A black hole is an object with a gravitational field so powerful that a region of space becomes cut off from the rest of the universe – no matter or radiation, including visible light that has entered the region can ever escape. The lack of escaping electromagnetic radiation renders the inside of black holes (beyond the event horizon) invisible, hence the name. However, black holes can be detectable if they interact with matter, e.g. by sucking in gas from an orbiting star. The gas spirals inward, heating up to very high temperatures and emitting large amounts of light, X-rays and Gamma rays in the process while still outside of the event horizon.Black holes are also thought to emit a weak form of thermal energy called Hawking radiation.
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. When an object is compressed enough for this to occur, collapse is unavoidable (it would take infinite strength to resist collapsing into a black hole). When an object passes within the event horizon at the boundary of the black hole, it is lost forever (it would take an infinite amount of effort for an object to climb out from inside the hole). Although the object would be reduced to a singularity, the information it carries is not lost.
While general relativity describes a black hole as a region of empty space with a point like 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. 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:
* Supermassive black holes that contain millions to billions of times the mass of the sun are believed to exist in the center of most galaxies, including our own Milky Way.
* Intermediate-mass black holes, whose size is measured in thousands of solar masses, may exist. Intermediate-mass black holes have been proposed as a possible power source for ultra-luminous X ray sources.
* Stellar-mass black holes have masses ranging from about 1.5-3.0 solar masses (the Tolman-Oppenheimer-Volkoff limit) to 15 solar masses. These black holes are created by the collapse of individual stars. Stars above about 20 solar masses may collapse to form black holes; the cores of lighter stars form neutron stars or white dwarf stars. In all cases some of the star's material is lost (blown away during the red giant stage for stars that turn into white dwarfs, or lost in a supernova explosion for stars that turn into neutron stars or black holes).
* Micro black holes, which have masses at which the effects of quantum mechanics are expected to become very important. This is usually assumed to be near the Planck mass. Alternatively, the term micro black hole or mini black hole may refer to any black hole with mass much less than that of a star. Black holes of this type have been proposed to have formed during the Big Bang (primordial black holes), but no such holes have been detected as of 2007.
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:
* The Big Bang, which produced pressures far larger than that of a supernova and therefore sufficient to produce primordial black holes without needing the powerful gravity fields of collapsing large stars.
* High-energy particle accelerators such as the Large Hadron Collider (LHC), if certain non-standard assumptions are correct (typically, an assumption of large extra dimensions). However, any black holes produced in such a manner will evaporate practically instantaneously, thus posing no danger to Earth.
A black hole is defined by the escape velocity that would have to be attained to escape from the gravitational pull exerted upon an object. For example, the escape velocity of earth is equal to 11km/s. Anything that wants to escape earth's gravitational pull must go at least 11 km/s, no matter what the thing is - a rocket ship or a baseball. The escape velocity of an object depends on how compact it is; that is, the ratio of its mass to radius. A black hole is an object so compact that, within a certain distance of it, even the speed of light is not fast enough to escape.
What makes it impossible to escape from black holes?
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.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.
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 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. 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.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. 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  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 - 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: - on the outside, an oblate spheroid which coincides with the event horizon at the poles and is noticeably wider around the "equator". This boundary is sometimes called the "ergosurface", but it is just a boundary and has no more solidity than the event horizon. At points exactly on the ergosurface, space-time is dragged around at the speed of light.
- on the inside, the outer event horizon.
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. |
