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One of the end points of gravitational collapse, in which the collapsing matter fades from view, leaving only a center of gravitational attraction behind. General relativity predicts that if a star of more than about 3 solar masses has completely burned its nuclear fuel, it should collapse to a configuration known as a black hole. The resulting object is independent of the properties of the matter that produced it and can be completely described by stating its mass, spin, and charge. The most striking feature of this object is the existence of a surface, called the horizon, which completely encloses the collapsed matter. The horizon is an ideal one-way membrane: that is, particles and light can go inward through the surface, but none can go outward. As a result, the object is dark, that is, black, and hides from view a finite region of space (a hole). See also
The possible formation of black holes depends critically on what other end points of stellar evolution are possible. There can always be chunks of cold matter which are stable, but their mass must be considerably less than that of the Sun. For masses on the order of a solar mass, only two stable configurations are known for cold, evolved matter. The first, the white dwarf, is supported against gravitational collapse by the same quantum forces that keep atoms from collapsing. However, these forces cannot support a star which has a mass in excess of about 1.2 solar masses. The second stable configuration, the neutron star, is supported against gravitational collapse by the same forces that keep the nucleus of an atom from collapsing. There is also a maximum mass for a neutron star, estimated to be between 1 and 3 solar masses.
It would appear from the theory that if a collapsing star of over 3 solar masses does not eject matter, it has no choice but to become a black hole. There are, of course, many stars with mass larger than 3 solar masses, and it is expected that a significant number of them will reach the collapse stage without having ejected sufficient matter to take them below the 3-solar-mass limit. Further, more massive stars evolve more rapidly, enhancing the rate of formation of black holes. It seems reasonable to conclude that a considerable number of black holes should exist in the universe.
The black hole solutions of general relativity, ignoring quantum-mechanical effects, are completely stable. Once massive black holes form, they will remain forever; and subsequent processes, for example, the accumulation of matter, only increase their size. Steven Hawking showed that when quantum effects are property taken into account, a black hole should emit thermal radiation, composed of all particles and quanta of radiation which exist. Since a radiating system loses energy and therefore loses mass, a black hole can shrink and decay if it is radiating faster than it is accumulating matter. However, for black holes formed from the collapse of stars, the ambient radiation incident on the black hole from other stars, and from the big bang itself, is much larger than the thermal radiation emitted by the black hole, implying that the black hole would not shrink. Even if the ambient radiation is shielded from the black hole, the time for the black hole to decay is much longer than the age of the universe, so that, in practice, black holes formed from collapse of a star are essentially as stable as they were thought to be before the Hawking radiation was predicted.
Because black holes themselves are unobservable, their existence must be inferred from their effect on other matter. Such is the case with the binary x-ray star system Cygnus X-l. There are a number of binary x-ray systems known. The model which best explains the data is one in which a fairly normal star is in mutual orbit about a very compact object. Because these two are so close, mass flows from the star onto an accreting disk about the compact object. As the mass in the disk spirals inward, it heats up by frictional forces. Because the central body is so compact, the matter heats to a temperature at which thermal x-rays are produced. The only compact objects known that could accomplish this are neutron stars and black holes. The existence of very short-time bursts of radiation also points to an object of small diameter, that is, compact. In some of these binary x-ray systems, there is also a regular pulsed component to the x-rays, indicating a rotating neutron star (by reasoning similar to that given for pulsars). In these systems, the compact object could not be a black hole because that would imply a more complicated structure than a black hole would allow. In other systems, however, there are only irregular pulsations or fluctuations; they are candidates for possible black holes.
The crucial evidence comes from the mass determination of the compact object. Because the inclination of the orbit is not known, a range of masses is found; however, there will be a typical mass obtained by assuming that the orbit is not in an extreme orientation. For three x-ray binaries, Cygnus X-1, LMC X-3, and A0620-00, the typical mass of the compact body is about 10 solar masses, much larger than the maximum mass of a neutron star. In fact, the compact objects in the first and third binary systems are more massive than the maximum mass of a neutron star, no matter what orientation the orbit is assumed to have. Assuming that general relativity is the correct theory of gravitation (and this assumption is now supported very well experimentally), there can be no compact objects of such a mass other than a black hole. In this sense it can now be said that black holes exist.
While the evidence is less direct and more model-dependent, there is growing acceptance of the idea that supermassive black holes exist at the cores of nuclei of active galaxies, including quasars and radio galaxies. Here, the black hole is assumed to interact with accreting matter in such a way as to provide a source of energy to power these ultraluminous objects.
Black holes are thought to exist in the nuclei of other galaxies as well, their presence not giving rise to amounts of radiation as spectacular as for active galactic nuclei only because of differing conditions near the black hole. In the Milky Way Galaxy, observations of the proper motions of stars within a fraction of a parsec of the galactic center demonstrate unambiguously that a central mass concentration of 2 × 106 solar masses is present in a region so compact that no explanation other than that of a central black hole is feasible. Similar, although less convincing, observations of the presence of central black holes have been made for several nearby galaxies. The existence of supermassive black holes is virtually certain. See also Astrophysics, high-energy; Binary star; X-ray astronomy.
For more information on black hole, visit Britannica.com.
Gravitational collapse begins when a star has depleted its steady sources of nuclear energy and can no longer produce the expansive force, a result of normal gas pressure, that supports the star against the compressive force of its own gravitation. As the star shrinks in size (and increases in density), it may assume one of several forms depending upon its mass. A less massive star may become a white dwarf, while a more massive one would become a supernova. If the mass is less than three times that of the sun, it will then form a neutron star. However, if the final mass of the remaining stellar core is more than three solar masses, as shown by the American physicists J. Robert Oppenheimer and Hartland S. Snyder in 1939, nothing remains to prevent the star from collapsing without limit to an indefinitely small size and infinitely large density, a point called the “singularity.”
At the point of singularity the effects of Einstein's general theory of relativity become paramount. According to this theory, space becomes curved in the vicinity of matter; the greater the concentration of matter, the greater the curvature. When the star (or supernova remnant) shrinks below a certain size determined by its mass, the extreme curvature of space seals off contact with the outside world. The place beyond which no radiation can escape is called the event horizon, and its radius is called the Schwarzschild radius after the German astronomer Karl Schwarzschild, who in 1916 postulated the existence of collapsed celestial objects that emit no radiation. For a star with a mass equal to that of the sun, this limit is a radius of only 1.86 mi (3.0 km). Even light cannot escape a black hole, but is turned back by the enormous pull of gravitation.
It is now believed that the origin of some black holes is nonstellar. Some astrophysicists suggest that immense volumes of interstellar matter can collect and collapse into supermassive black holes, such as are found at the center of some galaxies. The British physicist Stephen Hawking has postulated still another kind of nonstellar black hole. Called a primordial, or mini, black hole, it would have been created during the “big bang,” in which the universe was created (see cosmology). Unlike stellar black holes, primordial black holes create and emit elementary particles, called Hawking radiation, until they exhaust their energy and expire. It has also been suggested that the formation of black holes may be associated with intense gamma ray bursts. Beginning with a giant star collapsing on itself or the collision of two neutron stars, waves of radiation and subatomic particles are propelled outward from the nascent black hole and collide with one another, releasing the gamma radiation. Also released is longer-lasting electromagnetic radiation in the form of X rays, radio waves, and visible wavelengths that can be used to pinpoint the location of the disturbance.
Because light and other forms of energy and matter are permanently trapped inside a black hole, it can never be observed directly. However, a black hole can be detected by the effect of its gravitational field on nearby objects (e.g., if it is orbited by a visible star), during the collapse while it was forming, or by the X rays and radio frequency signals emitted by rapidly swirling matter being pulled into the black hole. A small number of possible black holes have been detected. The first discovered (1971) was Cygnus X-1, an X-ray source in the constellation Cygnus. In 1994 astronomers employing the Hubble Space Telescope announced that they had found conclusive evidence of a supermassive black hole in the M87 galaxy in the constellation Virgo. The first evidence (2002) of a binary black hole, two supermassive black holes circling one another, was detected in images from the orbiting Chandra X-ray Observatory. Located in the galaxy NGC6240, the pair are 3,000 light years apart, travel around each other at a speed of about 22,000 mph (35,415 km/hr), and have the mass of 100 million suns each. As the distance between them shrinks over 100 million years, the circling speed will increase until it approaches the speed of light, about 671 million mph (1080 million km/hr). The black holes will then collide spectacularly, spewing radiation and gravitational waves across the universe.
Bibliography
See S. W. Hawking, Black Holes and Baby Universes and Other Essays (1994); P. Strathern, The Big Idea: Hawking and Black Holes (1998); J. A. Wheeler, Geons, Black Holes, and Quantum Foam: A Life in Physics (1998); H. Falcke and F. W. Hehl, The Galactic Black Hole: Studies in High Energy Physics, Cosmology and Gravitation (2002).
Frances Tustin introduced the idea of black holes in her Autistic Barriers in Neurotic Patients (1986). The term was chosen by analogy with ideas in modern astrophysics, which has discovered zones of extraordinary density in the universe that are probably related to the condensation and fusion of several stars. Once formed, such hyperdense zones are thought to exert a sort of attraction upon other stars, which are thus at risk of plunging into the core of these vast concentrations of matter, which swallow them up and strip them of all individuality. It is not hard to see how the metaphor of a "black hole of the psyche" can help explain, or at least help us picture what happens at the core of the psyche of autistic children.
Indeed Tustin had already elaborated on a notion first proposed by Sydney Klein (1980), that of "autistic islands." And, most significantly, in her first book, Autism and Childhood Psychosis (1972), she had painstakingly recounted the case of John, who had described to her, on emerging from autism, what he himself called "the black hole w/the mechant piquant." What John was striving to verbalize in this way was all the pain and suffering he had felt on the occasion of far too brutal and premature a separation between the breast and the nipple, this at a time when nipple and mouth are inextricably conjoined (as described, albeit in a different way, by Piera Aulagnier, with her "complementary zone-object"). Naturally it is less a physical separation that is involved here than a mental one—or even, to be quite precise, the inscription in the psyche of the process of separation.
If, for one reason or another, this process turns out to be impossible or impeded, the child is liable to feel as if a part of him- or herself has been cut off.
This traumatic organization of the psyche leaves its mark in the shape of "autistic islands" which fail to become integrated into the cycles of deferred effects and historical time: Their massiveness and their intensity, in autistic children, are an obstacle to their becoming part of mental functioning, and they end up serving as pathological poles of attraction for a whole variety of psychic elements which accrete within their sphere of influence and thus become incapable of dispersing in a manner at once orderly and differentiated.
In the wake of Frances Tustin, the post-Kleinian tendency in psychoanalysis has made wide use of the concept of the black hole, extending it to nonpsychotic subjects in whom autistic islands are possible even if in such cases they are less significant and less serious in their implications.
Bibliography
Klein, Sydney. (1980). Autistic phenomena in neurotic patients. International Journal of Psycho-Analysis, 61 (2), 395-401.
Tustin, Frances. (1972). Autism and childhood psychosis. London: Hogarth; New York: Science House. Reprinted, London: Karnac, 1995.
——. (1986). Autistic barriers in neurotic patients. London: Karnac.
—BERNARD GOLSE
In astronomy, an object so massive that nothing, not even
| General relativity |
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| Key topics |
| Introduction to... Mathematical formulation of... |
| Fundamental concepts |
| Special relativity Equivalence principle World line · Riemannian geometry |
| Phenomena |
| Kepler problem · Lenses · Waves Frame-dragging · Geodetic effect |
| Equations |
| Linearized Gravity Post-Newtonian formalism Einstein field equations |
| Advanced theories |
| Kaluza-Klein Quantum gravity |
| Solutions |
| Schwarzschild Reissner-Nordström · Gödel |
| Scientists |
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Einstein · Minkowski · Eddington |
A black hole is a region of space in which the gravitational field is so powerful that nothing can escape after having fallen past the event horizon. The name comes from the fact that even electromagnetic radiation (e.g. light) is unable to escape, rendering the interior invisible. However, black holes can be detected if they interact with matter outside the event horizon, for example by drawing in gas from an orbiting star. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation in the process.[2][3][4]
While the idea of an object with
While general relativity describes a black hole as a region of empty space with a pointlike singularity at the center and an event horizon at the outer edge, the description changes when the effects of quantum mechanics are taken into account. Research on this subject indicates that, rather than holding captured matter forever, black holes may slowly leak a form of thermal energy called Hawking radiation.[5][6][7] However, the final, correct description of black holes, requiring a theory of quantum gravity, is unknown.
Black holes can have any mass. Since gravity increases in inverse proportion to volume, any quantity of matter that is sufficiently compressed will become a black hole. However, when black holes form naturally, only a few mass ranges are realistic.
Black holes can be divided into several size categories:
Astrophysicists expect to find stellar-mass and larger black holes, because a stellar mass black hole is formed by the gravitational collapse of a star of 20 or more solar masses at the end of its life, and can then act as a seed for the formation of a much larger black hole.
Micro black holes might be produced by:
General relativity describes mass as changing the shape of spacetime, and the shape of spacetime as describing how matter moves through space. For objects much less dense than black holes, this results in something similar to Newton's laws of gravity: objects with mass attract each other, but it's possible to define an escape velocity which allows a test object to leave the gravitational field of any large object. For objects as dense as black holes, this stops being the case. The effort required to leave the hole becomes infinite, with no escape velocity defined.
There are several ways of describing the situation that causes escape to be impossible. The difference between these descriptions is how space and time coordinates are drawn on spacetime (the choice of coordinates depends on the choice of observation point and on additional definitions used). One common description, based on the Schwarzschild description of black holes, is to consider the time axis in spacetime to point inwards towards the center of the black hole once the horizon is crossed.[8] Under these conditions, falling further into the hole is as inevitable as moving forward in time. A related description is to consider the future light cone of a test object near the hole (all possible paths the object or anything emitted by it could take, limited by the speed of light). As the object approaches the event horizon at the boundary of the black hole, the future light cone tilts inwards towards the horizon. When the test object passes the horizon, the cone tilts completely inward, and all possible paths lead into the hole.[9]
The "No hair" theorem states that black holes have only 3 independent internal properties: mass, angular momentum and electric charge. It is impossible to tell the difference between a black hole formed from a highly compressed mass of normal matter and one formed from, say, a highly compressed mass of anti-matter; in other words, any information about infalling matter or energy is destroyed. This is the black hole information paradox.
The theorem only works in some of the types of universe which the equations of general relativity allow, but this includes four-dimensional spacetimes with a zero or positive cosmological constant, which describes our universe at the classical level.
Despite the uncertainty about whether the "No Hair" theorem applies to our universe, astrophysicists currently classify black holes according to their angular momentum (non-zero angular momentum means the black hole is rotating) and electric charge:
| Non-rotating | Rotating | |
| Uncharged | Schwarzschild | Kerr |
| Charged | Reissner-Nordström | Kerr-Newman |
(All black holes have non-zero mass, so mass cannot be used for this type of "yes" / "no" classification)
Physicists do not expect that black holes with a significant electric charge will be formed in nature, because the electromagnetic repulsion which resists the compression of an electrically charged mass is about 40 orders of magnitude greater (about 1040 times greater) than the gravitational attraction which compresses the mass. So this article does not cover charged black holes in detail, but the Reissner-Nordström black hole and Kerr-Newman metric articles provide more information.
On the other hand astrophysicists expect that almost all black holes will rotate, because the stars from which they are formed rotate. In fact most black holes are expected to spin very rapidly, because they retain most of the angular momentum of the stars from which they were formed but concentrated into a much smaller radius. The same laws of angular momentum make skaters spin faster if they pull their arms closer to their bodies.
This article describes non-rotating, uncharged black holes first, because they are the simplest type.
This is the boundary of the region from which not even light can escape, but at the same time, light does not get sucked into the black hole. Stephen Hawking, in his book, A Brief History of Time, describes the event horizon as "the point of which light is just barely unable to escape (I like to think of it as being chased by the police but just barely managing to stay one step away!)." Another way to think of this is that the light is running on a spacetime "treadmill;" the light is moving away from the black hole at the rate of c, but the spacetime is being sucked into the black hole at the same rate, so the two cancel each other out, much like a treadmill. An observer at a safe distance would see a dull black sphere if the black hole was in a pure vacuum but in front of a light background such as a bright nebula. The event horizon is not a solid surface, and does not obstruct or slow down matter or radiation which is traveling towards the region within the event horizon.
The event horizon is the defining feature of a black hole - it is black because no light or other radiation can escape from inside it. So the event horizon hides whatever happens inside it and we can only calculate what happens by using the best theory available, which at present is general relativity.
The gravitational field outside the event horizon is identical to the field produced by any other spherically symmetric object of the same mass. The popular conception of black holes as "sucking" things in is false: objects can maintain an orbit around black holes indefinitely provided they stay outside the photon sphere. (described below)
According to general relativity, a black hole's mass is entirely compressed into a region with zero volume, which means its density and gravitational pull are infinite, and so is the curvature of space-time which it causes. These infinite values cause most physical equations, including those of general relativity, to stop working at the center of a black hole. So physicists call the zero-volume, infinitely dense region at the center of a black hole a "singularity".
The singularity in a non-rotating, uncharged black hole is a point, in other words it has zero length, width and height.
But there is an important uncertainty about this description: quantum mechanics is as well-supported by mathematics and experimental evidence as general relativity, and does not allow objects to have zero size - so quantum mechanics says the center of a black hole is not a singularity but just a very large mass compressed into the smallest possible volume. At present we have no well-established theory which combines quantum mechanics and general relativity; and the most promising candidate, string theory, also does not allow objects to have zero size.
The rest of this article will follow the predictions of general relativity, because quantum mechanics deals with very small-scale (sub-atomic) phenomena and general relativity is the best theory we have at present for explaining large-scale phenomena such as the behavior of masses similar to or larger than stars.
A non-rotating black hole's 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 larger than the radius of the event horizon. This may give the impression that a black hole will accumulate a 'shell' of captured photons which will grow in density indefinitely, but this is not true. No photon is likely to stay in this orbit for long, for two reasons. First, it is likely to interact with any infalling matter in the vicinity (being absorbed or scattered). Second, the orbit is dynamically unstable; small deviations from a perfectly circular path will grow into larger deviations very quickly, causing the photon to either escape or fall into the hole.
Other extremely compact objects such as neutron stars can also have photon spheres.[10] This follows from the fact that light "captured" by a photon sphere does not pass within the radius that would form the event horizon if the object were a black hole of the same mass, and therefore its behavior does not depend on the presence of an event horizon.
Space is not a pure vacuum - even interstellar space contains a few atoms of hydrogen per cubic centimeter.[11] The powerful gravity field of a black hole pulls this towards and then into the black hole. The gas nearest the event horizon forms a disk and, at this short range, the black hole's gravity is strong enough to compress the gas to a relatively high density. The pressure, friction and other mechanisms within the disk generate enormous energy (which causes the gases to turn into plasma (physics)) - in fact they convert matter to energy more efficiently than the nuclear fusion processes that power stars. As a result, the disk glows very brightly, although disks around black holes radiate mainly X-rays rather than visible light.
Accretion disks are not proof of the presence of black holes, because other massive, ultra-dense objects such as neutron stars and white dwarfs cause accretion disks to form and to behave in the same ways as those around black holes.
Rotating black holes share many of the features of non-rotating black holes - inability of light or anything else to escape from within their event horizons, accretion disks, etc. But general relativity predicts that rapid rotation of a large mass produces further distortions of space-time in addition to those which a non-rotating large mass produces, and these additional effects make rotating black holes strikingly different from non-rotating ones.
If two rotating black holes have the same mass but different rotation speeds, the inner event horizon of the faster-spinning black hole will have a larger radius and its outer event horizon will have a smaller radius than in the slower-spinning black hole. In the most extreme case the two event horizons have zero radius, the region hidden by them has zero size and therefore the object is not a black hole but a naked singularity. Many physicists think that some principle which has not yet been discovered prevents the existence of a naked singularity and therefore prevents a black hole from spinning fast enough to create one.
General relativity predicts that a rotating black hole has two photon spheres, one for each event horizon. A beam of light traveling in a direction opposite to the spin of the black hole will circularly orbit the hole at the outer photon sphere. A beam of light traveling in the same direction as the black hole's spin will circularly orbit at the inner photon sphere. This beam will then split itself in two and both pieces will move into the Hole.
A large, ultra-dense rotating mass creates an effect called frame-dragging, so that space-time is dragged around it in the direction of the rotation.
Rotating black holes have an ergosphere, a region bounded by:
Within the ergosphere space-time is dragged around faster than light - general relativity forbids material objects to travel faster than light (so does special relativity), but allows regions of space-time to move faster than light relative to other regions of space-time.
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.
General relativity predicts that a rotating black hole will have a ring singularity which lies in the plane of the "equator" and has zero width and thickness - but remember that quantum mechanics does not allow objects to have zero size in any dimension (their wavefunction must spread), so general relativity's prediction is only the best idea we have until someone devises a theory which combines general relativity and quantum mechanics.
Kerr's solution for the equations of general relativity predicts that:
If this is true, rotating black holes could theoretically provide the wormholes which often appear in science fiction. Unfortunately, it is unlikely that the internal properties of a rotating black hole are exactly as described by Kerr's solution[13] and it is not currently known whether the actual properties of a rotating black hole would provide a similar escape route for an object via the inner event horizon.
Even if this escape route is possible, it is unlikely to be useful because a spacecraft which followed that path would probably be distorted beyond recognition by spaghettification.
This section describes what happens when something falls into a non-rotating, uncharged black hole. The effects of rotating and charged black holes are more complicated but the final result is much the same - the falling object is absorbed (unless rotating black holes really can act as wormholes).
An object in any very strong gravitational field feels a tidal force stretching it in the direction of the object generating the gravitational field. This is because the inverse square law causes nearer parts of the stretched object to feel a stronger attraction than farther parts. Near black holes, the tidal force is expected to be strong enough to deform any object falling into it, even atoms or composite nucleons; this is called spaghettification.
The strength of the tidal force depends on how gravitational attraction changes with distance, rather than on the absolute force being felt. This means that small black holes cause spaghettification while infalling objects are still outside their event horizons, whereas objects falling into large, supermassive black holes may not be deformed or otherwise feel excessively large forces before passing the event horizon.
An object in a gravitational field experiences a slowing down of time, called gravitational time dilation, relative to observers outside the field. The observer will see that physical processes in the object, including clocks, appear to run slowly. As a test object approaches the event horizon, its gravitational time dilation (as measured by an observer far from the hole) would approach infinity.
From the viewpoint of a distant observer, an object falling into a black hole appears to slow down, approaching but never quite reaching the event horizon: and it appears to become redder and dimmer, because of the extreme gravitational red shift caused by the gravity of the black hole. Eventually, the falling object becomes so dim that it can no longer be seen, at a point just before it reaches the event horizon. All of this is a consequence of time dilation: the object's movement is one of the processes that appear to run slower and slower, and the time dilation effect is more significant than the acceleration due to gravity; the frequency of light from the object appears to decrease, making it look redder, because the light appears to complete fewer cycles per "tick" of the observer's clock; lower-frequency light has less energy and therefore appears dimmer.
From the viewpoint of the falling object, distant objects may appear either blue-shifted or red-shifted, depending on the falling object's trajectory. Light is blue-shifted by the gravity of the black hole, but is red-shifted by the velocity of the infalling object.
From the viewpoint of the falling object, nothing particularly special happens at the event horizon (apart from spaghettification due to tidal forces, if the black hole has relatively low mass). An infalling object takes a finite proper time to fall past the event horizon.
An outside observer, however, will never see an infalling object cross this surface. The object appears to halt just above the horizon, due to gravitational redshift, fading from view as its light is red-shifted and the rate at which it emits photons drops to approach zero. This doesn't mean that the object never crosses the horizon; instead, it means that light from the horizon-crossing event is delayed by a time that approaches infinity as the object approaches the horizon. The time of crossing depends on how the outside observer chooses to define space and time axes on spacetime near the horizon.
The object reaches the singularity at the center within a finite amount of proper time, as measured by the falling object. An observer on the falling object would continue to see objects outside the event horizon, blue-shifted or red-shifted depending on the falling object's trajectory. Objects closer to the singularity aren't seen, as all paths light could take from objects farther in point inwards towards the singularity.
The amount of proper time a faller experiences below the event horizon depends upon where they started from rest, with the maximum being for someone who starts from rest at the event horizon. A study in 2007 examined the effect of firing a rocket pack with the black hole, showing that this can only reduce the proper time of a person who starts from rest at the event horizon. However, for anyone else, a judicial burst of the rocket can extend the life time of the faller, but over doing it will again reduce the proper time experienced. However, this cannot prevent the inevitable collision with the central singularity.[14]
As an infalling object approaches the singularity, tidal forces acting on it approach
infinity. All components of the object, including atoms and
Stellar-mass black holes are formed in two ways:
Stars undergo gravitational collapse when they can no longer resist the pressure of their own gravity. 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 (Charles's law explains the connection between temperature and volume).
The collapse transforms the matter in the star's core into a denser state which forms one of the types of compact star. Which type of compact star is formed depends on the mass of the remnant, i.e. of the matter left to be compressed after the supernova (if one happened - see below) triggered by the collapse has blown away the outer layers.
Only the largest remnants, those exceeding 1.4 solar masses (known as the Chandrasekhar limit), generate enough pressure to produce black holes, because singularities are the most radically transformed state of matter known to physics (if you can still call it matter) and the force which resists this level of compression, neutron degeneracy pressure, is extremely strong. Remnants exceeding 5 solar masses are produced by stars which were over 20 solar masses before the collapse (the rest of the mass is usually blown into space by the supernova triggered by the collapse).
In stars which are too large to form white dwarfs, the collapse releases energy which usually produces a supernova, blowing the star's outer layers into space so that they form a spectacular nebula. But the supernova is a side-effect and does not directly contribute to producing a compact star. For example a few gamma ray bursts were expected to be followed by evidence of supernovae but this evidence did not appear,[16][17] and one explanation is that some very large stars can form black holes fast enough to swallow the whole star before the supernova blast can reach the surface.
There are two main ways in which black holes of larger than stellar mass can be formed:
No known process currently active in the universe can form black holes of less than stellar mass. This is because all present known black hole formation is through gravitational collapse, and the smallest mass which can collapse to form a black hole produces a hole approximately 1.5-3.0 times the mass of the sun (the Tolman-Oppenheimer-Volkoff limit). Smaller masses collapse to form white dwarf stars or neutron stars.
There are still a few ways in which smaller black holes might be formed, or might have formed in the past:
Hawking radiation is a theoretical process by which black holes can evaporate into nothing. As there is no experimental evidence to corroborate it and there are still some major questions about the theoretical basis of the process, there is still debate about whether Hawking radiation can enable black holes to evaporate.
Quantum mechanics says that even the purest vacuum is not completely empty but is instead a "sea" of energy (known as zero-point energy) which has wave-like fluctuations. We cannot observe this "sea" of energy directly because there is no lower energy level with which we can compare it. The Heisenberg uncertainty principle dictates that it is impossible to know the exact value of the mass-energy and position pairings. The fluctuations in this sea produce pairs of particles in which one is made of normal matter and the other is the corresponding antiparticle (special relativity proves mass-energy equivalence, i.e. that mass can be converted into energy and vice versa). Normally each would soon meet another instance of its antiparticle and the two would be totally converted into energy, restoring the overall matter-energy balance as it was before the pair of particles was created. The Hawking radiation theory suggests that, if such a pair of particles is created just outside the event horizon of a black hole, one of the two particles may fall into the black hole while the other escapes, because the two particles move in slightly different directions after their creation. From the point of view of an outside observer, the black hole has just emitted a particle and therefore the black hole has lost a minute amount of its mass.
If the Hawking radiation theory is correct, only the very smallest black holes are likely to evaporate in this way. For example a black hole with the mass of our Moon would gain as much energy (and therefore mass - mass-energy equivalence again) from cosmic microwave background radiation as it emits by Hawking radiation, and larger black holes will gain more energy (and mass) than they emit. To put this in perspective, the smallest black hole which can be created naturally at present is about 5 times the mass of our sun, so most black holes have much greater mass than our Moon.
Over time the cosmic microwave background radiation becomes weaker. Eventually it will be weak enough so that more Hawking radiation will be emitted than the energy of the background radiation being absorbed by the black hole. Through this process, even the largest black holes will eventually evaporate. However, this process may take nearly a googol years to complete.
Most accretion disks and gas jets are not clear proof that a stellar-mass black hole is present, because other massive, ultra-dense objects such as neutron stars and white dwarfs cause accretion disks and gas jets to form and to behave in the same ways as those around black holes. But they can often help by telling astronomers where it might be worth looking for a black hole.
On the other hand, extremely large accretion disks and gas jets may be good evidence for the presence of supermassive black holes, because as far as we know any mass large enough to power these
phenomena must be a black hole.
Steady X-ray and gamma ray emissions also do not prove that a black hole is present but can tell astronomers where it might be worth looking for one - and they have the advantage that they pass fairly easily through nebulae and gas clouds.
But strong, irregular emissions of X-rays, gamma rays and other electromagnetic radiation can help to prove that a massive, ultra-dense object is not a black hole, so that "black hole hunters" can move on to some other object. Neutron stars and other very dense stars have surfaces, and matter colliding with the surface at a high percentage of the speed of light will produce intense flares of radiation at irregular intervals. Black holes have no material surface, so the absence of irregular flares round a massive, ultra-dense object suggests that there is a good chance of finding a black hole there.
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[18] or by collisions between neutron stars,[19] and both types of event involve sufficient mass and pressure to produce black holes. But it appears that a collision between a neutron star and a black hole can also cause a GRB,[20] 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[21] so the black holes associated with them are actually billions of years old.
Some astrophysicists believe that some ultraluminous X-ray sources may be the accretion disks of intermediate-mass black holes.[22]
Quasars are thought to be caused by the accretion disks of supermassive black holes, since we know of nothing else which is powerful enough to produce such strong emissions. While X-rays and gamma rays have much higher frequencies and shorter wavelengths than visible light, quasars radiate mainly radio waves, which have lower frequencies and longer wavelengths than visible light.
Gravitational lensing is another phenomenon which can have other causes besides the presence of a black hole, because any very strong gravitational field bends light rays. The most spectacular examples produce multiple images of very distant objects by bending towards our telescopes light rays which would otherwise have gone in different directions. But these multiple-image effects are probably produced by distant galaxies. [Does not explain fully]
Some large celestial objects are almost certainly orbiting around black holes, and the principles behind this conclusion are surprisingly simple if we consider a circular orbit first (although all known astronomical orbits are elliptical):
Unfortunately, since the time of Johannes Kepler, astronomers have had to deal with the complications of real astronomy:
Although black holes cannot be dete
![Simulated view of a black hole in front of the Milky Way. The hole has 10 solar masses and is viewed from a distance of 600 km. An acceleration of about 400 million g is necessary to sustain this distance constantly.[1]](http://www.answers.com/main/Record2?a=NR&url=http%3A%2F%2Fcommons.wikimedia.org%2Fwiki%2FImage%3ABlack%2520Hole%2520Milkyway.jpg)