermassive black hole candidates.[83]

The evidence for stellar and supermassive black holes implies that in order for black holes not to form, general relativity must fail as a theory of gravity, perhaps due to the onset of quantum mechanical corrections. A much anticipated feature of a theory of quantum gravity is that it will not feature singularities or event horizons (and thus no black holes).[106] In recent years, much attention has been drawn by the fuzzball model in string theory. Based on calculations in specific situations in string theory, the proposal suggest that generically the individual states of a black hole solution do not have an event horizon or singularity (and can thus not really be considered to be a black hole), but that for a distant observer the statistical average of such states does appear just like an ordinary black hole in general relativity.[107]
Open questions
Entropy and thermodynamics
Further information: Black hole thermodynamics
If ultra-high-energy collisions of particles in a particle accelerator can create microscopic black holes, it is expected that all types of particles will be emitted by black hole evaporation, providing key evidence for any grand unified theory. Above are the high energy particles produced in a gold ion collision on the RHIC.

In 1971, Stephen Hawking showed under general conditions[Note 1] that the total area of the event horizons of any collection of classical black holes can never decrease, even if they collide and merge.[108] This result, now known as the second law of black hole mechanics, is remarkably similar to the second law of thermodynamics, which states that the total entropy of a system can never decrease. As with classical objects at absolute zero temperature, it was assumed that black holes had zero entropy. If this were the case, the second law of thermodynamics would be violated by entropy-laden matter entering the black hole, resulting in a decrease of the total entropy of the universe. Therefore, Jacob Bekenstein proposed that a black hole should have an entropy, and that it should be proportional to its horizon area.[109]

The link with the laws of thermodynamics was further strengthened by Hawking's discovery that quantum field theory predicts that a black hole radiates blackbody radiation at a constant temperature. This seemingly causes a violation of the second law of black hole mechanics, since the radiation will carry away energy from the black hole causing it to shrink. The radiation, however also carries away entropy, and it can be proven under general assumptions that the sum of the entropy of the matter surrounding the black hole and one quarter of the area of the horizon as measured in Planck units is in fact always increasing. This allows the formulation of the first law of black hole mechanics as an analogue of the first law of thermodynamics, with the mass acting as energy, the surface gravity as temperature and the area as entropy.[109]

One puzzling feature is that the entropy of a black hole scales with its area rather than with its volume, since entropy is normally an extensive quantity that scales linearly with the volume of the system. This odd property led 't Hooft and Susskind to propose the holographic principle, which suggests that anything that happens in volume of spacetime can be described by data on the boundary of that volume.[110]

Although general relativity can be used to perform a semi-classical calculation of black hole entropy, this situation is theoretically unsatisfying. In statistical mechanics, entropy is understood as counting the number of microscopic configurations of a system which have the same macroscopic qualities (such as mass, charge, pressure, etc.). Without a satisfactory theory of quantum gravity, one cannot perform such a computation for black holes. Some progress has been made in various approaches to quantum gravity. In 1995, Strominger and Vafa showed that counting the microstates of a specific supersymmetric black hole in string theory reproduced the Bekenstein–Hawking entropy.[111] Since then, similar results have been reported for different black holes both in string theory and in other approaches to quantum gravity like loop quantum gravity.[112]
Black hole unitarity
Main article: Black hole information paradox
Unsolved problems in physics Is physical information lost in black holes? Question mark2.svg

An open question in fundamental physics is the so-called information loss paradox, or black hole unitarity paradox. Classically, the laws of physics are the same run forward or in reverse (T-symmetry). Liouville's theorem dictates conservation of phase space volume, which can be thought of as "conservation of information", so there is some problem even in classical physics. In quantum mechanics, this corresponds to a vital property called unitarity, which has to do with the conservation of probability (it can also be thought of as a conservation of quantum phase space volume as expressed by the density matrix).[113]
See also

* Black holes in fiction
* Black string
* Kugelblitz (astrophysics)
* List of black holes
* Susskind-Hawking battle


He1523a.jpg Star portal

* Timeline of black hole physics
* White hole
* Wormhole

Notes

1. ^ In particular, he assumed that all matter satisfies the weak energy condition.

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105. ^ Hansson, J.; Sandin, F. (2005). "Preon stars: a new class of cosmic compact objects". Physics Letters B 616: 1. doi:10.1016/j.physletb.2005.04.034. edit
106. ^ Kiefer, C. (2006). "Quantum gravity: general introduction and recent developments". Annalen der Physik 15: 129–148. doi:10.1002/andp.200510175. edit
107. ^ Skenderis, K.; Taylor, M. (2008). "The fuzzball proposal for black holes". Physics Reports 467: 117. doi:10.1016/j.physrep.2008.08.001. edit
108. ^ Hawking, Stephen (1998). A Brief History of Time. New York: Bantam Books. ISBN 0-553-38016-8. [page needed]
109. ^ a b Wald (1999). "The Thermodynamics of Black Holes". arΧiv:gr-qc/9912119v2 [gr-qc].
110. ^ Gerard 't Hooft (2000). "The Holographic Principle". arΧiv:hep-th/0003004 [hep-th].
111. ^ Strominger, A.; Vafa, C. (1996). "Microscopic origin of the Bekenstein-Hawking entropy". Physics Letters B 379: 99. doi:10.1016/0370-2693(96)00345-0. edit
112. ^ Carlip, S. (2009). "Black Hole Thermodynamics and Statistical Mechanics". Lect.Notes Phys. 769: 89–12. doi:10.1007/978-3-540-88460-6_3. edit
113. ^ Hawking, Stephen. "Does God Play Dice?". http://www.hawking.org.uk/index.php/lectures/publiclectures/64. Retrieved 2009-03-14.

Further reading

Popular reading

* Ferguson, Kitty (1991). Black Holes in Space-Time. Watts Franklin. ISBN 0-531-12524-6. .
* Hawking, Stephen (1988). A Brief History of Time. Bantam Books, Inc. ISBN 0-553-38016-8. .
* Hawking, Stephen; Penrose, Roger (1996). The Nature of Space and Time. Princeton University Press. ISBN 0-691-03791-2. http://books.google.com/?id=LstaQTXP65cC. .
* Melia, Fulvio (2003). The Black Hole at the Center of Our Galaxy. Princeton U Press. ISBN 978-0-691-09505-9. .
* Melia, Fulvio (2003). The Edge of Infinity. Supermassive Black Holes in the Universe. Cambridge U Press. ISBN 978-0-521-81405-8. .
* Pickover, Clifford (1998). Black Holes: A Traveler's Guide. Wiley, John & Sons, Inc. ISBN 0-471-19704-1. .
* Stern, B. (2008). "Blackhole". http://www.wikilivres.info/wiki/Blackhole_%28Stern%29. , poem.
* Thorne, Kip S. (1994). Black Holes and Time Warps. Norton, W. W. & Company, Inc. ISBN 0-393-31276-3. .
* Wheeler, J. Craig (2007). Cosmic Catastrophes (2nd ed.). Cambridge University Press. ISBN 0-521-85714-7.

University textbooks and monographs

* Carroll, Sean M. (2004). Spacetime and Geometry. Addison Wesley. ISBN 0-8053-8732-3. , the lecture notes on which the book was based are available for free from Sean Carroll's website.
* Carter, B. (1973). "Black hole equilibrium states". In DeWitt, B.S.; DeWitt, C.. Black Holes. .
* Chandrasekhar, Subrahmanyan (1999). Mathematical Theory of Black Holes. Oxford University Press. ISBN 0-19-850370-9. .
* Frolov, V.P.; Novikov, I.D. (1998). Black hole physics. .
* Hawking, S.W.; Ellis, G.F.R. (1973). Large Scale Structure of space time. Cambridge University Press. ISBN 0521099064. http://books.google.com/?id=QagG_KI7Ll8C. .
* Melia, Fulvio (2007). The Galactic Supermassive Black Hole. Princeton U Press. ISBN 978-0-691-13129-0. .
* Taylor, Edwin F.; Wheeler, John Archibald (2000). Exploring Black Holes. Addison Wesley Longman. ISBN 0-201-38423-X. .
* Thorne, Kip S.; Misner, Charles; Wheeler, John (1973). Gravitation. W. H. Freeman and Company. ISBN 0-7167-0344-0. .
* Wald, Robert M. (1992). Space, Time, and Gravity: The Theory of the Big Bang and Black Holes. University of Chicago Press. ISBN 0-226-87029-4. .

Review papers

* Detweiler, S. (1981). "Resource letter BH-1: Black holes". American Journal of Physics 49 (5, pp): 394–400. doi:10.1119/1.12686.
* Gallo, E.; Marolf, D. (2009). "Resource Letter BH-2: Black Holes". American Journal of Physics 77: 294. doi:10.1119/1.3056569. arXiv:0806.2316. edit
* Hughes, Scott A. (2005). "Trust but verify: The case for astrophysical black holes". arΧiv:hep-ph/0511217v2 [hep-ph]. Lecture notes from 2005 SLAC Summer Institute.

External links
Wikimedia Commons has media related to: Black holes

* Stanford Encyclopedia of Philosophy: "Singularities and Black Holes" by Erik Curiel and Peter Bokulich.
* "Black hole" on Scholarpedia.
* Black Holes: Gravity's Relentless Pull - Interactive multimedia Web site about the physics and astronomy of black holes from the Space Telescope Science Institute
* FAQ on black holes
* "Schwarzschild Geometry" on Andrew Hamilton’s website
* UT Brownsville Group Simulates Spinning Black-Hole Binaries
* Advanced Mathematics of Black Hole Evaporation

Videos

* 16-year long study tracks stars orbiting Milky Way black hole
* Yale University Video Lecture: Introduction to Black Holes at Google Video.
* Movie of Black Hole Candidate from Max Planck Institute

News

* "Black Hole confirmed in Milky Way." Retrieved December 10, 2008
* Black Hole Research News

[Kool Bob Love]

A black hole is a region of space from which nothing, not even light, can escape. It is the result of the deformation of spacetime caused by a very compact mass. Around a black hole there is an undetectable surface which marks the point of no return, called an event horizon. It is called "black" because it absorbs all the light that hits it, reflecting nothing, just like a perfect black body in thermodynamics.[1] Quantum mechanics predicts that black holes also emit radiation like a black body with a finite temperature. This temperature decreases with the mass of the black hole, making it unlikely to observe this radiation for black holes of stellar mass.

Despite its invisible interior, a black hole can be observed through its interaction with other matter. A black hole can be inferred by tracking the movement of a group of stars that orbit a region in space. Alternatively, when gas falls into a stellar black hole from a companion star, the gas spirals inward, heating to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and Earth-orbiting telescopes.

Astronomers have identified numerous stellar black hole candidates, and have also found evidence of supermassive black holes at the center of galaxies. In 1998, astronomers found compelling evidence that a supermassive black hole of more than 2 million solar masses is located near the Sagittarius A* region in the center of the Milky Way galaxy, and more recent results using additional data find evidence that the supermassive black hole is more than 4 million solar masses.
Contents
[hide]

* 1 History
o 1.1 General relativity
o 1.2 Golden age
* 2 Properties and structure
o 2.1 Physical properties
o 2.2 Event horizon
o 2.3 Singularity
o 2.4 Photon sphere
o 2.5 Ergosphere
* 3 Formation and evolution
o 3.1 Gravitational collapse
+ 3.1.1 Primordial black holes in the Big Bang
o 3.2 High-energy collisions
o 3.3 Growth
o 3.4 Evaporation
* 4 Observational evidence
o 4.1 Accretion of matter
o 4.2 X-ray binaries
+ 4.2.1 Quiescence and advection-dominated accretion flow
+ 4.2.2 Quasi-periodic oscillations
o 4.3 Gamma ray bursts
o 4.4 Galactic nuclei
o 4.5 Gravitational lensing
o 4.6 Alternatives
* 5 Open questions
o 5.1 Entropy and thermodynamics
o 5.2 Black hole unitarity
* 6 See also
* 7 Notes
* 8 References
* 9 Further reading
* 10 External links

History
Schwarzschild black hole
Simulation of gravitational lensing by a black hole which distorts the image of a galaxy in the background (click here for larger animation)

The idea of a body so massive that even light could not escape was first put forward by geologist John Michell in a letter written to Henry Cavendish in 1783 to the Royal Society:

If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae, with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity.
—John Michell[2]

In 1796, mathematician Pierre-Simon Laplace promoted the same idea in the first and second editions of his book Exposition du système du Monde (it was removed from later editions).[3][4] Such "dark stars" were largely ignored in the nineteenth century, since it was not understood how a massless wave such as light could be influenced by gravity.[5]
General relativity

In 1915, Albert Einstein developed his theory of general relativity, having earlier shown that gravity does influence light's motion. A few months later, Karl Schwarzschild gave the solution for the gravitational field of a point mass and a spherical mass.[6] A few months after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution for the point mass and wrote more extensively about its properties.[7] This solution had a peculiar behaviour at what is now called the Schwarzschild radius, where it became singular, meaning that some of the terms in the Einstein equations became infinite. The nature of this surface was not quite understood at the time. In 1924, Arthur Eddington showed that the singularity disappeared after a change of coordinates (see Eddington coordinates), although it took until 1933 for Georges Lemaître to realize that this meant the singularity at the Schwarzschild radius was an unphysical coordinate singularity.[8]

In 1931, Subrahmanyan Chandrasekhar calculated, using general relativity, that a non-rotating body of electron-degenerate matter above 1.44 solar masses (the Chandrasekhar limit) would collapse.[citation needed] His arguments were opposed by many of his contemporaries like Eddington and Lev Landau, who argued that some yet unknown mechanism would stop the collapse.[9] They were partly correct: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star,[10] which is itself stable because of the Pauli exclusion principle. But in 1939, Robert Oppenheimer and others predicted that neutron stars above approximately three solar masses (the Tolman–Oppenheimer–Volkoff limit) would collapse into black holes for the reasons presented by Chandrasekhar, and concluded that no law of physics was likely to intervene and stop at least some stars from collapsing to black holes.[11]

Oppenheimer and his co-authors interpreted the singularity at the boundary of the Schwarzschild radius as indicating that this was the boundary of a bubble in which time stopped. This is a valid point of view for external observers, but not for infalling observers. Because of this property, the collapsed stars were called "frozen stars,"[12] because an outside observer would see the surface of the star frozen in time at the instant where its collapse takes it inside the Schwarzschild radius. This is a known property of modern black holes, but it must be emphasized that the light from the surface of the frozen star becomes redshifted very fast, turning the black hole black very quickly. Many physicists could not accept the idea of time standing still at the Schwarzschild radius, and there was little interest in the subject for over 20 years.
Golden age
See also: Golden age of general relativity

In 1958, David Finkelstein identified the Schwarzschild surface r = 2m [in geometrized units, i.e. 2Gm/c2, where r is the radius of the surface and m is the mass of the black hole] as an event horizon, "a perfect unidirectional membrane: causal influences can cross it in only one direction".[13] This did not strictly contradict Oppenheimer's results, but extended them to include the point of view of infalling observers. Finkelstein's solution extended the Schwarzschild solution for the future of observers falling into the black hole. A complete extension had already been found by Martin Kruskal, who was urged to publish it.[14]

These results came at the beginning of the golden age of general relativity, which is marked by general relativity and black holes becoming mainstream subjects of research. This process was helped by the discovery of pulsars in 1967,[15][16] which were within a few years shown to be rapidly rotating neutron stars. Until that time, neutron stars, like black holes, were regarded as just theoretical curiosities; but the discovery of pulsars showed their physical relevance and spurred a further interest in all types of compact objects that might be formed by gravitational collapse.

In this period more general black hole solutions where found. In 1963, Roy Kerr found the exact solution for a rotating black hole. Two years later Ezra T. Newman found the axisymmetric solution for a black hole which is both rotating and electrically charged.[17] Through the work of Werner Israel,[18] Brandon Carter,[19][20] and D. C. Robinson[21] the no-hair theorem emerged, stating that a stationary black hole solution is completely described by the three parameters of the Kerr–Newman metric; mass, angular momentum, and electric charge.[22]

For a long time, it was suspected that the strange features of the black hole solutions were pathological artefacts from the symmetry conditions imposed, and that the singularities would not appear in generic situations. This view was held in particular by Belinsky, Khalatnikov, and Lifshitz, who tried to prove that no singularities appear in generic solutions. However, in the late sixties Roger Penrose[23] and Stephen Hawking used global techniques to prove that singularities are generic.[24]

Work by James Bardeen, Jacob Bekenstein, Carter, and Hawking in the early 1970s led to the formulation of the laws of black hole mechanics.[25] These laws describe the behaviour of a black hole in close analogy to the laws of thermodynamics by relating mass to energy, area to entropy, and surface gravity to temperature. The analogy was completed when Hawking, in 1974, showed that quantum field theory predicts that black holes should radiate like a black body with a temperature proportional to the surface gravity of the black hole.[26]

The term "black hole" was first publicly used by John Wheeler during a lecture in 1967. Although he is usually credited with coining the phrase, he always insisted that it was suggested to him by somebody else. The first recorded use of the term is in a 1964 letter by Anne Ewing to the American Association for the Advancement of Science.[27] After Wheeler's use of the term, it was quickly adopted in general use.
Properties and structure

The no-hair theorem states that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, charge, and angular momentum.[22] Any two black holes that share the same values for these properties, or parameters, are indistinguishable according to classical (i.e. non-quantum) mechanics.

These properties are special because they are visible from outside the black hole. For example, a charged black hole repels other like charges just like any other charged object. Similarly, the total mass inside a sphere containing a black hole can be found by using the gravitational analog of Gauss's law, the ADM mass, far away from the black hole.[28] Likewise, the angular momentum can be measured from far away using frame dragging by the gravitomagnetic field.

When an object falls into a black hole, any information about the shape of the object or distribution of charge on it is evenly distributed along the horizon of the black hole, and is lost to outside observers. The behavior of the horizon in this situation is closely analogous to that of a conductive stretchy membrane with friction and electrical resistance, a dissipative system (see membrane paradigm).[29] This is different from other field theories like electromagnetism, which does not have any friction or resistivity at the microscopic level, because they are time-reversible. Because the black hole eventually achieves a stable state with only three parameters, there is no way to avoid losing information about the initial conditions: the gravitational and electric fields of the black hole give very little information about what went in. The information that is lost includes every quantity that cannot be measured far away from the black hole horizon, including the total baryon number, lepton number, and all the other nearly conserved pseudo-charges of particle physics. This behavior is so puzzling that it has been called the black hole information loss paradox.[30][31][32]
Physical properties

The simplest black hole has mass but neither electric charge nor angular momentum. These black holes are often referred to as Schwarzschild black holes after the physicist Karl Schwarzschild who discovered this solution in 1915.[6] According to Birkhoff's theorem, it is the only vacuum solution that is spherically symmetric.[33] This means that there is no observable difference between the gravitational field of such a black hole and that of any other spherical object of the same mass. The popular notion of a black hole "sucking in everything" in its surroundings is therefore only correct near the black hole horizon; far away, the external gravitational field is identical to that of any other body of the same mass.[34]

Solutions describing more general black holes also exist. Charged black holes are described by the Reissner-Nordström metric, while the Kerr metric describes a rotating black hole. The most general stationary black hole solution known is the Kerr-Newman metric, which describes a black hole with both charge and angular momentum.

While the mass of a black hole can take any positive value, the charge and angular momentum are constrained by the mass. In Planck units, the total electric charge Q and the total angular momentum J are expected to satisfy

Q^2+\left ( \tfrac{J}{M} \right )^2\le M^2\,

for a black hole of mass M. Black holes saturating this inequality are called extremal. Solutions of Einstein's equations violating the inequality exist, but do not have a horizon. These solutions have naked singularities and are deemed unphysical. The cosmic censorship hypothesis rules out the formation of such singularities through the gravitational collapse of realistic matter.[35] This is supported by numerical simulations.[36]

Due to the relatively large strength of the electromagnetic force, black holes forming from the collapse of stars are expected to retain the nearly neutral charge of the star. Rotation, however, is expected to be a common feature of compact objects, and the black-hole candidate binary X-ray source GRS 1915+105[37] appears to have an angular momentum near the maximum allowed value.
Class Mass Size
Supermassive black hole ~105–109 MSun ~0.001–10 AU
Intermediate-mass black hole ~103 MSun ~103 km = REarth
Stellar black hole ~10 MSun ~30 km
Micro black hole up to ~MMoon up to ~0.1 mm

Black holes are commonly classified according to their mass, independent of angular momentum J or electric charge Q. The size of a black hole, as determined by the radius of the event horizon, or Schwarzschild radius, is roughly proportional to the mass M through

r_\mathrm{sh} =\frac{2GM}{c^2} \approx 2.95\, \frac{M}{M_\mathrm{Sun}}~\mathrm{km,}

where rsh is the Schwarzschild radius and MSun is the mass of the Sun. This relation is exact only for black holes with zero charge and angular momentum, for more general black holes it can differ up to a factor of 2. The table on the right lists the various classes of black hole that are distinguished.
Event horizon
Main article: Event horizon
Image:BH-no-escape-1.svg
Far away from the black hole a particle can move in any direction. It is only restricted by the speed of light.
Image:BH-no-escape-2.svg
Closer to the black hole spacetime starts to deform. There are more paths going towards the black hole than paths moving away.
Image:BH-no-escape-3.svg
Inside of the event horizon all paths bring the particle closer to the center of the black hole. It is no longer possible for the particle to escape.

The defining feature of a black hole is the appearance of an event horizon—a boundary in spacetime through which matter and light can only pass inward towards the mass of the black hole. Nothing, not even light, can escape from inside the event horizon. The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if such an event occurred.[38]

As predicted by general relativity, the presence of a large mass deforms spacetime in such a way that the paths taken by particles bend towards the mass. At the event horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hole.[39]

To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole.[40] Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it.[41] At the same time, all processes on this object slow down causing emitted light to appear redder and dimmer, an effect known as gravitational redshift.[42] Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.

On the other hand, an observer falling into a black hole does not notice any of these effects as he crosses the event horizon. According to his own clock, he crosses the event horizon after a finite time, although he is unable to determine exactly when he crosses it, as it is impossible to determine the location of the event horizon from local observations.[43]

For a non rotating (static) black hole, the Schwarzschild radius delimits a spherical event horizon. The Schwarzschild radius of an object is proportional to the mass.[44] Rotating black holes have distorted, nonspherical event horizons. Since the event horizon is not a material surface but rather merely a mathematically defined demarcation boundary, nothing prevents matter or radiation from entering a black hole, only from exiting one. The description of black holes given by general relativity is known to be an approximation, and some scientists expect that quantum gravity effects will become significant near the vicinity of the event horizon.[45] This would allow observations of matter near a black hole's event horizon to be used to indirectly study general relativity and proposed extensions to it.
Singularity
Main article: Gravitational singularity

At the center of a black hole as described by general relativity lies a gravitational singularity, a region where the spacetime curvature becomes infinite.[46] For a non-rotating black hole this region takes the shape of a single point and for a rotating black hole it is smeared out to form a ring singularity lying in the plane of rotation.[47] In both cases the singular region has zero volume. It can also be shown that the singular region contains all the mass of the black hole solution.[48] The singular region can thus be thought of as having infinite density.

An observer falling into a Schwarzschild black hole (i.e. non-rotating and no charges) cannot avoid the singularity. Any attempt to do so will only shorten the time taken to get there.[49] When they reach the singularity, they are crushed to infinite density and their mass is added to the total of the black hole. Before that happens, they will have been torn apart by the growing tidal forces in a process sometimes referred to as spaghettification or the noodle effect.[50]

In the case of a charged (Reissner–Nordström) or rotating (Kerr) black hole it is possible to avoid the singularity. Extending these solutions as far as possible reveals the hypothetical possibility of exiting the black hole into a different spacetime with the black hole acting as a wormhole.[51] The possibility of traveling to another universe is however only theoretical, since any perturbation will destroy this possibility.[52] It also appears to be possible to follow closed timelike curves (going back to one's own past) around the Kerr singularity, which lead to problems with causality like the grandfather paradox.[53] It is expected that none of these peculiar effects would survive in a proper quantum mechanical treatment of rotating and charged black holes.[54]

The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory.[55] This breakdown, however, is expected; it occurs in a situation where quantum mechanical effects should describe these actions due to the extremely high density and therefore particle interactions. To date it has not been possible to combine quantum and gravitational effects into a single theory. It is generally expected that a theory of quantum gravity will feature black holes without singularities.[56][57]
Photon sphere
Main article: Photon sphere

The photon sphere is a spherical boundary of zero thickness such that photons moving along tangents to the sphere will be trapped in a circular orbit. For non-rotating black holes, the photon sphere has a radius 1.5 times the Schwarzschild radius. The orbits are dynamically unstable, hence any small perturbation (such as a particle of infalling matter) will grow over time, either setting it on an outward trajectory escaping the black hole or on an inward spiral eventually crossing the event horizon.

While light can still escape from inside the photon sphere, any light that crosses the photon sphere on an inbound trajectory will be captured by the black hole. Hence any light reaching an outside observer from inside the photon sphere must have been emitted by objects inside the photon sphere but still outside of the event horizon.

Other compact objects, such as neutron stars, can also have photon spheres.[58] This follows from the fact that the gravitational field of an object does not depend on its actual size, hence any object that is smaller than 1.5 times the Schwarzschild radius corresponding to its mass will indeed have a photon sphere.
Ergosphere
Main article: Ergosphere
The ergosphere is an oblate spheroid region outside of the event horizon, where objects cannot remain stationary.

Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still, called the ergosphere. This is the result of a process known as frame-dragging; general relativity predicts that any rotating mass will tend to slightly "drag" along the spacetime immediately surrounding it. Any object near the rotating mass will tend to start moving in the direction of rotation. For a rotating black hole this effect becomes so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still.[59]

The ergosphere of a black hole is bounded by the (outer) event horizon on the inside and an oblate spheroid, which coincides with the event horizon at the poles and is noticeably wider around the equator. The outer boundary is sometimes called the ergosurface.

Objects and radiation can escape normally from the ergosphere. Through the Penrose process, objects can emerge from the ergosphere with more energy than they entered. This energy is taken from the rotational energy of the black hole causing it to slow down.[60]
Formation and evolution

Considering the exotic nature of black holes, it may be natural to question if such bizarre objects could exist in nature or to suggest that they are merely pathological solutions to Einstein's equations. Einstein himself wrongly thought that black holes would not form, because he held that the angular momentum of collapsing particles would stabilize their motion at some radius.[61] This led the general relativity community to dismiss all results to the contrary for many years. However, a minority of relativists continued to contend that black holes were physical objects,[62] and by the end of the 1960s, they had persuaded the majority of researchers in the field that there is no obstacle to forming an event horizon.

Once an event horizon forms, Roger Penrose proved that a singularity will form somewhere inside it.[23] Shortly afterwards, Stephen Hawking showed that many cosmological solutions describing the Big Bang have singularities without scalar fields or other exotic matter (see Penrose-Hawking singularity theorems). The Kerr solution, the no-hair theorem and the laws of black hole thermodynamics showed that the physical properties of black holes were simple and comprehensible, making them respectable subjects for research.[63] The primary formation process for black holes is expected to be the gravitational collapse of heavy objects such as stars, but there are also more exotic processes that can lead to the production of black holes.
Gravitational collapse
Main article: Gravitational collapse

Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. For stars this usually occurs either because a star has too little "fuel" left to maintain its temperature through stellar nucleosynthesis, or because a star which would have been stable receives extra matter in a way which does not raise its core temperature. In either case the star's temperature is no longer high enough to prevent it from collapsing under its own weight (the ideal gas law explains the connection between pressure, temperature, and volume).[64]

The collapse may be stopped by the degeneracy pressure of the star's constituents, condensing the matter in an exotic denser state. The result is one of the various types of compact star. Which type of compact star is formed depends on the mass of the remnant — the matter left over after changes triggered by the collapse (such as supernova or pulsations leading to a planetary nebula) have blown away the outer layers. Note that this can be substantially less than the original star — remnants exceeding 5 solar masses are produced by stars which were over 20 solar masses before the collapse.[64]

If the mass of the remnant exceeds about 3–4 solar masses (the Tolman–Oppenheimer–Volkoff limit)—either because the original star was very heavy or because the remnant collected additional mass through accretion of matter—even the degeneracy pressure of neutrons is insufficient to stop the collapse. After this, no known mechanism (except possibly quark degeneracy pressure, see quark star) is powerful enough to stop the collapse and the object will inevitably collapse to a black hole.[64]

This gravitational collapse of heavy stars is assumed to be responsible for the formation of stellar mass black holes. Star formation in the young universe may have resulted in very heavy stars, which upon their collapse would have produced black holes of up to 103 solar masses. These heavy black holes could be the seeds of the supermassive black holes found in the centers of most galaxies.[65]

While most of the energy released during gravitational collapse is emitted very quickly, an outside observer does not actually see the end of this process. Even though the collapse takes a finite amount of time from the reference frame of infalling matter, a distant observer sees the infalling material slow and halt just above the event horizon, due to gravitational time dilation. Light from the collapsing material takes longer and longer to reach the observer, with the light emitted just before the event horizon forms delayed an infinite amount of time. Thus the external observer never sees the formation of the event horizon; instead, the collapsing material seems to become dimmer and increasingly red-shifted, eventually fading away.[66]
Primordial black holes in the Big Bang

Gravitational collapse requires great densities. In the current epoch of the universe these high densities are only found in stars, but in the early universe shortly after the big bang densities were much greater, possibly allowing for the creation of black holes. The high density alone is not enough to allow the formation of black holes since a uniform mass distribution will not allow the mass to bunch up. In order for primordial black holes to form in such a dense medium, there must be initial density perturbations which can then grow under their own gravity. Different models for the early universe vary widely in their predictions of the size of these perturbations. Various models predict the creation of black holes, ranging from a Planck mass to hundreds of thousands of solar masses.[67] Primordial black holes could thus account for the creation of any type of black hole.
High-energy collisions
A simulated event in the CMS detector, a collision in which a micro black hole may be created.

Gravitational collapse is not the only process that could create black holes. In principle, black holes could also be created in high-energy collisions that create sufficient density. However, to date, no such events have ever been detected either directly or indirectly as a deficiency of the mass balance in particle accelerator experiments.[68] This suggests that there must be a lower limit for the mass of black holes. Theoretically, this boundary is expected to lie around the Planck mass (mP = √ħc/G ≈ 1.2×1019 GeV/c2 ≈ 2.2×10−8 kg), where quantum effects are expected to make the theory of general relativity break down completely.[69] This would put the creation of black holes firmly out of reach of any high energy process occurring on or near the Earth. Certain developments in quantum gravity however suggest that the Planck mass could be much lower: some braneworld scenarios for example put it much lower, maybe even as low as 1 TeV/c2[70] This would make it possible for micro black holes to be created in the high energy collisions occurring when cosmic rays hit the Earth's atmosphere, or possibly in the new Large Hadron Collider at CERN. These theories are however very speculative, and the creation of black holes in these processes is deemed unlikely by many specialists.[71] Even if such micro black holes should be formed in these collisions, it is expected that they would evaporate in about 10−25 seconds, posing no threat to Earth[72]
Growth

Once a black hole has formed, it can continue to grow by absorbing additional matter. Any black hole will continually absorb gas and interstellar dust from its direct surroundings and omnipresent cosmic background radiation. This is the primary process through which supermassive black holes seem to have grown.[65] A similar process has been suggested for the formation of intermediate-mass black holes in globular clusters.[73]

Another possibility is for a black hole to merge with other objects such as stars or even other black holes. This is thought to have been important especially for the early development of supermassive black holes, which are thought to have formed from the coagulation of many smaller objects.[65] The process has also been proposed as the origin of some intermediate-mass black holes.[74][75]
Evaporation
Main article: Hawking radiation

In 1974, Stephen Hawking showed that black holes are not entirely black but emit small amounts of thermal radiation.[26] He got this result by applying quantum field theory in a static black hole background. The result of his calculations is that a black hole should emit particles in a perfect black body spectrum. This effect has become known as Hawking radiation. Since Hawking's result, many others have verified the effect through various methods.[76] If his theory of black hole radiation is correct, then black holes are expected to emit a thermal spectrum of radiation, and thereby lose mass, because according to the theory of relativity mass is just highly condensed energy (E = mc2).[26] Black holes will shrink and evaporate over time. The temperature of this spectrum (Hawking temperature) is proportional to the surface gravity of the black hole, which for a Schwarzschild black hole is inversely proportional to the mass. Large black holes, therefore, emit less radiation than small black holes.

A stellar black hole of one solar mass has a Hawking temperature of about 100 nanokelvins. This is far less than the 2.7 K temperature of the cosmic microwave background. Stellar mass (and larger) black holes receive more mass from the cosmic microwave background than they emit through Hawking radiation and will thus grow instead of shrink. To have a Hawking temperature larger than 2.7 K (and be able to evaporate), a black hole needs to be lighter than the Moon (and therefore a diameter of less than a tenth of a millimeter).[77]

On the other hand, if a black hole is very small the radiation effects are expected to become very strong. Even a black hole that is heavy compared to a human would evaporate in an instant. A black hole the weight of a car (~10−24 m) would only take a nanosecond to evaporate, during which time it would briefly have a luminosity more than 200 times that of the sun. Lighter black holes are expected to evaporate even faster, for example a black hole of mass 1 TeV/c2 would take less than 10−88 seconds to evaporate completely. Of course, for such a small black hole quantum gravitation effects are expected to play an important role and could even – although current developments in quantum gravity do not indicate so[78] – hypothetically make such a small black hole stable.[79]
Observational evidence

By their very nature, black holes do not directly emit any signals other than the hypothetical Hawking radiation; since the Hawking radiation for an astrophysical black hole is predicted to be very weak, this makes it impossible to directly detect astrophysical black holes from the Earth. A possible exception to the Hawking radiation being weak is the last stage of the evaporation of light (primordial) black holes; searches for such flashes in the past has proven unsuccessful and provides stringent limits on the possibility of existence of light primordial black holes.[80] NASA's Fermi Gamma-ray Space Telescope launched in 2008 will continue the search for these flashes.[81]

Astrophysicists searching for black holes thus have to rely on indirect observations. A black hole's existence can sometimes be inferred by observing its gravitational interactions with its surroundings.
Accretion of matter
See also: Accretion disc
Formation of extragalactic jets from a black hole's accretion disk

Due to conservation of angular momentum, gas falling into the gravitational well created by a massive object will typically form a disc-like structure around the object. Friction within the disc causes angular momentum to be transported outward allowing matter to fall further inward releasing potential energy and increasing the temperature of the gas.[82] In the case of compact objects such as white dwarfs, neutron stars, and black holes, the gas in the inner regions becomes so hot that it will emit vast amounts of radiation (mainly X-rays), which may be detected by telescopes. This process of accretion is one of the most efficient energy producing process known; up to 40% of the rest mass of the accreted material can be emitted in radiation.[82] (In nuclear fusion only about 0.7% of the rest mass will be emitted as energy.) In many cases, accretion discs are accompanied by relativistic jets emitted along the poles, which carry away much of the energy. The mechanism for the creation of these jets is currently not well understood.

As such many of the universe's more energetic phenomena have been attributed to the accretion of matter on black holes. In particular, active galactic nuclei and quasars are thought to be the accretion discs of supermassive black holes.[83] Similarly, X-ray binaries are thought to be binary star systems in which one of the two stars is a compact object accreting matter from its companion.[83] It has also been suggested that some ultraluminous X-ray sources may be the accretion disks of intermediate-mass black holes.[84]
X-ray binaries
See also: X-ray binary

X-ray binaries are binary star systems that are luminous in the X-ray part of the spectrum. These X-ray emissions are generally thought to be caused by one of the component stars being a compact object accreting matter from the other (regular) star. The presence of an ordinary star in such a system provides a unique opportunity for studying the central object and determining if it might be a black hole.
Artist impression of a binary system with an accretion disk around a compact object being fed by material from the companion star.

If such a system emits signals that can be directly traced back to the compact object, it cannot be a black hole. The absence of such a signal does, however, not exclude the possibility that the compact object is a neutron star. By studying the companion star it is often possible to obtain the orbital parameters of the system and obtain an estimate for the mass of the compact object. If this is much larger than the Tolman–Oppenheimer–Volkoff limit (that is, the maximum mass a neutron star can have before collapsing) then the object cannot be a neutron star and is generally expected to be a black hole.[83]

The first strong candidate for a black hole, Cygnus X-1, was discovered in this way by Charles Thomas Bolton[85] and Webster and Murdin[86] in 1972.[87][88] Some doubt, however, remained due to the uncertainties resultant from the companion star being much heavier than the candidate black hole.[83] Currently, better candidates for black holes are found in a class of X-ray binaries called soft X-ray transients.[83] In this class of system the companion star is relatively low mass allowing for more accurate estimates in the black hole mass. Moreover, these systems are only active in X-ray for several months once every 10–50 years. During the period of low X-ray emission (called quiescence), the accretion disc is extremely faint allowing for detailed observation of the companion star during this period. One of the best such candidates is V404 Cyg.
Quiescence and advection-dominated accretion flow

The faintness of the accretion disc during quiescence is thought to be caused by the flow entering a mode called an advection-dominated accretion flow (ADAF). In this mode, almost all the energy generated by friction in the disc is swept along with the flow instead of radiated away. If this model is correct, then it forms strong qualitative evidence for the presence of an event horizon.[89] Because, if the object at the center of the disc had a solid surface, it would emit large amounts of radiation as the highly energetic gas hits the surface, an effect that is observed for neutron stars in a similar state.[82]
Quasi-periodic oscillations
See also: Quasi-periodic oscillations

The X-ray emissions from accretion disks sometimes exhibit a flickering around certain frequencies. These signals are called quasi-periodic oscillations and are thought to be caused by material moving along the inner edge of the accretion disk (the innermost stable circular orbit). As such their frequency is linked to the mass of the compact object. They can thus be used as an alternative way to determine the mass of potential black holes.[90]
Gamma ray bursts

Intense but one-time gamma ray bursts (GRBs) may signal the birth of "new" black holes, because astrophysicists think that GRBs are caused either by the gravitational collapse of giant stars[91] or by collisions between neutron stars,[92] and both types of event involve sufficient mass and pressure to produce black holes. It appears that a collision between a neutron star and a black hole can also cause a GRB,[93] so a GRB is not proof that a "new" black hole has been formed. All known GRBs come from outside our own galaxy, and most come from billions of light years away[94] so the black holes associated with them are billions of years old.
Galactic nuclei
See also: Active galactic nucleus
The jet originating from the center of M87 in this image comes from an active galactic nucleus that may contain a supermassive black hole. Credit: Hubble Space Telescope/NASA/ESA.

It is now widely accepted that the center of every or at least nearly every galaxy contains a supermassive black hole.[95] The close observational correlation between the mass of this hole and the velocity dispersion of the host galaxy's bulge, known as the M-sigma relation, strongly suggests a connection between the formation of the black hole and the galaxy itself. [96]

For decades, astronomers have used the term "active galaxy" to describe galaxies with unusual characteristics, such as unusual spectral line emission and very strong radio emission.[97][98] However, theoretical and observational studies have shown that the active galactic nuclei (AGN) in these galaxies may contain supermassive black holes.[97][98] The models of these AGN consist of a central black hole that may be millions or billions of times more massive than the Sun; a disk of gas and dust called an accretion disk; and two jets that are perpendicular to the accretion disk.[98]

Although supermassive black holes are expected to be found in most AGN, only some galaxies' nuclei have been more carefully studied in attempts to both identify and measure the actual masses of the central supermassive black hole candidates. Some of the most notable galaxies with supermassive black hole candidates include the Andromeda Galaxy, M32, M87, NGC 3115, NGC 3377, NGC 4258, and the Sombrero Galaxy.[99]

Currently, the best evidence for a supermassive black hole comes from studying the proper motion of stars near the center of our own Milky Way.[100] Since 1995 astronomers have tracked the motion of 90 stars in a region called Sagittarius A*. By fitting their motion to Keplerian orbits they were able to infer in 1998 that 2.6 million solar masses must be contained in a volume with a radius of 0.02 lightyears.[101] Since then one of the stars—called S2—has completed a full orbit. From the orbital data they were able to place better constraints on the mass and size of the object causing the orbital motion of stars in the Sagittarius A* region, finding that there is a spherical mass of 4.3 million solar masses contained within a radius of less than 0.002 lightyears.[100] While this is more than 3000 times the Schwarzschild radius corresponding to that mass, it is at least consistent with the central object being a supermassive black hole, and no "realistic cluster [of stars] is physically tenable."[101]
Gravitational lensing
Further information: Gravitational lens

The deformation of spacetime around a massive object causes light rays to be deflected much like light passing through an optic lens. This phenomenon is known as gravitational lensing. Observations have been made of weak gravitational lensing, in which photons are deflected by only a few arcseconds. However, it has never been directly observed for a black hole.[102] One possibility for observing gravitational lensing by a black hole would be to observe stars in orbit around the black hole. There are several candidates for such an observation in orbit around Sagittarius A*.[102]
Alternatives

The evidence for stellar black holes strongly relies on the existence of an upper limit for the mass of a neutron star. The size of this limit heavily depends on the assumptions made about the properties of dense matter. New exotic phases of matter could push up this bound.[83] A phase of free quarks at high density might allow the existence of dense quark stars,[103] and some supersymmetric models predict the existence of Q stars.[104] Some extensions of the standard model posit the existence of preons as fundamental building blocks of quarks and leptons which could hypothetically form preon stars.[105] These hypothetical models could potentially explain a number of observations of stellar black hole candidates. However, it can be shown from general arguments in general relativity that any such object will have a maximum mass.[83]

Since the average density of a black hole inside its Schwarzschild radius is inversely proportional to the square of its mass, supermassive black holes are much less dense than stellar black holes (the average density of a large supermassive black hole is comparable to that of water).[83] Consequently, the physics of matter forming a supermassive black hole is much better understood and the possible alternative explanations for supermassive black hole observations are much more mundane. For example, a supermassive black hole could be modelled by a large cluster of very dark objects. However, typically such alternatives are not stable enough to explain the supermassive black hole candidates.[83]

The evidence for stellar and supermassive black holes implies that in order for black holes not to form, general relativity must fail as a theory of gravity, perhaps due to the onset of quantum mechanical corrections. A much anticipated feature of a theory of quantum gravity is that it will not feature singularities or event horizons (and thus no black holes).[106] In recent years, much attention has been drawn by the fuzzball model in string theory. Based on calculations in specific situations in string theory, the proposal suggest that generically the individual states of a black hole solution do not have an event horizon or singularity (and can thus not really be considered to be a black hole), but that for a distant observer the statistical average of such states does appear just like an ordinary black hole in general relativity.[107]
Open questions
Entropy and thermodynamics
Further information: Black hole thermodynamics
If ultra-high-energy collisions of particles in a particle accelerator can create microscopic black holes, it is expected that all types of particles will be emitted by black hole evaporation, providing key evidence for any grand unified theory. Above are the high energy particles produced in a gold ion collision on the RHIC.

In 1971, Stephen Hawking showed under general conditions[Note 1] that the total area of the event horizons of any collection of classical black holes can never decrease, even if they collide and merge.[108] This result, now known as the second law of black hole mechanics, is remarkably similar to the second law of thermodynamics, which states that the total entropy of a system can never decrease. As with classical objects at absolute zero temperature, it was assumed that black holes had zero entropy. If this were the case, the second law of thermodynamics would be violated by entropy-laden matter entering the black hole, resulting in a decrease of the total entropy of the universe. Therefore, Jacob Bekenstein proposed that a black hole should have an entropy, and that it should be proportional to its horizon area.[109]

The link with the laws of thermodynamics was further strengthened by Hawking's discovery that quantum field theory predicts that a black hole radiates blackbody radiation at a constant temperature. This seemingly causes a violation of the second law of black hole mechanics, since the radiation will carry away energy from the black hole causing it to shrink. The radiation, however also carries away entropy, and it can be proven under general assumptions that the sum of the entropy of the matter surrounding the black hole and one quarter of the area of the horizon as measured in Planck units is in fact always increasing. This allows the formulation of the first law of black hole mechanics as an analogue of the first law of thermodynamics, with the mass acting as energy, the surface gravity as temperature and the area as entropy.[109]

One puzzling feature is that the entropy of a black hole scales with its area rather than with its volume, since entropy is normally an extensive quantity that scales linearly with the volume of the system. This odd property led 't Hooft and Susskind to propose the holographic principle, which suggests that anything that happens in volume of spacetime can be described by data on the boundary of that volume.[110]

Although general relativity can be used to perform a semi-classical calculation of black hole entropy, this situation is theoretically unsatisfying. In statistical mechanics, entropy is understood as counting the number of microscopic configurations of a system which have the same macroscopic qualities (such as mass, charge, pressure, etc.). Without a satisfactory theory of quantum gravity, one cannot perform such a computation for black holes. Some progress has been made in various approaches to quantum gravity. In 1995, Strominger and Vafa showed that counting the microstates of a specific supersymmetric black hole in string theory reproduced the Bekenstein–Hawking entropy.[111] Since then, similar results have been reported for different black holes both in string theory and in other approaches to quantum gravity like loop quantum gravity.[112]
Black hole unitarity
Main article: Black hole information paradox
Unsolved problems in physics Is physical information lost in black holes? Question mark2.svg

An open question in fundamental physics is the so-called information loss paradox, or black hole unitarity paradox. Classically, the laws of physics are the same run forward or in reverse (T-symmetry). Liouville's theorem dictates conservation of phase space volume, which can be thought of as "conservation of information", so there is some problem even in classical physics. In quantum mechanics, this corresponds to a vital property called unitarity, which has to do with the conservation of probability (it can also be thought of as a conservation of quantum phase space volume as expressed by the density matrix).[113]
See also

* Black holes in fiction
* Black string
* Kugelblitz (astrophysics)
* List of black holes
* Susskind-Hawking battle


He1523a.jpg Star portal

* Timeline of black hole physics
* White hole
* Wormhole

Notes

1. ^ In particular, he assumed that all matter satisfies the weak energy condition.

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Further reading

Popular reading

* Ferguson, Kitty (1991). Black Holes in Space-Time. Watts Franklin. ISBN 0-531-12524-6. .
* Hawking, Stephen (1988). A Brief History of Time. Bantam Books, Inc. ISBN 0-553-38016-8. .
* Hawking, Stephen; Penrose, Roger (1996). The Nature of Space and Time. Princeton University Press. ISBN 0-691-03791-2. http://books.google.com/?id=LstaQTXP65cC. .
* Melia, Fulvio (2003). The Black Hole at the Center of Our Galaxy. Princeton U Press. ISBN 978-0-691-09505-9. .
* Melia, Fulvio (2003). The Edge of Infinity. Supermassive Black Holes in the Universe. Cambridge U Press. ISBN 978-0-521-81405-8. .
* Pickover, Clifford (1998). Black Holes: A Traveler's Guide. Wiley, John & Sons, Inc. ISBN 0-471-19704-1. .
* Stern, B. (2008). "Blackhole". http://www.wikilivres.info/wiki/Blackhole_%28Stern%29. , poem.
* Thorne, Kip S. (1994). Black Holes and Time Warps. Norton, W. W. & Company, Inc. ISBN 0-393-31276-3. .
* Wheeler, J. Craig (2007). Cosmic Catastrophes (2nd ed.). Cambridge University Press. ISBN 0-521-85714-7.

University textbooks and monographs

* Carroll, Sean M. (2004). Spacetime and Geometry. Addison Wesley. ISBN 0-8053-8732-3. , the lecture notes on which the book was based are available for free from Sean Carroll's website.
* Carter, B. (1973). "Black hole equilibrium states". In DeWitt, B.S.; DeWitt, C.. Black Holes. .
* Chandrasekhar, Subrahmanyan (1999). Mathematical Theory of Black Holes. Oxford University Press. ISBN 0-19-850370-9. .
* Frolov, V.P.; Novikov, I.D. (1998). Black hole physics. .
* Hawking, S.W.; Ellis, G.F.R. (1973). Large Scale Structure of space time. Cambridge University Press. ISBN 0521099064. http://books.google.com/?id=QagG_KI7Ll8C. .
* Melia, Fulvio (2007). The Galactic Supermassive Black Hole. Princeton U Press. ISBN 978-0-691-13129-0. .
* Taylor, Edwin F.; Wheeler, John Archibald (2000). Exploring Black Holes. Addison Wesley Longman. ISBN 0-201-38423-X. .
* Thorne, Kip S.; Misner, Charles; Wheeler, John (1973). Gravitation. W. H. Freeman and Company. ISBN 0-7167-0344-0. .
* Wald, Robert M. (1992). Space, Time, and Gravity: The Theory of the Big Bang and Black Holes. University of Chicago Press. ISBN 0-226-87029-4. .

Review papers

* Detweiler, S. (1981). "Resource letter BH-1: Black holes". American Journal of Physics 49 (5, pp): 394–400. doi:10.1119/1.12686.
* Gallo, E.; Marolf, D. (2009). "Resource Letter BH-2: Black Holes". American Journal of Physics 77: 294. doi:10.1119/1.3056569. arXiv:0806.2316. edit
* Hughes, Scott A. (2005). "Trust but verify: The case for astrophysical black holes". arΧiv:hep-ph/0511217v2 [hep-ph]. Lecture notes from 2005 SLAC Summer Institute.

External links
Wikimedia Commons has media related to: Black holes

* Stanford Encyclopedia of Philosophy: "Singularities and Black Holes" by Erik Curiel and Peter Bokulich.
* "Black hole" on Scholarpedia.
* Black Holes: Gravity's Relentless Pull - Interactive multimedia Web site about the physics and astronomy of black holes from the Space Telescope Science Institute
* FAQ on black holes
* "Schwarzschild Geometry" on Andrew Hamilton’s website
* UT Brownsville Group Simulates Spinning Black-Hole Binaries
* Advanced Mathematics of Black Hole Evaporation

Videos

* 16-year long study tracks stars orbiting Milky Way black hole
* Yale University Video Lecture: Introduction to Black Holes at Google Video.
* Movie of Black Hole Candidate from Max Planck Institute

News

* "Black Hole confirmed in Milky Way." Retrieved December 10, 2008
* Black Hole Research News

[polandprzem]

I've read all the thread and i must to say that I do agree with some point and disagree with another

[Kool Bob Love]

I've read all the thread and i must to say that I do agree with some point and disagree with another


Same.:wakeup

[Agloco]

A black hole is a region of space from which nothing, not even light, can escape. It is the result of the deformation of spacetime caused by a very compact mass. Around a black hole there is an undetectable surface which marks the point of no return, called an event horizon. It is called "black" because it absorbs all the light that hits it, reflecting nothing, just like a perfect black body in thermodynamics.[1] Quantum mechanics predicts that black holes also emit radiation like a black body with a finite temperature. This temperature decreases with the mass of the black hole, making it unlikely to observe this radiation for black holes of stellar mass.

Despite its invisible interior, a black hole can be observed through its interaction with other matter. A black hole can be inferred by tracking the movement of a group of stars that orbit a region in space. Alternatively, when gas falls into a stellar black hole from a companion star, the gas spirals inward, heating to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and Earth-orbiting telescopes.

Astronomers have identified numerous stellar black hole candidates, and have also found evidence of supermassive black holes at the center of galaxies. In 1998, astronomers found compelling evidence that a supermassive black hole of more than 2 million solar masses is located near the Sagittarius A* region in the center of the Milky Way galaxy, and more recent results using additional data find evidence that the supermassive black hole is more than 4 million solar masses.
Contents
[hide]

* 1 History
o 1.1 General relativity
o 1.2 Golden age
* 2 Properties and structure
o 2.1 Physical properties
o 2.2 Event horizon
o 2.3 Singularity
o 2.4 Photon sphere
o 2.5 Ergosphere
* 3 Formation and evolution
o 3.1 Gravitational collapse
+ 3.1.1 Primordial black holes in the Big Bang
o 3.2 High-energy collisions
o 3.3 Growth
o 3.4 Evaporation
* 4 Observational evidence
o 4.1 Accretion of matter
o 4.2 X-ray binaries
+ 4.2.1 Quiescence and advection-dominated accretion flow
+ 4.2.2 Quasi-periodic oscillations
o 4.3 Gamma ray bursts
o 4.4 Galactic nuclei
o 4.5 Gravitational lensing
o 4.6 Alternatives
* 5 Open questions
o 5.1 Entropy and thermodynamics
o 5.2 Black hole unitarity
* 6 See also
* 7 Notes
* 8 References
* 9 Further reading
* 10 External links

History
Schwarzschild black hole
Simulation of gravitational lensing by a black hole which distorts the image of a galaxy in the background (click here for larger animation)

The idea of a body so massive that even light could not escape was first put forward by geologist John Michell in a letter written to Henry Cavendish in 1783 to the Royal Society:

If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae, with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity.
—John Michell[2]

In 1796, mathematician Pierre-Simon Laplace promoted the same idea in the first and second editions of his book Exposition du système du Monde (it was removed from later editions).[3][4] Such "dark stars" were largely ignored in the nineteenth century, since it was not understood how a massless wave such as light could be influenced by gravity.[5]
General relativity

In 1915, Albert Einstein developed his theory of general relativity, having earlier shown that gravity does influence light's motion. A few months later, Karl Schwarzschild gave the solution for the gravitational field of a point mass and a spherical mass.[6] A few months after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution for the point mass and wrote more extensively about its properties.[7] This solution had a peculiar behaviour at what is now called the Schwarzschild radius, where it became singular, meaning that some of the terms in the Einstein equations became infinite. The nature of this surface was not quite understood at the time. In 1924, Arthur Eddington showed that the singularity disappeared after a change of coordinates (see Eddington coordinates), although it took until 1933 for Georges Lemaître to realize that this meant the singularity at the Schwarzschild radius was an unphysical coordinate singularity.[8]

In 1931, Subrahmanyan Chandrasekhar calculated, using general relativity, that a non-rotating body of electron-degenerate matter above 1.44 solar masses (the Chandrasekhar limit) would collapse.[citation needed] His arguments were opposed by many of his contemporaries like Eddington and Lev Landau, who argued that some yet unknown mechanism would stop the collapse.[9] They were partly correct: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star,[10] which is itself stable because of the Pauli exclusion principle. But in 1939, Robert Oppenheimer and others predicted that neutron stars above approximately three solar masses (the Tolman–Oppenheimer–Volkoff limit) would collapse into black holes for the reasons presented by Chandrasekhar, and concluded that no law of physics was likely to intervene and stop at least some stars from collapsing to black holes.[11]

Oppenheimer and his co-authors interpreted the singularity at the boundary of the Schwarzschild radius as indicating that this was the boundary of a bubble in which time stopped. This is a valid point of view for external observers, but not for infalling observers. Because of this property, the collapsed stars were called "frozen stars,"[12] because an outside observer would see the surface of the star frozen in time at the instant where its collapse takes it inside the Schwarzschild radius. This is a known property of modern black holes, but it must be emphasized that the light from the surface of the frozen star becomes redshifted very fast, turning the black hole black very quickly. Many physicists could not accept the idea of time standing still at the Schwarzschild radius, and there was little interest in the subject for over 20 years.
Golden age
See also: Golden age of general relativity

In 1958, David Finkelstein identified the Schwarzschild surface r = 2m [in geometrized units, i.e. 2Gm/c2, where r is the radius of the surface and m is the mass of the black hole] as an event horizon, "a perfect unidirectional membrane: causal influences can cross it in only one direction".[13] This did not strictly contradict Oppenheimer's results, but extended them to include the point of view of infalling observers. Finkelstein's solution extended the Schwarzschild solution for the future of observers falling into the black hole. A complete extension had already been found by Martin Kruskal, who was urged to publish it.[14]

These results came at the beginning of the golden age of general relativity, which is marked by general relativity and black holes becoming mainstream subjects of research. This process was helped by the discovery of pulsars in 1967,[15][16] which were within a few years shown to be rapidly rotating neutron stars. Until that time, neutron stars, like black holes, were regarded as just theoretical curiosities; but the discovery of pulsars showed their physical relevance and spurred a further interest in all types of compact objects that might be formed by gravitational collapse.

In this period more general black hole solutions where found. In 1963, Roy Kerr found the exact solution for a rotating black hole. Two years later Ezra T. Newman found the axisymmetric solution for a black hole which is both rotating and electrically charged.[17] Through the work of Werner Israel,[18] Brandon Carter,[19][20] and D. C. Robinson[21] the no-hair theorem emerged, stating that a stationary black hole solution is completely described by the three parameters of the Kerr–Newman metric; mass, angular momentum, and electric charge.[22]

For a long time, it was suspected that the strange features of the black hole solutions were pathological artefacts from the symmetry conditions imposed, and that the singularities would not appear in generic situations. This view was held in particular by Belinsky, Khalatnikov, and Lifshitz, who tried to prove that no singularities appear in generic solutions. However, in the late sixties Roger Penrose[23] and Stephen Hawking used global techniques to prove that singularities are generic.[24]

Work by James Bardeen, Jacob Bekenstein, Carter, and Hawking in the early 1970s led to the formulation of the laws of black hole mechanics.[25] These laws describe the behaviour of a black hole in close analogy to the laws of thermodynamics by relating mass to energy, area to entropy, and surface gravity to temperature. The analogy was completed when Hawking, in 1974, showed that quantum field theory predicts that black holes should radiate like a black body with a temperature proportional to the surface gravity of the black hole.[26]

The term "black hole" was first publicly used by John Wheeler during a lecture in 1967. Although he is usually credited with coining the phrase, he always insisted that it was suggested to him by somebody else. The first recorded use of the term is in a 1964 letter by Anne Ewing to the American Association for the Advancement of Science.[27] After Wheeler's use of the term, it was quickly adopted in general use.
Properties and structure

The no-hair theorem states that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, charge, and angular momentum.[22] Any two black holes that share the same values for these properties, or parameters, are indistinguishable according to classical (i.e. non-quantum) mechanics.

These properties are special because they are visible from outside the black hole. For example, a charged black hole repels other like charges just like any other charged object. Similarly, the total mass inside a sphere containing a black hole can be found by using the gravitational analog of Gauss's law, the ADM mass, far away from the black hole.[28] Likewise, the angular momentum can be measured from far away using frame dragging by the gravitomagnetic field.

When an object falls into a black hole, any information about the shape of the object or distribution of charge on it is evenly distributed along the horizon of the black hole, and is lost to outside observers. The behavior of the horizon in this situation is closely analogous to that of a conductive stretchy membrane with friction and electrical resistance, a dissipative system (see membrane paradigm).[29] This is different from other field theories like electromagnetism, which does not have any friction or resistivity at the microscopic level, because they are time-reversible. Because the black hole eventually achieves a stable state with only three parameters, there is no way to avoid losing information about the initial conditions: the gravitational and electric fields of the black hole give very little information about what went in. The information that is lost includes every quantity that cannot be measured far away from the black hole horizon, including the total baryon number, lepton number, and all the other nearly conserved pseudo-charges of particle physics. This behavior is so puzzling that it has been called the black hole information loss paradox.[30][31][32]
Physical properties

The simplest black hole has mass but neither electric charge nor angular momentum. These black holes are often referred to as Schwarzschild black holes after the physicist Karl Schwarzschild who discovered this solution in 1915.[6] According to Birkhoff's theorem, it is the only vacuum solution that is spherically symmetric.[33] This means that there is no observable difference between the gravitational field of such a black hole and that of any other spherical object of the same mass. The popular notion of a black hole "sucking in everything" in its surroundings is therefore only correct near the black hole horizon; far away, the external gravitational field is identical to that of any other body of the same mass.[34]

Solutions describing more general black holes also exist. Charged black holes are described by the Reissner-Nordström metric, while the Kerr metric describes a rotating black hole. The most general stationary black hole solution known is the Kerr-Newman metric, which describes a black hole with both charge and angular momentum.

While the mass of a black hole can take any positive value, the charge and angular momentum are constrained by the mass. In Planck units, the total electric charge Q and the total angular momentum J are expected to satisfy

Q^2+\left ( \tfrac{J}{M} \right )^2\le M^2\,

for a black hole of mass M. Black holes saturating this inequality are called extremal. Solutions of Einstein's equations violating the inequality exist, but do not have a horizon. These solutions have naked singularities and are deemed unphysical. The cosmic censorship hypothesis rules out the formation of such singularities through the gravitational collapse of realistic matter.[35] This is supported by numerical simulations.[36]

Due to the relatively large strength of the electromagnetic force, black holes forming from the collapse of stars are expected to retain the nearly neutral charge of the star. Rotation, however, is expected to be a common feature of compact objects, and the black-hole candidate binary X-ray source GRS 1915+105[37] appears to have an angular momentum near the maximum allowed value.
Class Mass Size
Supermassive black hole ~105–109 MSun ~0.001–10 AU
Intermediate-mass black hole ~103 MSun ~103 km = REarth
Stellar black hole ~10 MSun ~30 km
Micro black hole up to ~MMoon up to ~0.1 mm

Black holes are commonly classified according to their mass, independent of angular momentum J or electric charge Q. The size of a black hole, as determined by the radius of the event horizon, or Schwarzschild radius, is roughly proportional to the mass M through

r_\mathrm{sh} =\frac{2GM}{c^2} \approx 2.95\, \frac{M}{M_\mathrm{Sun}}~\mathrm{km,}

where rsh is the Schwarzschild radius and MSun is the mass of the Sun. This relation is exact only for black holes with zero charge and angular momentum, for more general black holes it can differ up to a factor of 2. The table on the right lists the various classes of black hole that are distinguished.
Event horizon
Main article: Event horizon
Image:BH-no-escape-1.svg
Far away from the black hole a particle can move in any direction. It is only restricted by the speed of light.
Image:BH-no-escape-2.svg
Closer to the black hole spacetime starts to deform. There are more paths going towards the black hole than paths moving away.
Image:BH-no-escape-3.svg
Inside of the event horizon all paths bring the particle closer to the center of the black hole. It is no longer possible for the particle to escape.

The defining feature of a black hole is the appearance of an event horizon—a boundary in spacetime through which matter and light can only pass inward towards the mass of the black hole. Nothing, not even light, can escape from inside the event horizon. The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if such an event occurred.[38]

As predicted by general relativity, the presence of a large mass deforms spacetime in such a way that the paths taken by particles bend towards the mass. At the event horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hole.[39]

To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole.[40] Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it.[41] At the same time, all processes on this object slow down causing emitted light to appear redder and dimmer, an effect known as gravitational redshift.[42] Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.

On the other hand, an observer falling into a black hole does not notice any of these effects as he crosses the event horizon. According to his own clock, he crosses the event horizon after a finite time, although he is unable to determine exactly when he crosses it, as it is impossible to determine the location of the event horizon from local observations.[43]

For a non rotating (static) black hole, the Schwarzschild radius delimits a spherical event horizon. The Schwarzschild radius of an object is proportional to the mass.[44] Rotating black holes have distorted, nonspherical event horizons. Since the event horizon is not a material surface but rather merely a mathematically defined demarcation boundary, nothing prevents matter or radiation from entering a black hole, only from exiting one. The description of black holes given by general relativity is known to be an approximation, and some scientists expect that quantum gravity effects will become significant near the vicinity of the event horizon.[45] This would allow observations of matter near a black hole's event horizon to be used to indirectly study general relativity and proposed extensions to it.
Singularity
Main article: Gravitational singularity

At the center of a black hole as described by general relativity lies a gravitational singularity, a region where the spacetime curvature becomes infinite.[46] For a non-rotating black hole this region takes the shape of a single point and for a rotating black hole it is smeared out to form a ring singularity lying in the plane of rotation.[47] In both cases the singular region has zero volume. It can also be shown that the singular region contains all the mass of the black hole solution.[48] The singular region can thus be thought of as having infinite density.

An observer falling into a Schwarzschild black hole (i.e. non-rotating and no charges) cannot avoid the singularity. Any attempt to do so will only shorten the time taken to get there.[49] When they reach the singularity, they are crushed to infinite density and their mass is added to the total of the black hole. Before that happens, they will have been torn apart by the growing tidal forces in a process sometimes referred to as spaghettification or the noodle effect.[50]

In the case of a charged (Reissner–Nordström) or rotating (Kerr) black hole it is possible to avoid the singularity. Extending these solutions as far as possible reveals the hypothetical possibility of exiting the black hole into a different spacetime with the black hole acting as a wormhole.[51] The possibility of traveling to another universe is however only theoretical, since any perturbation will destroy this possibility.[52] It also appears to be possible to follow closed timelike curves (going back to one's own past) around the Kerr singularity, which lead to problems with causality like the grandfather paradox.[53] It is expected that none of these peculiar effects would survive in a proper quantum mechanical treatment of rotating and charged black holes.[54]

The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory.[55] This breakdown, however, is expected; it occurs in a situation where quantum mechanical effects should describe these actions due to the extremely high density and therefore particle interactions. To date it has not been possible to combine quantum and gravitational effects into a single theory. It is generally expected that a theory of quantum gravity will feature black holes without singularities.[56][57]
Photon sphere
Main article: Photon sphere

The photon sphere is a spherical boundary of zero thickness such that photons moving along tangents to the sphere will be trapped in a circular orbit. For non-rotating black holes, the photon sphere has a radius 1.5 times the Schwarzschild radius. The orbits are dynamically unstable, hence any small perturbation (such as a particle of infalling matter) will grow over time, either setting it on an outward trajectory escaping the black hole or on an inward spiral eventually crossing the event horizon.

While light can still escape from inside the photon sphere, any light that crosses the photon sphere on an inbound trajectory will be captured by the black hole. Hence any light reaching an outside observer from inside the photon sphere must have been emitted by objects inside the photon sphere but still outside of the event horizon.

Other compact objects, such as neutron stars, can also have photon spheres.[58] This follows from the fact that the gravitational field of an object does not depend on its actual size, hence any object that is smaller than 1.5 times the Schwarzschild radius corresponding to its mass will indeed have a photon sphere.
Ergosphere
Main article: Ergosphere
The ergosphere is an oblate spheroid region outside of the event horizon, where objects cannot remain stationary.

Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still, called the ergosphere. This is the result of a process known as frame-dragging; general relativity predicts that any rotating mass will tend to slightly "drag" along the spacetime immediately surrounding it. Any object near the rotating mass will tend to start moving in the direction of rotation. For a rotating black hole this effect becomes so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still.[59]

The ergosphere of a black hole is bounded by the (outer) event horizon on the inside and an oblate spheroid, which coincides with the event horizon at the poles and is noticeably wider around the equator. The outer boundary is sometimes called the ergosurface.

Objects and radiation can escape normally from the ergosphere. Through the Penrose process, objects can emerge from the ergosphere with more energy than they entered. This energy is taken from the rotational energy of the black hole causing it to slow down.[60]
Formation and evolution

Considering the exotic nature of black holes, it may be natural to question if such bizarre objects could exist in nature or to suggest that they are merely pathological solutions to Einstein's equations. Einstein himself wrongly thought that black holes would not form, because he held that the angular momentum of collapsing particles would stabilize their motion at some radius.[61] This led the general relativity community to dismiss all results to the contrary for many years. However, a minority of relativists continued to contend that black holes were physical objects,[62] and by the end of the 1960s, they had persuaded the majority of researchers in the field that there is no obstacle to forming an event horizon.

Once an event horizon forms, Roger Penrose proved that a singularity will form somewhere inside it.[23] Shortly afterwards, Stephen Hawking showed that many cosmological solutions describing the Big Bang have singularities without scalar fields or other exotic matter (see Penrose-Hawking singularity theorems). The Kerr solution, the no-hair theorem and the laws of black hole thermodynamics showed that the physical properties of black holes were simple and comprehensible, making them respectable subjects for research.[63] The primary formation process for black holes is expected to be the gravitational collapse of heavy objects such as stars, but there are also more exotic processes that can lead to the production of black holes.
Gravitational collapse
Main article: Gravitational collapse

Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. For stars this usually occurs either because a star has too little "fuel" left to maintain its temperature through stellar nucleosynthesis, or because a star which would have been stable receives extra matter in a way which does not raise its core temperature. In either case the star's temperature is no longer high enough to prevent it from collapsing under its own weight (the ideal gas law explains the connection between pressure, temperature, and volume).[64]

The collapse may be stopped by the degeneracy pressure of the star's constituents, condensing the matter in an exotic denser state. The result is one of the various types of compact star. Which type of compact star is formed depends on the mass of the remnant — the matter left over after changes triggered by the collapse (such as supernova or pulsations leading to a planetary nebula) have blown away the outer layers. Note that this can be substantially less than the original star — remnants exceeding 5 solar masses are produced by stars which were over 20 solar masses before the collapse.[64]

If the mass of the remnant exceeds about 3–4 solar masses (the Tolman–Oppenheimer–Volkoff limit)—either because the original star was very heavy or because the remnant collected additional mass through accretion of matter—even the degeneracy pressure of neutrons is insufficient to stop the collapse. After this, no known mechanism (except possibly quark degeneracy pressure, see quark star) is powerful enough to stop the collapse and the object will inevitably collapse to a black hole.[64]

This gravitational collapse of heavy stars is assumed to be responsible for the formation of stellar mass black holes. Star formation in the young universe may have resulted in very heavy stars, which upon their collapse would have produced black holes of up to 103 solar masses. These heavy black holes could be the seeds of the supermassive black holes found in the centers of most galaxies.[65]

While most of the energy released during gravitational collapse is emitted very quickly, an outside observer does not actually see the end of this process. Even though the collapse takes a finite amount of time from the reference frame of infalling matter, a distant observer sees the infalling material slow and halt just above the event horizon, due to gravitational time dilation. Light from the collapsing material takes longer and longer to reach the observer, with the light emitted just before the event horizon forms delayed an infinite amount of time. Thus the external observer never sees the formation of the event horizon; instead, the collapsing material seems to become dimmer and increasingly red-shifted, eventually fading away.[66]
Primordial black holes in the Big Bang

Gravitational collapse requires great densities. In the current epoch of the universe these high densities are only found in stars, but in the early universe shortly after the big bang densities were much greater, possibly allowing for the creation of black holes. The high density alone is not enough to allow the formation of black holes since a uniform mass distribution will not allow the mass to bunch up. In order for primordial black holes to form in such a dense medium, there must be initial density perturbations which can then grow under their own gravity. Different models for the early universe vary widely in their predictions of the size of these perturbations. Various models predict the creation of black holes, ranging from a Planck mass to hundreds of thousands of solar masses.[67] Primordial black holes could thus account for the creation of any type of black hole.
High-energy collisions
A simulated event in the CMS detector, a collision in which a micro black hole may be created.

Gravitational collapse is not the only process that could create black holes. In principle, black holes could also be created in high-energy collisions that create sufficient density. However, to date, no such events have ever been detected either directly or indirectly as a deficiency of the mass balance in particle accelerator experiments.[68] This suggests that there must be a lower limit for the mass of black holes. Theoretically, this boundary is expected to lie around the Planck mass (mP = √ħc/G ≈ 1.2×1019 GeV/c2 ≈ 2.2×10−8 kg), where quantum effects are expected to make the theory of general relativity break down completely.[69] This would put the creation of black holes firmly out of reach of any high energy process occurring on or near the Earth. Certain developments in quantum gravity however suggest that the Planck mass could be much lower: some braneworld scenarios for example put it much lower, maybe even as low as 1 TeV/c2[70] This would make it possible for micro black holes to be created in the high energy collisions occurring when cosmic rays hit the Earth's atmosphere, or possibly in the new Large Hadron Collider at CERN. These theories are however very speculative, and the creation of black holes in these processes is deemed unlikely by many specialists.[71] Even if such micro black holes should be formed in these collisions, it is expected that they would evaporate in about 10−25 seconds, posing no threat to Earth[72]
Growth

Once a black hole has formed, it can continue to grow by absorbing additional matter. Any black hole will continually absorb gas and interstellar dust from its direct surroundings and omnipresent cosmic background radiation. This is the primary process through which supermassive black holes seem to have grown.[65] A similar process has been suggested for the formation of intermediate-mass black holes in globular clusters.[73]

Another possibility is for a black hole to merge with other objects such as stars or even other black holes. This is thought to have been important especially for the early development of supermassive black holes, which are thought to have formed from the coagulation of many smaller objects.[65] The process has also been proposed as the origin of some intermediate-mass black holes.[74][75]
Evaporation
Main article: Hawking radiation

In 1974, Stephen Hawking showed that black holes are not entirely black but emit small amounts of thermal radiation.[26] He got this result by applying quantum field theory in a static black hole background. The result of his calculations is that a black hole should emit particles in a perfect black body spectrum. This effect has become known as Hawking radiation. Since Hawking's result, many others have verified the effect through various methods.[76] If his theory of black hole radiation is correct, then black holes are expected to emit a thermal spectrum of radiation, and thereby lose mass, because according to the theory of relativity mass is just highly condensed energy (E = mc2).[26] Black holes will shrink and evaporate over time. The temperature of this spectrum (Hawking temperature) is proportional to the surface gravity of the black hole, which for a Schwarzschild black hole is inversely proportional to the mass. Large black holes, therefore, emit less radiation than small black holes.

A stellar black hole of one solar mass has a Hawking temperature of about 100 nanokelvins. This is far less than the 2.7 K temperature of the cosmic microwave background. Stellar mass (and larger) black holes receive more mass from the cosmic microwave background than they emit through Hawking radiation and will thus grow instead of shrink. To have a Hawking temperature larger than 2.7 K (and be able to evaporate), a black hole needs to be lighter than the Moon (and therefore a diameter of less than a tenth of a millimeter).[77]

On the other hand, if a black hole is very small the radiation effects are expected to become very strong. Even a black hole that is heavy compared to a human would evaporate in an instant. A black hole the weight of a car (~10−24 m) would only take a nanosecond to evaporate, during which time it would briefly have a luminosity more than 200 times that of the sun. Lighter black holes are expected to evaporate even faster, for example a black hole of mass 1 TeV/c2 would take less than 10−88 seconds to evaporate completely. Of course, for such a small black hole quantum gravitation effects are expected to play an important role and could even – although current developments in quantum gravity do not indicate so[78] – hypothetically make such a small black hole stable.[79]
Observational evidence

By their very nature, black holes do not directly emit any signals other than the hypothetical Hawking radiation; since the Hawking radiation for an astrophysical black hole is predicted to be very weak, this makes it impossible to directly detect astrophysical black holes from the Earth. A possible exception to the Hawking radiation being weak is the last stage of the evaporation of light (primordial) black holes; searches for such flashes in the past has proven unsuccessful and provides stringent limits on the possibility of existence of light primordial black holes.[80] NASA's Fermi Gamma-ray Space Telescope launched in 2008 will continue the search for these flashes.[81]

Astrophysicists searching for black holes thus have to rely on indirect observations. A black hole's existence can sometimes be inferred by observing its gravitational interactions with its surroundings.
Accretion of matter
See also: Accretion disc
Formation of extragalactic jets from a black hole's accretion disk

Due to conservation of angular momentum, gas falling into the gravitational well created by a massive object will typically form a disc-like structure around the object. Friction within the disc causes angular momentum to be transported outward allowing matter to fall further inward releasing potential energy and increasing the temperature of the gas.[82] In the case of compact objects such as white dwarfs, neutron stars, and black holes, the gas in the inner regions becomes so hot that it will emit vast amounts of radiation (mainly X-rays), which may be detected by telescopes. This process of accretion is one of the most efficient energy producing process known; up to 40% of the rest mass of the accreted material can be emitted in radiation.[82] (In nuclear fusion only about 0.7% of the rest mass will be emitted as energy.) In many cases, accretion discs are accompanied by relativistic jets emitted along the poles, which carry away much of the energy. The mechanism for the creation of these jets is currently not well understood.

As such many of the universe's more energetic phenomena have been attributed to the accretion of matter on black holes. In particular, active galactic nuclei and quasars are thought to be the accretion discs of supermassive black holes.[83] Similarly, X-ray binaries are thought to be binary star systems in which one of the two stars is a compact object accreting matter from its companion.[83] It has also been suggested that some ultraluminous X-ray sources may be the accretion disks of intermediate-mass black holes.[84]
X-ray binaries
See also: X-ray binary

X-ray binaries are binary star systems that are luminous in the X-ray part of the spectrum. These X-ray emissions are generally thought to be caused by one of the component stars being a compact object accreting matter from the other (regular) star. The presence of an ordinary star in such a system provides a unique opportunity for studying the central object and determining if it might be a black hole.
Artist impression of a binary system with an accretion disk around a compact object being fed by material from the companion star.

If such a system emits signals that can be directly traced back to the compact object, it cannot be a black hole. The absence of such a signal does, however, not exclude the possibility that the compact object is a neutron star. By studying the companion star it is often possible to obtain the orbital parameters of the system and obtain an estimate for the mass of the compact object. If this is much larger than the Tolman–Oppenheimer–Volkoff limit (that is, the maximum mass a neutron star can have before collapsing) then the object cannot be a neutron star and is generally expected to be a black hole.[83]

The first strong candidate for a black hole, Cygnus X-1, was discovered in this way by Charles Thomas Bolton[85] and Webster and Murdin[86] in 1972.[87][88] Some doubt, however, remained due to the uncertainties resultant from the companion star being much heavier than the candidate black hole.[83] Currently, better candidates for black holes are found in a class of X-ray binaries called soft X-ray transients.[83] In this class of system the companion star is relatively low mass allowing for more accurate estimates in the black hole mass. Moreover, these systems are only active in X-ray for several months once every 10–50 years. During the period of low X-ray emission (called quiescence), the accretion disc is extremely faint allowing for detailed observation of the companion star during this period. One of the best such candidates is V404 Cyg.
Quiescence and advection-dominated accretion flow

The faintness of the accretion disc during quiescence is thought to be caused by the flow entering a mode called an advection-dominated accretion flow (ADAF). In this mode, almost all the energy generated by friction in the disc is swept along with the flow instead of radiated away. If this model is correct, then it forms strong qualitative evidence for the presence of an event horizon.[89] Because, if the object at the center of the disc had a solid surface, it would emit large amounts of radiation as the highly energetic gas hits the surface, an effect that is observed for neutron stars in a similar state.[82]
Quasi-periodic oscillations
See also: Quasi-periodic oscillations

The X-ray emissions from accretion disks sometimes exhibit a flickering around certain frequencies. These signals are called quasi-periodic oscillations and are thought to be caused by material moving along the inner edge of the accretion disk (the innermost stable circular orbit). As such their frequency is linked to the mass of the compact object. They can thus be used as an alternative way to determine the mass of potential black holes.[90]
Gamma ray bursts

Intense but one-time gamma ray bursts (GRBs) may signal the birth of "new" black holes, because astrophysicists think that GRBs are caused either by the gravitational collapse of giant stars[91] or by collisions between neutron stars,[92] and both types of event involve sufficient mass and pressure to produce black holes. It appears that a collision between a neutron star and a black hole can also cause a GRB,[93] so a GRB is not proof that a "new" black hole has been formed. All known GRBs come from outside our own galaxy, and most come from billions of light years away[94] so the black holes associated with them are billions of years old.
Galactic nuclei
See also: Active galactic nucleus
The jet originating from the center of M87 in this image comes from an active galactic nucleus that may contain a supermassive black hole. Credit: Hubble Space Telescope/NASA/ESA.

It is now widely accepted that the center of every or at least nearly every galaxy contains a supermassive black hole.[95] The close observational correlation between the mass of this hole and the velocity dispersion of the host galaxy's bulge, known as the M-sigma relation, strongly suggests a connection between the formation of the black hole and the galaxy itself. [96]

For decades, astronomers have used the term "active galaxy" to describe galaxies with unusual characteristics, such as unusual spectral line emission and very strong radio emission.[97][98] However, theoretical and observational studies have shown that the active galactic nuclei (AGN) in these galaxies may contain supermassive black holes.[97][98] The models of these AGN consist of a central black hole that may be millions or billions of times more massive than the Sun; a disk of gas and dust called an accretion disk; and two jets that are perpendicular to the accretion disk.[98]

Although supermassive black holes are expected to be found in most AGN, only some galaxies' nuclei have been more carefully studied in attempts to both identify and measure the actual masses of the central supermassive black hole candidates. Some of the most notable galaxies with supermassive black hole candidates include the Andromeda Galaxy, M32, M87, NGC 3115, NGC 3377, NGC 4258, and the Sombrero Galaxy.[99]

Currently, the best evidence for a supermassive black hole comes from studying the proper motion of stars near the center of our own Milky Way.[100] Since 1995 astronomers have tracked the motion of 90 stars in a region called Sagittarius A*. By fitting their motion to Keplerian orbits they were able to infer in 1998 that 2.6 million solar masses must be contained in a volume with a radius of 0.02 lightyears.[101] Since then one of the stars—called S2—has completed a full orbit. From the orbital data they were able to place better constraints on the mass and size of the object causing the orbital motion of stars in the Sagittarius A* region, finding that there is a spherical mass of 4.3 million solar masses contained within a radius of less than 0.002 lightyears.[100] While this is more than 3000 times the Schwarzschild radius corresponding to that mass, it is at least consistent with the central object being a supermassive black hole, and no "realistic cluster [of stars] is physically tenable."[101]
Gravitational lensing
Further information: Gravitational lens

The deformation of spacetime around a massive object causes light rays to be deflected much like light passing through an optic lens. This phenomenon is known as gravitational lensing. Observations have been made of weak gravitational lensing, in which photons are deflected by only a few arcseconds. However, it has never been directly observed for a black hole.[102] One possibility for observing gravitational lensing by a black hole would be to observe stars in orbit around the black hole. There are several candidates for such an observation in orbit around Sagittarius A*.[102]
Alternatives

The evidence for stellar black holes strongly relies on the existence of an upper limit for the mass of a neutron star. The size of this limit heavily depends on the assumptions made about the properties of dense matter. New exotic phases of matter could push up this bound.[83] A phase of free quarks at high density might allow the existence of dense quark stars,[103] and some supersymmetric models predict the existence of Q stars.[104] Some extensions of the standard model posit the existence of preons as fundamental building blocks of quarks and leptons which could hypothetically form preon stars.[105] These hypothetical models could potentially explain a number of observations of stellar black hole candidates. However, it can be shown from general arguments in general relativity that any such object will have a maximum mass.[83]

Since the average density of a black hole inside its Schwarzschild radius is inversely proportional to the square of its mass, supermassive black holes are much less dense than stellar black holes (the average density of a large supermassive black hole is comparable to that of water).[83] Consequently, the physics of matter forming a supermassive black hole is much better understood and the possible alternative explanations for supermassive black hole observations are much more mundane. For example, a supermassive black hole could be modelled by a large cluster of very dark objects. However, typically such alternatives are not stable enough to explain the supermassive black hole candidates.[83]

The evidence for stellar and supermassive black holes implies that in order for black holes not to form, general relativity must fail as a theory of gravity, perhaps due to the onset of quantum mechanical corrections. A much anticipated feature of a theory of quantum gravity is that it will not feature singularities or event horizons (and thus no black holes).[106] In recent years, much attention has been drawn by the fuzzball model in string theory. Based on calculations in specific situations in string theory, the proposal suggest that generically the individual states of a black hole solution do not have an event horizon or singularity (and can thus not really be considered to be a black hole), but that for a distant observer the statistical average of such states does appear just like an ordinary black hole in general relativity.[107]
Open questions
Entropy and thermodynamics
Further information: Black hole thermodynamics
If ultra-high-energy collisions of particles in a particle accelerator can create microscopic black holes, it is expected that all types of particles will be emitted by black hole evaporation, providing key evidence for any grand unified theory. Above are the high energy particles produced in a gold ion collision on the RHIC.

In 1971, Stephen Hawking showed under general conditions[Note 1] that the total area of the event horizons of any collection of classical black holes can never decrease, even if they collide and merge.[108] This result, now known as the second law of black hole mechanics, is remarkably similar to the second law of thermodynamics, which states that the total entropy of a system can never decrease. As with classical objects at absolute zero temperature, it was assumed that black holes had zero entropy. If this were the case, the second law of thermodynamics would be violated by entropy-laden matter entering the black hole, resulting in a decrease of the total entropy of the universe. Therefore, Jacob Bekenstein proposed that a black hole should have an entropy, and that it should be proportional to its horizon area.[109]

The link with the laws of thermodynamics was further strengthened by Hawking's discovery that quantum field theory predicts that a black hole radiates blackbody radiation at a constant temperature. This seemingly causes a violation of the second law of black hole mechanics, since the radiation will carry away energy from the black hole causing it to shrink. The radiation, however also carries away entropy, and it can be proven under general assumptions that the sum of the entropy of the matter surrounding the black hole and one quarter of the area of the horizon as measured in Planck units is in fact always increasing. This allows the formulation of the first law of black hole mechanics as an analogue of the first law of thermodynamics, with the mass acting as energy, the surface gravity as temperature and the area as entropy.[109]

One puzzling feature is that the entropy of a black hole scales with its area rather than with its volume, since entropy is normally an extensive quantity that scales linearly with the volume of the system. This odd property led 't Hooft and Susskind to propose the holographic principle, which suggests that anything that happens in volume of spacetime can be described by data on the boundary of that volume.[110]

Although general relativity can be used to perform a semi-classical calculation of black hole entropy, this situation is theoretically unsatisfying. In statistical mechanics, entropy is understood as counting the number of microscopic configurations of a system which have the same macroscopic qualities (such as mass, charge, pressure, etc.). Without a satisfactory theory of quantum gravity, one cannot perform such a computation for black holes. Some progress has been made in various approaches to quantum gravity. In 1995, Strominger and Vafa showed that counting the microstates of a specific supersymmetric black hole in string theory reproduced the Bekenstein–Hawking entropy.[111] Since then, similar results have been reported for different black holes both in string theory and in other approaches to quantum gravity like loop quantum gravity.[112]
Black hole unitarity
Main article: Black hole information paradox
Unsolved problems in physics Is physical information lost in black holes? Question mark2.svg

An open question in fundamental physics is the so-called information loss paradox, or black hole unitarity paradox. Classically, the laws of physics are the same run forward or in reverse (T-symmetry). Liouville's theorem dictates conservation of phase space volume, which can be thought of as "conservation of information", so there is some problem even in classical physics. In quantum mechanics, this corresponds to a vital property called unitarity, which has to do with the conservation of probability (it can also be thought of as a conservation of quantum phase space volume as expressed by the density matrix).[113]
See also

* Black holes in fiction
* Black string
* Kugelblitz (astrophysics)
* List of black holes
* Susskind-Hawking battle


He1523a.jpg Star portal

* Timeline of black hole physics
* White hole
* Wormhole

Notes

1. ^ In particular, he assumed that all matter satisfies the weak energy condition.

References

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Further reading

Popular reading

* Ferguson, Kitty (1991). Black Holes in Space-Time. Watts Franklin. ISBN 0-531-12524-6. .
* Hawking, Stephen (1988). A Brief History of Time. Bantam Books, Inc. ISBN 0-553-38016-8. .
* Hawking, Stephen; Penrose, Roger (1996). The Nature of Space and Time. Princeton University Press. ISBN 0-691-03791-2. http://books.google.com/?id=LstaQTXP65cC. .
* Melia, Fulvio (2003). The Black Hole at the Center of Our Galaxy. Princeton U Press. ISBN 978-0-691-09505-9. .
* Melia, Fulvio (2003). The Edge of Infinity. Supermassive Black Holes in the Universe. Cambridge U Press. ISBN 978-0-521-81405-8. .
* Pickover, Clifford (1998). Black Holes: A Traveler's Guide. Wiley, John & Sons, Inc. ISBN 0-471-19704-1. .
* Stern, B. (2008). "Blackhole". http://www.wikilivres.info/wiki/Blackhole_%28Stern%29. , poem.
* Thorne, Kip S. (1994). Black Holes and Time Warps. Norton, W. W. & Company, Inc. ISBN 0-393-31276-3. .
* Wheeler, J. Craig (2007). Cosmic Catastrophes (2nd ed.). Cambridge University Press. ISBN 0-521-85714-7.

University textbooks and monographs

* Carroll, Sean M. (2004). Spacetime and Geometry. Addison Wesley. ISBN 0-8053-8732-3. , the lecture notes on which the book was based are available for free from Sean Carroll's website.
* Carter, B. (1973). "Black hole equilibrium states". In DeWitt, B.S.; DeWitt, C.. Black Holes. .
* Chandrasekhar, Subrahmanyan (1999). Mathematical Theory of Black Holes. Oxford University Press. ISBN 0-19-850370-9. .
* Frolov, V.P.; Novikov, I.D. (1998). Black hole physics. .
* Hawking, S.W.; Ellis, G.F.R. (1973). Large Scale Structure of space time. Cambridge University Press. ISBN 0521099064. http://books.google.com/?id=QagG_KI7Ll8C. .
* Melia, Fulvio (2007). The Galactic Supermassive Black Hole. Princeton U Press. ISBN 978-0-691-13129-0. .
* Taylor, Edwin F.; Wheeler, John Archibald (2000). Exploring Black Holes. Addison Wesley Longman. ISBN 0-201-38423-X. .
* Thorne, Kip S.; Misner, Charles; Wheeler, John (1973). Gravitation. W. H. Freeman and Company. ISBN 0-7167-0344-0. .
* Wald, Robert M. (1992). Space, Time, and Gravity: The Theory of the Big Bang and Black Holes. University of Chicago Press. ISBN 0-226-87029-4. .

Review papers

* Detweiler, S. (1981). "Resource letter BH-1: Black holes". American Journal of Physics 49 (5, pp): 394–400. doi:10.1119/1.12686.
* Gallo, E.; Marolf, D. (2009). "Resource Letter BH-2: Black Holes". American Journal of Physics 77: 294. doi:10.1119/1.3056569. arXiv:0806.2316. edit
* Hughes, Scott A. (2005). "Trust but verify: The case for astrophysical black holes". arΧiv:hep-ph/0511217v2 [hep-ph]. Lecture notes from 2005 SLAC Summer Institute.

External links
Wikimedia Commons has media related to: Black holes

* Stanford Encyclopedia of Philosophy: "Singularities and Black Holes" by Erik Curiel and Peter Bokulich.
* "Black hole" on Scholarpedia.
* Black Holes: Gravity's Relentless Pull - Interactive multimedia Web site about the physics and astronomy of black holes from the Space Telescope Science Institute
* FAQ on black holes
* "Schwarzschild Geometry" on Andrew Hamilton’s website
* UT Brownsville Group Simulates Spinning Black-Hole Binaries
* Advanced Mathematics of Black Hole Evaporation

Videos

* 16-year long study tracks stars orbiting Milky Way black hole
* Yale University Video Lecture: Introduction to Black Holes at Google Video.
* Movie of Black Hole Candidate from Max Planck Institute

News

* "Black Hole confirmed in Milky Way." Retrieved December 10, 2008
* Black Hole Research News

fify

[oligarchy]

been in 2 fights... sorta. they didnt last long.

once when i was like 14 cuz some annoying little mexican 7th grader was talking shit for no reason (i think he was talking simply for the sake of starting a fight with someone) and wouldnt leave me the fuck alone. so i shoved him and told him to fuck off. he swung at me, but he was so short he couldnt reach me, and i grabbed him and threw him in the locker pretty hard and told him to fuck off again, and he didnt do anything after that.

then a couple years ago at a house party, i was pretty buzzed, and a guy was drunk and for some reason was just being obnoxious. i told him to calm down cuz he was getting on peoples nerves. then he started threatening to kick my ass. i started to step up to him, but then i just walked away, because i didnt feel like dealing with it anymore and it wasnt worth it. but that got him acting all "crunk" so he kept talking shit to me. i just kinda stood there and didnt do anything, and normally i wouldnt care, but i think the alcohol got to me, and i walked up to him and knocked him senseless for a good 15 seconds or so. then when he came to, he was trying to come at me again, but i wasnt going to deal with it and i just went back inside. then later he broke down crying. i guess he is one of those guys who get all emotional when drunk or something.

i havent been in a fight that lasted very long though. like strike, i don't like fighting. i think its a pretty lame way to settle shit, especially since 98% of the shit people get into fights over, are gay reasons in the first place. the only reason i would ever fight someone is if im forced to physically defend myself, my family, or my friends. i never will be the aggressor in a fight though. my fighting teacher told me that i learn to fight so that i dont have to fight. wax on wax off bitches.

[oligarchy]

I almost forgot about 8 or 9 months ago I almost did get into a fight at a gym though. Some dumbass that had like a 14 year old kid guarding him kept abusing him. So after a couple scores, I came and doubled him from his blind side, and ended up blocking him. He tried doing the same thing the next trip down, and I blocked him again. The third time he got wise and dribbled out, which at that point I told the kid to switch with me. The guy tried to drive past me but couldnt, and like a moron, he killed his dribble, so I crowded him. He was considerably shorter than me, although a lot more muscular, but he wasnt able to get a pass or shot off with me on him, so like a bitch, he elbowed me in the face, then shot the ball and it hit front iron and he got the offensive rebound. he tried to pass it but i tipped it, and as i went to grab the ball, he elbowed me again. I told him to cut that mess out, and he got in my face and pointed at me, and told me to watch myself. I told him to cool off and stop throwing elbows, and then he lost it and slapped me (who the hell slaps people these days? for fucks sake if you are going to start a physical altercation with me, fucking punch me like a man). As I went at him, he ran backwards and had two of his friends grab me to hold me back. Those were the only guys that did, because everyone else wanted me to go after the guy, even his own teammates, cuz they all knew what a bitch move it was for him to hit me, then run backwards like Carmello, and tell his friends to grab me. I was extremely pissed, but cooled off and just played basketball. He didn't score a single point after that, as I swatted his shit a few more times, and I hit the game winner in his face, and told him to suck my dick. He didn't do anything cuz he looked like a total bitch at that point, and he knew it too.

funnier than that, a friend of mine was talking to him about it later, saying he was a bitch for what he did. a couple days later, the guy saw my friend at the gym again, and asked if they were cool. my friend said "whats done is done, but you're still a bitch-made ho". the guy said "good im glad your cool now. wanna play on my team?"

lol what a puss

[Cessation]

cool story bro

[koriwhat]

fuck the OP, this thread, and him being able to troll the spurs forum at will.

i'm against banning people and whatnot but shit is ridiculous these days on ST. always the same stupid fuckin' "lol" and "lmao" threads in here, keep that shit in the club, troll forum, or nba forum.

btw, did i mention, fuck you phillip!

[BanditHiro]

Proc. Natl. Acad. Sci. USA
Vol. 95, pp. 2355–2360, March 1998
Evolution
Modification of expression and cis-regulation of Hoxc8 in the
evolution of diverged axial morphology
HEINZ-GEORG BELTING*†, COODUVALLI S. SHASHIKANT*, AND FRANK H. RUDDLE*‡§
Departments of *Molecular, Cellular, and Developmental Biology and ‡Genetics, Yale University, POB 208103, New Haven, CT 06520
Contributed by Frank H. Ruddle, December 30, 1997
ABSTRACT Differential Hox gene expression between
vertebrate species has been implicated in the divergence of
axial morphology. To examine this relationship, we have
compared expression and transcriptional regulation of Hoxc8
in chicken and mouse. In both species, expression of Hoxc8 in
the paraxial mesoderm and neural tube is associated with
midthoracic and brachial identities, respectively. During embryogenesis,
there is a temporal delay in the activation of
Hoxc8 in chicken compared with mouse. As a result, chicken
Hoxc8 expression in the paraxial mesoderm is at a posterior
axial level, extending over a smaller domain compared with
mouse Hoxc8 expression. This finding is consistent with a
shorter thoracic region in chicken compared with mouse. In
addition, the chicken Hoxc8 early enhancer, differing from its
mouse counterpart in only a few specific nucleotides, directs
a reporter gene expression to a more posterior domain in
transgenic mouse embryos. These findings are consistent with
the concept that the diversification of axial morphology has
been achieved through changes in cis-regulation of developmental
control genes.
Genes that control axial patterning, such as Hox genes, are
highly conserved across the animal kingdom (1). However,
animals exhibit a high degree of diversity in the organization
of the primary body axis. This phenomenon may be caused by
conserved genetic programs having become variously modified
in different organisms (2–4). Differences in Hox gene
expression may contribute to an understanding of how modifications
of developmental programs generate axial diversity
between species (5–7).
Amniotes differ greatly in the number of segments contributing
to individual anatomic regions along the vertebral column
such as the cervical, thoracic, and lumbar regions. For
example, the vertebral column of the mouse consists of seven
cervical and 13 thoracic vertebrae, whereas the vertebral
column of chicken displays 14 cervical and seven thoracic
vertebrae. Comparisons between mouse and chicken show that
differences in axial morphology are associated with differences
in spatial domains of Hox gene expression (5, 6). For example,
Hoxc6 is expressed at the cervicalythoracic transition in both
mouse and chick, but at different relative levels along the
anteroposterior axis; likewise, Hoxc8 is expressed in the thoracic
region of both mouse and chicken (6).
Differences in expression patterns of Hox genes between
different species may be brought about by changes in components
of their transcriptional regulation, including changes in
cis-regulatory elements and trans-acting factors whose interactions
determine embryonic expression patterns of Hox
genes. However, experimental exchanges of Hox genes and
their cis-regulatory regions between different organisms have
demonstrated a high degree of functional conservation (8–24).
The expression of trans-acting factors of Hox genes probably
are largely retained in amniotes, setting up a pre-pattern that
provides positional coordinates along the body axis. We postulate
that subtle changes in cis-regulatory elements leading to
altered interactions with conserved trans-acting factors may
contribute to diverged expression patterns of Hox genes among
different species.
In previous studies, we identified cis-regulatory regions that
control different phases of mouse Hoxc8 expression (25–29).
The Hoxc8 early enhancer is involved in the establishment of
the anteroposterior expression domain of Hoxc8, consistent
with its role in the regionalization of the body axis (26, 29). The
early enhancer has been delimited to a 200-bp DNA fragment
by extensive deletion analyses in transgenic mice (26, 28). Five
partially redundant elements within this region act in combination
in determining early Hoxc8 expression (26, 28). A
survey among mammalian species reveals a remarkable conservation
of the nucleotide sequence of the early enhancer
(C.S.S., unpublished observations). Any difference in the
nucleotide sequence and activity of this highly conserved and
well-characterized enhancer may have strong implications on
the divergence of Hoxc8 expression between different species.
To test this hypothesis, we have studied Hoxc8 expression
during chicken and mouse embryogenesis and compared their
early enhancer regions for nucleotide sequence similarities,
and enhancer activities in transgenic mice. In this report we
provide evidence suggesting that transposition of Hoxc8 expression
between the two species is achieved by differential
activities of the Hoxc8 early enhancer.
MATERIALS AND METHODS
FVB mice (Taconics) were used for obtaining staged embryos
and for transgenic analysis. For mating, pairs were caged
together at noon, and the females were examined for the
presence of a vaginal plug the next morning, which was defined
as day 0.5. Fertilized eggs from white leghorn hens (SPAFAS)
were incubated at 37°C in an egg incubator. The chicken
embryos were staged as described (30).
Immunohistochemistry was performed with a mAb, C592y
7E, against the mouse Hoxc8 protein (29) as described (31)
with minor modifications.
For retrograde labeling experiments, mouse (10.5 day postcoital)

2-week incubation in PBS, 10 mM EDTA. Specimens were cut
on a vibratome at 80–100mm, and the image was captured with
a black-and-white charge-coupled device camera. The same
sections then were processed with Hoxc8 mAbs, and the
expression of Hoxc8 was compared with the DiI label, side by
side or by superimposing the captured images in pseudocolor.
Production of transgenic embryos, preparation of DNA for
microinjection, and staining for b-galactosidase have been
described (26). The reporter gene construct (construct A)
carrying 399-bp Hoxc8 early enhancer was described earlier
(28). A 151-bp chicken enhancer was isolated by PCR using
chicken genomic DNA as a template and synthetic oligonucleotide
primers designed from the mouse enhancer sequence
flanking the fragment. A 399-bp enhancer fragment in which
151 bp of the mouse sequence was replaced with the corresponding
chicken DNA fragment was generated by overlapping
PCR. The resulting fragment was cloned by ligation at
appropriate restriction sites in the polylinker sequence of
pHSF (26) to create a mouseychicken hybrid construct (construct
B).
RESULTS
Heterochronic Activation of Hoxc8. Previous studies by
Burke et al. (6), using RNA in situ procedures, showed that
axial levels of Hoxc8 expression differed between mouse and
chicken midgestation embryos. To determine the temporal
sequence of Hoxc8 expression during mouse and chicken
embryogenesis, we performed a comparative immunohistochemistry,
using a mAb raised against mouse Hoxc8 (29). The
mouse embryonic expression pattern is described in detail
elsewhere (29). Briefly, in the mouse, Hoxc8 first is detected
at the base of allantois in day 8 embryos having 6–7 somites
(Fig. 1A). At this stage, the neural tube is still open, and the
heart primordia have just formed (32). The earliest chicken
embryos examined, stages 10 and 11, are similar in their
developmental progression to day 8.0–8.5 mouse embryos. At
these stages, the major events of gastrulation have occurred,
and organogenesis is proceeding in the anterior portion of the
embryos. Embryos of both species possess a similar number
(8–12) of somites, and the degrees of the development of the
heart and nervous system are comparable at these stages.
However, in the chick, Hoxc8 protein was not detected in stages
10 or 12 (data not shown), but first was detected in the
posterior regions in stage 13 embryos having 18–19 somites
(Fig. 1E). At this stage, the neural tube, with the exception of
the caudal neuropore, is entirely closed, and the rostral
portions, including fore-, mid- and hindbrain, the optic cups,
and otic vesicles are formed. As in the mouse, chicken Hoxc8
expression at this stage was diffuse in all embryonic tissues
posterior to somite condensation. Thus, the onset of Hoxc8
expression in the chicken is developmentally delayed compared
with mouse.
Axial Levels of Hoxc8 Expression. A clear and differential
anterior boundary of Hoxc8 expression in the neural tube and
paraxial mesoderm is established at subsequent stages in the
mouse (day 8.5, Fig. 1B) and in the chicken embryos (stage 14,
Fig. 1F). In the mouse, at days 9.5 and 10.5, anterior bound-
FIG. 1. Expression of Hoxc8 during early mouse (A–D) and chicken (E and F) embryogenesis. The anterior boundary of expression in the neural
tube is indicated by arrowheads. (A) Embryo with 7–8 somites. (B) Embryo with 10–12 somites. The staining of the foregut in A and B is caused
by antibody trapping. (C) Embryo with 28–30 somites (9.5 day postcoital). (D) Embryo at 10.5 day p.c. Somite 17 is indicated in C and D. (E)
Chicken embryo at H&H stage 13 (18–19 somites). (F) Embryo at H&H stage 14 (20 somites). (G) Embryo at H&H stage 21. (H) At H&H stage
23, somite 22 is indicated in G and H. The staining of the brain and the allantois is caused by antibody trapping. a, allantois; e, eye; f, forelimb
bud, m, mesoderm; ov, otic vesicle; s, somite.
2356 Evolution: Belting et al. Proc. Natl. Acad. Sci. USA 95 (1998)

aries of expression of Hoxc8 were observed in the neural tube
at the level of 10th somite and in the paraxial mesoderm at the
level of 14th somite (Fig. 1 C and D). In the chick, however, at
stages 21 and 23, anterior boundaries of expression of Hoxc8
were observed in the neural tube at the level of the 18th somite
and in the paraxial mesoderm at the level of 20th somite. Thus,
the axial level of expression of Hoxc8 in chicken embryos is 6–8
somites more posterior than that observed in the mouse
embryos.
At these stages, in both mouse and chicken embryos, Hoxc8
expression declines in the tailbud region, thus defining posterior
boundaries of expression. However, these boundaries are
unlike the anterior boundaries of expression, having indistinct
margins. In the mouse, Hoxc8 expression in the neural tube
spans about six somite levels (from somites 10–15) and in the
paraxial mesoderm about 7–8 somite levels (somites 13–21,
with weaker levels of expression at somites 13 and 21). In the
chick, Hoxc8 expression in the neural tube spans about six
somite levels (somites 18–23; strong expression in somites
19–20) and in paraxial mesoderm about five somite levels
(somites 21–25). Thus Hoxc8 expression in paraxial mesoderm
is 2–3 somite levels shorter in the chicken compared with
mouse. Although the expression domain of Hoxc8 in the
paraxial mesoderm is smaller and more posterior compared
with mouse, the expression pattern is coincident with the
smaller thoracic region in chick. Thus, Hoxc8 expression in the
paraxial mesoderm is different not only with respect to absolute
axial levels but also in the number of expressing somites.
Early and Late Phases of Neural Tube Expression of Hoxc8.
In the mouse, two phases of Hoxc8 expression can be distinguished
in the neural tube, an early and a late phase (29). In
the early phase (day 8–9.5), Hoxc8 expression is found in most,
if not all, cells along the dorsoventral extent of the neural tube
(data not shown). In the late phase (after day 10.5), Hoxc8
expression is restricted to differentiating neurons, predominantly
in motor neurons in the ventrolateral region of the
neural tube (Fig. 1D). In the chick, similar phases of Hoxc8
expression in the neural tube are observed. In the early phase
(stage 13–15) Hoxc8 is distributed uniformly in the neural tube,
whereas in the late phase Hoxc8 is distributed predominantly
in the motor neurons in the ventrolateral region (Fig. 1H). The
distribution of Hoxc8 within the subregions of the spinal cord
at subsequent stages is very similar in both species.
Association of Hoxc8 Expression with Motor Neurons. To
determine whether the axial shift of Hoxc8 expression in the
neural tube of mouse and chicken corresponds to a transposition
of regional identity of spinal nerves, brachial motor
neurons were identified and tested for colocalization with
Hoxc8 protein. Brachial motor neurons of day 10.5 mouse and
stage 24 chicken embryos were retrograde-labeled with DiI as
described in Materials and Methods. The embryos were sectioned
serially, and the DiI signal was compared with the
distribution of Hoxc8 protein on the same sections. In both
mouse and chick, sections through the anterior brachial neural
tube at somite levels 9 and 17, respectively, showed no Hoxc8
expression (Fig. 2 A and D). At more posterior levels, (at
somite levels 11 in mouse and 19 in chick), however, DiI label
coincided with the domain of Hoxc8 expression (Fig. 2 B and
D). These findings were confirmed on horizontal sections
(data not shown). Thus, Hoxc8 expression in the central
nervous system is transposed according to the functional
identity of expressing motor neurons.
Hoxc8 Early Enhancer of Mouse and Chick. The mouse
Hoxc8 early enhancer is involved in the establishment of spatial
domains of Hoxc8 expression (25, 26, 29). We isolated the
chicken Hoxc8 early enhancer to test whether the difference in
the spatiotemporal pattern of chicken Hoxc8 expression compared
with mouse is caused by differences in the chicken
enhancer. Primers were designed, based on most conserved
regions of Hoxc8 early enhancer, to amplify a 151-bp fragment
of the chicken enhancer by PCR. This enhancer is highly
conserved between the mouse and chicken with respect to
structure and overall sequence similarity (Fig. 3A). The sequence
similarity over 151 bp is 80%. The critical elements
(Fig. 3A, A–E) required for the mouse enhancer activity in
FIG. 2. (A–D) Expression of Hoxc8 in the brachial region of the
neural tube. Cross-section through the neural tube of a mouse embryo
at the level of the ninth (A) and 11th (B) somite. (C and D)
Cross-sections through the neural tube of a chicken embryo at the level
of the 17th and 19th somites. Hoxc8 expression is shown in green, DiI
label in red and resulting overlap in yellow. (E and F) Expression of
mouse and chicken reporter genes in transgenic mouse embryos. (E)
Construct A (399-bp mouse sequence); B, construct B (399-bp mousechicken
hybrid construct). The arrowheads indicates somite 14. The
arrow indicates the anterior limit of reporter gene expression mediated
by construct B (F). f, forelimb bud; nc, notochord; r, ventral root; v,
ventral horn.
Evolution: Belting et al. Proc. Natl. Acad. Sci. USA 95 (1998) 2357


and chicken [Hamburger and Hamilton (H&H) 24]
embryos were fixed in 4% paraformaldehyde in PBS. Brachial
motor neurons were retrograde-labeled by placing finely
ground 1,19-dioctadecyl-3,3,39,39-tetramethylindocarbocyanine
(DiI) crystals into one severed forelimb bud and a 1- to
The publication costs of this article were defrayed in part by page charge
payment. This article must therefore be hereby marked ‘‘advertisement’’ in
accordance with 18 U.S.C. §1734 solely to indicate this fact.
© 1998 by The National Academy of Sciences 0027-8424y98y952355-6$2.00y0
PNAS is available online at http:yywww.pnas.org.
Abbreviations: DiI, 1,19-dioctadecyl-3,3,39,39-tetramethylindocarbocyanine;
H&H, Hamburger and Hamilton.
Data deposition: The sequence reported in this paper has been
deposited in the GenBank database (accession no. AJ223359).
†Present address: Institute for Biology I, University of Freiburg,
Hauptstr. 1, D-79104 Freiburg, Germany.
§To whom reprint requests should be addressed. e-mail: frank.ruddle@
yale.edu.
2355

transgenic mice are arranged identically in the chicken enhancer.
In addition to clusters of substitutions between the
known sites, there are several differences within and in proximity
to these elements.
The chicken enhancer was tested for its ability to direct the
expression of a reporter gene (hsp68-lacZ) in transgenic mice to
determine whether differences in its nucleotide sequence from
that of mouse affects enhancer activity. A 399-bp DNA fragment
containing the mouse early enhancer region (construct A, Fig.
3B) directs the reporter gene expression in day 9.5 embryos to the
neural tube and paraxial mesoderm at the level of somite 14 and
19, respectively (ref. 28 and Fig. 2E). From this construct, the
151-bp fragment of the mouse enhancer containing the critical
elements of the enhancer was replaced with the corresponding
sequences from the chick. The resulting construct (construct B,
Fig. 3B) directed the expression of the reporter gene in day 9.5
embryos to more posterior regions of the embryo in both neural
tube and mesoderm (Fig. 2F). Five founder embryos that were
generated displayed identical patterns. Expression in the neural
tube was consistently found posterior to the 18th somite, whereas
expression in the mesoderm was several somites posterior to that
in the neural tube. Thus, chicken Hoxc8 enhancer differing from
the mouse counterpart in a few nucleotides directs the reporter
gene expression to a different anterior boundary in the neural
tube and mesoderm. This posteriorization of the reporter gene
activity in the neural tube and mesoderm is consistent with the
more posterior localized expression of Hoxc8 in the chick.
DISCUSSION
The investigation of the genetic basis of morphological diversity
among animals now has become feasible because of the
identification of highly conserved regulatory genes that control
embryonic patterning and morphogenesis. To examine the
correlation of Hox gene expression and axial variation, we have
compared the spatiotemporal distribution of Hoxc8 in mouse
and chick. Our findings can be summarized as follows: Hoxc8
expression is modified in concert with variation in axial
morphology within the paraxial mesoderm and the neural tube
(Fig. 4). Posteriorization of Hoxc8 expression in chicken is
achieved through a temporal delay of activation compared
FIG. 3. The Hoxc8 early enhancer in mouse and chick. (A) Nucleotide sequence comparison of the critical region of chicken and mouse Hoxc8
early enhancers. One hundred fifty-one bp of the critical region of the early enhancer are shown. Five elements essential for the activity of the early
enhancer are indicated (A–E). (B) Design of mouse and mouse-chicken hybrid reporter constructs. Construct A consists of a 399-bp early Hoxc8
enhancer ligated to a mouse hsp68ylacZ reporter gene. Construct B was generated by replacing 151 bp of the critical enhancer region of the mouse
with that of chick.
FIG. 4. Schematic comparison of Hoxc8 expression in chicken and
mouse in relationship to morphological landmarks. Cervical, thoracic,
and lumbar regions of the vertebral column and the brachial region of
the neural tube are indicated. Brachial spinal nerves C6, C7, C8, and
T1 in mouse and C13, C14, C15, and T1 in chicken are shown. Shaded
region in somites and neural tube represent Hoxc8 expression. Regions
of highest expression are indicated in dark shades. The double-headed
arrow indicates the anteroposterior orientation of the body axis. a,
anterior; p, posterior; nt, neural tube; t, thoracic vertebrae; s, somites;
sn, spinal nerves; v, vertebrae.
2358 Evolution: Belting et al. Proc. Natl. Acad. Sci. USA 95 (1998)

with mouse. The comparison of chicken and mouse enhancer
elements shows that only a few nucleotide changes within the
critical region of the early Hoxc8 enhancer suffice to transpose
reporter gene expression to more posterior body regions.
The role of Hox genes in the regionalization of the nervous
system has been examined most closely in the hindbrain (33).
Within the hindbrain Hox genes are expressed in the same
segments in mouse and chick, reflecting that the organization,
in respect to the number of rhombomeres, has been conserved
between these two species (34–36). In contrast, the brachial
region of the spinal cord is transposed in these species and our
results show that the shift of Hoxc8 expression along the body
axis corresponds to this anatomical modification.
The different axial position and extent of Hoxc8 expression
in the segmental mesoderm reflects two major differences in
the axial organization of the vertebral column of chicken and
mouse. First, the relative expansion of the cervical region in
the chicken is reflected by a posteriorization of Hoxc8 expression.
Second, the overall reduction of thoracic segments is
reflected by a reduced number of somites expressing Hoxc8
compared with the mouse. This association of Hoxc8 expression
with regional morphology of vertebrae suggests a role in
the specification of midthoracic identity within the paraxial
mesoderm. The axial fate of somites already is established in
the presomitic mesoderm (37). Thus, the observation that
Hoxc8 is found at the correct place and at the right time in the
segmental plate and before somite condensation is consistent
with a role in the establishment of thoracic identity in the
paraxial mesoderm. Further support for a causal relationship
between Hoxc8 expression and midthoracic identity stems
from genetic analyses in mice. Ectopic expression of Hoxc8 in
somites of the lower thoracic and upper lumbar region leads to
anterior transformations within this region, including the
formation of lumbar ribs (38). Similarly, disruption of Hoxc9
causes a posterior expansion of Hoxc8 expression and the
appearance of supernumerary ribs as well (39). These results
also demonstrate that anatomical regions can be expanded by
an extension of Hox gene expression along the primary body
axis and agrees well with the finding that a larger expression
domain of Hoxc8 in the mouse, compared with the chick, is
linked to a higher number of thoracic vertebrae. This finding
suggests that axial variation among amniotes is not only
generated by axial shifts in the anterior expression boundaries
of Hox genes, but also by expansion or reduction of their
overall expression domains.
The axial variation in Hoxc8 expression may be caused by
changes in the transcriptional regulation of Hoxc8. Differences
in Hox gene expression could be caused by genetic changes in
cis elements andyor trans-acting factors. Changes in transregulation
are more difficult to study because of the multiplicity
of potential (and lack of bona fide) regulatory proteins.
Changes in cis-elements in this report were studied by comparing
a minimal sequence of the Hoxc8 early enhancer. The
nucleotide sequence of the early enhancer region is highly
conserved among mammals and the sequence of cis-acting
elements (A–E) are invariant. Compared with mammalian
Hoxc8 early enhancer sequence, the chicken enhancer sequence
showed more nucleotide sequence changes. Many of
the nucleotide differences were observed in the vicinity of the
genetically defined sites, A, D, and E. It is conceivable that
these and other specific nucleotide differences contribute
toward overall posteriorization of the reporter gene expression
mediated through the chicken enhancer. In the case of the
mouse enhancer, mutations at individual sites A, C, D, and E,
lead to posteriorization of the reporter gene expression (28).
The anterior extent of the reporter gene expression is determined
by combinatorial interactions at these elements. In
addition, nucleotide changes outside of the defined elements
in the chicken enhancer may be affecting hitherto undefined
cis-acting elements. A systematic exchange of nucleotide sequences
between mouse and chicken enhancers will pinpoint
critical nucleotides involved in the posteriorization of the
reporter gene expression directed by the chicken enhancer.
Comparative analysis of vertebrate cis-regulatory regions,
using reporter gene assays in transgenic mouse embryos, have
shown remarkable conservation of transcriptional regulation
of Hox genes (13–15, 17–19, 22). Many of these elements direct
reporter gene expression to similar spatial domains in transgenic
mouse embryos. On the other hand, a chicken Hoxb4
enhancer, although capable of directing expression of the
reporter gene to the correct anterior boundary in the neural
tube, directed expression to a more posterior boundary in the
mesoderm, suggesting a species-specific differences in the
enhancer activity (18). Transcriptional heterochrony also has
been suggested to be an important mechanism by which subtle
changes in temporal colinearity of Hox genes may result in the
evolution of body plans (40). A replacement of a conserved
mouse Hoxd11 regulatory region with its zebrafish counterpart
lead to a slightly premature activation of Hoxd11, leading to
rostral shift of its expression boundary and anterior transposition
of the sacrum (22).
In conclusion, we have shown that the Hoxc8 expression in
mouse and chicken is similar with respect to anatomical
features such as the brachial spinal nerves and the midthoracic
region of the vertebral column. However, significant differences
also exist in relational features of the body plan such as
the ratio of cervical and thoracic domains. We also show in
mouse transgenic experiments where we compare the early
enhancers of mouse and chick that chicken enhancer constructs
simulate a chicken relational pattern of expression.
Additional experiments will be required to determine the
specificity of nucleotide changes in the regulation of Hoxc8
expression pattern and correlated modifications of the body
plan.
We thank Charles Bieberich and Gunter Wagner for discussion of
earlier versions of this manuscript. This work has been supported by
National Institutes of Health Grant GM09966 to F.H.R.
1. Ruddle, F. H., Bartels, J. L., Bentley, K. L., Kappen, C., Murtha,
M. T. & Pendleton, J. W. (1994) Annu. Rev. Genet. 28, 423–442.
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502–508.
5. Gaunt, S. J. (1994) Int. J. Dev. Biol. 38, 549–552.
6. Burke, A. C., Nelson, C. E., Morgan, B. A. & Tabin, C. (1995)
Development (Cambridge, U.K.) 121, 333–346.
7. Averof, M. & Patel, N. H. Nature (London) 388, 682–686.
8. Malicki, J., Schughart, K. & McGinnis, W. (1990) Cell 63,
961–967.
9. McGinnis, N., Kuziora, M. A. & McGinnis, W. (1990) Cell 63,
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12. Zhao, J. J., Lazzarini, R. A. & Pick, L. (1993) Genes Dev. 7,
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Brenner, S. & Krumlauf, R. (1994) Nature (London) 370, 567–
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14. Studer, M., Popperl, H., Marshall, H., Kuroiwa, A. & Krumlauf,
R. (1994) Science 265, 1728–1732.
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C., Rigby, P., Krumlauf, R.&Brenner, S. (1995) Proc. Natl. Acad.
Sci. USA 92, 1684–1688.
16. Frasch, M., Chen, X. & Lufkin, T. (1995) Development (Cambridge,
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Evolution: Belting et al. Proc. Natl. Acad. Sci. USA 95 (1998) 2359

[Cessation]

lol calftats mad

[wontstartdumbthreads]

A black hole is a region of space from which nothing, not even light, can escape. It is the result of the deformation of spacetime caused by a very compact mass. Around a black hole there is an undetectable surface which marks the point of no return, called an event horizon. It is called "black" because it absorbs all the light that hits it, reflecting nothing, just like a perfect black body in thermodynamics.[1] Quantum mechanics predicts that black holes also emit radiation like a black body with a finite temperature. This temperature decreases with the mass of the black hole, making it unlikely to observe this radiation for black holes of stellar mass.

Despite its invisible interior, a black hole can be observed through its interaction with other matter. A black hole can be inferred by tracking the movement of a group of stars that orbit a region in space. Alternatively, when gas falls into a stellar black hole from a companion star, the gas spirals inward, heating to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and Earth-orbiting telescopes.

Astronomers have identified numerous stellar black hole candidates, and have also found evidence of supermassive black holes at the center of galaxies. In 1998, astronomers found compelling evidence that a supermassive black hole of more than 2 million solar masses is located near the Sagittarius A* region in the center of the Milky Way galaxy, and more recent results using additional data find evidence that the supermassive black hole is more than 4 million solar masses.
Contents
[hide]

* 1 History
o 1.1 General relativity
o 1.2 Golden age
* 2 Properties and structure
o 2.1 Physical properties
o 2.2 Event horizon
o 2.3 Singularity
o 2.4 Photon sphere
o 2.5 Ergosphere
* 3 Formation and evolution
o 3.1 Gravitational collapse
+ 3.1.1 Primordial black holes in the Big Bang
o 3.2 High-energy collisions
o 3.3 Growth
o 3.4 Evaporation
* 4 Observational evidence
o 4.1 Accretion of matter
o 4.2 X-ray binaries
+ 4.2.1 Quiescence and advection-dominated accretion flow
+ 4.2.2 Quasi-periodic oscillations
o 4.3 Gamma ray bursts
o 4.4 Galactic nuclei
o 4.5 Gravitational lensing
o 4.6 Alternatives
* 5 Open questions
o 5.1 Entropy and thermodynamics
o 5.2 Black hole unitarity
* 6 See also
* 7 Notes
* 8 References
* 9 Further reading
* 10 External links

History
Schwarzschild black hole
Simulation of gravitational lensing by a black hole which distorts the image of a galaxy in the background (click here for larger animation)

The idea of a body so massive that even light could not escape was first put forward by geologist John Michell in a letter written to Henry Cavendish in 1783 to the Royal Society:

If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae, with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity.
—John Michell[2]

In 1796, mathematician Pierre-Simon Laplace promoted the same idea in the first and second editions of his book Exposition du système du Monde (it was removed from later editions).[3][4] Such "dark stars" were largely ignored in the nineteenth century, since it was not understood how a massless wave such as light could be influenced by gravity.[5]
General relativity

In 1915, Albert Einstein developed his theory of general relativity, having earlier shown that gravity does influence light's motion. A few months later, Karl Schwarzschild gave the solution for the gravitational field of a point mass and a spherical mass.[6] A few months after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution for the point mass and wrote more extensively about its properties.[7] This solution had a peculiar behaviour at what is now called the Schwarzschild radius, where it became singular, meaning that some of the terms in the Einstein equations became infinite. The nature of this surface was not quite understood at the time. In 1924, Arthur Eddington showed that the singularity disappeared after a change of coordinates (see Eddington coordinates), although it took until 1933 for Georges Lemaître to realize that this meant the singularity at the Schwarzschild radius was an unphysical coordinate singularity.[8]

In 1931, Subrahmanyan Chandrasekhar calculated, using general relativity, that a non-rotating body of electron-degenerate matter above 1.44 solar masses (the Chandrasekhar limit) would collapse.[citation needed] His arguments were opposed by many of his contemporaries like Eddington and Lev Landau, who argued that some yet unknown mechanism would stop the collapse.[9] They were partly correct: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star,[10] which is itself stable because of the Pauli exclusion principle. But in 1939, Robert Oppenheimer and others predicted that neutron stars above approximately three solar masses (the Tolman–Oppenheimer–Volkoff limit) would collapse into black holes for the reasons presented by Chandrasekhar, and concluded that no law of physics was likely to intervene and stop at least some stars from collapsing to black holes.[11]

Oppenheimer and his co-authors interpreted the singularity at the boundary of the Schwarzschild radius as indicating that this was the boundary of a bubble in which time stopped. This is a valid point of view for external observers, but not for infalling observers. Because of this property, the collapsed stars were called "frozen stars,"[12] because an outside observer would see the surface of the star frozen in time at the instant where its collapse takes it inside the Schwarzschild radius. This is a known property of modern black holes, but it must be emphasized that the light from the surface of the frozen star becomes redshifted very fast, turning the black hole black very quickly. Many physicists could not accept the idea of time standing still at the Schwarzschild radius, and there was little interest in the subject for over 20 years.
Golden age
See also: Golden age of general relativity

In 1958, David Finkelstein identified the Schwarzschild surface r = 2m [in geometrized units, i.e. 2Gm/c2, where r is the radius of the surface and m is the mass of the black hole] as an event horizon, "a perfect unidirectional membrane: causal influences can cross it in only one direction".[13] This did not strictly contradict Oppenheimer's results, but extended them to include the point of view of infalling observers. Finkelstein's solution extended the Schwarzschild solution for the future of observers falling into the black hole. A complete extension had already been found by Martin Kruskal, who was urged to publish it.[14]

These results came at the beginning of the golden age of general relativity, which is marked by general relativity and black holes becoming mainstream subjects of research. This process was helped by the discovery of pulsars in 1967,[15][16] which were within a few years shown to be rapidly rotating neutron stars. Until that time, neutron stars, like black holes, were regarded as just theoretical curiosities; but the discovery of pulsars showed their physical relevance and spurred a further interest in all types of compact objects that might be formed by gravitational collapse.

In this period more general black hole solutions where found. In 1963, Roy Kerr found the exact solution for a rotating black hole. Two years later Ezra T. Newman found the axisymmetric solution for a black hole which is both rotating and electrically charged.[17] Through the work of Werner Israel,[18] Brandon Carter,[19][20] and D. C. Robinson[21] the no-hair theorem emerged, stating that a stationary black hole solution is completely described by the three parameters of the Kerr–Newman metric; mass, angular momentum, and electric charge.[22]

For a long time, it was suspected that the strange features of the black hole solutions were pathological artefacts from the symmetry conditions imposed, and that the singularities would not appear in generic situations. This view was held in particular by Belinsky, Khalatnikov, and Lifshitz, who tried to prove that no singularities appear in generic solutions. However, in the late sixties Roger Penrose[23] and Stephen Hawking used global techniques to prove that singularities are generic.[24]

Work by James Bardeen, Jacob Bekenstein, Carter, and Hawking in the early 1970s led to the formulation of the laws of black hole mechanics.[25] These laws describe the behaviour of a black hole in close analogy to the laws of thermodynamics by relating mass to energy, area to entropy, and surface gravity to temperature. The analogy was completed when Hawking, in 1974, showed that quantum field theory predicts that black holes should radiate like a black body with a temperature proportional to the surface gravity of the black hole.[26]

The term "black hole" was first publicly used by John Wheeler during a lecture in 1967. Although he is usually credited with coining the phrase, he always insisted that it was suggested to him by somebody else. The first recorded use of the term is in a 1964 letter by Anne Ewing to the American Association for the Advancement of Science.[27] After Wheeler's use of the term, it was quickly adopted in general use.
Properties and structure

The no-hair theorem states that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, charge, and angular momentum.[22] Any two black holes that share the same values for these properties, or parameters, are indistinguishable according to classical (i.e. non-quantum) mechanics.

These properties are special because they are visible from outside the black hole. For example, a charged black hole repels other like charges just like any other charged object. Similarly, the total mass inside a sphere containing a black hole can be found by using the gravitational analog of Gauss's law, the ADM mass, far away from the black hole.[28] Likewise, the angular momentum can be measured from far away using frame dragging by the gravitomagnetic field.

When an object falls into a black hole, any information about the shape of the object or distribution of charge on it is evenly distributed along the horizon of the black hole, and is lost to outside observers. The behavior of the horizon in this situation is closely analogous to that of a conductive stretchy membrane with friction and electrical resistance, a dissipative system (see membrane paradigm).[29] This is different from other field theories like electromagnetism, which does not have any friction or resistivity at the microscopic level, because they are time-reversible. Because the black hole eventually achieves a stable state with only three parameters, there is no way to avoid losing information about the initial conditions: the gravitational and electric fields of the black hole give very little information about what went in. The information that is lost includes every quantity that cannot be measured far away from the black hole horizon, including the total baryon number, lepton number, and all the other nearly conserved pseudo-charges of particle physics. This behavior is so puzzling that it has been called the black hole information loss paradox.[30][31][32]
Physical properties

The simplest black hole has mass but neither electric charge nor angular momentum. These black holes are often referred to as Schwarzschild black holes after the physicist Karl Schwarzschild who discovered this solution in 1915.[6] According to Birkhoff's theorem, it is the only vacuum solution that is spherically symmetric.[33] This means that there is no observable difference between the gravitational field of such a black hole and that of any other spherical object of the same mass. The popular notion of a black hole "sucking in everything" in its surroundings is therefore only correct near the black hole horizon; far away, the external gravitational field is identical to that of any other body of the same mass.[34]

Solutions describing more general black holes also exist. Charged black holes are described by the Reissner-Nordström metric, while the Kerr metric describes a rotating black hole. The most general stationary black hole solution known is the Kerr-Newman metric, which describes a black hole with both charge and angular momentum.

While the mass of a black hole can take any positive value, the charge and angular momentum are constrained by the mass. In Planck units, the total electric charge Q and the total angular momentum J are expected to satisfy

Q^2+\left ( \tfrac{J}{M} \right )^2\le M^2\,

for a black hole of mass M. Black holes saturating this inequality are called extremal. Solutions of Einstein's equations violating the inequality exist, but do not have a horizon. These solutions have naked singularities and are deemed unphysical. The cosmic censorship hypothesis rules out the formation of such singularities through the gravitational collapse of realistic matter.[35] This is supported by numerical simulations.[36]

Due to the relatively large strength of the electromagnetic force, black holes forming from the collapse of stars are expected to retain the nearly neutral charge of the star. Rotation, however, is expected to be a common feature of compact objects, and the black-hole candidate binary X-ray source GRS 1915+105[37] appears to have an angular momentum near the maximum allowed value.
Class Mass Size
Supermassive black hole ~105–109 MSun ~0.001–10 AU
Intermediate-mass black hole ~103 MSun ~103 km = REarth
Stellar black hole ~10 MSun ~30 km
Micro black hole up to ~MMoon up to ~0.1 mm

Black holes are commonly classified according to their mass, independent of angular momentum J or electric charge Q. The size of a black hole, as determined by the radius of the event horizon, or Schwarzschild radius, is roughly proportional to the mass M through

r_\mathrm{sh} =\frac{2GM}{c^2} \approx 2.95\, \frac{M}{M_\mathrm{Sun}}~\mathrm{km,}

where rsh is the Schwarzschild radius and MSun is the mass of the Sun. This relation is exact only for black holes with zero charge and angular momentum, for more general black holes it can differ up to a factor of 2. The table on the right lists the various classes of black hole that are distinguished.
Event horizon
Main article: Event horizon
Image:BH-no-escape-1.svg
Far away from the black hole a particle can move in any direction. It is only restricted by the speed of light.
Image:BH-no-escape-2.svg
Closer to the black hole spacetime starts to deform. There are more paths going towards the black hole than paths moving away.
Image:BH-no-escape-3.svg
Inside of the event horizon all paths bring the particle closer to the center of the black hole. It is no longer possible for the particle to escape.

The defining feature of a black hole is the appearance of an event horizon—a boundary in spacetime through which matter and light can only pass inward towards the mass of the black hole. Nothing, not even light, can escape from inside the event horizon. The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if such an event occurred.[38]

As predicted by general relativity, the presence of a large mass deforms spacetime in such a way that the paths taken by particles bend towards the mass. At the event horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hole.[39]

To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole.[40] Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it.[41] At the same time, all processes on this object slow down causing emitted light to appear redder and dimmer, an effect known as gravitational redshift.[42] Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.

On the other hand, an observer falling into a black hole does not notice any of these effects as he crosses the event horizon. According to his own clock, he crosses the event horizon after a finite time, although he is unable to determine exactly when he crosses it, as it is impossible to determine the location of the event horizon from local observations.[43]

For a non rotating (static) black hole, the Schwarzschild radius delimits a spherical event horizon. The Schwarzschild radius of an object is proportional to the mass.[44] Rotating black holes have distorted, nonspherical event horizons. Since the event horizon is not a material surface but rather merely a mathematically defined demarcation boundary, nothing prevents matter or radiation from entering a black hole, only from exiting one. The description of black holes given by general relativity is known to be an approximation, and some scientists expect that quantum gravity effects will become significant near the vicinity of the event horizon.[45] This would allow observations of matter near a black hole's event horizon to be used to indirectly study general relativity and proposed extensions to it.
Singularity
Main article: Gravitational singularity

At the center of a black hole as described by general relativity lies a gravitational singularity, a region where the spacetime curvature becomes infinite.[46] For a non-rotating black hole this region takes the shape of a single point and for a rotating black hole it is smeared out to form a ring singularity lying in the plane of rotation.[47] In both cases the singular region has zero volume. It can also be shown that the singular region contains all the mass of the black hole solution.[48] The singular region can thus be thought of as having infinite density.

An observer falling into a Schwarzschild black hole (i.e. non-rotating and no charges) cannot avoid the singularity. Any attempt to do so will only shorten the time taken to get there.[49] When they reach the singularity, they are crushed to infinite density and their mass is added to the total of the black hole. Before that happens, they will have been torn apart by the growing tidal forces in a process sometimes referred to as spaghettification or the noodle effect.[50]

In the case of a charged (Reissner–Nordström) or rotating (Kerr) black hole it is possible to avoid the singularity. Extending these solutions as far as possible reveals the hypothetical possibility of exiting the black hole into a different spacetime with the black hole acting as a wormhole.[51] The possibility of traveling to another universe is however only theoretical, since any perturbation will destroy this possibility.[52] It also appears to be possible to follow closed timelike curves (going back to one's own past) around the Kerr singularity, which lead to problems with causality like the grandfather paradox.[53] It is expected that none of these peculiar effects would survive in a proper quantum mechanical treatment of rotating and charged black holes.[54]

The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory.[55] This breakdown, however, is expected; it occurs in a situation where quantum mechanical effects should describe these actions due to the extremely high density and therefore particle interactions. To date it has not been possible to combine quantum and gravitational effects into a single theory. It is generally expected that a theory of quantum gravity will feature black holes without singularities.[56][57]
Photon sphere
Main article: Photon sphere

The photon sphere is a spherical boundary of zero thickness such that photons moving along tangents to the sphere will be trapped in a circular orbit. For non-rotating black holes, the photon sphere has a radius 1.5 times the Schwarzschild radius. The orbits are dynamically unstable, hence any small perturbation (such as a particle of infalling matter) will grow over time, either setting it on an outward trajectory escaping the black hole or on an inward spiral eventually crossing the event horizon.

While light can still escape from inside the photon sphere, any light that crosses the photon sphere on an inbound trajectory will be captured by the black hole. Hence any light reaching an outside observer from inside the photon sphere must have been emitted by objects inside the photon sphere but still outside of the event horizon.

Other compact objects, such as neutron stars, can also have photon spheres.[58] This follows from the fact that the gravitational field of an object does not depend on its actual size, hence any object that is smaller than 1.5 times the Schwarzschild radius corresponding to its mass will indeed have a photon sphere.
Ergosphere
Main article: Ergosphere
The ergosphere is an oblate spheroid region outside of the event horizon, where objects cannot remain stationary.

Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still, called the ergosphere. This is the result of a process known as frame-dragging; general relativity predicts that any rotating mass will tend to slightly "drag" along the spacetime immediately surrounding it. Any object near the rotating mass will tend to start moving in the direction of rotation. For a rotating black hole this effect becomes so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still.[59]

The ergosphere of a black hole is bounded by the (outer) event horizon on the inside and an oblate spheroid, which coincides with the event horizon at the poles and is noticeably wider around the equator. The outer boundary is sometimes called the ergosurface.

Objects and radiation can escape normally from the ergosphere. Through the Penrose process, objects can emerge from the ergosphere with more energy than they entered. This energy is taken from the rotational energy of the black hole causing it to slow down.[60]
Formation and evolution

Considering the exotic nature of black holes, it may be natural to question if such bizarre objects could exist in nature or to suggest that they are merely pathological solutions to Einstein's equations. Einstein himself wrongly thought that black holes would not form, because he held that the angular momentum of collapsing particles would stabilize their motion at some radius.[61] This led the general relativity community to dismiss all results to the contrary for many years. However, a minority of relativists continued to contend that black holes were physical objects,[62] and by the end of the 1960s, they had persuaded the majority of researchers in the field that there is no obstacle to forming an event horizon.

Once an event horizon forms, Roger Penrose proved that a singularity will form somewhere inside it.[23] Shortly afterwards, Stephen Hawking showed that many cosmological solutions describing the Big Bang have singularities without scalar fields or other exotic matter (see Penrose-Hawking singularity theorems). The Kerr solution, the no-hair theorem and the laws of black hole thermodynamics showed that the physical properties of black holes were simple and comprehensible, making them respectable subjects for research.[63] The primary formation process for black holes is expected to be the gravitational collapse of heavy objects such as stars, but there are also more exotic processes that can lead to the production of black holes.
Gravitational collapse
Main article: Gravitational collapse

Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. For stars this usually occurs either because a star has too little "fuel" left to maintain its temperature through stellar nucleosynthesis, or because a star which would have been stable receives extra matter in a way which does not raise its core temperature. In either case the star's temperature is no longer high enough to prevent it from collapsing under its own weight (the ideal gas law explains the connection between pressure, temperature, and volume).[64]

The collapse may be stopped by the degeneracy pressure of the star's constituents, condensing the matter in an exotic denser state. The result is one of the various types of compact star. Which type of compact star is formed depends on the mass of the remnant — the matter left over after changes triggered by the collapse (such as supernova or pulsations leading to a planetary nebula) have blown away the outer layers. Note that this can be substantially less than the original star — remnants exceeding 5 solar masses are produced by stars which were over 20 solar masses before the collapse.[64]

If the mass of the remnant exceeds about 3–4 solar masses (the Tolman–Oppenheimer–Volkoff limit)—either because the original star was very heavy or because the remnant collected additional mass through accretion of matter—even the degeneracy pressure of neutrons is insufficient to stop the collapse. After this, no known mechanism (except possibly quark degeneracy pressure, see quark star) is powerful enough to stop the collapse and the object will inevitably collapse to a black hole.[64]

This gravitational collapse of heavy stars is assumed to be responsible for the formation of stellar mass black holes. Star formation in the young universe may have resulted in very heavy stars, which upon their collapse would have produced black holes of up to 103 solar masses. These heavy black holes could be the seeds of the supermassive black holes found in the centers of most galaxies.[65]

While most of the energy released during gravitational collapse is emitted very quickly, an outside observer does not actually see the end of this process. Even though the collapse takes a finite amount of time from the reference frame of infalling matter, a distant observer sees the infalling material slow and halt just above the event horizon, due to gravitational time dilation. Light from the collapsing material takes longer and longer to reach the observer, with the light emitted just before the event horizon forms delayed an infinite amount of time. Thus the external observer never sees the formation of the event horizon; instead, the collapsing material seems to become dimmer and increasingly red-shifted, eventually fading away.[66]
Primordial black holes in the Big Bang

Gravitational collapse requires great densities. In the current epoch of the universe these high densities are only found in stars, but in the early universe shortly after the big bang densities were much greater, possibly allowing for the creation of black holes. The high density alone is not enough to allow the formation of black holes since a uniform mass distribution will not allow the mass to bunch up. In order for primordial black holes to form in such a dense medium, there must be initial density perturbations which can then grow under their own gravity. Different models for the early universe vary widely in their predictions of the size of these perturbations. Various models predict the creation of black holes, ranging from a Planck mass to hundreds of thousands of solar masses.[67] Primordial black holes could thus account for the creation of any type of black hole.
High-energy collisions
A simulated event in the CMS detector, a collision in which a micro black hole may be created.

Gravitational collapse is not the only process that could create black holes. In principle, black holes could also be created in high-energy collisions that create sufficient density. However, to date, no such events have ever been detected either directly or indirectly as a deficiency of the mass balance in particle accelerator experiments.[68] This suggests that there must be a lower limit for the mass of black holes. Theoretically, this boundary is expected to lie around the Planck mass (mP = √ħc/G ≈ 1.2×1019 GeV/c2 ≈ 2.2×10−8 kg), where quantum effects are expected to make the theory of general relativity break down completely.[69] This would put the creation of black holes firmly out of reach of any high energy process occurring on or near the Earth. Certain developments in quantum gravity however suggest that the Planck mass could be much lower: some braneworld scenarios for example put it much lower, maybe even as low as 1 TeV/c2[70] This would make it possible for micro black holes to be created in the high energy collisions occurring when cosmic rays hit the Earth's atmosphere, or possibly in the new Large Hadron Collider at CERN. These theories are however very speculative, and the creation of black holes in these processes is deemed unlikely by many specialists.[71] Even if such micro black holes should be formed in these collisions, it is expected that they would evaporate in about 10−25 seconds, posing no threat to Earth[72]
Growth

Once a black hole has formed, it can continue to grow by absorbing additional matter. Any black hole will continually absorb gas and interstellar dust from its direct surroundings and omnipresent cosmic background radiation. This is the primary process through which supermassive black holes seem to have grown.[65] A similar process has been suggested for the formation of intermediate-mass black holes in globular clusters.[73]

Another possibility is for a black hole to merge with other objects such as stars or even other black holes. This is thought to have been important especially for the early development of supermassive black holes, which are thought to have formed from the coagulation of many smaller objects.[65] The process has also been proposed as the origin of some intermediate-mass black holes.[74][75]
Evaporation
Main article: Hawking radiation

In 1974, Stephen Hawking showed that black holes are not entirely black but emit small amounts of thermal radiation.[26] He got this result by applying quantum field theory in a static black hole background. The result of his calculations is that a black hole should emit particles in a perfect black body spectrum. This effect has become known as Hawking radiation. Since Hawking's result, many others have verified the effect through various methods.[76] If his theory of black hole radiation is correct, then black holes are expected to emit a thermal spectrum of radiation, and thereby lose mass, because according to the theory of relativity mass is just highly condensed energy (E = mc2).[26] Black holes will shrink and evaporate over time. The temperature of this spectrum (Hawking temperature) is proportional to the surface gravity of the black hole, which for a Schwarzschild black hole is inversely proportional to the mass. Large black holes, therefore, emit less radiation than small black holes.

A stellar black hole of one solar mass has a Hawking temperature of about 100 nanokelvins. This is far less than the 2.7 K temperature of the cosmic microwave background. Stellar mass (and larger) black holes receive more mass from the cosmic microwave background than they emit through Hawking radiation and will thus grow instead of shrink. To have a Hawking temperature larger than 2.7 K (and be able to evaporate), a black hole needs to be lighter than the Moon (and therefore a diameter of less than a tenth of a millimeter).[77]

On the other hand, if a black hole is very small the radiation effects are expected to become very strong. Even a black hole that is heavy compared to a human would evaporate in an instant. A black hole the weight of a car (~10−24 m) would only take a nanosecond to evaporate, during which time it would briefly have a luminosity more than 200 times that of the sun. Lighter black holes are expected to evaporate even faster, for example a black hole of mass 1 TeV/c2 would take less than 10−88 seconds to evaporate completely. Of course, for such a small black hole quantum gravitation effects are expected to play an important role and could even – although current developments in quantum gravity do not indicate so[78] – hypothetically make such a small black hole stable.[79]
Observational evidence

By their very nature, black holes do not directly emit any signals other than the hypothetical Hawking radiation; since the Hawking radiation for an astrophysical black hole is predicted to be very weak, this makes it impossible to directly detect astrophysical black holes from the Earth. A possible exception to the Hawking radiation being weak is the last stage of the evaporation of light (primordial) black holes; searches for such flashes in the past has proven unsuccessful and provides stringent limits on the possibility of existence of light primordial black holes.[80] NASA's Fermi Gamma-ray Space Telescope launched in 2008 will continue the search for these flashes.[81]

Astrophysicists searching for black holes thus have to rely on indirect observations. A black hole's existence can sometimes be inferred by observing its gravitational interactions with its surroundings.
Accretion of matter
See also: Accretion disc
Formation of extragalactic jets from a black hole's accretion disk

Due to conservation of angular momentum, gas falling into the gravitational well created by a massive object will typically form a disc-like structure around the object. Friction within the disc causes angular momentum to be transported outward allowing matter to fall further inward releasing potential energy and increasing the temperature of the gas.[82] In the case of compact objects such as white dwarfs, neutron stars, and black holes, the gas in the inner regions becomes so hot that it will emit vast amounts of radiation (mainly X-rays), which may be detected by telescopes. This process of accretion is one of the most efficient energy producing process known; up to 40% of the rest mass of the accreted material can be emitted in radiation.[82] (In nuclear fusion only about 0.7% of the rest mass will be emitted as energy.) In many cases, accretion discs are accompanied by relativistic jets emitted along the poles, which carry away much of the energy. The mechanism for the creation of these jets is currently not well understood.

As such many of the universe's more energetic phenomena have been attributed to the accretion of matter on black holes. In particular, active galactic nuclei and quasars are thought to be the accretion discs of supermassive black holes.[83] Similarly, X-ray binaries are thought to be binary star systems in which one of the two stars is a compact object accreting matter from its companion.[83] It has also been suggested that some ultraluminous X-ray sources may be the accretion disks of intermediate-mass black holes.[84]
X-ray binaries
See also: X-ray binary

X-ray binaries are binary star systems that are luminous in the X-ray part of the spectrum. These X-ray emissions are generally thought to be caused by one of the component stars being a compact object accreting matter from the other (regular) star. The presence of an ordinary star in such a system provides a unique opportunity for studying the central object and determining if it might be a black hole.
Artist impression of a binary system with an accretion disk around a compact object being fed by material from the companion star.

If such a system emits signals that can be directly traced back to the compact object, it cannot be a black hole. The absence of such a signal does, however, not exclude the possibility that the compact object is a neutron star. By studying the companion star it is often possible to obtain the orbital parameters of the system and obtain an estimate for the mass of the compact object. If this is much larger than the Tolman–Oppenheimer–Volkoff limit (that is, the maximum mass a neutron star can have before collapsing) then the object cannot be a neutron star and is generally expected to be a black hole.[83]

The first strong candidate for a black hole, Cygnus X-1, was discovered in this way by Charles Thomas Bolton[85] and Webster and Murdin[86] in 1972.[87][88] Some doubt, however, remained due to the uncertainties resultant from the companion star being much heavier than the candidate black hole.[83] Currently, better candidates for black holes are found in a class of X-ray binaries called soft X-ray transients.[83] In this class of system the companion star is relatively low mass allowing for more accurate estimates in the black hole mass. Moreover, these systems are only active in X-ray for several months once every 10–50 years. During the period of low X-ray emission (called quiescence), the accretion disc is extremely faint allowing for detailed observation of the companion star during this period. One of the best such candidates is V404 Cyg.
Quiescence and advection-dominated accretion flow

The faintness of the accretion disc during quiescence is thought to be caused by the flow entering a mode called an advection-dominated accretion flow (ADAF). In this mode, almost all the energy generated by friction in the disc is swept along with the flow instead of radiated away. If this model is correct, then it forms strong qualitative evidence for the presence of an event horizon.[89] Because, if the object at the center of the disc had a solid surface, it would emit large amounts of radiation as the highly energetic gas hits the surface, an effect that is observed for neutron stars in a similar state.[82]
Quasi-periodic oscillations
See also: Quasi-periodic oscillations

The X-ray emissions from accretion disks sometimes exhibit a flickering around certain frequencies. These signals are called quasi-periodic oscillations and are thought to be caused by material moving along the inner edge of the accretion disk (the innermost stable circular orbit). As such their frequency is linked to the mass of the compact object. They can thus be used as an alternative way to determine the mass of potential black holes.[90]
Gamma ray bursts

Intense but one-time gamma ray bursts (GRBs) may signal the birth of "new" black holes, because astrophysicists think that GRBs are caused either by the gravitational collapse of giant stars[91] or by collisions between neutron stars,[92] and both types of event involve sufficient mass and pressure to produce black holes. It appears that a collision between a neutron star and a black hole can also cause a GRB,[93] so a GRB is not proof that a "new" black hole has been formed. All known GRBs come from outside our own galaxy, and most come from billions of light years away[94] so the black holes associated with them are billions of years old.
Galactic nuclei
See also: Active galactic nucleus
The jet originating from the center of M87 in this image comes from an active galactic nucleus that may contain a supermassive black hole. Credit: Hubble Space Telescope/NASA/ESA.

It is now widely accepted that the center of every or at least nearly every galaxy contains a supermassive black hole.[95] The close observational correlation between the mass of this hole and the velocity dispersion of the host galaxy's bulge, known as the M-sigma relation, strongly suggests a connection between the formation of the black hole and the galaxy itself. [96]

For decades, astronomers have used the term "active galaxy" to describe galaxies with unusual characteristics, such as unusual spectral line emission and very strong radio emission.[97][98] However, theoretical and observational studies have shown that the active galactic nuclei (AGN) in these galaxies may contain supermassive black holes.[97][98] The models of these AGN consist of a central black hole that may be millions or billions of times more massive than the Sun; a disk of gas and dust called an accretion disk; and two jets that are perpendicular to the accretion disk.[98]

Although supermassive black holes are expected to be found in most AGN, only some galaxies' nuclei have been more carefully studied in attempts to both identify and measure the actual masses of the central supermassive black hole candidates. Some of the most notable galaxies with supermassive black hole candidates include the Andromeda Galaxy, M32, M87, NGC 3115, NGC 3377, NGC 4258, and the Sombrero Galaxy.[99]

Currently, the best evidence for a supermassive black hole comes from studying the proper motion of stars near the center of our own Milky Way.[100] Since 1995 astronomers have tracked the motion of 90 stars in a region called Sagittarius A*. By fitting their motion to Keplerian orbits they were able to infer in 1998 that 2.6 million solar masses must be contained in a volume with a radius of 0.02 lightyears.[101] Since then one of the stars—called S2—has completed a full orbit. From the orbital data they were able to place better constraints on the mass and size of the object causing the orbital motion of stars in the Sagittarius A* region, finding that there is a spherical mass of 4.3 million solar masses contained within a radius of less than 0.002 lightyears.[100] While this is more than 3000 times the Schwarzschild radius corresponding to that mass, it is at least consistent with the central object being a supermassive black hole, and no "realistic cluster [of stars] is physically tenable."[101]
Gravitational lensing
Further information: Gravitational lens

The deformation of spacetime around a massive object causes light rays to be deflected much like light passing through an optic lens. This phenomenon is known as gravitational lensing. Observations have been made of weak gravitational lensing, in which photons are deflected by only a few arcseconds. However, it has never been directly observed for a black hole.[102] One possibility for observing gravitational lensing by a black hole would be to observe stars in orbit around the black hole. There are several candidates for such an observation in orbit around Sagittarius A*.[102]
Alternatives

The evidence for stellar black holes strongly relies on the existence of an upper limit for the mass of a neutron star. The size of this limit heavily depends on the assumptions made about the properties of dense matter. New exotic phases of matter could push up this bound.[83] A phase of free quarks at high density might allow the existence of dense quark stars,[103] and some supersymmetric models predict the existence of Q stars.[104] Some extensions of the standard model posit the existence of preons as fundamental building blocks of quarks and leptons which could hypothetically form preon stars.[105] These hypothetical models could potentially explain a number of observations of stellar black hole candidates. However, it can be shown from general arguments in general relativity that any such object will have a maximum mass.[83]

Since the average density of a black hole inside its Schwarzschild radius is inversely proportional to the square of its mass, supermassive black holes are much less dense than stellar black holes (the average density of a large supermassive black hole is comparable to that of water).[83] Consequently, the physics of matter forming a supermassive black hole is much better understood and the possible alternative explanations for supermassive black hole observations are much more mundane. For example, a supermassive black hole could be modelled by a large cluster of very dark objects. However, typically such alternatives are not stable enough to explain the supermassive black hole candidates.[83]

The evidence for stellar and supermassive black holes implies that in order for black holes not to form, general relativity must fail as a theory of gravity, perhaps due to the onset of quantum mechanical corrections. A much anticipated feature of a theory of quantum gravity is that it will not feature singularities or event horizons (and thus no black holes).[106] In recent years, much attention has been drawn by the fuzzball model in string theory. Based on calculations in specific situations in string theory, the proposal suggest that generically the individual states of a black hole solution do not have an event horizon or singularity (and can thus not really be considered to be a black hole), but that for a distant observer the statistical average of such states does appear just like an ordinary black hole in general relativity.[107]
Open questions
Entropy and thermodynamics
Further information: Black hole thermodynamics
If ultra-high-energy collisions of particles in a particle accelerator can create microscopic black holes, it is expected that all types of particles will be emitted by black hole evaporation, providing key evidence for any grand unified theory. Above are the high energy particles produced in a gold ion collision on the RHIC.

In 1971, Stephen Hawking showed under general conditions[Note 1] that the total area of the event horizons of any collection of classical black holes can never decrease, even if they collide and merge.[108] This result, now known as the second law of black hole mechanics, is remarkably similar to the second law of thermodynamics, which states that the total entropy of a system can never decrease. As with classical objects at absolute zero temperature, it was assumed that black holes had zero entropy. If this were the case, the second law of thermodynamics would be violated by entropy-laden matter entering the black hole, resulting in a decrease of the total entropy of the universe. Therefore, Jacob Bekenstein proposed that a black hole should have an entropy, and that it should be proportional to its horizon area.[109]

The link with the laws of thermodynamics was further strengthened by Hawking's discovery that quantum field theory predicts that a black hole radiates blackbody radiation at a constant temperature. This seemingly causes a violation of the second law of black hole mechanics, since the radiation will carry away energy from the black hole causing it to shrink. The radiation, however also carries away entropy, and it can be proven under general assumptions that the sum of the entropy of the matter surrounding the black hole and one quarter of the area of the horizon as measured in Planck units is in fact always increasing. This allows the formulation of the first law of black hole mechanics as an analogue of the first law of thermodynamics, with the mass acting as energy, the surface gravity as temperature and the area as entropy.[109]

One puzzling feature is that the entropy of a black hole scales with its area rather than with its volume, since entropy is normally an extensive quantity that scales linearly with the volume of the system. This odd property led 't Hooft and Susskind to propose the holographic principle, which suggests that anything that happens in volume of spacetime can be described by data on the boundary of that volume.[110]

Although general relativity can be used to perform a semi-classical calculation of black hole entropy, this situation is theoretically unsatisfying. In statistical mechanics, entropy is understood as counting the number of microscopic configurations of a system which have the same macroscopic qualities (such as mass, charge, pressure, etc.). Without a satisfactory theory of quantum gravity, one cannot perform such a computation for black holes. Some progress has been made in various approaches to quantum gravity. In 1995, Strominger and Vafa showed that counting the microstates of a specific supersymmetric black hole in string theory reproduced the Bekenstein–Hawking entropy.[111] Since then, similar results have been reported for different black holes both in string theory and in other approaches to quantum gravity like loop quantum gravity.[112]
Black hole unitarity
Main article: Black hole information paradox
Unsolved problems in physics Is physical information lost in black holes? Question mark2.svg

An open question in fundamental physics is the so-called information loss paradox, or black hole unitarity paradox. Classically, the laws of physics are the same run forward or in reverse (T-symmetry). Liouville's theorem dictates conservation of phase space volume, which can be thought of as "conservation of information", so there is some problem even in classical physics. In quantum mechanics, this corresponds to a vital property called unitarity, which has to do with the conservation of probability (it can also be thought of as a conservation of quantum phase space volume as expressed by the density matrix).[113]
See also

* Black holes in fiction
* Black string
* Kugelblitz (astrophysics)
* List of black holes
* Susskind-Hawking battle


He1523a.jpg Star portal

* Timeline of black hole physics
* White hole
* Wormhole

Notes

1. ^ In particular, he assumed that all matter satisfies the weak energy condition.

References

1. ^ Davies, P. C. W. (1978). "Thermodynamics of Black Holes". Rep. Prog. Phys. 41: 1313–1355. doi:10.1088/0034-4885/41/8/004. http://cosmos.asu.edu/publications/papers/ThermodynamicTheoryofBlackHoles%2034.pdf.
2. ^ Michell, J. (1784). "On the Means of Discovering the Distance, Magnitude, &c. of the Fixed Stars, in Consequence of the Diminution of the Velocity of Their Light, in Case Such a Diminution Should be Found to Take Place in any of Them, and Such Other Data Should be Procured from Observations, as Would be Farther Necessary for That Purpose". Phil. Trans. R. Soc. (London) (Philosophical Transactions of the Royal Society of London, Vol. 74) 74: 35–57. http://www.jstor.org/pss/106576.
3. ^ "Dark Stars (1783)". Thinkquest. 1999. http://library.thinkquest.org/25715/discovery/conceiving.htm#darkstars. Retrieved 2008-05-28.
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Further reading

Popular reading

* Ferguson, Kitty (1991). Black Holes in Space-Time. Watts Franklin. ISBN 0-531-12524-6. .
* Hawking, Stephen (1988). A Brief History of Time. Bantam Books, Inc. ISBN 0-553-38016-8. .
* Hawking, Stephen; Penrose, Roger (1996). The Nature of Space and Time. Princeton University Press. ISBN 0-691-03791-2. http://books.google.com/?id=LstaQTXP65cC. .
* Melia, Fulvio (2003). The Black Hole at the Center of Our Galaxy. Princeton U Press. ISBN 978-0-691-09505-9. .
* Melia, Fulvio (2003). The Edge of Infinity. Supermassive Black Holes in the Universe. Cambridge U Press. ISBN 978-0-521-81405-8. .
* Pickover, Clifford (1998). Black Holes: A Traveler's Guide. Wiley, John & Sons, Inc. ISBN 0-471-19704-1. .
* Stern, B. (2008). "Blackhole". http://www.wikilivres.info/wiki/Blackhole_%28Stern%29. , poem.
* Thorne, Kip S. (1994). Black Holes and Time Warps. Norton, W. W. & Company, Inc. ISBN 0-393-31276-3. .
* Wheeler, J. Craig (2007). Cosmic Catastrophes (2nd ed.). Cambridge University Press. ISBN 0-521-85714-7.

University textbooks and monographs

* Carroll, Sean M. (2004). Spacetime and Geometry. Addison Wesley. ISBN 0-8053-8732-3. , the lecture notes on which the book was based are available for free from Sean Carroll's website.
* Carter, B. (1973). "Black hole equilibrium states". In DeWitt, B.S.; DeWitt, C.. Black Holes. .
* Chandrasekhar, Subrahmanyan (1999). Mathematical Theory of Black Holes. Oxford University Press. ISBN 0-19-850370-9. .
* Frolov, V.P.; Novikov, I.D. (1998). Black hole physics. .
* Hawking, S.W.; Ellis, G.F.R. (1973). Large Scale Structure of space time. Cambridge University Press. ISBN 0521099064. http://books.google.com/?id=QagG_KI7Ll8C. .
* Melia, Fulvio (2007). The Galactic Supermassive Black Hole. Princeton U Press. ISBN 978-0-691-13129-0. .
* Taylor, Edwin F.; Wheeler, John Archibald (2000). Exploring Black Holes. Addison Wesley Longman. ISBN 0-201-38423-X. .
* Thorne, Kip S.; Misner, Charles; Wheeler, John (1973). Gravitation. W. H. Freeman and Company. ISBN 0-7167-0344-0. .
* Wald, Robert M. (1992). Space, Time, and Gravity: The Theory of the Big Bang and Black Holes. University of Chicago Press. ISBN 0-226-87029-4. .

Review papers

* Detweiler, S. (1981). "Resource letter BH-1: Black holes". American Journal of Physics 49 (5, pp): 394–400. doi:10.1119/1.12686.
* Gallo, E.; Marolf, D. (2009). "Resource Letter BH-2: Black Holes". American Journal of Physics 77: 294. doi:10.1119/1.3056569. arXiv:0806.2316. edit
* Hughes, Scott A. (2005). "Trust but verify: The case for astrophysical black holes". arΧiv:hep-ph/0511217v2 [hep-ph]. Lecture notes from 2005 SLAC Summer Institute.

External links
Wikimedia Commons has media related to: Black holes

* Stanford Encyclopedia of Philosophy: "Singularities and Black Holes" by Erik Curiel and Peter Bokulich.
* "Black hole" on Scholarpedia.
* Black Holes: Gravity's Relentless Pull - Interactive multimedia Web site about the physics and astronomy of black holes from the Space Telescope Science Institute
* FAQ on black holes
* "Schwarzschild Geometry" on Andrew Hamilton’s website
* UT Brownsville Group Simulates Spinning Black-Hole Binaries
* Advanced Mathematics of Black Hole Evaporation

Videos

* 16-year long study tracks stars orbiting Milky Way black hole
* Yale University Video Lecture: Introduction to Black Holes at Google Video.
* Movie of Black Hole Candidate from Max Planck Institute

News

* "Black Hole confirmed in Milky Way." Retrieved December 10, 2008
* Black Hole Research News

I took the liberty of putting this into Portugese.
Somtimes I get the tenses mixed up so my aplogies in advance.
Um buraco negro é uma região de espaço de que nada, não mesmo luz, pode escapar. É o resultado da deformação do spacetime causada por uma massa muito compacta. Em torno de um buraco negro há uma superfície undetectable que marque o ponto de nenhum retorno, chamada um horizonte de evento. É chamado " black" porque absorve toda a luz que batidas ele, não refletindo nada, apenas como um corpo preto perfeito no thermodynamics. [1] Os mecânicos do quantum prevêem que os buracos negros igualmente se emitem a radiação como um corpo preto com uma temperatura finita. Esta temperatura diminui com a massa do buraco negro, fazendo a pouco susceptível de observar esta radiação para buracos negros da massa estelar. Apesar de seu interior invisível, um buraco negro pode ser observado com sua interação com a outra matéria. Um buraco negro pode ser pressupor seguindo o movimento de um grupo de estrelas que orbitam uma região no espaço. Alternativamente, quando o gás cai em um buraco negro estelar de uma estrela de companheiro, o gás espirala para dentro, aquecendo-se muito às altas temperaturas e emitindo-se as grandes quantidades de radiação que podem ser detectadas dos telescópios earthbound e Terra-orbitando. Os astrónomos identificaram candidatos estelares numerosos do buraco negro, e igualmente encontraram a evidência de buracos negros supermassive no centro das galáxias. Em 1998, os astrónomos encontraram a evidência de obrigação que um buraco negro supermassive de mais de 2 milhão massas solares está encontrado perto da região de A* do Sagittarius no centro da galáxia da maneira leitosa, e uns resultados mais recentes que usam dados adicionais encontram a evidência que o buraco negro supermassive é mais de 4 milhão massas solares. Índices [couro cru] * 1 história relatividade geral do 1.1 época dourada do 1.2 * 2 propriedades e estruturas propriedades físicas do 2.1 horizonte de evento do 2.2 Singularity do 2.3 esfera do fotão do 2.4 o 2.5 Ergosphere * formação 3 e evolução colapso gravitacional do 3.1 + 3.1.1 buracos negros Primordial em Big Bang colisões alta-tensão do 3.2 crescimento do 3.3 evaporação do 3.4 * evidência 4 Observational aumento do 4.1 da matéria binários do raio X do 4.2 + Quiescence 4.2.1 e fluxo advecção-dominado do aumento + 4.2.2 oscilações Quase-periódicas estouros do raio de gama do 4.3 núcleos galácticos do 4.4 lensing gravitacional do 4.5 alternativas do 4.6 * 5 questões abertas entropia e thermodynamics do 5.1 unitarity do buraco negro do 5.2 * 6 vêem igualmente * 7 notas * 8 referências * leitura 9 mais adicional * 10 ligações externas História Buraco negro de Schwarzschild A simulação de lensing gravitacional por um buraco negro que distorça a imagem de uma galáxia no fundo (estala aqui para a animação maior) A idéia de um corpo tão maciço que mesmo a luz não poderia escapar foi propor primeiramente pelo geólogo John Michell em uma letra escrita a Henry Cavendish em 1783 à sociedade real: Se o semi-diameter de uma esfera da mesma densidade que o Sun era exceder isso do Sun na proporção de 500 a 1, um corpo que cai de uma altura infinita para ele adquiriria em sua maior velocidade de superfície do que aquele da luz, e conseqüentemente supor a luz para ser atraído pela mesma força em proporção a seus inertiae do vis, com outros corpos, toda a luz emissora de tal corpo seria feita para retornar para ela por sua própria gravidade apropriada. - John Michell [2] Em 1796, o matemático Pierre-Simon Laplace promoveu a mesma idéia nas primeiras e segundas edições de sua exposição du système du Monde do livro (foi removida de umas edições mais atrasadas). [3] [4] tal " stars" escuro; foram ignorados pela maior parte no 19o século, desde que não se compreendeu como uma onda massless tal como a luz poderia ser influenciada pela gravidade. [5] Relatividade geral Em 1915, Albert Einstein desenvolveu sua teoria da relatividade geral, mostrando mais cedo que a gravidade influencia light' movimento de s. Alguns meses mais tarde, Karl Schwarzschild deu a solução para o campo gravitacional de uma massa do ponto e de uma massa esférica. [6] Alguns meses após Schwarzschild, Johannes Droste, um estudante de Hendrik Lorentz, deu independente a mesma solução para a massa do ponto e escreveu-a mais extensivamente sobre suas propriedades. [7] Esta solução teve um comportamento peculiar em o que é chamado agora o raio de Schwarzschild, onde se tornou singular, significando que alguns dos termos nas equações de Einstein se tornaram infinitos. A natureza desta superfície não foi compreendida completamente naquele tempo. Em 1924, Arthur Eddington mostrou que o singularity desapareceu depois que uma mudança das coordenadas (veja coordenadas de Eddington), embora tomasse até 1933 para Georges Lemaître para realizar que esta significou o singularity no raio de Schwarzschild era um singularity coordenado unphysical. [8] Em 1931, Subrahmanyan Chandrasekhar calculou, usando a relatividade geral, que um corpo degiro da matéria elétron-degenerate acima de 1.44 massas solares (o limite de Chandrasekhar) desmoronaria. [citação necessário] seus argumentos foram opor por muitos de seus comtemporâneos como Eddington e Landau do lev, que discutiu que algum contudo o mecanismo desconhecido pararia o colapso. [9] Estavam em parte corretos: um anão branco ligeiramente mais maciço do que o limite de Chandrasekhar desmoronará em uma estrela de nêutron, [10] que sejam próprias estáveis por causa do princípio de exclusão de Pauli. Mas em 1939, Robert Oppenheimer e outro previram que as estrelas de nêutron acima de aproximadamente três massas solares (o limite de Tolman-Oppenheimer-Volkoff) desmoronariam em buracos negros para as razões apresentadas por Chandrasekhar, e concluíram que nenhuma lei de física era provável intervir pelo menos e parar algumas estrelas do desmoronamento aos buracos negros. [11] Oppenheimer e seus co-autores interpretaram o singularity no limite do raio de Schwarzschild porque indicando que este era o limite de uma bolha em que o tempo parou. Este é um ponto de vista válido para observadores externos, mas não para observadores infalling. Por causa desta propriedade, as estrelas desmoronadas foram chamadas " estrelas congeladas, " [12] porque um observador exterior veria a superfície da estrela congelada a tempo no o instante onde seu colapso a toma dentro do raio de Schwarzschild. Esta é uma propriedade conhecida de buracos negros modernos, mas deve-se emfatizar que a luz da superfície da estrela congelada se transforma muito rápida redshifted, girando o preto do buraco negro muito rapidamente. Muitos físicos não poderiam aceitar a idéia do tempo que está ainda no raio de Schwarzschild, e havia pouco interesse no assunto por mais de 20 anos. Época dourada Veja igualmente: Época dourada da relatividade geral Em 1958, David Finkelstein identificou o Schwarzschild de superfície r = 2m [em unidades geometrized, isto é 2Gm/c2, onde r é o raio da superfície e do m é a massa do buraco negro] como um horizonte de evento, " uma membrana unidireccional perfeita: as influências causais podem cruzá-la em somente um direction". [13] Isto não contradisse estritamente Oppenheimer' s resulta, mas estendido lhes para incluir o ponto de vista de observadores infalling. Finkelstein' a solução de s estendeu a solução de Schwarzschild para o futuro dos observadores que caem no buraco negro. Uma extensão completa tinha sido encontrada já por Martin Kruskal, que foi incitado a publicar. [14] Estes resultados vieram no início da época dourada da relatividade geral, que é marcada por assuntos se tornando da relatividade geral e do grosso da população dos buracos negros da pesquisa. Este processo foi ajudado pela descoberta dos pulsar em 1967, [15] [16] que se realizava dentro de alguns anos mostrados girar ràpida estrelas de nêutron. Até esse tempo, as estrelas de nêutron, como buracos negros, foram consideradas como apenas curiosidades teóricas; mas a descoberta dos pulsar mostrou sua relevância física e spurred um interesse mais adicional em todos os tipos de objetos compactos que puderam ser dados forma pelo colapso gravitacional. Em soluções mais gerais deste buraco negro do período onde encontrado. Em 1963, Roy Kerr encontrou a solução exata para um buraco negro de giro. Dois anos mais tarde de Ezra T. Newman encontraram a solução axisymmetric para um buraco negro que fosse ambos que giram e carregaram-na eletricamente. [17] Através do trabalho de Werner Israel, [18] de Brandon Carter, [19] [20] e de C.C. Robinson [21] o theorem do nenhum-cabelo emergeu, indic que uma solução estacionária do buraco negro está descrita completamente pelos três parâmetros do Kerr-Newman métrico; massa, impulso angular, e carga elétrica. [22] Por muito tempo, suspeitou-se que as características estranhas das soluções do buraco negro eram produtos manufacturados patológicos das condições da simetria impor, e que os singularities não apareceriam em situações genéricas. Esta vista foi prendida em particular por Belinsky, por Khalatnikov, e por Lifshitz, que tentou mostrar que nenhum singularities aparece em soluções genéricas. Entretanto, nos anos sessenta atrasados Roger Penrose [23] e em Hawking de Stephen usou técnicas globais para mostrar que os singularities são genéricos. [24] Trabalhe James Bardeen, Jacob Bekenstein, Carter, e Hawking no princípio dos anos 70 conduzido à formulação das leis de mecânicos do buraco negro. [25] Estas leis descrevem o comportamento de um buraco negro na analogia próxima às leis de thermodynamics relacionando a massa à energia, a área à entropia, e a gravidade de superfície à temperatura. A analogia foi terminada ao Hawking, em 1974, mostrado que a teoria de campo do quantum prevê que os buracos negros devem irradiar como um corpo preto com uma temperatura proporcional à gravidade de superfície do buraco negro. [26] O " do termo; hole" preto; era o primeiro usado publicamente por John Veículo com rodas durante uma leitura em 1967. Embora fosse creditado geralmente com a cunhagem da frase, insistiu sempre que lhe estêve sugerida por alguém mais. O uso primeiramente gravado do termo está em uma letra 1964 por Anne Ewing à associação americana para o avanço da ciência. [27] Após Wheeler' uso do termo, de s foi adotado rapidamente no uso geral. Propriedades e estrutura O theorem do nenhum-cabelo indic que, uma vez que consegue uma condição estável após a formação, um buraco negro tem somente três propriedades físicas independentes: massa, carga, e momentu angular

[Agloco]

fuck the OP, this thread, and him being able to troll the spurs forum at will.

i'm against banning people and whatnot but shit is ridiculous these days on ST. always the same stupid fuckin' "lol" and "lmao" threads in here, keep that shit in the club, troll forum, or nba forum.

btw, did i mention, fuck you phillip!

http://www.sptimes.com/News/022301/photos/sports-mark.jpg


Relax, and go get some more calf-ink.......

[UnWantedTheory]

I took the liberty of putting this into Portugese.
Somtimes I get the tenses mixed up so my aplogies in advance.
Um buraco negro é uma região de espaço de que nada, não mesmo luz, pode escapar. É o resultado da deformação do spacetime causada por uma massa muito compacta. Em torno de um buraco negro há uma superfície undetectable que marque o ponto de nenhum retorno, chamada um horizonte de evento. É chamado " black" porque absorve toda a luz que batidas ele, não refletindo nada, apenas como um corpo preto perfeito no thermodynamics. [1] Os mecânicos do quantum prevêem que os buracos negros igualmente se emitem a radiação como um corpo preto com uma temperatura finita. Esta temperatura diminui com a massa do buraco negro, fazendo a pouco susceptível de observar esta radiação para buracos negros da massa estelar. Apesar de seu interior invisível, um buraco negro pode ser observado com sua interação com a outra matéria. Um buraco negro pode ser pressupor seguindo o movimento de um grupo de estrelas que orbitam uma região no espaço. Alternativamente, quando o gás cai em um buraco negro estelar de uma estrela de companheiro, o gás espirala para dentro, aquecendo-se muito às altas temperaturas e emitindo-se as grandes quantidades de radiação que podem ser detectadas dos telescópios earthbound e Terra-orbitando. Os astrónomos identificaram candidatos estelares numerosos do buraco negro, e igualmente encontraram a evidência de buracos negros supermassive no centro das galáxias. Em 1998, os astrónomos encontraram a evidência de obrigação que um buraco negro supermassive de mais de 2 milhão massas solares está encontrado perto da região de A* do Sagittarius no centro da galáxia da maneira leitosa, e uns resultados mais recentes que usam dados adicionais encontram a evidência que o buraco negro supermassive é mais de 4 milhão massas solares. Índices [couro cru] * 1 história relatividade geral do 1.1 época dourada do 1.2 * 2 propriedades e estruturas propriedades físicas do 2.1 horizonte de evento do 2.2 Singularity do 2.3 esfera do fotão do 2.4 o 2.5 Ergosphere * formação 3 e evolução colapso gravitacional do 3.1 + 3.1.1 buracos negros Primordial em Big Bang colisões alta-tensão do 3.2 crescimento do 3.3 evaporação do 3.4 * evidência 4 Observational aumento do 4.1 da matéria binários do raio X do 4.2 + Quiescence 4.2.1 e fluxo advecção-dominado do aumento + 4.2.2 oscilações Quase-periódicas estouros do raio de gama do 4.3 núcleos galácticos do 4.4 lensing gravitacional do 4.5 alternativas do 4.6 * 5 questões abertas entropia e thermodynamics do 5.1 unitarity do buraco negro do 5.2 * 6 vêem igualmente * 7 notas * 8 referências * leitura 9 mais adicional * 10 ligações externas História Buraco negro de Schwarzschild A simulação de lensing gravitacional por um buraco negro que distorça a imagem de uma galáxia no fundo (estala aqui para a animação maior) A idéia de um corpo tão maciço que mesmo a luz não poderia escapar foi propor primeiramente pelo geólogo John Michell em uma letra escrita a Henry Cavendish em 1783 à sociedade real: Se o semi-diameter de uma esfera da mesma densidade que o Sun era exceder isso do Sun na proporção de 500 a 1, um corpo que cai de uma altura infinita para ele adquiriria em sua maior velocidade de superfície do que aquele da luz, e conseqüentemente supor a luz para ser atraído pela mesma força em proporção a seus inertiae do vis, com outros corpos, toda a luz emissora de tal corpo seria feita para retornar para ela por sua própria gravidade apropriada. - John Michell [2] Em 1796, o matemático Pierre-Simon Laplace promoveu a mesma idéia nas primeiras e segundas edições de sua exposição du système du Monde do livro (foi removida de umas edições mais atrasadas). [3] [4] tal " stars" escuro; foram ignorados pela maior parte no 19o século, desde que não se compreendeu como uma onda massless tal como a luz poderia ser influenciada pela gravidade. [5] Relatividade geral Em 1915, Albert Einstein desenvolveu sua teoria da relatividade geral, mostrando mais cedo que a gravidade influencia light' movimento de s. Alguns meses mais tarde, Karl Schwarzschild deu a solução para o campo gravitacional de uma massa do ponto e de uma massa esférica. [6] Alguns meses após Schwarzschild, Johannes Droste, um estudante de Hendrik Lorentz, deu independente a mesma solução para a massa do ponto e escreveu-a mais extensivamente sobre suas propriedades. [7] Esta solução teve um comportamento peculiar em o que é chamado agora o raio de Schwarzschild, onde se tornou singular, significando que alguns dos termos nas equações de Einstein se tornaram infinitos. A natureza desta superfície não foi compreendida completamente naquele tempo. Em 1924, Arthur Eddington mostrou que o singularity desapareceu depois que uma mudança das coordenadas (veja coordenadas de Eddington), embora tomasse até 1933 para Georges Lemaître para realizar que esta significou o singularity no raio de Schwarzschild era um singularity coordenado unphysical. [8] Em 1931, Subrahmanyan Chandrasekhar calculou, usando a relatividade geral, que um corpo degiro da matéria elétron-degenerate acima de 1.44 massas solares (o limite de Chandrasekhar) desmoronaria. [citação necessário] seus argumentos foram opor por muitos de seus comtemporâneos como Eddington e Landau do lev, que discutiu que algum contudo o mecanismo desconhecido pararia o colapso. [9] Estavam em parte corretos: um anão branco ligeiramente mais maciço do que o limite de Chandrasekhar desmoronará em uma estrela de nêutron, [10] que sejam próprias estáveis por causa do princípio de exclusão de Pauli. Mas em 1939, Robert Oppenheimer e outro previram que as estrelas de nêutron acima de aproximadamente três massas solares (o limite de Tolman-Oppenheimer-Volkoff) desmoronariam em buracos negros para as razões apresentadas por Chandrasekhar, e concluíram que nenhuma lei de física era provável intervir pelo menos e parar algumas estrelas do desmoronamento aos buracos negros. [11] Oppenheimer e seus co-autores interpretaram o singularity no limite do raio de Schwarzschild porque indicando que este era o limite de uma bolha em que o tempo parou. Este é um ponto de vista válido para observadores externos, mas não para observadores infalling. Por causa desta propriedade, as estrelas desmoronadas foram chamadas " estrelas congeladas, " [12] porque um observador exterior veria a superfície da estrela congelada a tempo no o instante onde seu colapso a toma dentro do raio de Schwarzschild. Esta é uma propriedade conhecida de buracos negros modernos, mas deve-se emfatizar que a luz da superfície da estrela congelada se transforma muito rápida redshifted, girando o preto do buraco negro muito rapidamente. Muitos físicos não poderiam aceitar a idéia do tempo que está ainda no raio de Schwarzschild, e havia pouco interesse no assunto por mais de 20 anos. Época dourada Veja igualmente: Época dourada da relatividade geral Em 1958, David Finkelstein identificou o Schwarzschild de superfície r = 2m [em unidades geometrized, isto é 2Gm/c2, onde r é o raio da superfície e do m é a massa do buraco negro] como um horizonte de evento, " uma membrana unidireccional perfeita: as influências causais podem cruzá-la em somente um direction". [13] Isto não contradisse estritamente Oppenheimer' s resulta, mas estendido lhes para incluir o ponto de vista de observadores infalling. Finkelstein' a solução de s estendeu a solução de Schwarzschild para o futuro dos observadores que caem no buraco negro. Uma extensão completa tinha sido encontrada já por Martin Kruskal, que foi incitado a publicar. [14] Estes resultados vieram no início da época dourada da relatividade geral, que é marcada por assuntos se tornando da relatividade geral e do grosso da população dos buracos negros da pesquisa. Este processo foi ajudado pela descoberta dos pulsar em 1967, [15] [16] que se realizava dentro de alguns anos mostrados girar ràpida estrelas de nêutron. Até esse tempo, as estrelas de nêutron, como buracos negros, foram consideradas como apenas curiosidades teóricas; mas a descoberta dos pulsar mostrou sua relevância física e spurred um interesse mais adicional em todos os tipos de objetos compactos que puderam ser dados forma pelo colapso gravitacional. Em soluções mais gerais deste buraco negro do período onde encontrado. Em 1963, Roy Kerr encontrou a solução exata para um buraco negro de giro. Dois anos mais tarde de Ezra T. Newman encontraram a solução axisymmetric para um buraco negro que fosse ambos que giram e carregaram-na eletricamente. [17] Através do trabalho de Werner Israel, [18] de Brandon Carter, [19] [20] e de C.C. Robinson [21] o theorem do nenhum-cabelo emergeu, indic que uma solução estacionária do buraco negro está descrita completamente pelos três parâmetros do Kerr-Newman métrico; massa, impulso angular, e carga elétrica. [22] Por muito tempo, suspeitou-se que as características estranhas das soluções do buraco negro eram produtos manufacturados patológicos das condições da simetria impor, e que os singularities não apareceriam em situações genéricas. Esta vista foi prendida em particular por Belinsky, por Khalatnikov, e por Lifshitz, que tentou mostrar que nenhum singularities aparece em soluções genéricas. Entretanto, nos anos sessenta atrasados Roger Penrose [23] e em Hawking de Stephen usou técnicas globais para mostrar que os singularities são genéricos. [24] Trabalhe James Bardeen, Jacob Bekenstein, Carter, e Hawking no princípio dos anos 70 conduzido à formulação das leis de mecânicos do buraco negro. [25] Estas leis descrevem o comportamento de um buraco negro na analogia próxima às leis de thermodynamics relacionando a massa à energia, a área à entropia, e a gravidade de superfície à temperatura. A analogia foi terminada ao Hawking, em 1974, mostrado que a teoria de campo do quantum prevê que os buracos negros devem irradiar como um corpo preto com uma temperatura proporcional à gravidade de superfície do buraco negro. [26] O " do termo; hole" preto; era o primeiro usado publicamente por John Veículo com rodas durante uma leitura em 1967. Embora fosse creditado geralmente com a cunhagem da frase, insistiu sempre que lhe estêve sugerida por alguém mais. O uso primeiramente gravado do termo está em uma letra 1964 por Anne Ewing à associação americana para o avanço da ciência. [27] Após Wheeler' uso do termo, de s foi adotado rapidamente no uso geral. Propriedades e estrutura O theorem do nenhum-cabelo indic que, uma vez que consegue uma condição estável após a formação, um buraco negro tem somente três propriedades físicas independentes: massa, carga, e momentu angular

[UnWantedTheory]

That was just beautiful.

[Solid D]

http://www.youtube.com/watch?v=oHg5SJYRHA0

[DJ Mbenga]

A black hole is a region of space from which nothing, not even light, can escape. It is the result of the deformation of spacetime caused by a very compact mass. Around a black hole there is an undetectable surface which marks the point of no return, called an event horizon. It is called "black" because it absorbs all the light that hits it, reflecting nothing, just like a perfect black body in thermodynamics.[1] Quantum mechanics predicts that black holes also emit radiation like a black body with a finite temperature. This temperature decreases with the mass of the black hole, making it unlikely to observe this radiation for black holes of stellar mass.

Despite its invisible interior, a black hole can be observed through its interaction with other matter. A black hole can be inferred by tracking the movement of a group of stars that orbit a region in space. Alternatively, when gas falls into a stellar black hole from a companion star, the gas spirals inward, heating to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and Earth-orbiting telescopes.

Astronomers have identified numerous stellar black hole candidates, and have also found evidence of supermassive black holes at the center of galaxies. In 1998, astronomers found compelling evidence that a supermassive black hole of more than 2 million solar masses is located near the Sagittarius A* region in the center of the Milky Way galaxy, and more recent results using additional data find evidence that the supermassive black hole is more than 4 million solar masses.
Contents
[hide]

* 1 History
o 1.1 General relativity
o 1.2 Golden age
* 2 Properties and structure
o 2.1 Physical properties
o 2.2 Event horizon
o 2.3 Singularity
o 2.4 Photon sphere
o 2.5 Ergosphere
* 3 Formation and evolution
o 3.1 Gravitational collapse
+ 3.1.1 Primordial black holes in the Big Bang
o 3.2 High-energy collisions
o 3.3 Growth
o 3.4 Evaporation
* 4 Observational evidence
o 4.1 Accretion of matter
o 4.2 X-ray binaries
+ 4.2.1 Quiescence and advection-dominated accretion flow
+ 4.2.2 Quasi-periodic oscillations
o 4.3 Gamma ray bursts
o 4.4 Galactic nuclei
o 4.5 Gravitational lensing
o 4.6 Alternatives
* 5 Open questions
o 5.1 Entropy and thermodynamics
o 5.2 Black hole unitarity
* 6 See also
* 7 Notes
* 8 References
* 9 Further reading
* 10 External links

History
Schwarzschild black hole
Simulation of gravitational lensing by a black hole which distorts the image of a galaxy in the background (click here for larger animation)

The idea of a body so massive that even light could not escape was first put forward by geologist John Michell in a letter written to Henry Cavendish in 1783 to the Royal Society:

If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae, with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity.
—John Michell[2]

In 1796, mathematician Pierre-Simon Laplace promoted the same idea in the first and second editions of his book Exposition du système du Monde (it was removed from later editions).[3][4] Such "dark stars" were largely ignored in the nineteenth century, since it was not understood how a massless wave such as light could be influenced by gravity.[5]
General relativity

In 1915, Albert Einstein developed his theory of general relativity, having earlier shown that gravity does influence light's motion. A few months later, Karl Schwarzschild gave the solution for the gravitational field of a point mass and a spherical mass.[6] A few months after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution for the point mass and wrote more extensively about its properties.[7] This solution had a peculiar behaviour at what is now called the Schwarzschild radius, where it became singular, meaning that some of the terms in the Einstein equations became infinite. The nature of this surface was not quite understood at the time. In 1924, Arthur Eddington showed that the singularity disappeared after a change of coordinates (see Eddington coordinates), although it took until 1933 for Georges Lemaître to realize that this meant the singularity at the Schwarzschild radius was an unphysical coordinate singularity.[8]

In 1931, Subrahmanyan Chandrasekhar calculated, using general relativity, that a non-rotating body of electron-degenerate matter above 1.44 solar masses (the Chandrasekhar limit) would collapse.[citation needed] His arguments were opposed by many of his contemporaries like Eddington and Lev Landau, who argued that some yet unknown mechanism would stop the collapse.[9] They were partly correct: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star,[10] which is itself stable because of the Pauli exclusion principle. But in 1939, Robert Oppenheimer and others predicted that neutron stars above approximately three solar masses (the Tolman–Oppenheimer–Volkoff limit) would collapse into black holes for the reasons presented by Chandrasekhar, and concluded that no law of physics was likely to intervene and stop at least some stars from collapsing to black holes.[11]

Oppenheimer and his co-authors interpreted the singularity at the boundary of the Schwarzschild radius as indicating that this was the boundary of a bubble in which time stopped. This is a valid point of view for external observers, but not for infalling observers. Because of this property, the collapsed stars were called "frozen stars,"[12] because an outside observer would see the surface of the star frozen in time at the instant where its collapse takes it inside the Schwarzschild radius. This is a known property of modern black holes, but it must be emphasized that the light from the surface of the frozen star becomes redshifted very fast, turning the black hole black very quickly. Many physicists could not accept the idea of time standing still at the Schwarzschild radius, and there was little interest in the subject for over 20 years.
Golden age
See also: Golden age of general relativity

In 1958, David Finkelstein identified the Schwarzschild surface r = 2m [in geometrized units, i.e. 2Gm/c2, where r is the radius of the surface and m is the mass of the black hole] as an event horizon, "a perfect unidirectional membrane: causal influences can cross it in only one direction".[13] This did not strictly contradict Oppenheimer's results, but extended them to include the point of view of infalling observers. Finkelstein's solution extended the Schwarzschild solution for the future of observers falling into the black hole. A complete extension had already been found by Martin Kruskal, who was urged to publish it.[14]

These results came at the beginning of the golden age of general relativity, which is marked by general relativity and black holes becoming mainstream subjects of research. This process was helped by the discovery of pulsars in 1967,[15][16] which were within a few years shown to be rapidly rotating neutron stars. Until that time, neutron stars, like black holes, were regarded as just theoretical curiosities; but the discovery of pulsars showed their physical relevance and spurred a further interest in all types of compact objects that might be formed by gravitational collapse.

In this period more general black hole solutions where found. In 1963, Roy Kerr found the exact solution for a rotating black hole. Two years later Ezra T. Newman found the axisymmetric solution for a black hole which is both rotating and electrically charged.[17] Through the work of Werner Israel,[18] Brandon Carter,[19][20] and D. C. Robinson[21] the no-hair theorem emerged, stating that a stationary black hole solution is completely described by the three parameters of the Kerr–Newman metric; mass, angular momentum, and electric charge.[22]

For a long time, it was suspected that the strange features of the black hole solutions were pathological artefacts from the symmetry conditions imposed, and that the singularities would not appear in generic situations. This view was held in particular by Belinsky, Khalatnikov, and Lifshitz, who tried to prove that no singularities appear in generic solutions. However, in the late sixties Roger Penrose[23] and Stephen Hawking used global techniques to prove that singularities are generic.[24]

Work by James Bardeen, Jacob Bekenstein, Carter, and Hawking in the early 1970s led to the formulation of the laws of black hole mechanics.[25] These laws describe the behaviour of a black hole in close analogy to the laws of thermodynamics by relating mass to energy, area to entropy, and surface gravity to temperature. The analogy was completed when Hawking, in 1974, showed that quantum field theory predicts that black holes should radiate like a black body with a temperature proportional to the surface gravity of the black hole.[26]

The term "black hole" was first publicly used by John Wheeler during a lecture in 1967. Although he is usually credited with coining the phrase, he always insisted that it was suggested to him by somebody else. The first recorded use of the term is in a 1964 letter by Anne Ewing to the American Association for the Advancement of Science.[27] After Wheeler's use of the term, it was quickly adopted in general use.
Properties and structure

The no-hair theorem states that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, charge, and angular momentum.[22] Any two black holes that share the same values for these properties, or parameters, are indistinguishable according to classical (i.e. non-quantum) mechanics.

These properties are special because they are visible from outside the black hole. For example, a charged black hole repels other like charges just like any other charged object. Similarly, the total mass inside a sphere containing a black hole can be found by using the gravitational analog of Gauss's law, the ADM mass, far away from the black hole.[28] Likewise, the angular momentum can be measured from far away using frame dragging by the gravitomagnetic field.

When an object falls into a black hole, any information about the shape of the object or distribution of charge on it is evenly distributed along the horizon of the black hole, and is lost to outside observers. The behavior of the horizon in this situation is closely analogous to that of a conductive stretchy membrane with friction and electrical resistance, a dissipative system (see membrane paradigm).[29] This is different from other field theories like electromagnetism, which does not have any friction or resistivity at the microscopic level, because they are time-reversible. Because the black hole eventually achieves a stable state with only three parameters, there is no way to avoid losing information about the initial conditions: the gravitational and electric fields of the black hole give very little information about what went in. The information that is lost includes every quantity that cannot be measured far away from the black hole horizon, including the total baryon number, lepton number, and all the other nearly conserved pseudo-charges of particle physics. This behavior is so puzzling that it has been called the black hole information loss paradox.[30][31][32]
Physical properties

The simplest black hole has mass but neither electric charge nor angular momentum. These black holes are often referred to as Schwarzschild black holes after the physicist Karl Schwarzschild who discovered this solution in 1915.[6] According to Birkhoff's theorem, it is the only vacuum solution that is spherically symmetric.[33] This means that there is no observable difference between the gravitational field of such a black hole and that of any other spherical object of the same mass. The popular notion of a black hole "sucking in everything" in its surroundings is therefore only correct near the black hole horizon; far away, the external gravitational field is identical to that of any other body of the same mass.[34]

Solutions describing more general black holes also exist. Charged black holes are described by the Reissner-Nordström metric, while the Kerr metric describes a rotating black hole. The most general stationary black hole solution known is the Kerr-Newman metric, which describes a black hole with both charge and angular momentum.

While the mass of a black hole can take any positive value, the charge and angular momentum are constrained by the mass. In Planck units, the total electric charge Q and the total angular momentum J are expected to satisfy

Q^2+\left ( \tfrac{J}{M} \right )^2\le M^2\,

for a black hole of mass M. Black holes saturating this inequality are called extremal. Solutions of Einstein's equations violating the inequality exist, but do not have a horizon. These solutions have naked singularities and are deemed unphysical. The cosmic censorship hypothesis rules out the formation of such singularities through the gravitational collapse of realistic matter.[35] This is supported by numerical simulations.[36]

Due to the relatively large strength of the electromagnetic force, black holes forming from the collapse of stars are expected to retain the nearly neutral charge of the star. Rotation, however, is expected to be a common feature of compact objects, and the black-hole candidate binary X-ray source GRS 1915+105[37] appears to have an angular momentum near the maximum allowed value.
Class Mass Size
Supermassive black hole ~105–109 MSun ~0.001–10 AU
Intermediate-mass black hole ~103 MSun ~103 km = REarth
Stellar black hole ~10 MSun ~30 km
Micro black hole up to ~MMoon up to ~0.1 mm

Black holes are commonly classified according to their mass, independent of angular momentum J or electric charge Q. The size of a black hole, as determined by the radius of the event horizon, or Schwarzschild radius, is roughly proportional to the mass M through

r_\mathrm{sh} =\frac{2GM}{c^2} \approx 2.95\, \frac{M}{M_\mathrm{Sun}}~\mathrm{km,}

where rsh is the Schwarzschild radius and MSun is the mass of the Sun. This relation is exact only for black holes with zero charge and angular momentum, for more general black holes it can differ up to a factor of 2. The table on the right lists the various classes of black hole that are distinguished.
Event horizon
Main article: Event horizon
Image:BH-no-escape-1.svg
Far away from the black hole a particle can move in any direction. It is only restricted by the speed of light.
Image:BH-no-escape-2.svg
Closer to the black hole spacetime starts to deform. There are more paths going towards the black hole than paths moving away.
Image:BH-no-escape-3.svg
Inside of the event horizon all paths bring the particle closer to the center of the black hole. It is no longer possible for the particle to escape.

The defining feature of a black hole is the appearance of an event horizon—a boundary in spacetime through which matter and light can only pass inward towards the mass of the black hole. Nothing, not even light, can escape from inside the event horizon. The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if such an event occurred.[38]

As predicted by general relativity, the presence of a large mass deforms spacetime in such a way that the paths taken by particles bend towards the mass. At the event horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hole.[39]

To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole.[40] Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it.[41] At the same time, all processes on this object slow down causing emitted light to appear redder and dimmer, an effect known as gravitational redshift.[42] Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.

On the other hand, an observer falling into a black hole does not notice any of these effects as he crosses the event horizon. According to his own clock, he crosses the event horizon after a finite time, although he is unable to determine exactly when he crosses it, as it is impossible to determine the location of the event horizon from local observations.[43]

For a non rotating (static) black hole, the Schwarzschild radius delimits a spherical event horizon. The Schwarzschild radius of an object is proportional to the mass.[44] Rotating black holes have distorted, nonspherical event horizons. Since the event horizon is not a material surface but rather merely a mathematically defined demarcation boundary, nothing prevents matter or radiation from entering a black hole, only from exiting one. The description of black holes given by general relativity is known to be an approximation, and some scientists expect that quantum gravity effects will become significant near the vicinity of the event horizon.[45] This would allow observations of matter near a black hole's event horizon to be used to indirectly study general relativity and proposed extensions to it.
Singularity
Main article: Gravitational singularity

At the center of a black hole as described by general relativity lies a gravitational singularity, a region where the spacetime curvature becomes infinite.[46] For a non-rotating black hole this region takes the shape of a single point and for a rotating black hole it is smeared out to form a ring singularity lying in the plane of rotation.[47] In both cases the singular region has zero volume. It can also be shown that the singular region contains all the mass of the black hole solution.[48] The singular region can thus be thought of as having infinite density.

An observer falling into a Schwarzschild black hole (i.e. non-rotating and no charges) cannot avoid the singularity. Any attempt to do so will only shorten the time taken to get there.[49] When they reach the singularity, they are crushed to infinite density and their mass is added to the total of the black hole. Before that happens, they will have been torn apart by the growing tidal forces in a process sometimes referred to as spaghettification or the noodle effect.[50]

In the case of a charged (Reissner–Nordström) or rotating (Kerr) black hole it is possible to avoid the singularity. Extending these solutions as far as possible reveals the hypothetical possibility of exiting the black hole into a different spacetime with the black hole acting as a wormhole.[51] The possibility of traveling to another universe is however only theoretical, since any perturbation will destroy this possibility.[52] It also appears to be possible to follow closed timelike curves (going back to one's own past) around the Kerr singularity, which lead to problems with causality like the grandfather paradox.[53] It is expected that none of these peculiar effects would survive in a proper quantum mechanical treatment of rotating and charged black holes.[54]

The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory.[55] This breakdown, however, is expected; it occurs in a situation where quantum mechanical effects should describe these actions due to the extremely high density and therefore particle interactions. To date it has not been possible to combine quantum and gravitational effects into a single theory. It is generally expected that a theory of quantum gravity will feature black holes without singularities.[56][57]
Photon sphere
Main article: Photon sphere

The photon sphere is a spherical boundary of zero thickness such that photons moving along tangents to the sphere will be trapped in a circular orbit. For non-rotating black holes, the photon sphere has a radius 1.5 times the Schwarzschild radius. The orbits are dynamically unstable, hence any small perturbation (such as a particle of infalling matter) will grow over time, either setting it on an outward trajectory escaping the black hole or on an inward spiral eventually crossing the event horizon.

While light can still escape from inside the photon sphere, any light that crosses the photon sphere on an inbound trajectory will be captured by the black hole. Hence any light reaching an outside observer from inside the photon sphere must have been emitted by objects inside the photon sphere but still outside of the event horizon.

Other compact objects, such as neutron stars, can also have photon spheres.[58] This follows from the fact that the gravitational field of an object does not depend on its actual size, hence any object that is smaller than 1.5 times the Schwarzschild radius corresponding to its mass will indeed have a photon sphere.
Ergosphere
Main article: Ergosphere
The ergosphere is an oblate spheroid region outside of the event horizon, where objects cannot remain stationary.

Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still, called the ergosphere. This is the result of a process known as frame-dragging; general relativity predicts that any rotating mass will tend to slightly "drag" along the spacetime immediately surrounding it. Any object near the rotating mass will tend to start moving in the direction of rotation. For a rotating black hole this effect becomes so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still.[59]

The ergosphere of a black hole is bounded by the (outer) event horizon on the inside and an oblate spheroid, which coincides with the event horizon at the poles and is noticeably wider around the equator. The outer boundary is sometimes called the ergosurface.

Objects and radiation can escape normally from the ergosphere. Through the Penrose process, objects can emerge from the ergosphere with more energy than they entered. This energy is taken from the rotational energy of the black hole causing it to slow down.[60]
Formation and evolution

Considering the exotic nature of black holes, it may be natural to question if such bizarre objects could exist in nature or to suggest that they are merely pathological solutions to Einstein's equations. Einstein himself wrongly thought that black holes would not form, because he held that the angular momentum of collapsing particles would stabilize their motion at some radius.[61] This led the general relativity community to dismiss all results to the contrary for many years. However, a minority of relativists continued to contend that black holes were physical objects,[62] and by the end of the 1960s, they had persuaded the majority of researchers in the field that there is no obstacle to forming an event horizon.

Once an event horizon forms, Roger Penrose proved that a singularity will form somewhere inside it.[23] Shortly afterwards, Stephen Hawking showed that many cosmological solutions describing the Big Bang have singularities without scalar fields or other exotic matter (see Penrose-Hawking singularity theorems). The Kerr solution, the no-hair theorem and the laws of black hole thermodynamics showed that the physical properties of black holes were simple and comprehensible, making them respectable subjects for research.[63] The primary formation process for black holes is expected to be the gravitational collapse of heavy objects such as stars, but there are also more exotic processes that can lead to the production of black holes.
Gravitational collapse
Main article: Gravitational collapse

Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. For stars this usually occurs either because a star has too little "fuel" left to maintain its temperature through stellar nucleosynthesis, or because a star which would have been stable receives extra matter in a way which does not raise its core temperature. In either case the star's temperature is no longer high enough to prevent it from collapsing under its own weight (the ideal gas law explains the connection between pressure, temperature, and volume).[64]

The collapse may be stopped by the degeneracy pressure of the star's constituents, condensing the matter in an exotic denser state. The result is one of the various types of compact star. Which type of compact star is formed depends on the mass of the remnant — the matter left over after changes triggered by the collapse (such as supernova or pulsations leading to a planetary nebula) have blown away the outer layers. Note that this can be substantially less than the original star — remnants exceeding 5 solar masses are produced by stars which were over 20 solar masses before the collapse.[64]

If the mass of the remnant exceeds about 3–4 solar masses (the Tolman–Oppenheimer–Volkoff limit)—either because the original star was very heavy or because the remnant collected additional mass through accretion of matter—even the degeneracy pressure of neutrons is insufficient to stop the collapse. After this, no known mechanism (except possibly quark degeneracy pressure, see quark star) is powerful enough to stop the collapse and the object will inevitably collapse to a black hole.[64]

This gravitational collapse of heavy stars is assumed to be responsible for the formation of stellar mass black holes. Star formation in the young universe may have resulted in very heavy stars, which upon their collapse would have produced black holes of up to 103 solar masses. These heavy black holes could be the seeds of the supermassive black holes found in the centers of most galaxies.[65]

While most of the energy released during gravitational collapse is emitted very quickly, an outside observer does not actually see the end of this process. Even though the collapse takes a finite amount of time from the reference frame of infalling matter, a distant observer sees the infalling material slow and halt just above the event horizon, due to gravitational time dilation. Light from the collapsing material takes longer and longer to reach the observer, with the light emitted just before the event horizon forms delayed an infinite amount of time. Thus the external observer never sees the formation of the event horizon; instead, the collapsing material seems to become dimmer and increasingly red-shifted, eventually fading away.[66]
Primordial black holes in the Big Bang

Gravitational collapse requires great densities. In the current epoch of the universe these high densities are only found in stars, but in the early universe shortly after the big bang densities were much greater, possibly allowing for the creation of black holes. The high density alone is not enough to allow the formation of black holes since a uniform mass distribution will not allow the mass to bunch up. In order for primordial black holes to form in such a dense medium, there must be initial density perturbations which can then grow under their own gravity. Different models for the early universe vary widely in their predictions of the size of these perturbations. Various models predict the creation of black holes, ranging from a Planck mass to hundreds of thousands of solar masses.[67] Primordial black holes could thus account for the creation of any type of black hole.
High-energy collisions
A simulated event in the CMS detector, a collision in which a micro black hole may be created.

Gravitational collapse is not the only process that could create black holes. In principle, black holes could also be created in high-energy collisions that create sufficient density. However, to date, no such events have ever been detected either directly or indirectly as a deficiency of the mass balance in particle accelerator experiments.[68] This suggests that there must be a lower limit for the mass of black holes. Theoretically, this boundary is expected to lie around the Planck mass (mP = √ħc/G ≈ 1.2×1019 GeV/c2 ≈ 2.2×10−8 kg), where quantum effects are expected to make the theory of general relativity break down completely.[69] This would put the creation of black holes firmly out of reach of any high energy process occurring on or near the Earth. Certain developments in quantum gravity however suggest that the Planck mass could be much lower: some braneworld scenarios for example put it much lower, maybe even as low as 1 TeV/c2[70] This would make it possible for micro black holes to be created in the high energy collisions occurring when cosmic rays hit the Earth's atmosphere, or possibly in the new Large Hadron Collider at CERN. These theories are however very speculative, and the creation of black holes in these processes is deemed unlikely by many specialists.[71] Even if such micro black holes should be formed in these collisions, it is expected that they would evaporate in about 10−25 seconds, posing no threat to Earth[72]
Growth

Once a black hole has formed, it can continue to grow by absorbing additional matter. Any black hole will continually absorb gas and interstellar dust from its direct surroundings and omnipresent cosmic background radiation. This is the primary process through which supermassive black holes seem to have grown.[65] A similar process has been suggested for the formation of intermediate-mass black holes in globular clusters.[73]

Another possibility is for a black hole to merge with other objects such as stars or even other black holes. This is thought to have been important especially for the early development of supermassive black holes, which are thought to have formed from the coagulation of many smaller objects.[65] The process has also been proposed as the origin of some intermediate-mass black holes.[74][75]
Evaporation
Main article: Hawking radiation

In 1974, Stephen Hawking showed that black holes are not entirely black but emit small amounts of thermal radiation.[26] He got this result by applying quantum field theory in a static black hole background. The result of his calculations is that a black hole should emit particles in a perfect black body spectrum. This effect has become known as Hawking radiation. Since Hawking's result, many others have verified the effect through various methods.[76] If his theory of black hole radiation is correct, then black holes are expected to emit a thermal spectrum of radiation, and thereby lose mass, because according to the theory of relativity mass is just highly condensed energy (E = mc2).[26] Black holes will shrink and evaporate over time. The temperature of this spectrum (Hawking temperature) is proportional to the surface gravity of the black hole, which for a Schwarzschild black hole is inversely proportional to the mass. Large black holes, therefore, emit less radiation than small black holes.

A stellar black hole of one solar mass has a Hawking temperature of about 100 nanokelvins. This is far less than the 2.7 K temperature of the cosmic microwave background. Stellar mass (and larger) black holes receive more mass from the cosmic microwave background than they emit through Hawking radiation and will thus grow instead of shrink. To have a Hawking temperature larger than 2.7 K (and be able to evaporate), a black hole needs to be lighter than the Moon (and therefore a diameter of less than a tenth of a millimeter).[77]

On the other hand, if a black hole is very small the radiation effects are expected to become very strong. Even a black hole that is heavy compared to a human would evaporate in an instant. A black hole the weight of a car (~10−24 m) would only take a nanosecond to evaporate, during which time it would briefly have a luminosity more than 200 times that of the sun. Lighter black holes are expected to evaporate even faster, for example a black hole of mass 1 TeV/c2 would take less than 10−88 seconds to evaporate completely. Of course, for such a small black hole quantum gravitation effects are expected to play an important role and could even – although current developments in quantum gravity do not indicate so[78] – hypothetically make such a small black hole stable.[79]
Observational evidence

By their very nature, black holes do not directly emit any signals other than the hypothetical Hawking radiation; since the Hawking radiation for an astrophysical black hole is predicted to be very weak, this makes it impossible to directly detect astrophysical black holes from the Earth. A possible exception to the Hawking radiation being weak is the last stage of the evaporation of light (primordial) black holes; searches for such flashes in the past has proven unsuccessful and provides stringent limits on the possibility of existence of light primordial black holes.[80] NASA's Fermi Gamma-ray Space Telescope launched in 2008 will continue the search for these flashes.[81]

Astrophysicists searching for black holes thus have to rely on indirect observations. A black hole's existence can sometimes be inferred by observing its gravitational interactions with its surroundings.
Accretion of matter
See also: Accretion disc
Formation of extragalactic jets from a black hole's accretion disk

Due to conservation of angular momentum, gas falling into the gravitational well created by a massive object will typically form a disc-like structure around the object. Friction within the disc causes angular momentum to be transported outward allowing matter to fall further inward releasing potential energy and increasing the temperature of the gas.[82] In the case of compact objects such as white dwarfs, neutron stars, and black holes, the gas in the inner regions becomes so hot that it will emit vast amounts of radiation (mainly X-rays), which may be detected by telescopes. This process of accretion is one of the most efficient energy producing process known; up to 40% of the rest mass of the accreted material can be emitted in radiation.[82] (In nuclear fusion only about 0.7% of the rest mass will be emitted as energy.) In many cases, accretion discs are accompanied by relativistic jets emitted along the poles, which carry away much of the energy. The mechanism for the creation of these jets is currently not well understood.

As such many of the universe's more energetic phenomena have been attributed to the accretion of matter on black holes. In particular, active galactic nuclei and quasars are thought to be the accretion discs of supermassive black holes.[83] Similarly, X-ray binaries are thought to be binary star systems in which one of the two stars is a compact object accreting matter from its companion.[83] It has also been suggested that some ultraluminous X-ray sources may be the accretion disks of intermediate-mass black holes.[84]
X-ray binaries
See also: X-ray binary

X-ray binaries are binary star systems that are luminous in the X-ray part of the spectrum. These X-ray emissions are generally thought to be caused by one of the component stars being a compact object accreting matter from the other (regular) star. The presence of an ordinary star in such a system provides a unique opportunity for studying the central object and determining if it might be a black hole.
Artist impression of a binary system with an accretion disk around a compact object being fed by material from the companion star.

If such a system emits signals that can be directly traced back to the compact object, it cannot be a black hole. The absence of such a signal does, however, not exclude the possibility that the compact object is a neutron star. By studying the companion star it is often possible to obtain the orbital parameters of the system and obtain an estimate for the mass of the compact object. If this is much larger than the Tolman–Oppenheimer–Volkoff limit (that is, the maximum mass a neutron star can have before collapsing) then the object cannot be a neutron star and is generally expected to be a black hole.[83]

The first strong candidate for a black hole, Cygnus X-1, was discovered in this way by Charles Thomas Bolton[85] and Webster and Murdin[86] in 1972.[87][88] Some doubt, however, remained due to the uncertainties resultant from the companion star being much heavier than the candidate black hole.[83] Currently, better candidates for black holes are found in a class of X-ray binaries called soft X-ray transients.[83] In this class of system the companion star is relatively low mass allowing for more accurate estimates in the black hole mass. Moreover, these systems are only active in X-ray for several months once every 10–50 years. During the period of low X-ray emission (called quiescence), the accretion disc is extremely faint allowing for detailed observation of the companion star during this period. One of the best such candidates is V404 Cyg.
Quiescence and advection-dominated accretion flow

The faintness of the accretion disc during quiescence is thought to be caused by the flow entering a mode called an advection-dominated accretion flow (ADAF). In this mode, almost all the energy generated by friction in the disc is swept along with the flow instead of radiated away. If this model is correct, then it forms strong qualitative evidence for the presence of an event horizon.[89] Because, if the object at the center of the disc had a solid surface, it would emit large amounts of radiation as the highly energetic gas hits the surface, an effect that is observed for neutron stars in a similar state.[82]
Quasi-periodic oscillations
See also: Quasi-periodic oscillations

The X-ray emissions from accretion disks sometimes exhibit a flickering around certain frequencies. These signals are called quasi-periodic oscillations and are thought to be caused by material moving along the inner edge of the accretion disk (the innermost stable circular orbit). As such their frequency is linked to the mass of the compact object. They can thus be used as an alternative way to determine the mass of potential black holes.[90]
Gamma ray bursts

Intense but one-time gamma ray bursts (GRBs) may signal the birth of "new" black holes, because astrophysicists think that GRBs are caused either by the gravitational collapse of giant stars[91] or by collisions between neutron stars,[92] and both types of event involve sufficient mass and pressure to produce black holes. It appears that a collision between a neutron star and a black hole can also cause a GRB,[93] so a GRB is not proof that a "new" black hole has been formed. All known GRBs come from outside our own galaxy, and most come from billions of light years away[94] so the black holes associated with them are billions of years old.
Galactic nuclei
See also: Active galactic nucleus
The jet originating from the center of M87 in this image comes from an active galactic nucleus that may contain a supermassive black hole. Credit: Hubble Space Telescope/NASA/ESA.

It is now widely accepted that the center of every or at least nearly every galaxy contains a supermassive black hole.[95] The close observational correlation between the mass of this hole and the velocity dispersion of the host galaxy's bulge, known as the M-sigma relation, strongly suggests a connection between the formation of the black hole and the galaxy itself. [96]

For decades, astronomers have used the term "active galaxy" to describe galaxies with unusual characteristics, such as unusual spectral line emission and very strong radio emission.[97][98] However, theoretical and observational studies have shown that the active galactic nuclei (AGN) in these galaxies may contain supermassive black holes.[97][98] The models of these AGN consist of a central black hole that may be millions or billions of times more massive than the Sun; a disk of gas and dust called an accretion disk; and two jets that are perpendicular to the accretion disk.[98]

Although supermassive black holes are expected to be found in most AGN, only some galaxies' nuclei have been more carefully studied in attempts to both identify and measure the actual masses of the central supermassive black hole candidates. Some of the most notable galaxies with supermassive black hole candidates include the Andromeda Galaxy, M32, M87, NGC 3115, NGC 3377, NGC 4258, and the Sombrero Galaxy.[99]

Currently, the best evidence for a supermassive black hole comes from studying the proper motion of stars near the center of our own Milky Way.[100] Since 1995 astronomers have tracked the motion of 90 stars in a region called Sagittarius A*. By fitting their motion to Keplerian orbits they were able to infer in 1998 that 2.6 million solar masses must be contained in a volume with a radius of 0.02 lightyears.[101] Since then one of the stars—called S2—has completed a full orbit. From the orbital data they were able to place better constraints on the mass and size of the object causing the orbital motion of stars in the Sagittarius A* region, finding that there is a spherical mass of 4.3 million solar masses contained within a radius of less than 0.002 lightyears.[100] While this is more than 3000 times the Schwarzschild radius corresponding to that mass, it is at least consistent with the central object being a supermassive black hole, and no "realistic cluster [of stars] is physically tenable."[101]
Gravitational lensing
Further information: Gravitational lens

The deformation of spacetime around a massive object causes light rays to be deflected much like light passing through an optic lens. This phenomenon is known as gravitational lensing. Observations have been made of weak gravitational lensing, in which photons are deflected by only a few arcseconds. However, it has never been directly observed for a black hole.[102] One possibility for observing gravitational lensing by a black hole would be to observe stars in orbit around the black hole. There are several candidates for such an observation in orbit around Sagittarius A*.[102]
Alternatives

The evidence for stellar black holes strongly relies on the existence of an upper limit for the mass of a neutron star. The size of this limit heavily depends on the assumptions made about the properties of dense matter. New exotic phases of matter could push up this bound.[83] A phase of free quarks at high density might allow the existence of dense quark stars,[103] and some supersymmetric models predict the existence of Q stars.[104] Some extensions of the standard model posit the existence of preons as fundamental building blocks of quarks and leptons which could hypothetically form preon stars.[105] These hypothetical models could potentially explain a number of observations of stellar black hole candidates. However, it can be shown from general arguments in general relativity that any such object will have a maximum mass.[83]

Since the average density of a black hole inside its Schwarzschild radius is inversely proportional to the square of its mass, supermassive black holes are much less dense than stellar black holes (the average density of a large supermassive black hole is comparable to that of water).[83] Consequently, the physics of matter forming a supermassive black hole is much better understood and the possible alternative explanations for supermassive black hole observations are much more mundane. For example, a supermassive black hole could be modelled by a large cluster of very dark objects. However, typically such alternatives are not stable enough to explain the supermassive black hole candidates.[83]

The evidence for stellar and supermassive black holes implies that in order for black holes not to form, general relativity must fail as a theory of gravity, perhaps due to the onset of quantum mechanical corrections. A much anticipated feature of a theory of quantum gravity is that it will not feature singularities or event horizons (and thus no black holes).[106] In recent years, much attention has been drawn by the fuzzball model in string theory. Based on calculations in specific situations in string theory, the proposal suggest that generically the individual states of a black hole solution do not have an event horizon or singularity (and can thus not really be considered to be a black hole), but that for a distant observer the statistical average of such states does appear just like an ordinary black hole in general relativity.[107]
Open questions
Entropy and thermodynamics
Further information: Black hole thermodynamics
If ultra-high-energy collisions of particles in a particle accelerator can create microscopic black holes, it is expected that all types of particles will be emitted by black hole evaporation, providing key evidence for any grand unified theory. Above are the high energy particles produced in a gold ion collision on the RHIC.

In 1971, Stephen Hawking showed under general conditions[Note 1] that the total area of the event horizons of any collection of classical black holes can never decrease, even if they collide and merge.[108] This result, now known as the second law of black hole mechanics, is remarkably similar to the second law of thermodynamics, which states that the total entropy of a system can never decrease. As with classical objects at absolute zero temperature, it was assumed that black holes had zero entropy. If this were the case, the second law of thermodynamics would be violated by entropy-laden matter entering the black hole, resulting in a decrease of the total entropy of the universe. Therefore, Jacob Bekenstein proposed that a black hole should have an entropy, and that it should be proportional to its horizon area.[109]

The link with the laws of thermodynamics was further strengthened by Hawking's discovery that quantum field theory predicts that a black hole radiates blackbody radiation at a constant temperature. This seemingly causes a violation of the second law of black hole mechanics, since the radiation will carry away energy from the black hole causing it to shrink. The radiation, however also carries away entropy, and it can be proven under general assumptions that the sum of the entropy of the matter surrounding the black hole and one quarter of the area of the horizon as measured in Planck units is in fact always increasing. This allows the formulation of the first law of black hole mechanics as an analogue of the first law of thermodynamics, with the mass acting as energy, the surface gravity as temperature and the area as entropy.[109]

One puzzling feature is that the entropy of a black hole scales with its area rather than with its volume, since entropy is normally an extensive quantity that scales linearly with the volume of the system. This odd property led 't Hooft and Susskind to propose the holographic principle, which suggests that anything that happens in volume of spacetime can be described by data on the boundary of that volume.[110]

Although general relativity can be used to perform a semi-classical calculation of black hole entropy, this situation is theoretically unsatisfying. In statistical mechanics, entropy is understood as counting the number of microscopic configurations of a system which have the same macroscopic qualities (such as mass, charge, pressure, etc.). Without a satisfactory theory of quantum gravity, one cannot perform such a computation for black holes. Some progress has been made in various approaches to quantum gravity. In 1995, Strominger and Vafa showed that counting the microstates of a specific supersymmetric black hole in string theory reproduced the Bekenstein–Hawking entropy.[111] Since then, similar results have been reported for different black holes both in string theory and in other approaches to quantum gravity like loop quantum gravity.[112]
Black hole unitarity
Main article: Black hole information paradox
Unsolved problems in physics Is physical information lost in black holes? Question mark2.svg

An open question in fundamental physics is the so-called information loss paradox, or black hole unitarity paradox. Classically, the laws of physics are the same run forward or in reverse (T-symmetry). Liouville's theorem dictates conservation of phase space volume, which can be thought of as "conservation of information", so there is some problem even in classical physics. In quantum mechanics, this corresponds to a vital property called unitarity, which has to do with the conservation of probability (it can also be thought of as a conservation of quantum phase space volume as expressed by the density matrix).[113]
See also

* Black holes in fiction
* Black string
* Kugelblitz (astrophysics)
* List of black holes
* Susskind-Hawking battle


He1523a.jpg Star portal

* Timeline of black hole physics
* White hole
* Wormhole

Notes

1. ^ In particular, he assumed that all matter satisfies the weak energy condition.

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Further reading

Popular reading

* Ferguson, Kitty (1991). Black Holes in Space-Time. Watts Franklin. ISBN 0-531-12524-6. .
* Hawking, Stephen (1988). A Brief History of Time. Bantam Books, Inc. ISBN 0-553-38016-8. .
* Hawking, Stephen; Penrose, Roger (1996). The Nature of Space and Time. Princeton University Press. ISBN 0-691-03791-2. http://books.google.com/?id=LstaQTXP65cC. .
* Melia, Fulvio (2003). The Black Hole at the Center of Our Galaxy. Princeton U Press. ISBN 978-0-691-09505-9. .
* Melia, Fulvio (2003). The Edge of Infinity. Supermassive Black Holes in the Universe. Cambridge U Press. ISBN 978-0-521-81405-8. .
* Pickover, Clifford (1998). Black Holes: A Traveler's Guide. Wiley, John & Sons, Inc. ISBN 0-471-19704-1. .
* Stern, B. (2008). "Blackhole". http://www.wikilivres.info/wiki/Blackhole_%28Stern%29. , poem.
* Thorne, Kip S. (1994). Black Holes and Time Warps. Norton, W. W. & Company, Inc. ISBN 0-393-31276-3. .
* Wheeler, J. Craig (2007). Cosmic Catastrophes (2nd ed.). Cambridge University Press. ISBN 0-521-85714-7.

University textbooks and monographs

* Carroll, Sean M. (2004). Spacetime and Geometry. Addison Wesley. ISBN 0-8053-8732-3. , the lecture notes on which the book was based are available for free from Sean Carroll's website.
* Carter, B. (1973). "Black hole equilibrium states". In DeWitt, B.S.; DeWitt, C.. Black Holes. .
* Chandrasekhar, Subrahmanyan (1999). Mathematical Theory of Black Holes. Oxford University Press. ISBN 0-19-850370-9. .
* Frolov, V.P.; Novikov, I.D. (1998). Black hole physics. .
* Hawking, S.W.; Ellis, G.F.R. (1973). Large Scale Structure of space time. Cambridge University Press. ISBN 0521099064. http://books.google.com/?id=QagG_KI7Ll8C. .
* Melia, Fulvio (2007). The Galactic Supermassive Black Hole. Princeton U Press. ISBN 978-0-691-13129-0. .
* Taylor, Edwin F.; Wheeler, John Archibald (2000). Exploring Black Holes. Addison Wesley Longman. ISBN 0-201-38423-X. .
* Thorne, Kip S.; Misner, Charles; Wheeler, John (1973). Gravitation. W. H. Freeman and Company. ISBN 0-7167-0344-0. .
* Wald, Robert M. (1992). Space, Time, and Gravity: The Theory of the Big Bang and Black Holes. University of Chicago Press. ISBN 0-226-87029-4. .

Review papers

* Detweiler, S. (1981). "Resource letter BH-1: Black holes". American Journal of Physics 49 (5, pp): 394–400. doi:10.1119/1.12686.
* Gallo, E.; Marolf, D. (2009). "Resource Letter BH-2: Black Holes". American Journal of Physics 77: 294. doi:10.1119/1.3056569. arXiv:0806.2316. edit
* Hughes, Scott A. (2005). "Trust but verify: The case for astrophysical black holes". arΧiv:hep-ph/0511217v2 [hep-ph]. Lecture notes from 2005 SLAC Summer Institute.

External links
Wikimedia Commons has media related to: Black holes

* Stanford Encyclopedia of Philosophy: "Singularities and Black Holes" by Erik Curiel and Peter Bokulich.
* "Black hole" on Scholarpedia.
* Black Holes: Gravity's Relentless Pull - Interactive multimedia Web site about the physics and astronomy of black holes from the Space Telescope Science Institute
* FAQ on black holes
* "Schwarzschild Geometry" on Andrew Hamilton’s website
* UT Brownsville Group Simulates Spinning Black-Hole Binaries
* Advanced Mathematics of Black Hole Evaporation

Videos

* 16-year long study tracks stars orbiting Milky Way black hole
* Yale University Video Lecture: Introduction to Black Holes at Google Video.
* Movie of Black Hole Candidate from Max Planck Institute

News

* "Black Hole confirmed in Milky Way." Retrieved December 10, 2008
* Black Hole Research News

riveting tale chap

[SpursWoman]

proc. Natl. Acad. Sci. Usa
vol. 95, pp. 2355–2360, march 1998
evolution
modification of expression and cis-regulation of hoxc8 in the
evolution of diverged axial morphology
heinz-georg belting*†, cooduvalli s. Shashikant*, and frank h. Ruddle*‡§
departments of *molecular, cellular, and developmental biology and ‡genetics, yale university, pob 208103, new haven, ct 06520
contributed by frank h. Ruddle, december 30, 1997
abstract differential hox gene expression between
vertebrate species has been implicated in the divergence of
axial morphology. To examine this relationship, we have
compared expression and transcriptional regulation of hoxc8
in chicken and mouse. In both species, expression of hoxc8 in
the paraxial mesoderm and neural tube is associated with
midthoracic and brachial identities, respectively. During embryogenesis,
there is a temporal delay in the activation of
hoxc8 in chicken compared with mouse. As a result, chicken
hoxc8 expression in the paraxial mesoderm is at a posterior
axial level, extending over a smaller domain compared with
mouse hoxc8 expression. This finding is consistent with a
shorter thoracic region in chicken compared with mouse. In
addition, the chicken hoxc8 early enhancer, differing from its
mouse counterpart in only a few specific nucleotides, directs
a reporter gene expression to a more posterior domain in
transgenic mouse embryos. These findings are consistent with
the concept that the diversification of axial morphology has
been achieved through changes in cis-regulation of developmental
control genes.
Genes that control axial patterning, such as hox genes, are
highly conserved across the animal kingdom (1). However,
animals exhibit a high degree of diversity in the organization
of the primary body axis. This phenomenon may be caused by
conserved genetic programs having become variously modified
in different organisms (2–4). Differences in hox gene
expression may contribute to an understanding of how modifications
of developmental programs generate axial diversity
between species (5–7).
Amniotes differ greatly in the number of segments contributing
to individual anatomic regions along the vertebral column
such as the cervical, thoracic, and lumbar regions. For
example, the vertebral column of the mouse consists of seven
cervical and 13 thoracic vertebrae, whereas the vertebral
column of chicken displays 14 cervical and seven thoracic
vertebrae. Comparisons between mouse and chicken show that
differences in axial morphology are associated with differences
in spatial domains of hox gene expression (5, 6). For example,
hoxc6 is expressed at the cervicalythoracic transition in both
mouse and chick, but at different relative levels along the
anteroposterior axis; likewise, hoxc8 is expressed in the thoracic
region of both mouse and chicken (6).
Differences in expression patterns of hox genes between
different species may be brought about by changes in components
of their transcriptional regulation, including changes in
cis-regulatory elements and trans-acting factors whose interactions
determine embryonic expression patterns of hox
genes. However, experimental exchanges of hox genes and
their cis-regulatory regions between different organisms have
demonstrated a high degree of functional conservation (8–24).
The expression of trans-acting factors of hox genes probably
are largely retained in amniotes, setting up a pre-pattern that
provides positional coordinates along the body axis. We postulate
that subtle changes in cis-regulatory elements leading to
altered interactions with conserved trans-acting factors may
contribute to diverged expression patterns of hox genes among
different species.
In previous studies, we identified cis-regulatory regions that
control different phases of mouse hoxc8 expression (25–29).
The hoxc8 early enhancer is involved in the establishment of
the anteroposterior expression domain of hoxc8, consistent
with its role in the regionalization of the body axis (26, 29). The
early enhancer has been delimited to a 200-bp dna fragment
by extensive deletion analyses in transgenic mice (26, 28). Five
partially redundant elements within this region act in combination
in determining early hoxc8 expression (26, 28). A
survey among mammalian species reveals a remarkable conservation
of the nucleotide sequence of the early enhancer
(c.s.s., unpublished observations). Any difference in the
nucleotide sequence and activity of this highly conserved and
well-characterized enhancer may have strong implications on
the divergence of hoxc8 expression between different species.
To test this hypothesis, we have studied hoxc8 expression
during chicken and mouse embryogenesis and compared their
early enhancer regions for nucleotide sequence similarities,
and enhancer activities in transgenic mice. In this report we
provide evidence suggesting that transposition of hoxc8 expression
between the two species is achieved by differential
activities of the hoxc8 early enhancer.
Materials and methods
fvb mice (taconics) were used for obtaining staged embryos
and for transgenic analysis. For mating, pairs were caged
together at noon, and the females were examined for the
presence of a vaginal plug the next morning, which was defined
as day 0.5. Fertilized eggs from white leghorn hens (spafas)
were incubated at 37°c in an egg incubator. The chicken
embryos were staged as described (30).
Immunohistochemistry was performed with a mab, c592y
7e, against the mouse hoxc8 protein (29) as described (31)
with minor modifications.
For retrograde labeling experiments, mouse (10.5 day postcoital)

2-week incubation in pbs, 10 mm edta. Specimens were cut
on a vibratome at 80–100mm, and the image was captured with
a black-and-white charge-coupled device camera. The same
sections then were processed with hoxc8 mabs, and the
expression of hoxc8 was compared with the dii label, side by
side or by superimposing the captured images in pseudocolor.
Production of transgenic embryos, preparation of dna for
microinjection, and staining for b-galactosidase have been
described (26). The reporter gene construct (construct a)
carrying 399-bp hoxc8 early enhancer was described earlier
(28). A 151-bp chicken enhancer was isolated by pcr using
chicken genomic dna as a template and synthetic oligonucleotide
primers designed from the mouse enhancer sequence
flanking the fragment. A 399-bp enhancer fragment in which
151 bp of the mouse sequence was replaced with the corresponding
chicken dna fragment was generated by overlapping
pcr. The resulting fragment was cloned by ligation at
appropriate restriction sites in the polylinker sequence of
phsf (26) to create a mouseychicken hybrid construct (construct
b).
Results
heterochronic activation of hoxc8. Previous studies by
burke et al. (6), using rna in situ procedures, showed that
axial levels of hoxc8 expression differed between mouse and
chicken midgestation embryos. To determine the temporal
sequence of hoxc8 expression during mouse and chicken
embryogenesis, we performed a comparative immunohistochemistry,
using a mab raised against mouse hoxc8 (29). The
mouse embryonic expression pattern is described in detail
elsewhere (29). Briefly, in the mouse, hoxc8 first is detected
at the base of allantois in day 8 embryos having 6–7 somites
(fig. 1a). At this stage, the neural tube is still open, and the
heart primordia have just formed (32). The earliest chicken
embryos examined, stages 10 and 11, are similar in their
developmental progression to day 8.0–8.5 mouse embryos. At
these stages, the major events of gastrulation have occurred,
and organogenesis is proceeding in the anterior portion of the
embryos. Embryos of both species possess a similar number
(8–12) of somites, and the degrees of the development of the
heart and nervous system are comparable at these stages.
However, in the chick, hoxc8 protein was not detected in stages
10 or 12 (data not shown), but first was detected in the
posterior regions in stage 13 embryos having 18–19 somites
(fig. 1e). At this stage, the neural tube, with the exception of
the caudal neuropore, is entirely closed, and the rostral
portions, including fore-, mid- and hindbrain, the optic cups,
and otic vesicles are formed. As in the mouse, chicken hoxc8
expression at this stage was diffuse in all embryonic tissues
posterior to somite condensation. Thus, the onset of hoxc8
expression in the chicken is developmentally delayed compared
with mouse.
Axial levels of hoxc8 expression. A clear and differential
anterior boundary of hoxc8 expression in the neural tube and
paraxial mesoderm is established at subsequent stages in the
mouse (day 8.5, fig. 1b) and in the chicken embryos (stage 14,
fig. 1f). In the mouse, at days 9.5 and 10.5, anterior bound-
fig. 1. Expression of hoxc8 during early mouse (a–d) and chicken (e and f) embryogenesis. The anterior boundary of expression in the neural
tube is indicated by arrowheads. (a) embryo with 7–8 somites. (b) embryo with 10–12 somites. The staining of the foregut in a and b is caused
by antibody trapping. (c) embryo with 28–30 somites (9.5 day postcoital). (d) embryo at 10.5 day p.c. Somite 17 is indicated in c and d. (e)
chicken embryo at h&h stage 13 (18–19 somites). (f) embryo at h&h stage 14 (20 somites). (g) embryo at h&h stage 21. (h) at h&h stage
23, somite 22 is indicated in g and h. The staining of the brain and the allantois is caused by antibody trapping. A, allantois; e, eye; f, forelimb
bud, m, mesoderm; ov, otic vesicle; s, somite.
2356 evolution: Belting et al. Proc. Natl. Acad. Sci. Usa 95 (1998)

aries of expression of hoxc8 were observed in the neural tube
at the level of 10th somite and in the paraxial mesoderm at the
level of 14th somite (fig. 1 c and d). In the chick, however, at
stages 21 and 23, anterior boundaries of expression of hoxc8
were observed in the neural tube at the level of the 18th somite
and in the paraxial mesoderm at the level of 20th somite. Thus,
the axial level of expression of hoxc8 in chicken embryos is 6–8
somites more posterior than that observed in the mouse
embryos.
At these stages, in both mouse and chicken embryos, hoxc8
expression declines in the tailbud region, thus defining posterior
boundaries of expression. However, these boundaries are
unlike the anterior boundaries of expression, having indistinct
margins. In the mouse, hoxc8 expression in the neural tube
spans about six somite levels (from somites 10–15) and in the
paraxial mesoderm about 7–8 somite levels (somites 13–21,
with weaker levels of expression at somites 13 and 21). In the
chick, hoxc8 expression in the neural tube spans about six
somite levels (somites 18–23; strong expression in somites
19–20) and in paraxial mesoderm about five somite levels
(somites 21–25). Thus hoxc8 expression in paraxial mesoderm
is 2–3 somite levels shorter in the chicken compared with
mouse. Although the expression domain of hoxc8 in the
paraxial mesoderm is smaller and more posterior compared
with mouse, the expression pattern is coincident with the
smaller thoracic region in chick. Thus, hoxc8 expression in the
paraxial mesoderm is different not only with respect to absolute
axial levels but also in the number of expressing somites.
Early and late phases of neural tube expression of hoxc8.
In the mouse, two phases of hoxc8 expression can be distinguished
in the neural tube, an early and a late phase (29). In
the early phase (day 8–9.5), hoxc8 expression is found in most,
if not all, cells along the dorsoventral extent of the neural tube
(data not shown). In the late phase (after day 10.5), hoxc8
expression is restricted to differentiating neurons, predominantly
in motor neurons in the ventrolateral region of the
neural tube (fig. 1d). In the chick, similar phases of hoxc8
expression in the neural tube are observed. In the early phase
(stage 13–15) hoxc8 is distributed uniformly in the neural tube,
whereas in the late phase hoxc8 is distributed predominantly
in the motor neurons in the ventrolateral region (fig. 1h). The
distribution of hoxc8 within the subregions of the spinal cord
at subsequent stages is very similar in both species.
Association of hoxc8 expression with motor neurons. To
determine whether the axial shift of hoxc8 expression in the
neural tube of mouse and chicken corresponds to a transposition
of regional identity of spinal nerves, brachial motor
neurons were identified and tested for colocalization with
hoxc8 protein. Brachial motor neurons of day 10.5 mouse and
stage 24 chicken embryos were retrograde-labeled with dii as
described in materials and methods. The embryos were sectioned
serially, and the dii signal was compared with the
distribution of hoxc8 protein on the same sections. In both
mouse and chick, sections through the anterior brachial neural
tube at somite levels 9 and 17, respectively, showed no hoxc8
expression (fig. 2 a and d). At more posterior levels, (at
somite levels 11 in mouse and 19 in chick), however, dii label
coincided with the domain of hoxc8 expression (fig. 2 b and
d). These findings were confirmed on horizontal sections
(data not shown). Thus, hoxc8 expression in the central
nervous system is transposed according to the functional
identity of expressing motor neurons.
Hoxc8 early enhancer of mouse and chick. The mouse
hoxc8 early enhancer is involved in the establishment of spatial
domains of hoxc8 expression (25, 26, 29). We isolated the
chicken hoxc8 early enhancer to test whether the difference in
the spatiotemporal pattern of chicken hoxc8 expression compared
with mouse is caused by differences in the chicken
enhancer. Primers were designed, based on most conserved
regions of hoxc8 early enhancer, to amplify a 151-bp fragment
of the chicken enhancer by pcr. This enhancer is highly
conserved between the mouse and chicken with respect to
structure and overall sequence similarity (fig. 3a). The sequence
similarity over 151 bp is 80%. The critical elements
(fig. 3a, a–e) required for the mouse enhancer activity in
fig. 2. (a–d) expression of hoxc8 in the brachial region of the
neural tube. Cross-section through the neural tube of a mouse embryo
at the level of the ninth (a) and 11th (b) somite. (c and d)
cross-sections through the neural tube of a chicken embryo at the level
of the 17th and 19th somites. Hoxc8 expression is shown in green, dii
label in red and resulting overlap in yellow. (e and f) expression of
mouse and chicken reporter genes in transgenic mouse embryos. (e)
construct a (399-bp mouse sequence); b, construct b (399-bp mousechicken
hybrid construct). The arrowheads indicates somite 14. The
arrow indicates the anterior limit of reporter gene expression mediated
by construct b (f). F, forelimb bud; nc, notochord; r, ventral root; v,
ventral horn.
Evolution: Belting et al. Proc. Natl. Acad. Sci. Usa 95 (1998) 2357


and chicken [hamburger and hamilton (h&h) 24]
embryos were fixed in 4% paraformaldehyde in pbs. Brachial
motor neurons were retrograde-labeled by placing finely
ground 1,19-dioctadecyl-3,3,39,39-tetramethylindocarbocyanine
(dii) crystals into one severed forelimb bud and a 1- to
the publication costs of this article were defrayed in part by page charge
payment. This article must therefore be hereby marked ‘‘advertisement’’ in
accordance with 18 u.s.c. §1734 solely to indicate this fact.
© 1998 by the national academy of sciences 0027-8424y98y952355-6$2.00y0
pnas is available online at http:yywww.pnas.org.
Abbreviations: Dii, 1,19-dioctadecyl-3,3,39,39-tetramethylindocarbocyanine;
h&h, hamburger and hamilton.
Data deposition: The sequence reported in this paper has been
deposited in the genbank database (accession no. Aj223359).
†present address: Institute for biology i, university of freiburg,
hauptstr. 1, d-79104 freiburg, germany.
§to whom reprint requests should be addressed. E-mail: Frank.ruddle@
yale.edu.
2355

transgenic mice are arranged identically in the chicken enhancer.
In addition to clusters of substitutions between the
known sites, there are several differences within and in proximity
to these elements.
The chicken enhancer was tested for its ability to direct the
expression of a reporter gene (hsp68-lacz) in transgenic mice to
determine whether differences in its nucleotide sequence from
that of mouse affects enhancer activity. A 399-bp dna fragment
containing the mouse early enhancer region (construct a, fig.
3b) directs the reporter gene expression in day 9.5 embryos to the
neural tube and paraxial mesoderm at the level of somite 14 and
19, respectively (ref. 28 and fig. 2e). From this construct, the
151-bp fragment of the mouse enhancer containing the critical
elements of the enhancer was replaced with the corresponding
sequences from the chick. The resulting construct (construct b,
fig. 3b) directed the expression of the reporter gene in day 9.5
embryos to more posterior regions of the embryo in both neural
tube and mesoderm (fig. 2f). Five founder embryos that were
generated displayed identical patterns. Expression in the neural
tube was consistently found posterior to the 18th somite, whereas
expression in the mesoderm was several somites posterior to that
in the neural tube. Thus, chicken hoxc8 enhancer differing from
the mouse counterpart in a few nucleotides directs the reporter
gene expression to a different anterior boundary in the neural
tube and mesoderm. This posteriorization of the reporter gene
activity in the neural tube and mesoderm is consistent with the
more posterior localized expression of hoxc8 in the chick.
Discussion
the investigation of the genetic basis of morphological diversity
among animals now has become feasible because of the
identification of highly conserved regulatory genes that control
embryonic patterning and morphogenesis. To examine the
correlation of hox gene expression and axial variation, we have
compared the spatiotemporal distribution of hoxc8 in mouse
and chick. Our findings can be summarized as follows: Hoxc8
expression is modified in concert with variation in axial
morphology within the paraxial mesoderm and the neural tube
(fig. 4). Posteriorization of hoxc8 expression in chicken is
achieved through a temporal delay of activation compared
fig. 3. The hoxc8 early enhancer in mouse and chick. (a) nucleotide sequence comparison of the critical region of chicken and mouse hoxc8
early enhancers. One hundred fifty-one bp of the critical region of the early enhancer are shown. Five elements essential for the activity of the early
enhancer are indicated (a–e). (b) design of mouse and mouse-chicken hybrid reporter constructs. Construct a consists of a 399-bp early hoxc8
enhancer ligated to a mouse hsp68ylacz reporter gene. Construct b was generated by replacing 151 bp of the critical enhancer region of the mouse
with that of chick.
Fig. 4. Schematic comparison of hoxc8 expression in chicken and
mouse in relationship to morphological landmarks. Cervical, thoracic,
and lumbar regions of the vertebral column and the brachial region of
the neural tube are indicated. Brachial spinal nerves c6, c7, c8, and
t1 in mouse and c13, c14, c15, and t1 in chicken are shown. Shaded
region in somites and neural tube represent hoxc8 expression. Regions
of highest expression are indicated in dark shades. The double-headed
arrow indicates the anteroposterior orientation of the body axis. A,
anterior; p, posterior; nt, neural tube; t, thoracic vertebrae; s, somites;
sn, spinal nerves; v, vertebrae.
2358 evolution: Belting et al. Proc. Natl. Acad. Sci. Usa 95 (1998)

with mouse. The comparison of chicken and mouse enhancer
elements shows that only a few nucleotide changes within the
critical region of the early hoxc8 enhancer suffice to transpose
reporter gene expression to more posterior body regions.
The role of hox genes in the regionalization of the nervous
system has been examined most closely in the hindbrain (33).
Within the hindbrain hox genes are expressed in the same
segments in mouse and chick, reflecting that the organization,
in respect to the number of rhombomeres, has been conserved
between these two species (34–36). In contrast, the brachial
region of the spinal cord is transposed in these species and our
results show that the shift of hoxc8 expression along the body
axis corresponds to this anatomical modification.
The different axial position and extent of hoxc8 expression
in the segmental mesoderm reflects two major differences in
the axial organization of the vertebral column of chicken and
mouse. First, the relative expansion of the cervical region in
the chicken is reflected by a posteriorization of hoxc8 expression.
Second, the overall reduction of thoracic segments is
reflected by a reduced number of somites expressing hoxc8
compared with the mouse. This association of hoxc8 expression
with regional morphology of vertebrae suggests a role in
the specification of midthoracic identity within the paraxial
mesoderm. The axial fate of somites already is established in
the presomitic mesoderm (37). Thus, the observation that
hoxc8 is found at the correct place and at the right time in the
segmental plate and before somite condensation is consistent
with a role in the establishment of thoracic identity in the
paraxial mesoderm. Further support for a causal relationship
between hoxc8 expression and midthoracic identity stems
from genetic analyses in mice. Ectopic expression of hoxc8 in
somites of the lower thoracic and upper lumbar region leads to
anterior transformations within this region, including the
formation of lumbar ribs (38). Similarly, disruption of hoxc9
causes a posterior expansion of hoxc8 expression and the
appearance of supernumerary ribs as well (39). These results
also demonstrate that anatomical regions can be expanded by
an extension of hox gene expression along the primary body
axis and agrees well with the finding that a larger expression
domain of hoxc8 in the mouse, compared with the chick, is
linked to a higher number of thoracic vertebrae. This finding
suggests that axial variation among amniotes is not only
generated by axial shifts in the anterior expression boundaries
of hox genes, but also by expansion or reduction of their
overall expression domains.
The axial variation in hoxc8 expression may be caused by
changes in the transcriptional regulation of hoxc8. Differences
in hox gene expression could be caused by genetic changes in
cis elements andyor trans-acting factors. Changes in transregulation
are more difficult to study because of the multiplicity
of potential (and lack of bona fide) regulatory proteins.
Changes in cis-elements in this report were studied by comparing
a minimal sequence of the hoxc8 early enhancer. The
nucleotide sequence of the early enhancer region is highly
conserved among mammals and the sequence of cis-acting
elements (a–e) are invariant. Compared with mammalian
hoxc8 early enhancer sequence, the chicken enhancer sequence
showed more nucleotide sequence changes. Many of
the nucleotide differences were observed in the vicinity of the
genetically defined sites, a, d, and e. It is conceivable that
these and other specific nucleotide differences contribute
toward overall posteriorization of the reporter gene expression
mediated through the chicken enhancer. In the case of the
mouse enhancer, mutations at individual sites a, c, d, and e,
lead to posteriorization of the reporter gene expression (28).
The anterior extent of the reporter gene expression is determined
by combinatorial interactions at these elements. In
addition, nucleotide changes outside of the defined elements
in the chicken enhancer may be affecting hitherto undefined
cis-acting elements. A systematic exchange of nucleotide sequences
between mouse and chicken enhancers will pinpoint
critical nucleotides involved in the posteriorization of the
reporter gene expression directed by the chicken enhancer.
Comparative analysis of vertebrate cis-regulatory regions,
using reporter gene assays in transgenic mouse embryos, have
shown remarkable conservation of transcriptional regulation
of hox genes (13–15, 17–19, 22). Many of these elements direct
reporter gene expression to similar spatial domains in transgenic
mouse embryos. On the other hand, a chicken hoxb4
enhancer, although capable of directing expression of the
reporter gene to the correct anterior boundary in the neural
tube, directed expression to a more posterior boundary in the
mesoderm, suggesting a species-specific differences in the
enhancer activity (18). Transcriptional heterochrony also has
been suggested to be an important mechanism by which subtle
changes in temporal colinearity of hox genes may result in the
evolution of body plans (40). A replacement of a conserved
mouse hoxd11 regulatory region with its zebrafish counterpart
lead to a slightly premature activation of hoxd11, leading to
rostral shift of its expression boundary and anterior transposition
of the sacrum (22).
In conclusion, we have shown that the hoxc8 expression in
mouse and chicken is similar with respect to anatomical
features such as the brachial spinal nerves and the midthoracic
region of the vertebral column. However, significant differences
also exist in relational features of the body plan such as
the ratio of cervical and thoracic domains. We also show in
mouse transgenic experiments where we compare the early
enhancers of mouse and chick that chicken enhancer constructs
simulate a chicken relational pattern of expression.
Additional experiments will be required to determine the
specificity of nucleotide changes in the regulation of hoxc8
expression pattern and correlated modifications of the body
plan.
We thank charles bieberich and gunter wagner for discussion of
earlier versions of this manuscript. This work has been supported by
national institutes of health grant gm09966 to f.h.r.
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Sci. Usa 92, 1684–1688.
16. Frasch, m., chen, x. & lufkin, t. (1995) development (cambridge,
u.k.) 121, 957–974.
17. Knittel, t., kessel, m., kim, m. H. & gruss, p. (1995) development
(cambridge, u.k.) 121, 1077–1088.
Evolution: Belting et al. Proc. Natl. Acad. Sci. Usa 95 (1998) 2359


+1

[TimDunkem]

A black hole is a region of space from which nothing, not even light, can escape. It is the result of the deformation of spacetime caused by a very compact mass. Around a black hole there is an undetectable surface which marks the point of no return, called an event horizon. It is called "black" because it absorbs all the light that hits it, reflecting nothing, just like a perfect black body in thermodynamics.[1] Quantum mechanics predicts that black holes also emit radiation like a black body with a finite temperature. This temperature decreases with the mass of the black hole, making it unlikely to observe this radiation for black holes of stellar mass.

Despite its invisible interior, a black hole can be observed through its interaction with other matter. A black hole can be inferred by tracking the movement of a group of stars that orbit a region in space. Alternatively, when gas falls into a stellar black hole from a companion star, the gas spirals inward, heating to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and Earth-orbiting telescopes.

Astronomers have identified numerous stellar black hole candidates, and have also found evidence of supermassive black holes at the center of galaxies. In 1998, astronomers found compelling evidence that a supermassive black hole of more than 2 million solar masses is located near the Sagittarius A* region in the center of the Milky Way galaxy, and more recent results using additional data find evidence that the supermassive black hole is more than 4 million solar masses.
Contents
[hide]

* 1 History
o 1.1 General relativity
o 1.2 Golden age
* 2 Properties and structure
o 2.1 Physical properties
o 2.2 Event horizon
o 2.3 Singularity
o 2.4 Photon sphere
o 2.5 Ergosphere
* 3 Formation and evolution
o 3.1 Gravitational collapse
+ 3.1.1 Primordial black holes in the Big Bang
o 3.2 High-energy collisions
o 3.3 Growth
o 3.4 Evaporation
* 4 Observational evidence
o 4.1 Accretion of matter
o 4.2 X-ray binaries
+ 4.2.1 Quiescence and advection-dominated accretion flow
+ 4.2.2 Quasi-periodic oscillations
o 4.3 Gamma ray bursts
o 4.4 Galactic nuclei
o 4.5 Gravitational lensing
o 4.6 Alternatives
* 5 Open questions
o 5.1 Entropy and thermodynamics
o 5.2 Black hole unitarity
* 6 See also
* 7 Notes
* 8 References
* 9 Further reading
* 10 External links

History
Schwarzschild black hole
Simulation of gravitational lensing by a black hole which distorts the image of a galaxy in the background (click here for larger animation)

The idea of a body so massive that even light could not escape was first put forward by geologist John Michell in a letter written to Henry Cavendish in 1783 to the Royal Society:

If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae, with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity.
—John Michell[2]

In 1796, mathematician Pierre-Simon Laplace promoted the same idea in the first and second editions of his book Exposition du système du Monde (it was removed from later editions).[3][4] Such "dark stars" were largely ignored in the nineteenth century, since it was not understood how a massless wave such as light could be influenced by gravity.[5]
General relativity

In 1915, Albert Einstein developed his theory of general relativity, having earlier shown that gravity does influence light's motion. A few months later, Karl Schwarzschild gave the solution for the gravitational field of a point mass and a spherical mass.[6] A few months after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution for the point mass and wrote more extensively about its properties.[7] This solution had a peculiar behaviour at what is now called the Schwarzschild radius, where it became singular, meaning that some of the terms in the Einstein equations became infinite. The nature of this surface was not quite understood at the time. In 1924, Arthur Eddington showed that the singularity disappeared after a change of coordinates (see Eddington coordinates), although it took until 1933 for Georges Lemaître to realize that this meant the singularity at the Schwarzschild radius was an unphysical coordinate singularity.[8]

In 1931, Subrahmanyan Chandrasekhar calculated, using general relativity, that a non-rotating body of electron-degenerate matter above 1.44 solar masses (the Chandrasekhar limit) would collapse.[citation needed] His arguments were opposed by many of his contemporaries like Eddington and Lev Landau, who argued that some yet unknown mechanism would stop the collapse.[9] They were partly correct: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star,[10] which is itself stable because of the Pauli exclusion principle. But in 1939, Robert Oppenheimer and others predicted that neutron stars above approximately three solar masses (the Tolman–Oppenheimer–Volkoff limit) would collapse into black holes for the reasons presented by Chandrasekhar, and concluded that no law of physics was likely to intervene and stop at least some stars from collapsing to black holes.[11]

Oppenheimer and his co-authors interpreted the singularity at the boundary of the Schwarzschild radius as indicating that this was the boundary of a bubble in which time stopped. This is a valid point of view for external observers, but not for infalling observers. Because of this property, the collapsed stars were called "frozen stars,"[12] because an outside observer would see the surface of the star frozen in time at the instant where its collapse takes it inside the Schwarzschild radius. This is a known property of modern black holes, but it must be emphasized that the light from the surface of the frozen star becomes redshifted very fast, turning the black hole black very quickly. Many physicists could not accept the idea of time standing still at the Schwarzschild radius, and there was little interest in the subject for over 20 years.
Golden age
See also: Golden age of general relativity

In 1958, David Finkelstein identified the Schwarzschild surface r = 2m [in geometrized units, i.e. 2Gm/c2, where r is the radius of the surface and m is the mass of the black hole] as an event horizon, "a perfect unidirectional membrane: causal influences can cross it in only one direction".[13] This did not strictly contradict Oppenheimer's results, but extended them to include the point of view of infalling observers. Finkelstein's solution extended the Schwarzschild solution for the future of observers falling into the black hole. A complete extension had already been found by Martin Kruskal, who was urged to publish it.[14]

These results came at the beginning of the golden age of general relativity, which is marked by general relativity and black holes becoming mainstream subjects of research. This process was helped by the discovery of pulsars in 1967,[15][16] which were within a few years shown to be rapidly rotating neutron stars. Until that time, neutron stars, like black holes, were regarded as just theoretical curiosities; but the discovery of pulsars showed their physical relevance and spurred a further interest in all types of compact objects that might be formed by gravitational collapse.

In this period more general black hole solutions where found. In 1963, Roy Kerr found the exact solution for a rotating black hole. Two years later Ezra T. Newman found the axisymmetric solution for a black hole which is both rotating and electrically charged.[17] Through the work of Werner Israel,[18] Brandon Carter,[19][20] and D. C. Robinson[21] the no-hair theorem emerged, stating that a stationary black hole solution is completely described by the three parameters of the Kerr–Newman metric; mass, angular momentum, and electric charge.[22]

For a long time, it was suspected that the strange features of the black hole solutions were pathological artefacts from the symmetry conditions imposed, and that the singularities would not appear in generic situations. This view was held in particular by Belinsky, Khalatnikov, and Lifshitz, who tried to prove that no singularities appear in generic solutions. However, in the late sixties Roger Penrose[23] and Stephen Hawking used global techniques to prove that singularities are generic.[24]

Work by James Bardeen, Jacob Bekenstein, Carter, and Hawking in the early 1970s led to the formulation of the laws of black hole mechanics.[25] These laws describe the behaviour of a black hole in close analogy to the laws of thermodynamics by relating mass to energy, area to entropy, and surface gravity to temperature. The analogy was completed when Hawking, in 1974, showed that quantum field theory predicts that black holes should radiate like a black body with a temperature proportional to the surface gravity of the black hole.[26]

The term "black hole" was first publicly used by John Wheeler during a lecture in 1967. Although he is usually credited with coining the phrase, he always insisted that it was suggested to him by somebody else. The first recorded use of the term is in a 1964 letter by Anne Ewing to the American Association for the Advancement of Science.[27] After Wheeler's use of the term, it was quickly adopted in general use.
Properties and structure

The no-hair theorem states that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, charge, and angular momentum.[22] Any two black holes that share the same values for these properties, or parameters, are indistinguishable according to classical (i.e. non-quantum) mechanics.

These properties are special because they are visible from outside the black hole. For example, a charged black hole repels other like charges just like any other charged object. Similarly, the total mass inside a sphere containing a black hole can be found by using the gravitational analog of Gauss's law, the ADM mass, far away from the black hole.[28] Likewise, the angular momentum can be measured from far away using frame dragging by the gravitomagnetic field.

When an object falls into a black hole, any information about the shape of the object or distribution of charge on it is evenly distributed along the horizon of the black hole, and is lost to outside observers. The behavior of the horizon in this situation is closely analogous to that of a conductive stretchy membrane with friction and electrical resistance, a dissipative system (see membrane paradigm).[29] This is different from other field theories like electromagnetism, which does not have any friction or resistivity at the microscopic level, because they are time-reversible. Because the black hole eventually achieves a stable state with only three parameters, there is no way to avoid losing information about the initial conditions: the gravitational and electric fields of the black hole give very little information about what went in. The information that is lost includes every quantity that cannot be measured far away from the black hole horizon, including the total baryon number, lepton number, and all the other nearly conserved pseudo-charges of particle physics. This behavior is so puzzling that it has been called the black hole information loss paradox.[30][31][32]
Physical properties

The simplest black hole has mass but neither electric charge nor angular momentum. These black holes are often referred to as Schwarzschild black holes after the physicist Karl Schwarzschild who discovered this solution in 1915.[6] According to Birkhoff's theorem, it is the only vacuum solution that is spherically symmetric.[33] This means that there is no observable difference between the gravitational field of such a black hole and that of any other spherical object of the same mass. The popular notion of a black hole "sucking in everything" in its surroundings is therefore only correct near the black hole horizon; far away, the external gravitational field is identical to that of any other body of the same mass.[34]

Solutions describing more general black holes also exist. Charged black holes are described by the Reissner-Nordström metric, while the Kerr metric describes a rotating black hole. The most general stationary black hole solution known is the Kerr-Newman metric, which describes a black hole with both charge and angular momentum.

While the mass of a black hole can take any positive value, the charge and angular momentum are constrained by the mass. In Planck units, the total electric charge Q and the total angular momentum J are expected to satisfy

Q^2+\left ( \tfrac{J}{M} \right )^2\le M^2\,

for a black hole of mass M. Black holes saturating this inequality are called extremal. Solutions of Einstein's equations violating the inequality exist, but do not have a horizon. These solutions have naked singularities and are deemed unphysical. The cosmic censorship hypothesis rules out the formation of such singularities through the gravitational collapse of realistic matter.[35] This is supported by numerical simulations.[36]

Due to the relatively large strength of the electromagnetic force, black holes forming from the collapse of stars are expected to retain the nearly neutral charge of the star. Rotation, however, is expected to be a common feature of compact objects, and the black-hole candidate binary X-ray source GRS 1915+105[37] appears to have an angular momentum near the maximum allowed value.
Class Mass Size
Supermassive black hole ~105–109 MSun ~0.001–10 AU
Intermediate-mass black hole ~103 MSun ~103 km = REarth
Stellar black hole ~10 MSun ~30 km
Micro black hole up to ~MMoon up to ~0.1 mm

Black holes are commonly classified according to their mass, independent of angular momentum J or electric charge Q. The size of a black hole, as determined by the radius of the event horizon, or Schwarzschild radius, is roughly proportional to the mass M through

r_\mathrm{sh} =\frac{2GM}{c^2} \approx 2.95\, \frac{M}{M_\mathrm{Sun}}~\mathrm{km,}

where rsh is the Schwarzschild radius and MSun is the mass of the Sun. This relation is exact only for black holes with zero charge and angular momentum, for more general black holes it can differ up to a factor of 2. The table on the right lists the various classes of black hole that are distinguished.
Event horizon
Main article: Event horizon
Image:BH-no-escape-1.svg
Far away from the black hole a particle can move in any direction. It is only restricted by the speed of light.
Image:BH-no-escape-2.svg
Closer to the black hole spacetime starts to deform. There are more paths going towards the black hole than paths moving away.
Image:BH-no-escape-3.svg
Inside of the event horizon all paths bring the particle closer to the center of the black hole. It is no longer possible for the particle to escape.

The defining feature of a black hole is the appearance of an event horizon—a boundary in spacetime through which matter and light can only pass inward towards the mass of the black hole. Nothing, not even light, can escape from inside the event horizon. The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if such an event occurred.[38]

As predicted by general relativity, the presence of a large mass deforms spacetime in such a way that the paths taken by particles bend towards the mass. At the event horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hole.[39]

To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole.[40] Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it.[41] At the same time, all processes on this object slow down causing emitted light to appear redder and dimmer, an effect known as gravitational redshift.[42] Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.

On the other hand, an observer falling into a black hole does not notice any of these effects as he crosses the event horizon. According to his own clock, he crosses the event horizon after a finite time, although he is unable to determine exactly when he crosses it, as it is impossible to determine the location of the event horizon from local observations.[43]

For a non rotating (static) black hole, the Schwarzschild radius delimits a spherical event horizon. The Schwarzschild radius of an object is proportional to the mass.[44] Rotating black holes have distorted, nonspherical event horizons. Since the event horizon is not a material surface but rather merely a mathematically defined demarcation boundary, nothing prevents matter or radiation from entering a black hole, only from exiting one. The description of black holes given by general relativity is known to be an approximation, and some scientists expect that quantum gravity effects will become significant near the vicinity of the event horizon.[45] This would allow observations of matter near a black hole's event horizon to be used to indirectly study general relativity and proposed extensions to it.
Singularity
Main article: Gravitational singularity

At the center of a black hole as described by general relativity lies a gravitational singularity, a region where the spacetime curvature becomes infinite.[46] For a non-rotating black hole this region takes the shape of a single point and for a rotating black hole it is smeared out to form a ring singularity lying in the plane of rotation.[47] In both cases the singular region has zero volume. It can also be shown that the singular region contains all the mass of the black hole solution.[48] The singular region can thus be thought of as having infinite density.

An observer falling into a Schwarzschild black hole (i.e. non-rotating and no charges) cannot avoid the singularity. Any attempt to do so will only shorten the time taken to get there.[49] When they reach the singularity, they are crushed to infinite density and their mass is added to the total of the black hole. Before that happens, they will have been torn apart by the growing tidal forces in a process sometimes referred to as spaghettification or the noodle effect.[50]

In the case of a charged (Reissner–Nordström) or rotating (Kerr) black hole it is possible to avoid the singularity. Extending these solutions as far as possible reveals the hypothetical possibility of exiting the black hole into a different spacetime with the black hole acting as a wormhole.[51] The possibility of traveling to another universe is however only theoretical, since any perturbation will destroy this possibility.[52] It also appears to be possible to follow closed timelike curves (going back to one's own past) around the Kerr singularity, which lead to problems with causality like the grandfather paradox.[53] It is expected that none of these peculiar effects would survive in a proper quantum mechanical treatment of rotating and charged black holes.[54]

The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory.[55] This breakdown, however, is expected; it occurs in a situation where quantum mechanical effects should describe these actions due to the extremely high density and therefore particle interactions. To date it has not been possible to combine quantum and gravitational effects into a single theory. It is generally expected that a theory of quantum gravity will feature black holes without singularities.[56][57]
Photon sphere
Main article: Photon sphere

The photon sphere is a spherical boundary of zero thickness such that photons moving along tangents to the sphere will be trapped in a circular orbit. For non-rotating black holes, the photon sphere has a radius 1.5 times the Schwarzschild radius. The orbits are dynamically unstable, hence any small perturbation (such as a particle of infalling matter) will grow over time, either setting it on an outward trajectory escaping the black hole or on an inward spiral eventually crossing the event horizon.

While light can still escape from inside the photon sphere, any light that crosses the photon sphere on an inbound trajectory will be captured by the black hole. Hence any light reaching an outside observer from inside the photon sphere must have been emitted by objects inside the photon sphere but still outside of the event horizon.

Other compact objects, such as neutron stars, can also have photon spheres.[58] This follows from the fact that the gravitational field of an object does not depend on its actual size, hence any object that is smaller than 1.5 times the Schwarzschild radius corresponding to its mass will indeed have a photon sphere.
Ergosphere
Main article: Ergosphere
The ergosphere is an oblate spheroid region outside of the event horizon, where objects cannot remain stationary.

Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still, called the ergosphere. This is the result of a process known as frame-dragging; general relativity predicts that any rotating mass will tend to slightly "drag" along the spacetime immediately surrounding it. Any object near the rotating mass will tend to start moving in the direction of rotation. For a rotating black hole this effect becomes so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still.[59]

The ergosphere of a black hole is bounded by the (outer) event horizon on the inside and an oblate spheroid, which coincides with the event horizon at the poles and is noticeably wider around the equator. The outer boundary is sometimes called the ergosurface.

Objects and radiation can escape normally from the ergosphere. Through the Penrose process, objects can emerge from the ergosphere with more energy than they entered. This energy is taken from the rotational energy of the black hole causing it to slow down.[60]
Formation and evolution

Considering the exotic nature of black holes, it may be natural to question if such bizarre objects could exist in nature or to suggest that they are merely pathological solutions to Einstein's equations. Einstein himself wrongly thought that black holes would not form, because he held that the angular momentum of collapsing particles would stabilize their motion at some radius.[61] This led the general relativity community to dismiss all results to the contrary for many years. However, a minority of relativists continued to contend that black holes were physical objects,[62] and by the end of the 1960s, they had persuaded the majority of researchers in the field that there is no obstacle to forming an event horizon.

Once an event horizon forms, Roger Penrose proved that a singularity will form somewhere inside it.[23] Shortly afterwards, Stephen Hawking showed that many cosmological solutions describing the Big Bang have singularities without scalar fields or other exotic matter (see Penrose-Hawking singularity theorems). The Kerr solution, the no-hair theorem and the laws of black hole thermodynamics showed that the physical properties of black holes were simple and comprehensible, making them respectable subjects for research.[63] The primary formation process for black holes is expected to be the gravitational collapse of heavy objects such as stars, but there are also more exotic processes that can lead to the production of black holes.
Gravitational collapse
Main article: Gravitational collapse

Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. For stars this usually occurs either because a star has too little "fuel" left to maintain its temperature through stellar nucleosynthesis, or because a star which would have been stable receives extra matter in a way which does not raise its core temperature. In either case the star's temperature is no longer high enough to prevent it from collapsing under its own weight (the ideal gas law explains the connection between pressure, temperature, and volume).[64]

The collapse may be stopped by the degeneracy pressure of the star's constituents, condensing the matter in an exotic denser state. The result is one of the various types of compact star. Which type of compact star is formed depends on the mass of the remnant — the matter left over after changes triggered by the collapse (such as supernova or pulsations leading to a planetary nebula) have blown away the outer layers. Note that this can be substantially less than the original star — remnants exceeding 5 solar masses are produced by stars which were over 20 solar masses before the collapse.[64]

If the mass of the remnant exceeds about 3–4 solar masses (the Tolman–Oppenheimer–Volkoff limit)—either because the original star was very heavy or because the remnant collected additional mass through accretion of matter—even the degeneracy pressure of neutrons is insufficient to stop the collapse. After this, no known mechanism (except possibly quark degeneracy pressure, see quark star) is powerful enough to stop the collapse and the object will inevitably collapse to a black hole.[64]

This gravitational collapse of heavy stars is assumed to be responsible for the formation of stellar mass black holes. Star formation in the young universe may have resulted in very heavy stars, which upon their collapse would have produced black holes of up to 103 solar masses. These heavy black holes could be the seeds of the supermassive black holes found in the centers of most galaxies.[65]

While most of the energy released during gravitational collapse is emitted very quickly, an outside observer does not actually see the end of this process. Even though the collapse takes a finite amount of time from the reference frame of infalling matter, a distant observer sees the infalling material slow and halt just above the event horizon, due to gravitational time dilation. Light from the collapsing material takes longer and longer to reach the observer, with the light emitted just before the event horizon forms delayed an infinite amount of time. Thus the external observer never sees the formation of the event horizon; instead, the collapsing material seems to become dimmer and increasingly red-shifted, eventually fading away.[66]
Primordial black holes in the Big Bang

Gravitational collapse requires great densities. In the current epoch of the universe these high densities are only found in stars, but in the early universe shortly after the big bang densities were much greater, possibly allowing for the creation of black holes. The high density alone is not enough to allow the formation of black holes since a uniform mass distribution will not allow the mass to bunch up. In order for primordial black holes to form in such a dense medium, there must be initial density perturbations which can then grow under their own gravity. Different models for the early universe vary widely in their predictions of the size of these perturbations. Various models predict the creation of black holes, ranging from a Planck mass to hundreds of thousands of solar masses.[67] Primordial black holes could thus account for the creation of any type of black hole.
High-energy collisions
A simulated event in the CMS detector, a collision in which a micro black hole may be created.

Gravitational collapse is not the only process that could create black holes. In principle, black holes could also be created in high-energy collisions that create sufficient density. However, to date, no such events have ever been detected either directly or indirectly as a deficiency of the mass balance in particle accelerator experiments.[68] This suggests that there must be a lower limit for the mass of black holes. Theoretically, this boundary is expected to lie around the Planck mass (mP = √ħc/G ≈ 1.2×1019 GeV/c2 ≈ 2.2×10−8 kg), where quantum effects are expected to make the theory of general relativity break down completely.[69] This would put the creation of black holes firmly out of reach of any high energy process occurring on or near the Earth. Certain developments in quantum gravity however suggest that the Planck mass could be much lower: some braneworld scenarios for example put it much lower, maybe even as low as 1 TeV/c2[70] This would make it possible for micro black holes to be created in the high energy collisions occurring when cosmic rays hit the Earth's atmosphere, or possibly in the new Large Hadron Collider at CERN. These theories are however very speculative, and the creation of black holes in these processes is deemed unlikely by many specialists.[71] Even if such micro black holes should be formed in these collisions, it is expected that they would evaporate in about 10−25 seconds, posing no threat to Earth[72]
Growth

Once a black hole has formed, it can continue to grow by absorbing additional matter. Any black hole will continually absorb gas and interstellar dust from its direct surroundings and omnipresent cosmic background radiation. This is the primary process through which supermassive black holes seem to have grown.[65] A similar process has been suggested for the formation of intermediate-mass black holes in globular clusters.[73]

Another possibility is for a black hole to merge with other objects such as stars or even other black holes. This is thought to have been important especially for the early development of supermassive black holes, which are thought to have formed from the coagulation of many smaller objects.[65] The process has also been proposed as the origin of some intermediate-mass black holes.[74][75]
Evaporation
Main article: Hawking radiation

In 1974, Stephen Hawking showed that black holes are not entirely black but emit small amounts of thermal radiation.[26] He got this result by applying quantum field theory in a static black hole background. The result of his calculations is that a black hole should emit particles in a perfect black body spectrum. This effect has become known as Hawking radiation. Since Hawking's result, many others have verified the effect through various methods.[76] If his theory of black hole radiation is correct, then black holes are expected to emit a thermal spectrum of radiation, and thereby lose mass, because according to the theory of relativity mass is just highly condensed energy (E = mc2).[26] Black holes will shrink and evaporate over time. The temperature of this spectrum (Hawking temperature) is proportional to the surface gravity of the black hole, which for a Schwarzschild black hole is inversely proportional to the mass. Large black holes, therefore, emit less radiation than small black holes.

A stellar black hole of one solar mass has a Hawking temperature of about 100 nanokelvins. This is far less than the 2.7 K temperature of the cosmic microwave background. Stellar mass (and larger) black holes receive more mass from the cosmic microwave background than they emit through Hawking radiation and will thus grow instead of shrink. To have a Hawking temperature larger than 2.7 K (and be able to evaporate), a black hole needs to be lighter than the Moon (and therefore a diameter of less than a tenth of a millimeter).[77]

On the other hand, if a black hole is very small the radiation effects are expected to become very strong. Even a black hole that is heavy compared to a human would evaporate in an instant. A black hole the weight of a car (~10−24 m) would only take a nanosecond to evaporate, during which time it would briefly have a luminosity more than 200 times that of the sun. Lighter black holes are expected to evaporate even faster, for example a black hole of mass 1 TeV/c2 would take less than 10−88 seconds to evaporate completely. Of course, for such a small black hole quantum gravitation effects are expected to play an important role and could even – although current developments in quantum gravity do not indicate so[78] – hypothetically make such a small black hole stable.[79]
Observational evidence

By their very nature, black holes do not directly emit any signals other than the hypothetical Hawking radiation; since the Hawking radiation for an astrophysical black hole is predicted to be very weak, this makes it impossible to directly detect astrophysical black holes from the Earth. A possible exception to the Hawking radiation being weak is the last stage of the evaporation of light (primordial) black holes; searches for such flashes in the past has proven unsuccessful and provides stringent limits on the possibility of existence of light primordial black holes.[80] NASA's Fermi Gamma-ray Space Telescope launched in 2008 will continue the search for these flashes.[81]

Astrophysicists searching for black holes thus have to rely on indirect observations. A black hole's existence can sometimes be inferred by observing its gravitational interactions with its surroundings.
Accretion of matter
See also: Accretion disc
Formation of extragalactic jets from a black hole's accretion disk

Due to conservation of angular momentum, gas falling into the gravitational well created by a massive object will typically form a disc-like structure around the object. Friction within the disc causes angular momentum to be transported outward allowing matter to fall further inward releasing potential energy and increasing the temperature of the gas.[82] In the case of compact objects such as white dwarfs, neutron stars, and black holes, the gas in the inner regions becomes so hot that it will emit vast amounts of radiation (mainly X-rays), which may be detected by telescopes. This process of accretion is one of the most efficient energy producing process known; up to 40% of the rest mass of the accreted material can be emitted in radiation.[82] (In nuclear fusion only about 0.7% of the rest mass will be emitted as energy.) In many cases, accretion discs are accompanied by relativistic jets emitted along the poles, which carry away much of the energy. The mechanism for the creation of these jets is currently not well understood.

As such many of the universe's more energetic phenomena have been attributed to the accretion of matter on black holes. In particular, active galactic nuclei and quasars are thought to be the accretion discs of supermassive black holes.[83] Similarly, X-ray binaries are thought to be binary star systems in which one of the two stars is a compact object accreting matter from its companion.[83] It has also been suggested that some ultraluminous X-ray sources may be the accretion disks of intermediate-mass black holes.[84]
X-ray binaries
See also: X-ray binary

X-ray binaries are binary star systems that are luminous in the X-ray part of the spectrum. These X-ray emissions are generally thought to be caused by one of the component stars being a compact object accreting matter from the other (regular) star. The presence of an ordinary star in such a system provides a unique opportunity for studying the central object and determining if it might be a black hole.
Artist impression of a binary system with an accretion disk around a compact object being fed by material from the companion star.

If such a system emits signals that can be directly traced back to the compact object, it cannot be a black hole. The absence of such a signal does, however, not exclude the possibility that the compact object is a neutron star. By studying the companion star it is often possible to obtain the orbital parameters of the system and obtain an estimate for the mass of the compact object. If this is much larger than the Tolman–Oppenheimer–Volkoff limit (that is, the maximum mass a neutron star can have before collapsing) then the object cannot be a neutron star and is generally expected to be a black hole.[83]

The first strong candidate for a black hole, Cygnus X-1, was discovered in this way by Charles Thomas Bolton[85] and Webster and Murdin[86] in 1972.[87][88] Some doubt, however, remained due to the uncertainties resultant from the companion star being much heavier than the candidate black hole.[83] Currently, better candidates for black holes are found in a class of X-ray binaries called soft X-ray transients.[83] In this class of system the companion star is relatively low mass allowing for more accurate estimates in the black hole mass. Moreover, these systems are only active in X-ray for several months once every 10–50 years. During the period of low X-ray emission (called quiescence), the accretion disc is extremely faint allowing for detailed observation of the companion star during this period. One of the best such candidates is V404 Cyg.
Quiescence and advection-dominated accretion flow

The faintness of the accretion disc during quiescence is thought to be caused by the flow entering a mode called an advection-dominated accretion flow (ADAF). In this mode, almost all the energy generated by friction in the disc is swept along with the flow instead of radiated away. If this model is correct, then it forms strong qualitative evidence for the presence of an event horizon.[89] Because, if the object at the center of the disc had a solid surface, it would emit large amounts of radiation as the highly energetic gas hits the surface, an effect that is observed for neutron stars in a similar state.[82]
Quasi-periodic oscillations
See also: Quasi-periodic oscillations

The X-ray emissions from accretion disks sometimes exhibit a flickering around certain frequencies. These signals are called quasi-periodic oscillations and are thought to be caused by material moving along the inner edge of the accretion disk (the innermost stable circular orbit). As such their frequency is linked to the mass of the compact object. They can thus be used as an alternative way to determine the mass of potential black holes.[90]
Gamma ray bursts

Intense but one-time gamma ray bursts (GRBs) may signal the birth of "new" black holes, because astrophysicists think that GRBs are caused either by the gravitational collapse of giant stars[91] or by collisions between neutron stars,[92] and both types of event involve sufficient mass and pressure to produce black holes. It appears that a collision between a neutron star and a black hole can also cause a GRB,[93] so a GRB is not proof that a "new" black hole has been formed. All known GRBs come from outside our own galaxy, and most come from billions of light years away[94] so the black holes associated with them are billions of years old.
Galactic nuclei
See also: Active galactic nucleus
The jet originating from the center of M87 in this image comes from an active galactic nucleus that may contain a supermassive black hole. Credit: Hubble Space Telescope/NASA/ESA.

It is now widely accepted that the center of every or at least nearly every galaxy contains a supermassive black hole.[95] The close observational correlation between the mass of this hole and the velocity dispersion of the host galaxy's bulge, known as the M-sigma relation, strongly suggests a connection between the formation of the black hole and the galaxy itself. [96]

For decades, astronomers have used the term "active galaxy" to describe galaxies with unusual characteristics, such as unusual spectral line emission and very strong radio emission.[97][98] However, theoretical and observational studies have shown that the active galactic nuclei (AGN) in these galaxies may contain supermassive black holes.[97][98] The models of these AGN consist of a central black hole that may be millions or billions of times more massive than the Sun; a disk of gas and dust called an accretion disk; and two jets that are perpendicular to the accretion disk.[98]

Although supermassive black holes are expected to be found in most AGN, only some galaxies' nuclei have been more carefully studied in attempts to both identify and measure the actual masses of the central supermassive black hole candidates. Some of the most notable galaxies with supermassive black hole candidates include the Andromeda Galaxy, M32, M87, NGC 3115, NGC 3377, NGC 4258, and the Sombrero Galaxy.[99]

Currently, the best evidence for a supermassive black hole comes from studying the proper motion of stars near the center of our own Milky Way.[100] Since 1995 astronomers have tracked the motion of 90 stars in a region called Sagittarius A*. By fitting their motion to Keplerian orbits they were able to infer in 1998 that 2.6 million solar masses must be contained in a volume with a radius of 0.02 lightyears.[101] Since then one of the stars—called S2—has completed a full orbit. From the orbital data they were able to place better constraints on the mass and size of the object causing the orbital motion of stars in the Sagittarius A* region, finding that there is a spherical mass of 4.3 million solar masses contained within a radius of less than 0.002 lightyears.[100] While this is more than 3000 times the Schwarzschild radius corresponding to that mass, it is at least consistent with the central object being a supermassive black hole, and no "realistic cluster [of stars] is physically tenable."[101]
Gravitational lensing
Further information: Gravitational lens

The deformation of spacetime around a massive object causes light rays to be deflected much like light passing through an optic lens. This phenomenon is known as gravitational lensing. Observations have been made of weak gravitational lensing, in which photons are deflected by only a few arcseconds. However, it has never been directly observed for a black hole.[102] One possibility for observing gravitational lensing by a black hole would be to observe stars in orbit around the black hole. There are several candidates for such an observation in orbit around Sagittarius A*.[102]
Alternatives

The evidence for stellar black holes strongly relies on the existence of an upper limit for the mass of a neutron star. The size of this limit heavily depends on the assumptions made about the properties of dense matter. New exotic phases of matter could push up this bound.[83] A phase of free quarks at high density might allow the existence of dense quark stars,[103] and some supersymmetric models predict the existence of Q stars.[104] Some extensions of the standard model posit the existence of preons as fundamental building blocks of quarks and leptons which could hypothetically form preon stars.[105] These hypothetical models could potentially explain a number of observations of stellar black hole candidates. However, it can be shown from general arguments in general relativity that any such object will have a maximum mass.[83]

Since the average density of a black hole inside its Schwarzschild radius is inversely proportional to the square of its mass, supermassive black holes are much less dense than stellar black holes (the average density of a large supermassive black hole is comparable to that of water).[83] Consequently, the physics of matter forming a supermassive black hole is much better understood and the possible alternative explanations for supermassive black hole observations are much more mundane. For example, a supermassive black hole could be modelled by a large cluster of very dark objects. However, typically such alternatives are not stable enough to explain the supermassive black hole candidates.[83]

The evidence for stellar and supermassive black holes implies that in order for black holes not to form, general relativity must fail as a theory of gravity, perhaps due to the onset of quantum mechanical corrections. A much anticipated feature of a theory of quantum gravity is that it will not feature singularities or event horizons (and thus no black holes).[106] In recent years, much attention has been drawn by the fuzzball model in string theory. Based on calculations in specific situations in string theory, the proposal suggest that generically the individual states of a black hole solution do not have an event horizon or singularity (and can thus not really be considered to be a black hole), but that for a distant observer the statistical average of such states does appear just like an ordinary black hole in general relativity.[107]
Open questions
Entropy and thermodynamics
Further information: Black hole thermodynamics
If ultra-high-energy collisions of particles in a particle accelerator can create microscopic black holes, it is expected that all types of particles will be emitted by black hole evaporation, providing key evidence for any grand unified theory. Above are the high energy particles produced in a gold ion collision on the RHIC.

In 1971, Stephen Hawking showed under general conditions[Note 1] that the total area of the event horizons of any collection of classical black holes can never decrease, even if they collide and merge.[108] This result, now known as the second law of black hole mechanics, is remarkably similar to the second law of thermodynamics, which states that the total entropy of a system can never decrease. As with classical objects at absolute zero temperature, it was assumed that black holes had zero entropy. If this were the case, the second law of thermodynamics would be violated by entropy-laden matter entering the black hole, resulting in a decrease of the total entropy of the universe. Therefore, Jacob Bekenstein proposed that a black hole should have an entropy, and that it should be proportional to its horizon area.[109]

The link with the laws of thermodynamics was further strengthened by Hawking's discovery that quantum field theory predicts that a black hole radiates blackbody radiation at a constant temperature. This seemingly causes a violation of the second law of black hole mechanics, since the radiation will carry away energy from the black hole causing it to shrink. The radiation, however also carries away entropy, and it can be proven under general assumptions that the sum of the entropy of the matter surrounding the black hole and one quarter of the area of the horizon as measured in Planck units is in fact always increasing. This allows the formulation of the first law of black hole mechanics as an analogue of the first law of thermodynamics, with the mass acting as energy, the surface gravity as temperature and the area as entropy.[109]

One puzzling feature is that the entropy of a black hole scales with its area rather than with its volume, since entropy is normally an extensive quantity that scales linearly with the volume of the system. This odd property led 't Hooft and Susskind to propose the holographic principle, which suggests that anything that happens in volume of spacetime can be described by data on the boundary of that volume.[110]

Although general relativity can be used to perform a semi-classical calculation of black hole entropy, this situation is theoretically unsatisfying. In statistical mechanics, entropy is understood as counting the number of microscopic configurations of a system which have the same macroscopic qualities (such as mass, charge, pressure, etc.). Without a satisfactory theory of quantum gravity, one cannot perform such a computation for black holes. Some progress has been made in various approaches to quantum gravity. In 1995, Strominger and Vafa showed that counting the microstates of a specific supersymmetric black hole in string theory reproduced the Bekenstein–Hawking entropy.[111] Since then, similar results have been reported for different black holes both in string theory and in other approaches to quantum gravity like loop quantum gravity.[112]
Black hole unitarity
Main article: Black hole information paradox
Unsolved problems in physics Is physical information lost in black holes? Question mark2.svg

An open question in fundamental physics is the so-called information loss paradox, or black hole unitarity paradox. Classically, the laws of physics are the same run forward or in reverse (T-symmetry). Liouville's theorem dictates conservation of phase space volume, which can be thought of as "conservation of information", so there is some problem even in classical physics. In quantum mechanics, this corresponds to a vital property called unitarity, which has to do with the conservation of probability (it can also be thought of as a conservation of quantum phase space volume as expressed by the density matrix).[113]
See also

* Black holes in fiction
* Black string
* Kugelblitz (astrophysics)
* List of black holes
* Susskind-Hawking battle


He1523a.jpg Star portal

* Timeline of black hole physics
* White hole
* Wormhole

Notes

1. ^ In particular, he assumed that all matter satisfies the weak energy condition.

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100. ^ a b Gillessen, S.; Eisenhauer, F.; Trippe, S.; Alexander, T.; Genzel, R.; Martins, F.; Ott, T. (2009). "Monitoring Stellar Orbits Around the Massive Black Hole in the Galactic Center". Astrophysical Journal 692: 1075–1109. doi:10.1088/0004-637X/692/2/1075. arXiv:0810.4674. edit
101. ^ a b Ghez, A. M.; Klein, B. L.; Morris, M.; Becklin, E. E. (1998). "High Proper‐Motion Stars in the Vicinity of Sagittarius A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy". The Astrophysical Journal 509: 678. doi:10.1086/306528. edit
102. ^ a b Bozza, Valerio (2009). "Gravitational Lensing by Black Holes". arΧiv:0911.2187 [gr-qc].
103. ^ Kovacs; Cheng; Harko (2009). "Can stellar mass black holes be quark stars?". arΧiv:0908.2672 [astro-ph.HE].
104. ^ Alexander Kusenko (2006). "Properties and signatures of supersymmetric Q-balls". arΧiv:hep-ph/0612159 [hep-ph].
105. ^ Hansson, J.; Sandin, F. (2005). "Preon stars: a new class of cosmic compact objects". Physics Letters B 616: 1. doi:10.1016/j.physletb.2005.04.034. edit
106. ^ Kiefer, C. (2006). "Quantum gravity: general introduction and recent developments". Annalen der Physik 15: 129–148. doi:10.1002/andp.200510175. edit
107. ^ Skenderis, K.; Taylor, M. (2008). "The fuzzball proposal for black holes". Physics Reports 467: 117. doi:10.1016/j.physrep.2008.08.001. edit
108. ^ Hawking, Stephen (1998). A Brief History of Time. New York: Bantam Books. ISBN 0-553-38016-8. [page needed]
109. ^ a b Wald (1999). "The Thermodynamics of Black Holes". arΧiv:gr-qc/9912119v2 [gr-qc].
110. ^ Gerard 't Hooft (2000). "The Holographic Principle". arΧiv:hep-th/0003004 [hep-th].
111. ^ Strominger, A.; Vafa, C. (1996). "Microscopic origin of the Bekenstein-Hawking entropy". Physics Letters B 379: 99. doi:10.1016/0370-2693(96)00345-0. edit
112. ^ Carlip, S. (2009). "Black Hole Thermodynamics and Statistical Mechanics". Lect.Notes Phys. 769: 89–12. doi:10.1007/978-3-540-88460-6_3. edit
113. ^ Hawking, Stephen. "Does God Play Dice?". http://www.hawking.org.uk/index.php/lectures/publiclectures/64. Retrieved 2009-03-14.

Further reading

Popular reading

* Ferguson, Kitty (1991). Black Holes in Space-Time. Watts Franklin. ISBN 0-531-12524-6. .
* Hawking, Stephen (1988). A Brief History of Time. Bantam Books, Inc. ISBN 0-553-38016-8. .
* Hawking, Stephen; Penrose, Roger (1996). The Nature of Space and Time. Princeton University Press. ISBN 0-691-03791-2. http://books.google.com/?id=LstaQTXP65cC. .
* Melia, Fulvio (2003). The Black Hole at the Center of Our Galaxy. Princeton U Press. ISBN 978-0-691-09505-9. .
* Melia, Fulvio (2003). The Edge of Infinity. Supermassive Black Holes in the Universe. Cambridge U Press. ISBN 978-0-521-81405-8. .
* Pickover, Clifford (1998). Black Holes: A Traveler's Guide. Wiley, John & Sons, Inc. ISBN 0-471-19704-1. .
* Stern, B. (2008). "Blackhole". http://www.wikilivres.info/wiki/Blackhole_%28Stern%29. , poem.
* Thorne, Kip S. (1994). Black Holes and Time Warps. Norton, W. W. & Company, Inc. ISBN 0-393-31276-3. .
* Wheeler, J. Craig (2007). Cosmic Catastrophes (2nd ed.). Cambridge University Press. ISBN 0-521-85714-7.

University textbooks and monographs

* Carroll, Sean M. (2004). Spacetime and Geometry. Addison Wesley. ISBN 0-8053-8732-3. , the lecture notes on which the book was based are available for free from Sean Carroll's website.
* Carter, B. (1973). "Black hole equilibrium states". In DeWitt, B.S.; DeWitt, C.. Black Holes. .
* Chandrasekhar, Subrahmanyan (1999). Mathematical Theory of Black Holes. Oxford University Press. ISBN 0-19-850370-9. .
* Frolov, V.P.; Novikov, I.D. (1998). Black hole physics. .
* Hawking, S.W.; Ellis, G.F.R. (1973). Large Scale Structure of space time. Cambridge University Press. ISBN 0521099064. http://books.google.com/?id=QagG_KI7Ll8C. .
* Melia, Fulvio (2007). The Galactic Supermassive Black Hole. Princeton U Press. ISBN 978-0-691-13129-0. .
* Taylor, Edwin F.; Wheeler, John Archibald (2000). Exploring Black Holes. Addison Wesley Longman. ISBN 0-201-38423-X. .
* Thorne, Kip S.; Misner, Charles; Wheeler, John (1973). Gravitation. W. H. Freeman and Company. ISBN 0-7167-0344-0. .
* Wald, Robert M. (1992). Space, Time, and Gravity: The Theory of the Big Bang and Black Holes. University of Chicago Press. ISBN 0-226-87029-4. .

Review papers

* Detweiler, S. (1981). "Resource letter BH-1: Black holes". American Journal of Physics 49 (5, pp): 394–400. doi:10.1119/1.12686.
* Gallo, E.; Marolf, D. (2009). "Resource Letter BH-2: Black Holes". American Journal of Physics 77: 294. doi:10.1119/1.3056569. arXiv:0806.2316. edit
* Hughes, Scott A. (2005). "Trust but verify: The case for astrophysical black holes". arΧiv:hep-ph/0511217v2 [hep-ph]. Lecture notes from 2005 SLAC Summer Institute.

External links
Wikimedia Commons has media related to: Black holes

* Stanford Encyclopedia of Philosophy: "Singularities and Black Holes" by Erik Curiel and Peter Bokulich.
* "Black hole" on Scholarpedia.
* Black Holes: Gravity's Relentless Pull - Interactive multimedia Web site about the physics and astronomy of black holes from the Space Telescope Science Institute
* FAQ on black holes
* "Schwarzschild Geometry" on Andrew Hamilton’s website
* UT Brownsville Group Simulates Spinning Black-Hole Binaries
* Advanced Mathematics of Black Hole Evaporation

Videos

* 16-year long study tracks stars orbiting Milky Way black hole
* Yale University Video Lecture: Introduction to Black Holes at Google Video.
* Movie of Black Hole Candidate from Max Planck Institute

News

* "Black Hole confirmed in Milky Way." Retrieved December 10, 2008
* Black Hole Research News

Good read.

Thanks.

[UnWantedTheory]

Mental retardation (MR) is a generalized disorder, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors (http://en.wikipedia.org/wiki/Adaptive_behavior) that appears before adulthood. It has historically been defined as an Intelligence Quotient (http://en.wikipedia.org/wiki/Intelligence_Quotient) score under 70.[1] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-0) Once focused almost entirely on cognition (http://en.wikipedia.org/wiki/Cognition), the definition now includes both a component relating to mental functioning and one relating to individuals' functional skills in their environment. As a result, a person with a below-average intelligence quotient (http://en.wikipedia.org/w/index.php?title=Below-average_intelligence_quotient&action=edit&redlink=1) (BAIQ) may not be considered mentally retarded. Syndromic mental retardation is intellectual deficits associated with other medical and behavioral signs and symptoms (http://en.wikipedia.org/wiki/Signs_and_symptoms). Non-syndromic mental retardation is intellectual deficits that appear without other abnormalities.
Mental retardation is a subtype of intellectual disability, and that term is now preferred by most advocates in most English-speaking countries as a euphemism (http://en.wikipedia.org/wiki/Euphemism) for mental retardation. However, intellectual disability is a broader concept, and includes intellectual deficits that are too mild to properly qualify as mental retardation, too specific (as in specific learning disability (http://en.wikipedia.org/wiki/Specific_learning_disability)), or acquired later in life, through acquired brain injuries (http://en.wikipedia.org/wiki/Acquired_brain_injuries) or neurodegenerative diseases (http://en.wikipedia.org/wiki/Neurodegenerative_diseases) like dementia (http://en.wikipedia.org/wiki/Dementia). Intellectual disabilities may appear at any age.
Developmental disability (http://en.wikipedia.org/wiki/Developmental_disability) is any disability that is due to problems with growth and development (http://en.wikipedia.org/wiki/Human_development_(biology)). This term encompasses many congenital medical conditions (http://en.wikipedia.org/wiki/Congenital_medical_condition) that have no mental or intellectual components, although it, too, is sometimes used as a euphemism for mental retardation.


Among children, the cause is unknown for one-third to one-half of cases. Down syndrome (http://en.wikipedia.org/wiki/Down_syndrome), fetal alcohol syndrome (http://en.wikipedia.org/wiki/Fetal_alcohol_syndrome) and Fragile X syndrome (http://en.wikipedia.org/wiki/Fragile_X_syndrome) are the three most common inborn causes. However, doctors have found many other causes. The most common are:

Genetic (http://en.wikipedia.org/wiki/Genetics) conditions. Sometimes disability is caused by abnormal genes (http://en.wikipedia.org/wiki/Genes) inherited from parents, errors when genes combine, or other reasons. The most prevalent genetic conditions (http://en.wikipedia.org/wiki/Genetic_disorder) include Down syndrome (http://en.wikipedia.org/wiki/Down_syndrome), Klinefelter's syndrome (http://en.wikipedia.org/wiki/Klinefelter%27s_syndrome), Fragile X syndrome (http://en.wikipedia.org/wiki/Fragile_X_syndrome), Neurofibromatosis (http://en.wikipedia.org/wiki/Neurofibromatosis), congenital (http://en.wikipedia.org/wiki/Congenital_disorder) hypothyroidism (http://en.wikipedia.org/wiki/Hypothyroidism), Williams syndrome (http://en.wikipedia.org/wiki/Williams_syndrome), Phenylketonuria (http://en.wikipedia.org/wiki/Phenylketonuria) (PKU),Prader-Willi syndrome (http://en.wikipedia.org/wiki/Prader-Willi_syndrome), Hyperphosphatasia with mental retardation syndrome (http://en.wikipedia.org/wiki/Hyperphosphatasia_with_mental_retardation_syndrome ). Other genetic conditions include Phelan-McDermid syndrome (22q13del) (http://en.wikipedia.org/wiki/22q13_deletion_syndrome), Mowat-Wilson syndrome (http://en.wikipedia.org/wiki/Mowat-Wilson_syndrome), genetic ciliopathy (http://en.wikipedia.org/wiki/Ciliopathy),[2] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-1) and Siderius type X-linked mental retardation (http://en.wikipedia.org/wiki/X-Linked_mental_retardation) (OMIM (http://en.wikipedia.org/wiki/Mendelian_Inheritance_in_Man) 300263 (http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=300263)) as caused by mutations in the PHF8 (http://en.wikipedia.org/wiki/PHF8) gene (OMIM (http://en.wikipedia.org/wiki/Mendelian_Inheritance_in_Man) 300560 (http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=300560)).[3] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-2)[4] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-3) In the rarest of cases, abnormalities with the X (http://en.wikipedia.org/wiki/X_chromosome) or Y chromosome (http://en.wikipedia.org/wiki/Y_chromosome) may also cause disability. 48, XXXX (http://en.wikipedia.org/wiki/48,_XXXX) and 49, XXXXX (http://en.wikipedia.org/wiki/49,_XXXXX) syndrome affect a small number of girls worldwide, while boys may be affected by 47, XYY (http://en.wikipedia.org/wiki/XYY), 49, XXXXY (http://en.wikipedia.org/wiki/XXXXY), or 49, XYYYY.
Problems during pregnancy (http://en.wikipedia.org/wiki/Pregnancy). Mental disability can result when the fetus (http://en.wikipedia.org/wiki/Fetus) does not develop properly. For example, there may be a problem with the way the fetus' cells divide as it grows. A woman who drinks alcohol (http://en.wikipedia.org/wiki/Alcohol) (see fetal alcohol syndrome (http://en.wikipedia.org/wiki/Fetal_alcohol_syndrome)) or gets an infection like rubella (http://en.wikipedia.org/wiki/Rubella) during pregnancy may also have a baby with mental disability.
Problems at birth. If a baby has problems during labor and birth, such as not getting enough oxygen (http://en.wikipedia.org/wiki/Oxygen), he or she may have developmental disability due to brain damage.
Exposure to certain types of disease (http://en.wikipedia.org/wiki/Disease) or toxins (http://en.wikipedia.org/wiki/Toxins). Diseases like whooping cough (http://en.wikipedia.org/wiki/Whooping_cough), measles (http://en.wikipedia.org/wiki/Measles), or meningitis (http://en.wikipedia.org/wiki/Meningitis) can cause mental disability if medical care (http://en.wikipedia.org/wiki/Health_care) is delayed or inadequate. Exposure to poisons (http://en.wikipedia.org/wiki/Poison) like lead (http://en.wikipedia.org/wiki/Lead) or mercury (http://en.wikipedia.org/wiki/Mercury_(element)) may also affect mental ability.
Iodine deficiency (http://en.wikipedia.org/wiki/Iodine_deficiency), affecting approximately 2 billion people worldwide, is the leading preventable cause of mental disability in areas of the developing world (http://en.wikipedia.org/wiki/Third_World) where iodine deficiency is endemic (http://en.wikipedia.org/wiki/Endemic_(epidemiology)). Iodine deficiency also causes goiter (http://en.wikipedia.org/wiki/Goitre), an enlargement of the thyroid gland (http://en.wikipedia.org/wiki/Thyroid_gland). More common than full-fledged cretinism (http://en.wikipedia.org/wiki/Cretinism), as retardation caused by severe iodine deficiency is called, is mild impairment of intelligence. Certain areas of the world due to natural deficiency and governmental inaction are severely affected. India (http://en.wikipedia.org/wiki/India) is the most outstanding, with 500 million suffering from deficiency, 54 million from goiter, and 2 million from cretinism. Among other nations affected by iodine deficiency, China (http://en.wikipedia.org/wiki/Peoples_Republic_of_China) and Kazakhstan (http://en.wikipedia.org/wiki/Kazakhstan) have instituted widespread iodization programs, whereas, as of 2006, Russia (http://en.wikipedia.org/wiki/Russia) had not.[5] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-4)
Malnutrition (http://en.wikipedia.org/wiki/Malnutrition) is a common cause of reduced intelligence in parts of the world affected by famine (http://en.wikipedia.org/wiki/Famine), such as Ethiopia (http://en.wikipedia.org/wiki/Ethiopia).[6] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-5)
Absence of the arcuate fasciculus (http://en.wikipedia.org/wiki/Arcuate_fasciculus).[7] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-6)
Diagnosis

According to the latest edition of the Diagnostic and Statistical Manual of Mental Disorders (http://en.wikipedia.org/wiki/Diagnostic_and_Statistical_Manual_of_Mental_Disord ers) (DSM-IV),[8] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-7) three criteria must be met for a diagnosis of mental retardation: an IQ (http://en.wikipedia.org/wiki/IQ) below 70, significant limitations in two or more areas of adaptive behavior (http://en.wikipedia.org/wiki/Adaptive_behavior) (as measured by an adaptive behavior rating scale, i.e. communication, self-help skills, interpersonal skills (http://en.wikipedia.org/wiki/Interpersonal_skills), and more), and evidence that the limitations became apparent before the age of 18.
It is formally diagnosed by professional assessment of intelligence and adaptive behavior.
IQ below 70

The first English-language IQ (http://en.wikipedia.org/wiki/Intelligence_quotient) test, the Terman-Binet (http://en.wikipedia.org/wiki/Stanford-Binet), was adapted from an instrument used to measure potential to achieve developed by Binet in France. Terman translated the test and employed it as a means to measure intellectual capacity based on oral language (http://en.wikipedia.org/wiki/Language), vocabulary, numerical reasoning, memory, motor speed and analysis skills. The mean score on the currently available IQ tests is 100, with a standard deviation of 15 (WAIS/WISC-IV) or 16 (Stanford-Binet). Sub-average intelligence is generally considered to be present when an individual scores two standard deviations below the test mean. Factors other than cognitive ability (depression, anxiety, etc.) can contribute to low IQ scores; it is important for the evaluator to rule them out prior to concluding that measured IQ is "significantly below average".
The following ranges, based on Standard Scores of intelligence tests, reflect the categories of the American Association of Mental Retardation, the Diagnostic and Statistical Manual of Mental Disorders-IV-TR, and the International Classification of Diseases-10[citation needed (http://en.wikipedia.org/wiki/Wikipedia:Citation_needed)]:
ClassIQProfound mental retardationBelow 20Severe mental retardation20–34Moderate mental retardation35–49Mild mental retardation50–69Borderline intellectual functioning (http://en.wikipedia.org/wiki/Borderline_intellectual_functioning)70–84
Since the diagnosis is not based only on IQ scores, but must also take into consideration a person's adaptive functioning, the diagnosis is not made rigidly. It encompasses intellectual scores, adaptive functioning scores from an adaptive behavior rating scale based on descriptions of known abilities provided by someone familiar with the person, and also the observations of the assessment examiner who is able to find out directly from the person what he or she can understand, communicate, and the like.
Significant limitations in two or more areas of adaptive behavior



Adaptive behavior, or adaptive functioning, refers to the skills needed to live independently (or at the minimally acceptable level for age). To assess adaptive behavior, professionals compare the functional abilities of a child to those of other children of similar age. To measure adaptive behavior, professionals use structured interviews, with which they systematically elicit information about persons' functioning in the community from people who know them well. There are many adaptive behavior scales, and accurate assessment of the quality of someone's adaptive behavior requires clinical judgment as well. Certain skills are important to adaptive behavior, such as:

Daily living skills (http://en.wikipedia.org/wiki/Daily_living_skills), such as getting dressed, using the bathroom, and feeding oneself
Communication (http://en.wikipedia.org/wiki/Communication) skills, such as understanding what is said and being able to answer
Social skills (http://en.wikipedia.org/wiki/Social_skills) with peers, family (http://en.wikipedia.org/wiki/Family) members, spouses, adults, and others
Evidence that the limitations became apparent in childhood

This third condition is used to distinguish mental retardation from dementing (http://en.wikipedia.org/wiki/Dementing) conditions such as Alzheimer's disease (http://en.wikipedia.org/wiki/Alzheimer%27s_disease) or due to traumatic injuries with attendant brain damage.
Management

By most definitions mental retardation is more accurately considered a disability rather than a disease. MR can be distinguished in many ways from mental illness (http://en.wikipedia.org/wiki/Mental_illness), such as schizophrenia (http://en.wikipedia.org/wiki/Schizophrenia) or depression (http://en.wikipedia.org/wiki/Clinical_depression). Currently, there is no "cure" for an established disability, though with appropriate support and teaching, most individuals can learn to do many things.
There are thousands of agencies around the world that provide assistance for people with developmental disabilities. They include state-run, for-profit, and non-profit, privately run agencies. Within one agency there could be departments that include fully staffed residential homes, day rehabilitation programs that approximate schools, workshops wherein people with disabilities can obtain jobs, programs that assist people with developmental disabilities in obtaining jobs in the community, programs that provide support for people with developmental disabilities who have their own apartments, programs that assist them with raising their children, and many more. There are also many agencies and programs for parents of children with developmental disabilities.
Beyond that there are specific programs that people with developmental disabilities can take part in wherein they learn basic life skills. These "goals" may take a much longer amount of time for them to accomplish, but the ultimate goal is independence. This may be anything from independence in tooth brushing to an independent residence. People with developmental disabilities learn throughout their lives and can obtain many new skills even late in life with the help of their families, caregivers, clinicians and the people who coordinate the efforts of all of these people.
Although there is no specific medication for mental retardation, many people with developmental disabilities have further medical complications and may take several medications. For example autisic children with developmental delay may utilize anti-psychotics or mood stabilizers to help with behavior. Use of psychotropic medications such as benzodiazepines (http://en.wikipedia.org/wiki/Benzodiazepines) in people with mental retardation requires monitoring and vigilance as side effects occur commonly and are often misdiagnosed as behavioural and psychiatric problems.[9] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-8)
History of the condition

Intellectual disabilities of all kinds have been documented under a variety of names throughout history. Throughout much of human history, society was unkind to those with any type of disability, and people with intellectual disabilities were commonly viewed as burdens on their families.
Greek and Roman philosophers, who valued reasoning abilities, disparaged people with intellectual disabilities as barely human.[10] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-Parnell-9) The oldest physiological view of mental retardation is in the writings of Hippocrates (http://en.wikipedia.org/wiki/Hippocrates) in the late fifth century BCE, who believed that it was caused by an imbalance in the four humors (http://en.wikipedia.org/wiki/Four_humors) in the brain.
Until the Enlightenment (http://en.wikipedia.org/wiki/Age_of_Enlightenment) in Europe, care and asylum was provided by families and the church (in monasteries and other religious communities), focusing on the provision of basic physical needs such as food, shelter and clothing. Negative stereotypes were prominent in social attitudes of the time. During the Renaissance (http://en.wikipedia.org/wiki/Renaissance) communities would sometimes send away those who were developmentally delayed in boats. These "ships of fools" (http://en.wikipedia.org/wiki/Ship_of_fools) would then show up at another harbour, only to be sent away to the next community.
In the 13th century, England declared people with intellectual disabilities to be incapable of making decisions or managing their affairs.[10] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-Parnell-9) Guardianships (http://en.wikipedia.org/wiki/Guardianship) were created to take over their financial affairs.
In the 17th century, Thomas Willis (http://en.wikipedia.org/wiki/Thomas_Willis) provided the first description of intellectual disabilities as a disease (http://en.wikipedia.org/wiki/Disease).[10] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-Parnell-9) He believed that it was caused by structural problems in the brain. According to Willis, the anatomical problems could be either an inborn condition or acquired later in life.
In the 18th and 19th centuries, housing and care moved away from families and towards an asylum model. People were placed by, or removed from, their families (usually in infancy) and housed in large professional institutions, many of which were self-sufficient through the labor of the residents. Some of these institutions provided a very basic level of education (such as differentiation between colors and basic word recognition and numeracy), but most continued to focus solely on the provision of basic needs (http://en.wikipedia.org/wiki/Basic_needs) of food, clothing, and shelter. Conditions in such institutions varied widely, but the support provided was generally non-individualized, with aberrant behavior and low levels of economic productivity regarded as a burden to society. Heavy tranquilization and assembly line methods of support were the norm, and the medical model of disability (http://en.wikipedia.org/wiki/Medical_model_of_disability) prevailed. Services were provided based on the relative ease to the provider, not based on the needs of the individual.
In the late 19th century, in response to Charles Darwin's On the Origin of Species (http://en.wikipedia.org/wiki/On_the_Origin_of_Species), Francis Galton (http://en.wikipedia.org/wiki/Francis_Galton) proposed selective breeding of humans to reduce intellectual disabilities.[10] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-Parnell-9) Early in the twentieth century the eugenics (http://en.wikipedia.org/wiki/Eugenics) movement became popular throughout the world. This led to the forced sterilization and prohibition of marriage in most of the developed world and later used by Hitler (http://en.wikipedia.org/wiki/Hitler) as rationale for the mass murder of mentally challenged individuals during the holocaust (http://en.wikipedia.org/wiki/Holocaust). Eugenics was later abandoned as an evil violation of human rights, and the practice of forced sterilization and prohibition from marriage was discontinued by most of the developed world by the mid 20th century.
In 1905, Alfred Binet (http://en.wikipedia.org/wiki/Alfred_Binet) produced the first standardized test (http://en.wikipedia.org/wiki/Standardized_test) for measuring intelligence in children.[10] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-Parnell-9)
Although ancient Roman law had declared people with mental retardation to be incapable of the deliberate intent to harm (http://en.wikipedia.org/wiki/Mens_rea) that was necessary for a person to commit a crime, during the 1920s, Western society believed they were morally degenerate.[10] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-Parnell-9)
Ignoring the prevailing attitude, Civitans (http://en.wikipedia.org/wiki/Civitan_International) adopted service to the developmentally disabled as a major organizational emphasis in 1952. Their earliest efforts included workshops for special education teachers and daycamps for disabled children, all at a time when such training and programs were almost nonexistent.[11] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-10) The segregation of people with developmental disabilities wasn't widely questioned by academics or policy-makers until the 1969 publication of Wolf Wolfensberger (http://en.wikipedia.org/wiki/Wolf_Wolfensberger)'s seminal work "The Origin and Nature of Our Institutional Models",[12] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-11) drawing on some of the ideas proposed by SG Howe 100 years earlier. This book posited that society characterizes people with disabilities as deviant (http://en.wikipedia.org/wiki/Deviant), sub-human and burdens of charity, resulting in the adoption of that "deviant" role. Wolfensberger argued that this dehumanization, and the segregated institutions that result from it, ignored the potential productive contributions that all people can make to society. He pushed for a shift in policy and practice that recognized the human needs of "retardates" and provided the same basic human rights as for the rest of the population.
The publication of this book may be regarded as the first move towards the widespread adoption of the social model of disability (http://en.wikipedia.org/wiki/Social_model_of_disability) in regard to these types of disabilities, and was the impetus for the development of government strategies for desegregation. Successful lawsuits (http://en.wikipedia.org/wiki/Lawsuit) against governments and an increasing awareness of human rights and self-advocacy also contributed to this process, resulting in the passing in the U.S. of the Civil Rights of Institutionalized Persons Act in 1980.
From the 1960s to the present, most states have moved towards the elimination of segregated institutions. Normalization (http://en.wikipedia.org/wiki/Normalization_(people_with_disabilities)) and deinstitutionalization (http://en.wikipedia.org/wiki/Deinstitutionalization) are dominant.[10] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-Parnell-9) Along with the work of Wolfensberger and others including Gunnar and Rosemary Dybwad,[13] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-12) a number of scandalous revelations around the horrific conditions within state institutions created public outrage that led to change to a more community-based method of providing services.[14] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-13)
By the mid-1970s, most governments had committed to de-institutionalization, and had started preparing for the wholesale movement of people into the general community, in line with the principles of normalization (http://en.wikipedia.org/wiki/Normalisation_(people_with_disabilities)). In most countries, this was essentially complete by the late 1990s, although the debate over whether or not to close institutions persists in some states, including Massachusetts.[15] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-14)
In the past, lead poisoning (http://en.wikipedia.org/wiki/Lead_poisoning) and infectious diseases were significant causes of intellectual disabilities. Some causes of mental retardation are decreasing, as medical advances, such as vaccination, increases. Other causes are increasing, perhaps due to rising maternal age, which is associated with several syndromic forms of mental retardation.
Along with the changes in terminology, and the downward drift in acceptability of the old terms, institutions of all kinds have had to repeatedly change their names. This affects the names of schools, hospitals, societies, government departments, and academic journals. For example, the Midlands Institute of Mental Subnormality became the British Institute of Mental Handicap and is now the British Institute of Learning Disability. This phenomenon is shared with mental health (http://en.wikipedia.org/wiki/Mental_health) and motor disabilities, and seen to a lesser degree in sensory disabilities.
Several traditional terms denoting varying degrees of mental deficiency long predate psychiatry (http://en.wikipedia.org/wiki/Psychiatry). All terms have been subjected to the euphemism treadmill (http://en.wikipedia.org/wiki/Euphemism_treadmill). In common usage, these terms are simple forms of abuse. They are often encountered in old documents such as books, academic papers, and census (http://en.wikipedia.org/wiki/Census) forms (for example, the British census of 1901 has a column heading including the terms imbecile and feeble-minded (http://en.wikipedia.org/wiki/Feeble-minded)).
Negative connotations associated with these numerous terms for mental retardation reflect society's attitude about the condition. There are competing desires among elements of society, some of whom seek neutral medical terms, and others who want to use such terms as weapons with which to abuse people.[16] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-mhcinc-15)
Today, the term "retarded" is slowly being replaced by new words like "special" or "challenged." The term "developmental delay" is popular among caretakers and parents of individuals with mental retardation. Using the word "delay" is preferred over "disability" by many people, because the former term encapsulates the core deficit that creates mental retardation in the first place. Delay suggests that a person has been held back from their potential, rather than someone who has been disabled.[citation needed (http://en.wikipedia.org/wiki/Wikipedia:Citation_needed)]
Usage has changed over the years, and differed from country to country, which needs to be borne in mind when looking at older books and papers. For example, "mental retardation" in some contexts covers the whole field, but previously applied to what is now the mild MR group. "Feeble-minded" used to mean mild MR in the UK, and once applied in the US to the whole field. "Borderline MR" is not currently defined, but the term may be used to apply to people with IQs in the 70s. People with IQs of 70 to 85 used to be eligible for special consideration in the US public education system on grounds of mental retardation.[citation needed (http://en.wikipedia.org/wiki/Wikipedia:Citation_needed)]

Cretin (http://en.wikipedia.org/wiki/Cretinism) is the oldest and comes from a dialectal French word for Christian.[17] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-16) The implication was that people with significant intellectual or developmental disabilities were "still human" (or "still Christian") and deserved to be treated with basic human dignity (http://en.wikipedia.org/wiki/Human_dignity). Individuals with the condition were considered to be incapable of sinning, thus "christ-like" in their disposition. This term is not used in scientific endeavors since the middle of the 20th century and is generally considered a term of abuse. "Cretinism" is also used as an obsolescent term to refer to the condition of congenital hypothyroidism (http://en.wikipedia.org/wiki/Hypothyroidism), in which there is some degree of mental retardation.
Amentia has a long history, mostly associated with dementia. The difference between amentia and dementia was originally defined by time of onset. Amentia was the term used to describe an individual who developed deficits in mental functioning early in life, while dementia described individuals who develop mental deficiencies as adults. During the 1890s, amentia was used to describe someone who was born with mental deficiencies. By 1912, ament was a classification lumping "idiots, imbeciles, and feeble minded" individuals in a category separate from a dement classification, in which the onset is later in life.[16] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-mhcinc-15)
Idiot (http://en.wikipedia.org/wiki/Idiot_(usage)) indicated the greatest degree of intellectual disability, where the mental age (http://en.wikipedia.org/wiki/Mental_age) is two years or less, and the person cannot guard himself or herself against common physical dangers. The term was gradually replaced by the term profound mental retardation.
Imbecile (http://en.wikipedia.org/wiki/Imbecile) indicated an intellectual disability less extreme than idiocy and not necessarily inherited. It is now usually subdivided into two categories, known as severe mental retardation and moderate mental retardation.
Moron (http://en.wikipedia.org/wiki/Moron_(psychology)) was defined by the American Association for the Study of the Feeble-minded (http://en.wikipedia.org/wiki/Feeble-minded) in 1910, following work by Henry H. Goddard (http://en.wikipedia.org/wiki/Henry_H._Goddard), as the term for an adult (http://en.wikipedia.org/wiki/Adult) with a mental age (http://en.wikipedia.org/wiki/Mental_age) between eight and twelve; mild mental retardation is now the term for this condition. Alternative definitions of these terms based on IQ were also used. This group was known in UK law from 1911 to 1959/60 as "feeble-minded".
Mongolism was a medical term used to identify someone with Down syndrome (http://en.wikipedia.org/wiki/Down_syndrome). The Mongolian People's Republic (http://en.wikipedia.org/wiki/Mongolian_People%27s_Republic) requested that the medical community cease use of the term as a description of mental retardation. Their request was granted in the 1960s, when the World Health Organization agreed that the term should cease being used within the medical community.[16] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-mhcinc-15)
In the field of special education (http://en.wikipedia.org/wiki/Special_education), educable (or "educable mentally retarded") refers to MR students with IQs of approximately 50–75 who can progress academically to a late elementary level. Trainable (or "trainable mentally retarded") refers to students whose IQs fall below 50 but who are still capable of learning personal hygiene (http://en.wikipedia.org/wiki/Hygiene) and other living skills in a sheltered setting, such as a group home. In many areas, these terms have been replaced by use of "severe" and "moderate" mental retardation. While the names change, the meaning stays roughly the same in practice.
Retarded comes from the Latin retardare, "to make slow, delay, keep back, or hinder." The term was recorded in 1426 as a "fact or action of making slower in movement or time." The first record of retarded in relation to being mentally slow was in 1895. The term retarded was used to replace terms like idiot, moron, and imbecile because it was not a derogatory term. By the 1960s, however, the term had taken on a partially derogatory meaning as well.[16] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-mhcinc-15) The noun "retard" is particularly seen as pejorative; as of 2010, the Special Olympics (http://en.wikipedia.org/wiki/Special_Olympics), Best Buddies (http://en.wikipedia.org/wiki/Best_Buddies) and over 100 other organizations are striving to help eliminate the use of the "r-word" (analogous to the "n-word (http://en.wikipedia.org/wiki/N-word)") in everyday conversation.[18] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-17)[19] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-18)
The term "mental retardation" is a diagnostic term denoting the group of disconnected categories of mental functioning such as "idiot (http://en.wikipedia.org/wiki/Idiot)", "imbecile (http://en.wikipedia.org/wiki/Imbecile)", and "moron (http://en.wikipedia.org/wiki/Moron_(psychology))" derived from early IQ tests, which acquired pejorative (http://en.wikipedia.org/wiki/Pejorative) connotations in popular discourse. The term "mental retardation" acquired pejorative and shameful connotations over the last few decades due to the use of the words "retarded" and "retard" as insults. This may have contributed to its replacement with euphemisms such as "mentally challenged" or "intellectually disabled". While "developmental disability" may be considered to subsume other disorders (see below), "developmental disability" and "developmental delay" (for people under the age of 18), are generally considered more acceptable terms than "mental retardation".



In North America (http://en.wikipedia.org/wiki/North_America) mental retardation is subsumed into the broader term developmental disability, which also includes epilepsy (http://en.wikipedia.org/wiki/Epilepsy), autism (http://en.wikipedia.org/wiki/Autism), cerebral palsy (http://en.wikipedia.org/wiki/Cerebral_palsy) and other disorders that develop during the developmental period (birth to age 18.) Because service provision is tied to the designation developmental disability (http://en.wikipedia.org/wiki/Developmental_disability), it is used by many parents, direct support professionals, and physicians. In the United States, however, in school-based settings, the more specific term mental retardation is still typically used, and is one of 13 categories of disability under which children may be identified for special education services under Public Law 108-446.
The phrase intellectual disability is increasingly being used as a synonym for people with significantly below-average cognitive ability.[20] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-19) These terms are sometimes used as a means of separating general intellectual limitations from specific, limited deficits as well as indicating that it is not an emotional or psychological disability. Intellectual disability may also used to describe the outcome of traumatic brain injury (http://en.wikipedia.org/wiki/Traumatic_brain_injury) or lead poisoning (http://en.wikipedia.org/wiki/Lead_poisoning) or dementing (http://en.wikipedia.org/wiki/Dementing) conditions such as Alzheimer's disease (http://en.wikipedia.org/wiki/Alzheimer%27s_disease). It is not specific to congenital disorders such as Down syndrome (http://en.wikipedia.org/wiki/Down_syndrome).
The American Association on Mental Retardation (http://en.wikipedia.org/wiki/American_Association_on_Mental_Retardation) continued to use the term mental retardation until 2006.[21] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-20) In June 2006 its members voted to change the name of the organization to the "American Association on Intellectual and Developmental Disabilities," rejecting the options to become the AAID or AADD. Part of the rationale for the double name was that many members worked with people with pervasive developmental disorders (http://en.wikipedia.org/wiki/Pervasive_developmental_disorder), most of whom do not have mental retardation.[22] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-21)


In the UK, "mental handicap" had become the common medical term, replacing "mental subnormality" in Scotland and "mental deficiency" in England and Wales, until Stephen Dorrell (http://en.wikipedia.org/wiki/Stephen_Dorrell), Secretary of State for Health (http://en.wikipedia.org/wiki/Secretary_of_State_for_Health) for the United Kingdom from 1995–97, changed the NHS (http://en.wikipedia.org/wiki/National_Health_Service)'s designation to "learning disability (http://en.wikipedia.org/wiki/Learning_disability)." The new term is not yet widely understood, and is often taken to refer to problems affecting schoolwork (the American usage), which are known in the UK as "learning difficulties." British social workers (http://en.wikipedia.org/wiki/Social_work) may use "learning difficulty" to refer to both people with MR and those with conditions such as dyslexia (http://en.wikipedia.org/wiki/Dyslexia).[23] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-22) In education, "learning difficulties" is applied to a wide range of conditions: "specific learning difficulty" may refer to dyslexia (http://en.wikipedia.org/wiki/Dyslexia), dyscalculia (http://en.wikipedia.org/wiki/Dyscalculia) or dyspraxia (http://en.wikipedia.org/wiki/Dyspraxia), while "moderate learning difficulties", "severe learning difficulties" and "profound learning difficulties" refer to more significant impairments.[24] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-23)[25] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-24)
In England and Wales between 1983 and 2008 the Mental Health Act 1983 (http://en.wikipedia.org/wiki/Mental_Health_Act_1983) defined "mental impairment" and "severe mental impairment" as "a state of arrested or incomplete development of mind which includes significant/severe impairment of intelligence and social functioning and is associated with abnormally aggressive or seriously irresponsible conduct on the part of the person concerned."[26] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-25) As behavior was involved, these were not necessarily permanent conditions: they were defined for the purpose of authorizing detention in hospital or guardianship. The term Mental Impairment was removed from the Act in November 2008, but the grounds for detention remained. However, English statute law uses "mental impairment" elsewhere in a less well-defined manner—e.g. to allow exemption from taxes—implying that mental retardation without any behavioral problems is what is meant.
A BBC (http://en.wikipedia.org/wiki/BBC) poll conducted in the United Kingdom came to the conclusion that 'retard' was the most offensive disability-related word.[27] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-26) On the reverse side of that, when a contestant on Celebrity Big Brother (http://en.wikipedia.org/wiki/Celebrity_Big_Brother) live used the phrase "walking like a retard", despite complaints from the public and the charity Mencap (http://en.wikipedia.org/wiki/Mencap), the communications regulator Ofcom (http://en.wikipedia.org/wiki/Ofcom) did not uphold the complaint saying "it was not used in an offensive context [...] and had been used light-heartedly". It was however noted that two previous similar complaints from other shows were upheld.[28] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-27)


The term "mental retardation" is still used in Australia (http://en.wikipedia.org/wiki/Australia); however, "intellectual disability" is now the preferred and more commonly used descriptor.[29] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-28)[30] (http://en.wikipedia.org/wiki/Intellectual_disability#cite_note-29)


People with such disabilities are often not seen as full citizens of society. Person-centered planning and approaches are seen as methods of addressing the continued labelling and exclusion of socially devalued people, such as people with disabilities, encouraging a focus on the person as someone with capacities and gifts, as well as support needs.
There has been a change in the perception of people with intellectual disabilities over the past generation. People with intellectual disabilities were routinely excluded from public education, or educated away from other typically developing children. This changed in 1975 with the Individuals with Disabilities Education Act. (http://en.wikipedia.org/w/index.php?title=Individuals_with_Disabilities_Educ ation_Act.&action=edit&redlink=1) This mandated that all children be educated, and that be educated with typically developing children. [1] (http://www2.ed.gov/policy/speced/leg/idea/history.html) This gave children the same education and opportunities as their non-disabled children received, giving these children with intellectual disabilities not only comprable skills to their peers, but the same dreams and aspirations as their non disabled peers. [2] (http://www.ncbi.nlm.nih.gov/pubmed/16672037)
These educated people with intellectual disabilities are now working together to change the perception of people with intellectual disabilities in the society. They are part of a new movement called the self advocacy movement (http://en.wikipedia.org/w/index.php?title=Self_advocacy_movement&action=edit&redlink=1). Some of these organizations have been effective in lobbying the legislature to make changes to laws that effect people with intellectual disabilities. [3] (http://www.massadvocatesstandingstrong.org/index.php?option=com_content&view=category&layout=blog&id=1&Itemid=2) The campaign led by people with intellectual disabilities to ban the “R” word, [4] (http://www.r-word.org/) succeed and Federal Legislation was signed into law by President Obama. [5] (http://frwebgate.access.gpo.gov/cgi-bin/getdoc.cgi?dbname=111_cong_bills&docid=f:s2781enr.txt.pdf)
This has also led to the concept and policy of self direction (http://en.wikipedia.org/wiki/Self_direction), allowing people with intellectual disabilities to make decisions about their own lives.
See also


Secondary handicap (http://en.wikipedia.org/wiki/Secondary_handicap)
References


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I thought this might be relevant. Those who are worried for the OP please read.

[phxspurfan]

__VQX2Xn7tI

[maddnezz]

lol bonner

lol jefferson

lol griffin

lol clippers

lol spurs

lol fail
LOL at your :lobt::lobt::lobt::lobt:........wait, that's not right. YOU CAN SUCK IT! YOU, CUBAN, JET AND THE FUNKY GERMAN !:ihit

[Phillip]

fuck the OP, this thread, and him being able to troll the spurs forum at will.

i'm against banning people and whatnot but shit is ridiculous these days on ST. always the same stupid fuckin' "lol" and "lmao" threads in here, keep that shit in the club, troll forum, or nba forum.

btw, did i mention, fuck you phillip!

there is a reason i get away with it

i make only 1 thread for each loss

and successfully troll morons like you who get mad. aka the morons that kori and timvp dont like anyways, and would probably rather them get trolled, than ban me from making these threads

plus i am the creator of the LMAO threads

[Horse]

lol bonner

lol jefferson

lol griffin

lol clippers

lol spurs

lol fail
Not really sure how you can talk shit. We still have the better record, you finally get the phantom call on Manu to beat us in 2006 and blow it against a miami team we would've destroyed. I understand your win against us was your championship and the closest you'll ever get but get a life!:flag:

[spurraider21]

lol faggot