HOME WEB NEWS IMAGES CLASSIFIEDS YELLOW PAGESPOLLS - SURVEYS WIKI COUNTRIES PHOTOS US UK INDIA
Cities Stocks Weather Movies Coupons Travel Blogs Health Horoscope Maps ASIA EUROPE Classifieds Yellow Pages
 sweet honey, user comments, honey board, National honey, honey has | ||
|
Cosmetic Surgery Cost -, Cosmetic Laser Surgery | ||
|
music videos, questions with, All things, rights reserved, hip hop | ||
 1
|
For other uses, see Black hole (disambiguation).
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."Step by Step into a Black Hole"
| General relativity | ||||||||||||
| Einstein field equations | ||||||||||||
| Introduction to... Mathematical formulation of...
| ||||||||||||
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.NASA/Goddard Space Flight Center: "Gamma-rays from Black Holes and Neutron Stars".Max-Planck-Gesellschaft October 28, 2006, "Discovery Of Gamma Rays From The Edge Of A Black Hole".Milky Way Black Hole May Be a Colossal \'Particle Accelerator\'.
While the idea of an object with gravity strong enough to prevent light from escaping was proposed in the 18th century, black holes, as presently understood, are described by Einstein\'s theory of general relativity, developed in 1916. This theory predicts that when a large enough amount of mass is present within a sufficiently small region of space, all paths through space are warped inwards towards the center of the volume, forcing all matter and radiation to fall inward.
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.Hawking, Stephen (1974). "Black Hole Explosions". Nature 248: pp. 30-31. McDonald, Kirk T. (1998). "Hawking-Unruh Radiation and Radiation of a Uniformly Accelerated Charge". Princeton University: p. 1. Hawking, Stephen; Penrose, Roger (2000). The Nature of Space and Time, New Ed edition, Princeton University Press, p. 44. ISBN 978-0691050843. However, the final, correct description of black holes, requiring a theory of quantum gravity, is unknown.
Black holes can have any mass. Since the gravitational force of a body on itself, at the surface of a body of any shape, increases in inverse proportion to its characteristic lengthscale squared (as volume-2/3 ), an object of any shape and mass that is sufficiently compressed will collapse under its own gravity and form 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 definable.
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.Kaufmann, William J. III (1979)). Black Holes and Warped Spacetime. W H Freeman & Co (Sd). ISBN 0-7167-1153-2. 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.http://www.phy.syr.edu/courses/modules/LIGHTCONE/schwarzschild.html Schwarzschild\'s Spacetime: Introducing the Black Hole
The "No hair" theorem states that black holes have only 3 independent internal properties: mass, angular momentum and electric charge. As a consequence 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 other information (apart from mass, angular momentum and charge) about infalling matter or energy is seemingly 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 able 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 disc 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 that 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, excluding Hawking radiation. 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), and also ignoring the effects of gravitational radiation, which causes orbiting objects to lose energy, similar to the effect of electromagnetic radiation.
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 that 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 it 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 that 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 that 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 due to light\'s enormous speed; 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.Nemiroff, R. J.. Journey to a strong gravity neutron star. Retrieved on 2006-03-25. 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.
An artist view taken from the Hubble Space Telescope website showing an accretion disk around the black hole. The friction from the gas generates a massive amount of heat. The heated gas emits X-rays.
Space is not a pure vacuum - even interstellar space contains a few atoms of hydrogen per cubic centimeter.Cai, Da We (2000). Density of Outer Space. The Physics Factbook. Retrieved on 2007-07-11. 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) - 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.
Two important surfaces around a rotating black hole. The inner sphere is the static limit (the event horizon). It is the inner boundary of a region called the ergosphere. The oval-shaped surface, touching the event horizon at the poles, is the outer boundary of the ergosphere. Within the ergosphere a particle is forced (dragging of space and time) to rotate and may gain energy at the cost of the rotational energy of the black hole (Penrose process).
Rotating black holes share many of the features of non-rotating black holes—the 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 that a non-rotating large mass produces; and these additional effects make rotating black holes strikingly different from non-rotating ones.
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 that 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 solutionarXiv:gr-qc/9902008 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 outside 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, as well as redder.
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. An infalling object takes a finite proper time (i.e. measured by its own clock) 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 judicious burst of the rocket can extend the lifetime of the faller, but overdoing it will again reduce the proper time experienced. However, this cannot prevent the inevitable collision with the central singularity.Lewis, G. F. and Kwan, J. (2007). "No Way Back: Maximizing Survival Time Below the Schwarzschild Event Horizon". Publications of the Astronomical Society of Australia 24 (2): 46-52.
As an infalling object approaches the singularity, tidal forces acting on it approach infinity. All components of the object, including atoms and subatomic particles, are torn away from each other before striking the singularity. At the singularity itself, effects are unknown; a theory of quantum gravity is needed to accurately describe events near it. Regardless, as soon as an object passes within the hole\'s event horizon, it is lost to the outside universe. An observer far from the hole simply sees the hole\'s mass, charge, and angular momentum change slightly, to reflect the addition of the infalling object\'s matter. After the event horizon all is unknown. Anything that passes this point cannot be retrieved to study.
Astrophysical black holes are characterized by two parameters: their mass and their angular momentum (or spin). The mass parameter M is equivalent to a characteristic length GM/c2=1.48km(M/M0) , or a characteristic timescale GM/c³=4.93 x 10-6(M/M0) , where M0 denotes the mass of the Sun. These scales, for example, give the order of magnitude of the radii and periods of near-hole orbits. The timescale also applies to the process in which a developing horizon settles into its asymptotically stationary form. For a stellar mass hole this is of order 10-5 sec , while for a supermassive hole of 108 M0 , it is thousands of seconds.
For Schwarzschild holes, and approximately for Kerr holes, the horizon is at radius RH=2GM/c². At the horizon the "acceleration of gravity" has no meaning, since a falling observer cannot stop at the horizon to be weighed. What is relevant at the horizon is the tidal stresses that stretch and distort the falling observer. This tidal stretching is given by the same expression, the gradient of the gravitational acceleration, as in Newtonian theory: 2GM/RH3=c6/(4G2M2) .
In the case of a solar mass black hole the tidal stress (acceleration per unit length) is enormous at the horizon, on the order of : 3 x 109(M/M0)2 sec-2 : that is, a person would experience a differential gravitational field of about 109 Earth gravities, enough to rip apart ordinary materials. For a supermassive hole, by contrast, the tidal force at the horizon is smaller by a typical factor 1010-16 and would be easily survivable. However, at the central singularity, deep inside the event horizon, the tidal stress is infinite. In addition to its mass M, the Kerr spacetime is described with a spin parameter \'a\' defined by the dimensionless expression a/M= cJ/GM2 where J is the angular momentum of the hole. For the Sun (based on surface rotation) this number is about 0.2, and is much larger for many stars. Since angular momentum is ubiquitous in astrophysics, and since it is expected to be approximately conserved during collapse and black hole formation, astrophysical holes are expected to have significant values of a/M , from several tenths up to and approaching unity.
The value of a/M can be unity (an "extreme" Kerr hole), but it cannot be greater than unity. In the mathematics of general relativity, exceeding this limit replaces the event horizon with an inner boundary on the spacetime where tidal forces become infinite. Because this singularity is "visible" to observers, rather than hidden behind a horizon, as in a black hole, it is called a naked singularity. Toy models and heuristic arguments suggest that as a/M approaches unity it becomes more and more difficult to add angular momentum. The conjecture that such mechanisms will always keep a/M below unity is called cosmic censorship.
The inclusion of angular momentum changes details of the description of the horizon, so that, for example, the horizon area becomes Horizon area= 4πG2/c4[{M+(M²-a²)1/2}²+a²]
This modification of the Schwarzschild (a=0) result is not significant until a/M becomes very close to unity. For this reason, good estimates can be made in many astrophysical scenarios with a ignored.
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 (the ideal gas law explains the connection between pressure, 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 - 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.
Only the largest remnants, those exceeding a particular limit (the Tolman-Oppenheimer-Volkoff limit, not to be confused with the Chandrasekhar limit), generate enough pressure to produce black holes, because black holes are the most radically transformed state of matter known to physics, and the force which resists this level of compression, neutron degeneracy pressure, is extremely strong. But any remnant this size will never be able to stop collapsing, and when its outer radius falls below its Schwarzschild radius, the transition to black hole is complete.
The collapse process for stars producing remnants this size releases energy which usually produces a supernova, blowing the star\'s outer layers into space so that they form a spectacular nebula (this sort of nebula is called a supernova remnant). But the supernova is a side-effect and does not directly contribute to producing the black hole (or other type of compact star). For example a few gamma ray bursts were expected to be followed by evidence of supernovae but this evidence did not appear.Fynbo et al. (2006). "No supernovae associated with two long-duration gamma ray bursts". Nature 444: 1047-1049. Astronomy.com - New type of cosmic explosion found One possible explanation is that some very large stars can form black holes fast enough to swallow the supernova blast wave before it can reach the surface of the star.
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.
Larger black holes evaporate. If the initial mass of the hole was stellar mass, the time required for it to lose most of its mass via Hawking evaporation is much longer than the age of the universe, so small black holes are not expected to have formed by this method yet.
The Big Bang produced sufficient pressure to form smaller black holes without the need for anything resembling a star. None of these hypothesized primordial black holes have been detected.
In principle, a sufficiently energetic collision within a very powerful Particle accelerator could produce a micro black hole. In practice, this is expected to require energies comparable to the Planck energy, which is vastly beyond the capability of any present, planned, or expected future particle accelerator to produce. Some speculative models allow the formation of black holes at much lower energies. This would allow production of extremely short-lived black holes in terrestrial particle accelerators. No evidence of this type of black hole production has been presented as of 2007.
See Micro black hole escaping from a particle accelerator
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 Fluctuation (thermodynamics). 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 starsBloom, J.S., Kulkarni, S. R., & Djorgovski, S. G. (2002). "The Observed Offset Distribution of Gamma-Ray Bursts from Their Host Galaxies: A Robust Clue to the Nature of the Progenitors". Astronomical Journal 123: 1111-1148. or by collisions between neutron stars,Blinnikov, S., et al. (1984). "Exploding Neutron Stars in Close Binaries". Soviet Astronomy Letters 10: 177. 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,Lattimer, J. M. and Schramm, D. N. (1976). "The tidal disruption of neutron stars by black holes in close binaries". Astrophysical Journal 210: 549. 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 awayPaczynski, B. (1995). "How Far Away Are Gamma-Ray Bursters?". Publications of the Astronomical Society of the Pacific 107: 1167. 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.Winter, L.M., Mushotzky, R.F. and Reynolds, C.S. (2005, revised 2006). "XMM-Newton Archival Study of the ULX Population in Nearby Galaxies". Astrophysical Journal 649: 730.
Quasars are thought to be the accretion disks of supermassive black holes, since no other known object is powerful enough to produce such strong emissions. Quasars produce strong emission across the electromagnetic spectrum, including UV, X-rays and gamma-rays and are visible at tremendous distances due to their high luminosity. Between 5 and 25% of quasars are "radio loud," so called because of their powerful radio emission.Jiang, L., Fan, X., Ivezić, Ž., Richards, G.~T., Schneider, D.~P., Strauss, M.~A., Kelly, B.~C. (2007). "The Radio-Loud Fraction of Quasars is a Strong Function of Redshift and Optical Luminosity". Astrophysical Journal 656: 680-690.
Gravitational lensing distorts the image around a black hole in front of the Large Magellanic Cloud (simulated view)
A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is "bent" around a massive object (such as a black hole) between the source object and the observer. The process is known as gravitational lensing, and is one of the predictions of Albert Einstein\'s general theory of relativity. According to this theory, mass "warps" space-time to create gravitational fields and therefore bend light as a result.
A source image behind the lens may appear as multiple images to the observer. In cases where the source, massive lensing object, and the observer lie in a straight line, the source will appear as a ring behind the massive object.
Gravitational lensing can be caused by objects other than black holes, because any very strong gravitational field will bend light rays. Some of these multiple-image effects are probably produced by distant galaxies.
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 closed 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 detected directly, many observational studies have provided substantial evidence for black holes. Black holes may be divided into three classes of objects:
Further details are given below.
The jet originating from the center of M87 in this image comes from an active galactic nucleus that may contain a supermassive black hole. Credit: Hubble Space Telescope/NASA/ESA.
According to the American Astronomical Society, every large galaxy has a supermassive black hole at its center. The black hole’s mass is proportional to the mass of the host galaxy, suggesting that the two are linked very closely. The Hubble and ground-based telescopes in Hawaii were used in a large survey of galaxies.
For decades, astronomers have used the term "active galaxy" to describe galaxies with unusual characteristics, such as unusual spectral line emission and very strong radio emission.J. H. Krolik (1999). Active Galactic Nuclei. Princeton, New Jersey: Princeton University Press. ISBN 0-691-01151-6. L. S. Sparke, J. S. Gallagher III (2000). Galaxies in the Universe: An Introduction. Cambridge: Cambridge University Press. ISBN 0-521-59704-4. However, theoretical and observational studies have shown that the active galactic nuclei (AGN) in these galaxies may contain supermassive black holes. The models of these AGN consist of a central black hole that may be millions or billions of times more massive than the Sun; a disk of gas and dust called an accretion disk; and two jets that are perpendicular to the accretion disk.
Although supermassive black holes are expected to be found in most AGN, only some galaxies\' nuclei have been more carefully studied in attempts to both identify and measure the actual masses of the central supermassive black hole candidates. Some of the most notable galaxies with supermassive black hole candidates include the Andromeda Galaxy, M32, M87, NGC 3115, NGC 3377, NGC 4258, and the Sombrero Galaxy.
Astronomers are confident that our own Milky Way galaxy has a supermassive black hole at its center, in a region called Sagittarius A*:
In 2002, the Hubble Space Telescope produced observations indicating that globular clusters named M15 and G1 may contain intermediate-mass black holes. (December 2002) "Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15. II. Kinematic Analysis and Dynamical Modeling". The Astronomical Journal 124 (6): 3270-3288. Retrieved on 2007-10-31. Hubble Discovers Black Holes in Unexpected Places (September 17, 2002). Retrieved on 2007-10-31. This interpretation is based on the sizes and periods of the orbits of the stars in the globular clusters. But the Hubble evidence is not conclusive, since a group of neutron stars could cause similar observations. Until recent discoveries, many astronomers thought that the complex gravitational interactions in globular clusters would eject newly-formed black holes.
In November 2004 a team of astronomers reported the discovery of the first well-confirmed intermediate-mass black hole in our Galaxy, orbiting three light-years from Sagittarius A*. This black hole of 1,300 solar masses is within a cluster of seven stars, possibly the remnant of a massive star cluster that has been stripped down by the Galactic Centre.Second black hole found at the centre of our Galaxy. News@Nature.com. Retrieved on 2006-03-25.The nature of the Galactic Center source IRS 13 revealed by high spatial resolution in the infrared. Retrieved on 2007-01-07. This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars.
In January 2007, researchers at the University of Southampton in the United Kingdom reported finding a black hole, possibly of about 400 solar masses, in a globular cluster associated with a galaxy named NGC 4472, some 55 million light-years away.Black hole found in ancient lair. Retrieved on 2007-01-07.
Artist\'s impression of a binary system consisting of a black hole and a main sequence star. The black hole is drawing matter from the main sequence star via an accretion disk around it, and some of this matter forms a gas jet.
Our Milky Way galaxy contains several probable stellar-mass black holes which are closer to us than the supermassive black hole in the Sagittarius A* region. These candidates are all members of X-ray binary systems in which the denser object draws matter from its partner via an accretion disk. The probable black holes in these pairs range from three to more than a dozen solar masses.J. Casares: Observational evidence for stellar mass black holes. PreprintM.R. Garcia et al.: Resolved Jets and Long Period Black Hole Novae. Preprint The most distant stellar-mass black hole ever observed is a member of a binary system located in the Messier 33 galaxy.Ker Than, SPACE.com: Monster black hole busts theory
In theory there is no smallest size for a black hole. Once created, it has the properties of a black hole. Stephen Hawking theorized that primordial black holes could evaporate and become even tinier, i.e. micro black holes. Searches for evaporating primordial black holes are proposed for the GLAST satellite to be launched in 2008. However, if micro black holes can be created by other means, such as by cosmic ray impacts or in colliders, that does not imply that they must evaporate.
The formation of black hole analogs on Earth in particle accelerators has been reported,Lab fireball \'may be black hole\'. BBC News (17 March 2005). Retrieved on 2006-03-25.. These black hole analogs are not the same as gravitational black holes, but they are vital testing grounds for quantum theories of gravity.
They act like black holes because of the correspondence between the theory of the strong nuclear force, which has nothing to do with gravity, and the quantum theory of gravity. They are similar because both are described by string theory. So the formation and disintegration of a fireball in quark gluon plasma can be interpreted in black hole language. The fireball at the Relativistic Heavy Ion Collider [RHIC] is a phenomenon which is closely analogous to a black hole, and many of its physical properties can be correctly predicted using this analogy. The fireball, however, is not a gravitational object. It is presently unknown whether the much more energetic Large Hadron Collider [LHC] would be capable of producing the speculative large extra dimension micro black hole, as many theorists have suggested.
The Newtonian conceptions of Michell and Laplace are often referred to as "dark stars" to distinguish them from the "black holes" of general relativity.
The concept of a body so massive that even light could not escape was put forward by the geologist John Michell in a letter written to Henry Cavendish in 1783 and published by the Royal Society.J. Michell, Phil. Trans. Roy. Soc., 74 (1784) 35-57.
| “ | 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. | ” |
This assumes that light is influenced by gravity in the same way as massive objects.
In 1796, the 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).
The idea of black holes was largely ignored in the nineteenth century, since light was then thought to be a massless wave and therefore not influenced by gravity. Unlike a modern black hole, the object behind the horizon is assumed to be stable against collapse.
In 1915, Albert Einstein developed the theory of gravity called general relativity, having earlier shown that gravity does influence light (although light has zero rest mass, it is not the rest mass that is the source of gravity but the energy). A few months later, Karl Schwarzschild gave the solution for the gravitational field of a point mass and a spherical mass,K. Schwarzschild, Sitzungsber.Preuss.Akad.Wiss.Berlin (Math.Phys.), (1916) 189-196K. Schwarzschild, Sitzungsber.Preuss.Akad.Wiss.Berlin (Math.Phys.), (1916) 424-434 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 Lorentz, independently gave the same solution for the point mass a few months after Schwarzschild and wrote more extensively about its properties.