In particular, the discovery opened up a profound paradox that aims at the heart of why general relativity and quantum mechanics are so hard to reconcile. According to relativity theory, information about what falls into a black hole is forever lost. If the hole evaporates, however, what happens to the information contained within?
Hawking suggested that black holes completely evaporate, destroying the information--in contradiction with the tenets of quantum mechanics. Destruction of information conflicts with the law of energy conservation, making this scenario implausible.
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