In the 1700's, the French mathematician Laplace proposed that if the gravitational field of a body was sufficiently intense, even light might somehow be prevented from escaping from it and it would appear dark. It wasn't until Albert Einstein developed the theory of general relativity in 1915, and physicists began to study his equations in detail that Laplace's speculations began to take shape in certain specific mathematical 'solutions' to Einstein's equations for gravity.

A black hole is a region of space-time into which matter has collapsed, and out of which light may not escape. Unlike the prediction by Laplace using ordinary Newtonian physics, this escape horizon has a very sharp boundary in space called the Event Horizon. There is a precise mathematical prediction of the radius of this horizon which for objects that do not rotate, depends only on the mass of the body that has fallen through its event horizon. It is given by the formula

                        2 G M
             R    =    --------
                          2
                         c

where c = 300,000 kilometers/sec and G is the constant of gravity. For a body with the mass of the sun (2 x 10^30 kilograms) the horizon radius is 2.7 kilometers. For a body the mass of a small mountain, the black hole size is about that of a proton. The earth as a black hole would be as large as a ping pong ball.

Black holes that actually form in our universe from the collapse of very massive stars after supernova explosions, actually form objects that do not become true black holes until an infinite amount of time passes for those of us outside watching. This is because the mathematical 'black hole' solution only applies to the end state of such objects after they have settled down. Because of the gravitational time dilation effect, although someone riding on the surface of a collapsing body will judge this to take less than a second, we will see this process take thousands and even millions of years. The star's surface will actually 'wink out' very quickly and cause the object to become black, but the actual surface is still just outside the horizon as seen by us, and the last few centimeters of travel takes a very long time to finish as seen by us outside watching.

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