Definition
A black hole is a region of space from which nothing, not even light, can escape (according to the general theory of relativity).
Basics
A black hole is a celestial object of such extremely intense gravity that it attracts everything near it and prevents everything, including light, from escaping. The term was first used in reference to a star in the last phases of gravitational collapse.
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. In some cases, nothing remains to prevent the star from collapsing without limit to an indefinitely small size and infinitely large density, to create a black hole.
At this point the effects of Einstein's general theory of relativity become paramount. According to this theory, space becomes curved in the vicinity of matter (this is the meaning of gravity); the greater the concentration of matter, the greater the curvature (the greater the gravity). When the star 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 even not light.
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.
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 could 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, although none of the discoveries has been conclusive.
What is a black hole, really?
How does the gravity get out of the black hole?
What happens to you if you fall into a black hole?
If you go too fast do you become a black hole?
What is Hawking Radiation?
The black hole information loss problem
Is the Big Bang a black hole?
Topics of Interest
According to the general theory of relativity, a black hole is a region of space from which nothing, including light, can escape. It is the result of the deformation of spacetime caused by a very compact mass. Around a black hole there is an undetectable surface which marks the point of no return, called an event horizon. It is called "black" because it absorbs all the light that comes towards it, reflecting nothing, just like a perfect black body in thermodynamics. Under the theory of quantum mechanics black holes possess a temperature and emit Hawking radiation.
Despite its invisible interior, a black hole can be observed through its interaction with other matter. A black hole can be inferred by tracking the movement of a group of stars that orbit a region in space. Alternatively, when gas falls into a stellar black hole from a companion star, the gas spirals inward, heating to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and earth-orbiting telescopes.
The defining feature of a black hole is the appearance of an event horizon; a boundary in spacetime beyond which events cannot affect an outside observer. As predicted by general relativity, the presence of a mass deforms spacetime in such a way that the paths particles take tend towards the mass. At the event horizon of a black hole this deformation becomes so strong that there are no more paths that lead away from the black hole. Once a particle is inside the horizon, moving into the hole is as inevitable as moving forward in time (and can actually be thought of as equivalent to doing so).
A supermassive black hole is the largest type of black hole in a galaxy, on the order of hundreds of thousands to billions of solar masses. Most, if not all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.
In astrophysics, a white hole is the hypothetical time reversal of a black hole. While a black hole acts as an attractor, drawing in any matter that crosses the event horizon, a white hole acts as a source that ejects matter from its event horizon. The sign of the acceleration is invariant (unchanged) under time reversal, so both black and white holes attract matter. The only potential difference between them is in the behavior at the horizon.
The black hole information paradox results from the combination of quantum mechanics and general relativity. It suggests that physical information could "disappear" in a black hole, allowing many physical states to evolve into precisely the same state. This is a contentious subject since it violates a commonly assumed tenet of science—that in principle complete information about a physical system at one point in time should determine its state at any other time.
Micro black holes are tiny hypothetical black holes, also called quantum mechanical black holes or mini black holes, for which quantum mechanical effects play an important role.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)
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