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Suppose I throw
a computer into a black hole. This computer's hard disc contains a great deal
of usefull (and useless) information. Once the computer falls below the
"point of no return", the information on this hard disc is lost for
ever to the outside world. This is not a problem since in principle, if I
wanted it badly enough, I can fall down the black hole after it, and retrieve
it. But now we know that black holes are not stable: they evaporate.
Moreover, this evaporation occurs due to microscopic processes just outside
the "horizon", and it cannot know about anything what has already
fallen through the horizon. Thus it cannot contain any information about what
is inside: the radiation that it emits carries no information. We call it
pure heat, or thermal radiation. The second picture indicates the black hole
after it has evaporated a little: the surrounding universe is a bit hotter,
and the black hole, which has lost energy, is correspondingly smaller. If we
follow this process to its logical conclusion, what we have at the end is no
black hole, only thermal radiation filling the universe. The information on
the hard disc has irrevocably disappeared along with the black hole, and
there is no way to retrieve it, even in principle. In physics, such
information loss is unacceptable: it means among other things that the future
cannot be predicted by knowing the past. There is no apparent correlation
between the thermal radiation that fills the Universe, and the state of the
Universe (i.e. the hard disc) before it was thrown into the black hole.
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