  Almheiri et al. have emphasized that otherwise reasonable beliefs about black
hole evaporation are incompatible with the monogamy of quantum entanglement, a
general property of quantum mechanics. We investigate the final-state
projection model of black hole evaporation proposed by Horowitz and Maldacena,
pointing out that this model admits cloning of quantum states and polygamous
entanglement, allowing unitarity of the evaporation process to be reconciled
with smoothness of the black hole event horizon. Though the model seems to
require carefully tuned dynamics to ensure exact unitarity of the black hole
S-matrix, for a generic final-state boundary condition the deviations from
unitarity are exponentially small in the black hole entropy; furthermore
observers inside black holes need not detect any deviations from standard
quantum mechanics. Though measurements performed inside old black holes could
potentially produce causality-violating phenomena, the computational complexity
of decoding the Hawking radiation may render the causality violation
unobservable. Final-state projection models illustrate how inviolable
principles of standard quantum mechanics might be circumvented in a theory of
quantum gravity.

High Energy Physics - Theory;Quantum Physics