Imagine that your home is broken into by a group of physicists with mischief on their minds. They grab your collection of books and CD’s, but instead of just making off with them, they crunch them together to make a black hole. (Applied physicists, obviously.) In the old days, you might have been quite despondent, thinking that all of the information in your collection of music and literature had been lost forever. However, as we all know by now, almost thirty years ago Hawking showed that black holes don’t just sit there, they emit radiation, and in the process of emission they lose energy and eventually evaporate away completely (if you don’t keep putting extra energy in). So now you might think that you could be very clever, and recover the information that you thought was lost: just observe absolutely every particle emitted in the Hawking radiation, and use your knowledge of what came out of the black hole to reconstruct what went in. In practice this would be a bit far-fetched, but in principle this is exactly what you could do if, for example, the pranksters had just set your collection on fire instead of collapsing it to a black hole. Physicists tend to believe that information is never really lost in physical processes, even if it gets re-arranged into less useful forms.
But black holes are different, sadly. To the best of our knowledge, there is no correlation between what went into making the black hole and what kind of radiation comes out. Indeed, there are reasonable-sounding arguments that there can’t be any such correlation. In that case, the information originally present in your books and CD’s has truly been lost.
Of course, we haven’t seen any black holes up close and evaporating, so these are all thought experiments thus far. But it bugs people to no end to think that evaporating black holes violate such a cherished notion of physics; this conundrum is known as the black hole information loss paradox. For a long time we didn’t even know whether the information could somehow be stored in a black hole, much less retrieved; more recently, however, string theorists have shown that (in many cases) that the amount of information in the black hole really is the same as contained in the stuff that went into making it. And there have been a couple of very clever proposals recently, one of which might turn out to be on the right track: Horowitz and Maldacena have suggested that the Hawking radiation that falls into the black hole is carefully arranged to cancel out the information of the infalling matter, effectively transferring it to the outgoing radiation; while Mathur has suggested that we need to dramatically change our ideas about what the interior of the black hole is like, enough so that the information is actually sitting close to the surface where perhaps it can escape more easily.
I bring this up because I have secret inside information which I now feel empowered to reveal, since the rumors seem to be public anyway. Stephen Hawking has asked to give a plenary talk at a big upcoming conference, where he says he will announce a solution to the information loss problem. As a member of the scientific organizing committee of GR17, I got the email request from our chief organizer; was there any chance we would say “no”? But actually I doubt very much that Hawking will simply announce a solution that everyone will agree with. Theoretical physics just doesn’t work that way. Even if he has a clever idea, people will have to wrestle with it themselves before it would catch on. Also, Hawking isn’t always right; for a long time he has been insisting that information really is lost, which is certainly a minority viewpoint. Unfortunately I won’t be at the conference, which is July 18-23 in Dublin. But I’m sure we’ll be seeing news reports about Hawking’s talk; the media love him, for good reason.