  Hawking's 1974 calculation of thermal emission from a classical black hole
led to his 1976 proposal that information may be lost from our universe as a
pure quantum state collapses gravitationally into a black hole, which then
evaporates completely into a mixed state of thermal radiation. Another
possibility is that the information is not lost, but is stored in a remnant of
the evaporating black hole. A third idea is that the information comes out in
nonthermal correlations within the Hawking radiation, which would be expected
to occur at too slow a rate, or be too spread out, to be revealed by any
nonperturbative calculation.
