Slide 21 of 22
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.