Cite as: doi:10.26081/K6434N

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Black holes highlight a problem with our understanding of quantum
gravity: using a semiclassical approximation, they appear to have too
many internal states. New light has been shed recently by “replica
wormholes”, indicating that non-perturbative semiclassical effects can
resolve this problem. But this result is somewhat indirect, so the fate
of the extra apparent states remains mysterious. I’ll discuss replica
wormholes more directly from the perspective of the Hilbert space of
black holes, as a modification to the inner product. I give three
applications: (1) a simple qubit model of a black hole with unitary
evaporation, a Page curve and a smooth horizon, (2) a counting of black
hole states which makes the fate of the extra states manifest, and (3)
an interpretation of replica wormholes as arising from an extended
gauging of diffeomorphisms.
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