One of the great controversies of modern physics, Stephen Hawking’s ‘black hole paradox’, considers what happens to the information contained in anything dropped into a black hole. The matter disappears from our universe; the black hole increases in mass, but that tells us nothing about the nature of what has gone in; it, and the information it carries, appear to have disappeared from the universe. In a seminal paper in 1974 Hawking showed that black holes emit a form of radiation, which will eventually cause the black hole to dissipate, returning its mass/energy to the universe, But, according to Hawking, the nature of this radiation is the same regardless of what has gone into the black hole; the information is not returned to the universe. This goes against one of the fundamental tenets of physics, the law of the conservation of information.
Much effort has gone into solving this paradox, and, according to a recent article in New Scientist by Leah Crane, it is effectively solved (although, as usual, all the details have still to be worked out). Hawking was wrong, on this account, and information is conserved after all.
Hawking himself began to have second thoughts, in 2004 presenting calculations suggesting that information could re-emerge from a black hole, but in a mangled and unrecognisable form. Subsequent analyses have recognised that the paradox cannot be resolved by treating black holes, as Hawking initially did, in terms of classical physics, considering quantum effects only as subsidiary influences on the boundary of the black hole. To resolve the paradox, researchers have had to treat the whole black hole system, and indeed the universe with which it interacts, as quantum entities. And when this is done, it can be shown that quantum effects do allow information to emerge in a recognisable form. Accessing it, and reconstructing what went in, are not likely to be possible in practice, but in theory at least information is conserved even in this most extreme circumstance.
The Occasional Informationist 