Thursday, June 21, 2007


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 (see the black hole information paradox).

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. The final, correct description of black holes, requiring a theory of quantum gravity, is unknown.

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