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is necessary to sustain this distance constantly. "Step by Step into a Black Hole" ]] A black hole is a region of space whose Gravitational Field is so powerful that nothing can escape it once it has fallen past a certain point, called the Event Horizon . The name comes from the fact that even Electromagnetic Radiation (i.e. 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.123 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 inwardly. 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 slowly leak a form of thermal energy called Hawking Radiation .456 However, the final, correct description of black holes, requiring a theory of Quantum Gravity , is unknown. SIZES OF BLACK HOLES 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:
A black hole is defined by the velocity that would have to be attained to escape from its gravitational pull, which is termed the Escape Velocity . Within some distance from a black hole, this velocity would be greater than the Speed Of Light - in other words infinite energy would be required to accelerate away from the black hole. For example, the escape velocity of the Earth at the surface is equal to 11 km/s. For an object to escape the Earth's gravitational pull at the surface ''without applying additional energy'' (i.e. unpowered), ignoring the effects of Drag , it must go at least 11 km/s, regardless of its mass or density. On the other hand, the escape velocity at the surface of a gravitational body is related to its density - the ratio of its mass to radius - since the velocity required diminishes as one moves away from the center of mass. It is theoretically possible for objects with very small masses to be so dense that light couldn't escape, within a correspondingly small radius, however black holes are usually postulated as objects on the scale of the mass of stars or much greater. WHAT MAKES IT IMPOSSIBLE TO ESCAPE FROM BLACK HOLES? 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.7 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 DO BLACK HOLES HAVE "NO HAIR"? See Also: No hair theorem The 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 . TYPES OF BLACK HOLES 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 : (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. MAJOR FEATURES OF NON-ROTATING, UNCHARGED BLACK HOLES Event horizon This is the boundary of the region from which not even light can escape. 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) Singularity at a single point 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 photon sphere A non-rotating black hole's Photon Sphere is a spherical boundary of zero thickness such that photons moving along Tangent s 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. 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.8 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. Accretion disk Space is not a pure Vacuum - even interstellar space contains a few atoms of hydrogen per cubic centimeter.9 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 - 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. MAJOR FEATURES OF ROTATING BLACK HOLES See Also: Rotating black hole . 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 - 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. Two event horizons 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. Two photon spheres 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. Ergosphere 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. Ring-shaped singularity 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. Possibility of escaping from a rotating black hole s of various Schwarzschild solutions. Time is the vertical dimension, space is horizontal, and light travels at 45° angles. Paths less than 45° to the horizontal are forbidden by special relativity, but rotating black holes allow for travel to future "universes"]] 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 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 . WHAT HAPPENS WHEN SOMETHING FALLS INTO A BLACK HOLE? 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 ). Spaghettification 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; 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 Horizon s, whereas objects falling into large, Supermassive Black Hole s may not be deformed or otherwise feel excessively large forces before passing the event horizon. Before the falling object crosses 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 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. As the object passes through the event horizon From the viewpoint of the falling object, nothing particularly special happens at the event horizon (apart from spaghettification due to Tidal Force s, 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 Photon s 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. Inside the event 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.
Hitting the singularity As an infalling object approaches the singularity, Tidal Force s acting on it approach infinity. All components of the object, including Atom s 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 to reflect the addition of the new object's matter. After the event horizon all is unknown. Anything that passes this point cannot be retrieved to study. Many people believe that the matter is extremely compacted. Stephen Hawking made a theory that the matter disappeared into the universe, defying the laws of physics. He later revised this theory to say that the disappearing matter was compensated by Parallel Universes without black holes, saying, in the end, the matter was not lost. FORMATION AND EVAPORATION Formation of stellar-mass black holes Stellar-mass Black Hole s 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).
Formation of larger black holes There are two main ways in which black holes of larger than stellar mass can be formed:
Formation of smaller black holes No known process currently active in the universe can form black holes of less than stellar mass. This is because all present 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 Star s. There are still a few ways in which smaller black holes might be formed, or might have formed in the past: Evaporation 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. TECHNIQUES FOR FINDING BLACK HOLES Accretion disks and gas jets ]] Most Accretion Disk s and Gas Jets are not clear proof that a Stellar-mass Black Hole is present, because other massive, ultra-dense objects such as Neutron Star s and White Dwarf s 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 Hole s, because as far as we know any mass large enough to power these phenomena must be a black hole. Strong radiation emissions 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 Nebula e and gas clouds. But strong, irregular emissions of X-ray s, Gamma Ray s 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.
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http://adsabsharvardedu/cgi-bin/nph-data_querybibcode=1984SvAL10177B&db_key=AST&link_type=ABSTRACT&high=4322390bbe18728 |
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author |
Blinnikov, S, ''et al'' |
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date |
1984 |
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title |
Exploding Neutron Stars in Close Binaries |
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journal |
Soviet Astronomy Letters |
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volume |
10 |
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pages |
177 |
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