"It has been a long-standing challege for to construct a theory of quantum gravity. String theory is the leading candidate for a quantum theory of gravity. General Relativity has the seeds of its own destruction in it, since smooth can evolve into singular field configurations ... Classically this is not a problem if the singularities are hidden behind s ... since this means that nothing can come out from the region containing the singularity. However, Hawking showed, under very general assumptions, that quantum mechanics implies that black holes emit particles ... In his approximation this radiation is exactly thermal and contains no information about the state of the black hole. This leads to the , since particles can fall in carrying information but what comes out is featureless thermal radiation ... Hawking argued that this would lead to non-unitary evolution, so that one of the basic principles of quantum mechanics would have to be modified."