welcome to space glossory
In general relativity, a black hole is a region of space in which the gravitational field is so powerful that nothing, not even light, can escape. The black hole has a one-way surface, called an event horizon, into which objects can fall, but out of which nothing can come. 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 analysis of black holes shows them to possess a temperature and Hawking radiation.
Despite its invisible interior, a black hole can reveal its presence through 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 which looks empty. Alternatively, one can see gas falling into a relatively small black hole, from a companion star. This gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and earth-orbiting telescopes. Such observations have resulted in the scientific consensus that, barring
In general relativity, a black hole is a region of space in which the gravitational field is so powerful that nothing, not even light, can escape. The black hole has a one-way surface, called an event horizon, into which objects can fall, but out of which nothing can come. 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 analysis of black holes shows them to possess a temperature and Hawking radiation.
Despite its invisible interior, a black hole can reveal its presence through 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 which looks empty. Alternatively, one can see gas falling into a relatively small black hole, from a companion star. This gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and earth-orbiting telescopes. Such observations have resulted in the scientific consensus that, barring
Introduction and terminology A black hole is often defined as an object whose escape velocity exceeds the speed of light. This picture is qualitatively wrong, but provides a way of understanding the order of magnitude for the black hole radius.
The escape velocity is the minimum speed at which an object needs to travel so as to escape a source of gravity without falling back into orbit before stopping. On the Earth, the escape velocity is equal to 11.2 km/s, so no matter what the object is, whether a bullet or a baseball, it must go at least 11.2 km/s to avoid falling back to the Earth's surface. To calculate the escape velocity in Newtonian mechanics, consider a heavy object of mass M centered at the origin. A second object with mass m starting at distance r from the origin with speed v, trying to escape to infinity, needs to have just enough kinetic energy to make up for the negative gravitational potential energy, with nothing left over:
(where G is the gravitational constant). That way, as it gets closer to it has less and less kinetic energy, finally ending up at infinity with zero speed.
This relation gives the critical escape velocity v in terms of M and r. But it also says that for each value of v and M, there is a critical value of r so that a particle with speed v is just able to escape:
When the velocity is equal to the speed of light, this gives the radius of a hypothetical Newtonian dark star, a Newtonian body from which a particle moving at the speed of light cannot escape. In the most commonly used convention for the value of the radius of a black hole, the radius of the event horizon is equal to this Newtonian value.
In general relativity, the coordinate r is not completely straightforward to define due to the curved nature of space-time and the choice of different coordinates. For this result to be true, the value of r should be defined so that the surface area A of a sphere of radius r in the curved space time is still given by the formula A = 4πr2. This definition of r only makes sense when the gravitational field is spherically symmetric, so that there are concentric spheres on which the gravitational field is constant.
The velocity necessary to escape from an object's gravitational field (called the object's escape velocity) depends on how dense the object is; that is, the ratio of its mass to its volume. A black hole forms when an object is so dense that, within a certain distance of it, even light is not fast enough to escape, since the speed of light is slower than the black hole's escape velocity. Unlike in Newtonian gravity, in general relativity, light going away from a black hole doesn't slow down and turn around. The Schwarzschild radius is still the last distance from which light can escape to infinity, but outgoing light which starts at the Schwarzschild radius doesn't go out and come back, it just stays there. Inside the Schwarzschild radius, everything must move inward, getting crushed somehow at the center.
In general relativity, the black hole's mass can be thought of as concentrated at a singularity, which can be a point, a ring, a light-ray, or a sphere; the exact details are not currently well understood in all circumstances. Surrounding the singularity is a spherical boundary called the event horizon. The event horizon marks the 'point of no return,' a boundary beyond which matter and radiation inevitably fall inwards, towards the singularity. The distance from the singularity at the center to the event horizon is the size of the black hole, and is equal to twice the mass in units where G and c equal 1.
The radius of a black hole of mass equal to that of the Sun is about 3 km. At distances much larger than this, the black hole has the exact same total gravitational attraction as any other body of the same mass, just like the sun. So if the sun were replaced by a black hole of the same mass, the orbits of the planets would remain unchanged.
There are several types of black holes, characterized by their typical size. When they form as a result of the gravitational collapse of a star, they are called stellar black holes. Black holes found at the center of galaxies have a mass up to several billion solar masses and are called supermassive black holes. Between these two scales, it has been hypothesized intermediate black holes with a mass of several thousand solar masses. Black holes with very small masses, hypthesized to have formed early in the history of the Universe, during the Big Bang, might also exist, and are referred to as primordial black holes. Their existence is, at present, not confirmed.
It is impossible to directly observe a black hole. However, it is possible to infer its presence by its gravitational action on the surrounding environment, particularly with microquasars and active galactic nuclei, where material falling into a nearby black hole is significantly heated and emits a large amount of X-ray radiation. This observation method allows astronomers to detect their existence. The only objects that agree with these observations and are consistent within the framework of general relativity are black holes.
History Simulation of Gravitational lensing by a black hole which distorts the image of a galaxy in the background. (Click for larger animation.) The idea of a body so massive that even light could not escape was 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 light was then thought to be a massless wave and therefore not influenced by gravity. Unlike the modern black hole concept, the object behind the horizon is assumed to be stable against collapse.
In 1915, Albert Einstein developed his general theory of relativity, having earlier shown that gravity does in fact 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,[5] showing that a black hole could theoretically exist. The Schwarzschild radius is now known to be the radius of the event horizon of a non-rotating black hole, but this was not well understood at that time, for example Schwarzschild himself thought it was not physical. Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution for the point mass a few months after Schwarzschild and wrote more extensively about its properties.
In 1930, astrophysicist 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. His arguments were opposed by Arthur Eddington, who believed that something would inevitably stop the collapse. Eddington was partly correct: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star. But in 1939, Robert Oppenheimer and others predicted that stars above approximately three solar masses (the Tolman-Oppenheimer-Volkoff limit) would collapse into black holes for the reasons presented by Chandrasekhar.[6]
Oppenheimer and his co-authors used Schwarzschild's system of coordinates (the only coordinates available in 1939), which produced mathematical singularities at the Schwarzschild radius, in other words some of the terms in the equations became infinite at the Schwarzschild radius. This was interpreted as indicating that the Schwarzschild radius 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 briefly known as "frozen stars,"[citation needed] 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.
In 1958, David Finkelstein introduced the concept of the event horizon by presenting Eddington-Finkelstein coordinates, which enabled him to show that "The Schwarzschild surface r = 2 m is not a singularity, but that it acts as a perfect unidirectional membrane: causal influences can cross it in only one direction".[7] This did not strictly contradict Oppenheimer's results, but extended them to include the point of view of infalling observers. All theories up to this point, including Finkelstein's, covered only non-rotating black holes.
In 1963, Roy Kerr found the exact solution for a rotating black hole. The rotating singularity of this solution was a ring, and not a point. A short while later, Roger Penrose was able to prove that singularities occur inside any black hole.
In 1967, astronomers discovered pulsars,[8][9] and within a few years could show that the known pulsars were rapidly rotating neutron stars. Until that time, neutron stars were also regarded as just theoretical curiosities. So the discovery of pulsars awakened interest in all types of ultra-dense objects that might be formed by gravitational collapse.
Physicist John Wheeler is widely credited with coining the term black hole in his 1967 public lecture Our Universe: the Known and Unknown, as an alternative to the more cumbersome "gravitationally completely collapsed star." However, Wheeler insisted that someone else at the conference had coined the term and he had merely adopted it as useful shorthand. The term was also cited in a 1964 letter by Anne Ewing to the AAAS:
According to Einstein’s general theory of relativity, as mass is added to a degenerate star a sudden collapse will take place and the intense gravitational field of the star will close in on itself. Such a star then forms a "black hole" in the universe. —Ann Ewing, letter to AAAS[10] Properties and features The No hair theorem states that, once it settles down, a black hole has only three independent physical properties: mass, charge, and angular momentum.[11] Any two black holes that share the same values for these properties, or parameters, are classically indistinguishable.
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, despite the fact that photons, the particles responsible for electric and magnetic forces, cannot escape from the interior region. The reason is Gauss's law, the total electric flux going out of a big sphere always stays the same, and measures the total charge inside the sphere. When charge falls into a black hole, electric field lines still remain, poking out of the horizon, and these field lines conserve the total charge of all the infalling matter. The electric field lines eventually spread out evenly over the surface of the black hole, settling down to a uniform field-line density on the surface. The black hole acts in this regard like a classical conducting sphere with a definite resistivity.[12]
Similarly, the total mass inside a sphere containing a black hole can be found by using the gravitational analog of Gauss's law, far away from the black hole. Likewise, the angular momentum can be measured from far away using frame dragging by the gravitomagnetic field.
When a black hole swallows any form of matter, its horizon oscillates like a stretchy membrane with friction, a dissipative system, until it settles down to a simple final state. This is different from other field theories like electromagnetism or gauge theory, which never have any friction or resistivity, because they are time reversible. Because the black hole eventually settles down into a final 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.[13][14][15]
The loss of information in black holes is puzzling even classically, because general relativity is a Lagrangian theory, which superficially appears to be time reversible and Hamiltonian. But because of the horizon, a black hole is not time reversible: matter can enter but it cannot escape. The time reverse of a classical black hole has been called a white hole, although entropy considerations and quantum mechanics suggest that white holes are just the same as black holes.
The no-hair theorem makes some assumptions about the nature of our universe and the matter it contains, and other assumptions lead to different conclusions. For example, if Magnetic monopoles exist, as predicted by some theories,[16] the magnetic charge would be a fourth parameter for a classical black hole.
Counterexamples to the no-hair theorem are known for the following cases:
Classification By physical properties The simplest black hole has mass but neither 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.[5] It was the first non-trivial exact solution to the Einstein field equations to be discovered, and according to Birkhoff's theorem, the only vacuum solution that is spherically symmetric.[20] 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 essentially like that of ordinary massive bodies.[21]
More general black hole solutions were discovered later in the 20th century. The Reissner-Nordström metric describes a black hole with electric charge, while the Kerr metric yields a rotating black hole. The more generally known stationary black hole solution, the Kerr-Newman metric, describes 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 natural units , the total charge and the total angular momentum are expected to satisfy
for a black hole of mass M.
Black holes saturating this inequality are called extremal. Solutions of Einstein's equations violating the inequality do exist, but do not have a horizon. These solutions have naked singularities and are deemed unphysical, as the cosmic censorship hypothesis rules out such singularities due to the generic gravitational collapse of realistic matter.[22] This is supported by numerical simulations.[23]
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[24] appears to have an angular momentum near the maximum allowed value.
By mass Class Mass Size Supermassive black hole ~105–109 MSun ~0.001–10 AU Intermediate-mass black hole ~103 MSun ~103 km = REarth Stellar-mass ~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 . The size of a black hole, as determined by the radius of the event horizon, or Schwarzschild radius, is proportional to the mass through
where is the Schwarzschild radius and is the mass of the Sun. A black hole's size and mass are thus simply related independent of rotation. According to this criterion, black holes are classed as:
Far away from the black hole a particle can move in any direction. It is only restricted by the speed of light.
Closer to the black hole spacetime starts to deform. There are more paths going towards the black hole than paths moving away.
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 beyond which events cannot affect an outside observer. As predicted by general relativity, the presence of a mass deforms spacetime in such a way that the paths particles take tend towards the mass. At the event horizon of a black hole this deformation becomes so strong that there are no more paths that lead away from the black hole.[31] Once a particle is inside the horizon, moving into the hole is as inevitable as moving forward in time (and can actually be thought of as equivalent to doing so).
To a distant observer clocks near a black hole appear to tick more slowly than those further away from the black hole.[32] Due to this effect (known as gravitational time dilation) the distant observer will see an object falling into a black hole slow down as it approaches the event horizon, taking an infinite time to reach it.[33] At the same time all processes on this object slow down causing emitted light to appear redder and dimmer, an effect known as gravitational red shift.[34] Eventually, the falling object becomes so dim that it can no longer be seen, at a point just before it reaches the event horizon.
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.[35] 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 it is expected that quantum gravity effects become significant near the vicinity of the event horizon.[36] This allows observations of matter in the vicinity of a black hole's event horizon to be used to indirectly study general relativity and proposed extensions to it.
Though black holes themselves may not radiate energy, electromagnetic radiation and matter particles may be radiated from just outside the event horizon via Hawking radiation.[37]
Singularity Main article: Gravitational singularity At the center of a black hole lies the singularity, where matter is crushed to infinite density, the pull of gravity is infinitely strong, and spacetime has infinite curvature.[38] This means that a black hole's mass becomes entirely compressed into a region with zero volume.[39] This zero-volume, infinitely dense region at the center of a black hole is called a gravitational singularity.
The singularity of a non-rotating black hole has zero length, width, and height; a rotating black hole is smeared out to form a ring shape lying in the plane of rotation.[40] The ring still has no thickness and hence no volume.
The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory.[41] 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.[42][43]
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.[44] 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.
The ergosphere of a black hole is bounded by:
Objects and radiation (including light) can stay in orbit within the ergosphere without falling to the center. But they cannot hover (remain stationary, as seen by an external observer), because that would require them to move backwards faster than light relative to their own regions of space-time, which are moving faster than light relative to an external observer.
Objects and radiation can also escape from the ergosphere. In fact the Penrose process predicts that objects will sometimes fly out of the ergosphere, obtaining the energy for this by "stealing" some of the black hole's rotational energy. If a large total mass of objects escapes in this way, the black hole will spin more slowly and may even stop spinning eventually.
Formation and evolution Considering the exotic nature of black holes, it may be natural to question if such bizarre objects could actually 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.[45] 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,[46] 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. Shortly afterwards, Stephen Hawking showed that many cosmological solutions describing the big bang have singularities, in the absence of 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.[47] 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, or because a star which would have been stable receives a lot of 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).
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.
If the mass of the remnant exceeds ~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.
This gravitational collapse of heavy stars is assumed to be responsible for the formation of most (if not all) stellar mass black holes.
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.[48] 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.[49] 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 (~1019 GeV/c2 = ~2 × 10-8 kg), where quantum effects are expected to make the theory of general relativity break down completely.[citation needed] 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 this bound could be much lower. Some braneworld scenarios for example put the Planck mass much lower, may be even as low as 1 TeV/c2.[50] 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.
Growth Once a black hole has formed, it can continue to grow by absorbing additional matter. Any black hole will continually absorb interstellar dust from its direct surroundings and omnipresent cosmic background radiation, but neither of these processes should significantly affect the mass of a stellar black hole. More significant contributions can occur when the black hole formed in a binary star system. After formation the black hole can then leech significant amounts of matter from its companion.
Much larger contributions can be obtained when a black hole merges with other stars or compact objects. The supermassive black holes suspected in the center of most galaxies are expected to have formed from the coagulation of many smaller objects. The process has also been proposed as the origin of some intermediate-mass black holes.
As an object approaches the event horizon, the horizon near the object bulges up and swallows the object. Shortly thereafter the increase in radius (due to the extra mass) is distributed evenly around the hole.
Evaporation Main article: Hawking radiation In 1974, Stephen Hawking showed that black holes are not entirely black but emit small amounts of thermal radiation.[51] 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.[52] 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).[51] 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 in turn is inversely proportional to the mass. Large black holes, therefore, emit less radiation than small black holes.
A stellar black hole of 5 solar masses has a Hawking temperature of about 12 nanokelvins. This is far less than the 2.7 K produced by 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. In order 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).
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 – hypothetically make such a small black hole stable.
The escape velocity is the minimum speed at which an object needs to travel so as to escape a source of gravity without falling back into orbit before stopping. On the Earth, the escape velocity is equal to 11.2 km/s, so no matter what the object is, whether a bullet or a baseball, it must go at least 11.2 km/s to avoid falling back to the Earth's surface. To calculate the escape velocity in Newtonian mechanics, consider a heavy object of mass M centered at the origin. A second object with mass m starting at distance r from the origin with speed v, trying to escape to infinity, needs to have just enough kinetic energy to make up for the negative gravitational potential energy, with nothing left over:
(where G is the gravitational constant). That way, as it gets closer to it has less and less kinetic energy, finally ending up at infinity with zero speed.
This relation gives the critical escape velocity v in terms of M and r. But it also says that for each value of v and M, there is a critical value of r so that a particle with speed v is just able to escape:
When the velocity is equal to the speed of light, this gives the radius of a hypothetical Newtonian dark star, a Newtonian body from which a particle moving at the speed of light cannot escape. In the most commonly used convention for the value of the radius of a black hole, the radius of the event horizon is equal to this Newtonian value.
In general relativity, the coordinate r is not completely straightforward to define due to the curved nature of space-time and the choice of different coordinates. For this result to be true, the value of r should be defined so that the surface area A of a sphere of radius r in the curved space time is still given by the formula A = 4πr2. This definition of r only makes sense when the gravitational field is spherically symmetric, so that there are concentric spheres on which the gravitational field is constant.
The velocity necessary to escape from an object's gravitational field (called the object's escape velocity) depends on how dense the object is; that is, the ratio of its mass to its volume. A black hole forms when an object is so dense that, within a certain distance of it, even light is not fast enough to escape, since the speed of light is slower than the black hole's escape velocity. Unlike in Newtonian gravity, in general relativity, light going away from a black hole doesn't slow down and turn around. The Schwarzschild radius is still the last distance from which light can escape to infinity, but outgoing light which starts at the Schwarzschild radius doesn't go out and come back, it just stays there. Inside the Schwarzschild radius, everything must move inward, getting crushed somehow at the center.
In general relativity, the black hole's mass can be thought of as concentrated at a singularity, which can be a point, a ring, a light-ray, or a sphere; the exact details are not currently well understood in all circumstances. Surrounding the singularity is a spherical boundary called the event horizon. The event horizon marks the 'point of no return,' a boundary beyond which matter and radiation inevitably fall inwards, towards the singularity. The distance from the singularity at the center to the event horizon is the size of the black hole, and is equal to twice the mass in units where G and c equal 1.
The radius of a black hole of mass equal to that of the Sun is about 3 km. At distances much larger than this, the black hole has the exact same total gravitational attraction as any other body of the same mass, just like the sun. So if the sun were replaced by a black hole of the same mass, the orbits of the planets would remain unchanged.
There are several types of black holes, characterized by their typical size. When they form as a result of the gravitational collapse of a star, they are called stellar black holes. Black holes found at the center of galaxies have a mass up to several billion solar masses and are called supermassive black holes. Between these two scales, it has been hypothesized intermediate black holes with a mass of several thousand solar masses. Black holes with very small masses, hypthesized to have formed early in the history of the Universe, during the Big Bang, might also exist, and are referred to as primordial black holes. Their existence is, at present, not confirmed.
It is impossible to directly observe a black hole. However, it is possible to infer its presence by its gravitational action on the surrounding environment, particularly with microquasars and active galactic nuclei, where material falling into a nearby black hole is significantly heated and emits a large amount of X-ray radiation. This observation method allows astronomers to detect their existence. The only objects that agree with these observations and are consistent within the framework of general relativity are black holes.
History Simulation of Gravitational lensing by a black hole which distorts the image of a galaxy in the background. (Click for larger animation.) The idea of a body so massive that even light could not escape was 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 light was then thought to be a massless wave and therefore not influenced by gravity. Unlike the modern black hole concept, the object behind the horizon is assumed to be stable against collapse.
In 1915, Albert Einstein developed his general theory of relativity, having earlier shown that gravity does in fact 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,[5] showing that a black hole could theoretically exist. The Schwarzschild radius is now known to be the radius of the event horizon of a non-rotating black hole, but this was not well understood at that time, for example Schwarzschild himself thought it was not physical. Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution for the point mass a few months after Schwarzschild and wrote more extensively about its properties.
In 1930, astrophysicist 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. His arguments were opposed by Arthur Eddington, who believed that something would inevitably stop the collapse. Eddington was partly correct: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star. But in 1939, Robert Oppenheimer and others predicted that stars above approximately three solar masses (the Tolman-Oppenheimer-Volkoff limit) would collapse into black holes for the reasons presented by Chandrasekhar.[6]
Oppenheimer and his co-authors used Schwarzschild's system of coordinates (the only coordinates available in 1939), which produced mathematical singularities at the Schwarzschild radius, in other words some of the terms in the equations became infinite at the Schwarzschild radius. This was interpreted as indicating that the Schwarzschild radius 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 briefly known as "frozen stars,"[citation needed] 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.
In 1958, David Finkelstein introduced the concept of the event horizon by presenting Eddington-Finkelstein coordinates, which enabled him to show that "The Schwarzschild surface r = 2 m is not a singularity, but that it acts as a perfect unidirectional membrane: causal influences can cross it in only one direction".[7] This did not strictly contradict Oppenheimer's results, but extended them to include the point of view of infalling observers. All theories up to this point, including Finkelstein's, covered only non-rotating black holes.
In 1963, Roy Kerr found the exact solution for a rotating black hole. The rotating singularity of this solution was a ring, and not a point. A short while later, Roger Penrose was able to prove that singularities occur inside any black hole.
In 1967, astronomers discovered pulsars,[8][9] and within a few years could show that the known pulsars were rapidly rotating neutron stars. Until that time, neutron stars were also regarded as just theoretical curiosities. So the discovery of pulsars awakened interest in all types of ultra-dense objects that might be formed by gravitational collapse.
Physicist John Wheeler is widely credited with coining the term black hole in his 1967 public lecture Our Universe: the Known and Unknown, as an alternative to the more cumbersome "gravitationally completely collapsed star." However, Wheeler insisted that someone else at the conference had coined the term and he had merely adopted it as useful shorthand. The term was also cited in a 1964 letter by Anne Ewing to the AAAS:
According to Einstein’s general theory of relativity, as mass is added to a degenerate star a sudden collapse will take place and the intense gravitational field of the star will close in on itself. Such a star then forms a "black hole" in the universe. —Ann Ewing, letter to AAAS[10] Properties and features The No hair theorem states that, once it settles down, a black hole has only three independent physical properties: mass, charge, and angular momentum.[11] Any two black holes that share the same values for these properties, or parameters, are classically indistinguishable.
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, despite the fact that photons, the particles responsible for electric and magnetic forces, cannot escape from the interior region. The reason is Gauss's law, the total electric flux going out of a big sphere always stays the same, and measures the total charge inside the sphere. When charge falls into a black hole, electric field lines still remain, poking out of the horizon, and these field lines conserve the total charge of all the infalling matter. The electric field lines eventually spread out evenly over the surface of the black hole, settling down to a uniform field-line density on the surface. The black hole acts in this regard like a classical conducting sphere with a definite resistivity.[12]
Similarly, the total mass inside a sphere containing a black hole can be found by using the gravitational analog of Gauss's law, far away from the black hole. Likewise, the angular momentum can be measured from far away using frame dragging by the gravitomagnetic field.
When a black hole swallows any form of matter, its horizon oscillates like a stretchy membrane with friction, a dissipative system, until it settles down to a simple final state. This is different from other field theories like electromagnetism or gauge theory, which never have any friction or resistivity, because they are time reversible. Because the black hole eventually settles down into a final 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.[13][14][15]
The loss of information in black holes is puzzling even classically, because general relativity is a Lagrangian theory, which superficially appears to be time reversible and Hamiltonian. But because of the horizon, a black hole is not time reversible: matter can enter but it cannot escape. The time reverse of a classical black hole has been called a white hole, although entropy considerations and quantum mechanics suggest that white holes are just the same as black holes.
The no-hair theorem makes some assumptions about the nature of our universe and the matter it contains, and other assumptions lead to different conclusions. For example, if Magnetic monopoles exist, as predicted by some theories,[16] the magnetic charge would be a fourth parameter for a classical black hole.
Counterexamples to the no-hair theorem are known for the following cases:
- In spacetime dimensions higher than four
- In the presence of non-abelian Yang-Mills fields
- For discrete gauge symmetries.
- Some non-minimally coupled scalar fields[17]
- When scalars can be topologically twisted, as in the case of skyrmions
- In modified theories of gravity, different from Einstein’s general relativity.
Classification By physical properties The simplest black hole has mass but neither 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.[5] It was the first non-trivial exact solution to the Einstein field equations to be discovered, and according to Birkhoff's theorem, the only vacuum solution that is spherically symmetric.[20] 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 essentially like that of ordinary massive bodies.[21]
More general black hole solutions were discovered later in the 20th century. The Reissner-Nordström metric describes a black hole with electric charge, while the Kerr metric yields a rotating black hole. The more generally known stationary black hole solution, the Kerr-Newman metric, describes 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 natural units , the total charge and the total angular momentum are expected to satisfy
for a black hole of mass M.
Black holes saturating this inequality are called extremal. Solutions of Einstein's equations violating the inequality do exist, but do not have a horizon. These solutions have naked singularities and are deemed unphysical, as the cosmic censorship hypothesis rules out such singularities due to the generic gravitational collapse of realistic matter.[22] This is supported by numerical simulations.[23]
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[24] appears to have an angular momentum near the maximum allowed value.
By mass Class Mass Size Supermassive black hole ~105–109 MSun ~0.001–10 AU Intermediate-mass black hole ~103 MSun ~103 km = REarth Stellar-mass ~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 . The size of a black hole, as determined by the radius of the event horizon, or Schwarzschild radius, is proportional to the mass through
where is the Schwarzschild radius and is the mass of the Sun. A black hole's size and mass are thus simply related independent of rotation. According to this criterion, black holes are classed as:
- Supermassive – contain hundreds of thousands to billions of solar masses, and are thought to exist in the center of most galaxies,[25][26] including the Milky Way.[27] They are thought to be responsible for active galactic nuclei, and presumably form either from the coalescence of smaller black holes, or by the accretion of stars and gas onto them. The largest known supermassive black hole is located in OJ 287 weighing in at 18 billion solar masses.[28]
- Intermediate – contain thousands of solar masses. They have been proposed as a possible power source for ultraluminous X-ray sources.[29] There is no known mechanism for them to form directly, so they likely form via collisions of lower mass black holes, either in the dense stellar cores of globular clusters or galaxies.[citation needed] Such creation events should produce intense bursts of gravitational waves, which may be observed soon. The boundary between super- and intermediate-mass black holes is a matter of convention. Their lower mass limit, the maximum mass for direct formation of a single black hole from collapse of a massive star, is poorly known at present, but is thought to be somewhere well below 200 solar masses.
- Stellar-mass – have masses ranging from a lower limit of about 1.4–3 solar masses (1.4 is the Chandrasekhar limit and 3 is the Tolman-Oppenheimer-Volkoff limit for the maximum mass of neutron stars) up to perhaps 15–20 solar masses. They are created by the collapse of individual stars, or by the coalescence (inevitable, due to gravitational radiation) of binary neutron stars. Stars may form with initial masses up to about 100 solar masses, or in the distant past, possibly even higher, but these shed most of their outer massive layers during earlier phases of their evolution, either blown away in stellar winds during the red giant, AGB, and Wolf-Rayet stages, or expelled in supernova explosions for stars that turn into neutron stars or black holes. Being known mostly by theoretical models for late-stage stellar evolution, the upper limit for the mass of stellar-mass black holes is somewhat uncertain at present. The cores of still lighter stars form white dwarfs.
- Micro – (also mini black holes) have masses much less than that of a star. At these sizes, quantum mechanics is expected to take effect. There is no known mechanism for them to form via normal processes of stellar evolution, but certain inflationary scenarios predict their production during the early stages of the evolution of the universe.[citation needed] According to some theories of quantum gravity they may also be produced in the highly energetic reaction produced by cosmic rays hitting the atmosphere or even in particle accelerators such as the Large Hadron Collider.[citation needed] The theory of Hawking radiation predicts that such black holes will evaporate in bright flashes of gamma radiation. NASA's Fermi Gamma-ray Space Telescope satellite (formerly GLAST) launched in 2008 is searching for such flashes.[30]
Far away from the black hole a particle can move in any direction. It is only restricted by the speed of light.
Closer to the black hole spacetime starts to deform. There are more paths going towards the black hole than paths moving away.
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 beyond which events cannot affect an outside observer. As predicted by general relativity, the presence of a mass deforms spacetime in such a way that the paths particles take tend towards the mass. At the event horizon of a black hole this deformation becomes so strong that there are no more paths that lead away from the black hole.[31] Once a particle is inside the horizon, moving into the hole is as inevitable as moving forward in time (and can actually be thought of as equivalent to doing so).
To a distant observer clocks near a black hole appear to tick more slowly than those further away from the black hole.[32] Due to this effect (known as gravitational time dilation) the distant observer will see an object falling into a black hole slow down as it approaches the event horizon, taking an infinite time to reach it.[33] At the same time all processes on this object slow down causing emitted light to appear redder and dimmer, an effect known as gravitational red shift.[34] Eventually, the falling object becomes so dim that it can no longer be seen, at a point just before it reaches the event horizon.
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.[35] 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 it is expected that quantum gravity effects become significant near the vicinity of the event horizon.[36] This allows observations of matter in the vicinity of a black hole's event horizon to be used to indirectly study general relativity and proposed extensions to it.
Though black holes themselves may not radiate energy, electromagnetic radiation and matter particles may be radiated from just outside the event horizon via Hawking radiation.[37]
Singularity Main article: Gravitational singularity At the center of a black hole lies the singularity, where matter is crushed to infinite density, the pull of gravity is infinitely strong, and spacetime has infinite curvature.[38] This means that a black hole's mass becomes entirely compressed into a region with zero volume.[39] This zero-volume, infinitely dense region at the center of a black hole is called a gravitational singularity.
The singularity of a non-rotating black hole has zero length, width, and height; a rotating black hole is smeared out to form a ring shape lying in the plane of rotation.[40] The ring still has no thickness and hence no volume.
The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory.[41] 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.[42][43]
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.[44] 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.
The ergosphere of a black hole is bounded by:
- On the outside, an oblate spheroid, which coincides with the event horizon at the poles and is noticeably wider around the "equator." This boundary is sometimes called the "ergosurface," but it is just a boundary and has no more solidity than the event horizon. At points exactly on the ergosurface, spacetime is "dragged around at the speed of light."
- On the inside, the (outer) event horizon.
Objects and radiation (including light) can stay in orbit within the ergosphere without falling to the center. But they cannot hover (remain stationary, as seen by an external observer), because that would require them to move backwards faster than light relative to their own regions of space-time, which are moving faster than light relative to an external observer.
Objects and radiation can also escape from the ergosphere. In fact the Penrose process predicts that objects will sometimes fly out of the ergosphere, obtaining the energy for this by "stealing" some of the black hole's rotational energy. If a large total mass of objects escapes in this way, the black hole will spin more slowly and may even stop spinning eventually.
Formation and evolution Considering the exotic nature of black holes, it may be natural to question if such bizarre objects could actually 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.[45] 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,[46] 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. Shortly afterwards, Stephen Hawking showed that many cosmological solutions describing the big bang have singularities, in the absence of 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.[47] 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, or because a star which would have been stable receives a lot of 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).
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.
If the mass of the remnant exceeds ~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.
This gravitational collapse of heavy stars is assumed to be responsible for the formation of most (if not all) stellar mass black holes.
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.[48] 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.[49] 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 (~1019 GeV/c2 = ~2 × 10-8 kg), where quantum effects are expected to make the theory of general relativity break down completely.[citation needed] 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 this bound could be much lower. Some braneworld scenarios for example put the Planck mass much lower, may be even as low as 1 TeV/c2.[50] 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.
Growth Once a black hole has formed, it can continue to grow by absorbing additional matter. Any black hole will continually absorb interstellar dust from its direct surroundings and omnipresent cosmic background radiation, but neither of these processes should significantly affect the mass of a stellar black hole. More significant contributions can occur when the black hole formed in a binary star system. After formation the black hole can then leech significant amounts of matter from its companion.
Much larger contributions can be obtained when a black hole merges with other stars or compact objects. The supermassive black holes suspected in the center of most galaxies are expected to have formed from the coagulation of many smaller objects. The process has also been proposed as the origin of some intermediate-mass black holes.
As an object approaches the event horizon, the horizon near the object bulges up and swallows the object. Shortly thereafter the increase in radius (due to the extra mass) is distributed evenly around the hole.
Evaporation Main article: Hawking radiation In 1974, Stephen Hawking showed that black holes are not entirely black but emit small amounts of thermal radiation.[51] 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.[52] 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).[51] 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 in turn is inversely proportional to the mass. Large black holes, therefore, emit less radiation than small black holes.
A stellar black hole of 5 solar masses has a Hawking temperature of about 12 nanokelvins. This is far less than the 2.7 K produced by 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. In order 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).
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 – hypothetically make such a small black hole stable.
