It is said that fact is sometimes stranger than fiction, and nowhere is that more than in the case of black holes. Black holes are stranger than anything dreamed up by science-fiction writers, but they are firmly matters of science facts. The scientific community was slow to realize that massive stars could collapse in on themselves, under their own gravity, and to consider how the objects left behind would behave. Albert Einstein even wrote a paper in 1939 claiming that stars could not collapse under gravity because matter could not be compressed beyond a certain point. Many scientists shared Einstein's gut feeling. The principle exception was the American scientist John Wheeler, who in many ways is the hero of the black hole story. In his work in 1950s and 1960s, he emphasized that many stars would eventually collapse, and pointed out the problems that possibility posed for theoretical physics.He also foresaw many of the properties of the objects which collapsed stars become - that is, black holes.
During most of the life of a normal stars, over many of years, it will support itself against its own gravity by thermal pressure, caused by nuclear processes which convert hydrogen into helium.
Eventually, however, the star will exhaust its nuclear fuel. The stars will now contract. In some cases, it may be able to support itself as a 'white dwarf' stars. However, Subrahmanyan Chandrasekhar showed in 1930 that the maximum mass of a white dwarf stars is about 1.4 times of the sun. A similar maximum mass was calculated by Soviet physicist Lev Landau for a stars made entirely of neutrons. What, then, would be the fate of those countless stars with grater mass than a white dwarf or neutron star when they had exhausted their nuclear fuel? The problem was investigated by Robert Oppenheimer, of later atom bomb fame. In a couple of papers in 1939, with George Volkoff and Hartland Snyder, he showed that such a star could not be supported by outward pressure; and that, if you take pressure out of the calculation, a uniform spherically systematic symmetric star would contract to a single point of infinite density. Such a point is called a singularity.
All our theories of space are formulated on the assumption that space-time is smooth and nearly flat, so they break down at the singularity, where the curvature of space-time is infinite. In fact, the singularity marks the end of time itself. That is what Einstein found so objectionable. Then the Second World War intervened. Most of scientists, including Robert Oppenheimer, switched their attention to nuclear physics, and the issue of gravitational collapse was largely forgotten. Interest in the subject revived with the discovery of distant objects called quasars. They were bright, despite being very distant. Nuclear process could not account for their energy output, because they release only a tiny amount of their rest mass a pure energy. The only alternative was gravitational energy, released by gravitational collapse. Thus gravitational collapses of stars were rediscovered.
It was already clear that a uniform spherical stars would contract to a point of infinite density, a singularity. The Einstein equation don't work at singularity. This means that at this point of infinite density, one can't predict the feature, which in turn implies that something strange could happen whenever a stars collapsed. We wouldn't be affected by the breakdown of prediction if the singularities were naked, that is,if they were not shielded from the outside.
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