Bill Unruh's KITP seminar on information loss and energy non-conservation stimulates a vigorous discussion of one of the key aspects of the black hole information problem.
The question is whether pure states can evolve into mixed states (decohere) without energy loss. The answer is of course yes, but many (perhaps most) black hole researchers are under the impression that a famous paper by Banks, Peskin and Susskind showed that information loss due to black hole evaporation must lead to observable (even catastrophic) energy non-conservation. (Theorists who accept the BPS argument all believe that the information is somehow encoded in the outgoing Hawking radiation, which may indeed be true, but it doesn't follow from BPS!) Unruh and Wald refuted this claim (to my satisfaction) years ago, but the subject is still controversial. The video at the link is entertaining even if you're not an expert on this topic. Bill's counterexamples are quite elementary.
Unfortunately, I missed this talk as I was giving a colloquium at UC Irvine the same day. At lunch the day before I urged Bill to "lay down the law" ;-)
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