Macroscopic superpositions and black hole unitarity
We discuss the black hole information problem, including the recent claim that unitarity requires a horizon firewall, emphasizing the role of decoherence and macroscopic superpositions. We consider the formation and evaporation of a large black hole as a quantum amplitude, and note that during intermediate stages (e.g., after the Page time), the amplitude is a superposition of macroscopically distinct (and decohered) spacetimes, with the black hole itself in different positions on different branches. Small but semiclassical observers (who are themselves part of the quantum amplitude) that fall into the hole on one branch will miss it entirely on other branches and instead reach future infinity. This observation can reconcile the subjective experience of an infalling observer with unitarity. We also discuss implications for the nice slice formulation of the information problem, and to complementarity.
Two good introductions to horizon firewalls and AMPS, by John Preskill and Joe Polchinski.
Earlier posts on this blog of related interest: here, here and here. From discussion at the third link (relevant, I claim, to AMPS):
Hawking claimed bh's could make a pure state evolve to a mixed state. But decoherence does this all the time, FAPP. To tell whether it is caused by the bh rather than decoherence, one needs to turn off (defeat) the latter. One has to go beyond FAPP!In the paper I cite a famous article by Bell, Against Measurement, which appeared in Physics World in 1990, and which emphasizes the distinction between actual pure to mixed state evolution, and its apparent, or FAPP, counterpart (caused by decoherence). This distinction is central to an understanding of quantum foundations. The article can be a bit hard to find so I am including the link above.
FAPP = Bell's term = "For All Practical Purposes"
Slides from an elementary lecture: black holes, entropy and information.