Preposterous Universe

Wednesday, July 21, 2004
Hawking speaks

As anticipated, Stephen Hawking gave his talk on black hole information loss at the GR17 conference in Dublin today; newspaper stories are already popping up, although they don't tell us much we didn't already know. I'll try to have some incisive commentary soon; in the meantime, why not just get right to the heart of the matter? Here are the press release and transcript for Hawking's talk. Judge for yourselves! At least, if you're up on the background reading about Euclidean quantum gravity and the AdS/CFT correspondence. Thanks to Dennis Overbye of the New York Times for forwarding the material.

Update: Peter Woit has a parsing of what Hawking is trying to say. I think the most direct paragraph is probably this one:
So in the end, everyone was right, in a way. Information is lost in topologically non trivial metrics, like the eternal black hole. On the other hand, information is preserved in topologically trivial metrics. The confusion and paradox arose because people thought classically, in terms of a single topology for spacetime. It was either R4, or a black hole. But the Feynman sum over histories, allows it to be both at once. One can not tell which topology contributed the observation, any more than one can tell which slit the electron went through, in the two slits experiment. All that observation at infinity can determine, is that there is a unitary mapping from initial states, to final, and that information is not lost.
The idea seems to be that, so far as information measured at infinity is concerned, when we integrate over all possible geometries the relevant ones are those that don't have black holes at all, merely apparent horizons. Some evidence for this point of view is adduced from AdS/CFT (the connection, first discovered by Juan Maldacena, between certain configurations of quantum gravity and certain field theories in one less spacetime dimension).

When we ultimately agree on the resolution of the information paradox, this idea may very well be part of the story. For the moment, it doesn't seem like a very practical suggestion, to say the least; it amounts to a promise that, if only we could actually do the path integral for Euclidean quantum gravity, all the information would be preserved and end up in the outgoing Hawking radiation. It remains the case that most people are quite skeptical that we will ever make sense of the Euclidean path integral, especially in the semi-classical regime Hawking makes use of. So one way or another, there's still a lot of work to be done!

Ideas on culture, science, politics.
Sean Carroll

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