What is a Black Hole?

A black hole is a region of space in which the gravitational field is so powerful that nothing, including electromagnetic radiation (for example visible light), can escape its pull after having fallen past its event horizon. The term derives from the fact that absorption of visible light renders the hole's interior invisible, and indistinguishable from the black space around it.

Despite its invisible interior, a black hole may reveal its presence through interaction with matter orbiting the event horizon. For example, a black hole may be perceived by tracking the movement of a group of stars that orbit its center.

Alternatively, one may observe gas (from a nearby star, for instance) that has been drawn into the black hole. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and earth-orbiting telescopes. Such observations have resulted in the general scientific consensus that—barring a breakdown in our understanding of nature—black holes do exist in our universe.

The idea of an object with gravity strong enough to prevent light from escaping was proposed in 1783 by John Michell an amateur British astronomer. In 1795, Pierre-Simon Laplace, a French physicist independently came to the same conclusion. However, such "Newtonian black holes" are very different from black holes in general relativity. They prevent only light from escaping (not, for example, a rocket ship) and only in certain Newtonian models of light (such as an emission theory).

Black holes, as currently understood, are described by the general theory of relativity. This theory predicts that when a large enough amount of mass is present in a sufficiently small region of space, all paths through space are warped inwards towards the center of the volume, preventing all matter and radiation within it from escaping.

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. Research indicates that, rather than holding captured matter forever, black holes may slowly leak a form of thermal energy called Hawking radiation and may well have a finite life. The as yet unknown theory of quantum gravity is believed to give the fully correct description of black holes.

a black hole as it spins..

image of a black hole in the outerspace

a black hole quasar NASA