Unitarity From a Smooth Horizon?

TL;DR

This paper explores the compatibility of unitarity with smooth horizons in black holes, proposing that nonlocal effects or ensemble interpretations may resolve apparent conflicts in semiclassical gravity and holography.

Contribution

It introduces a novel bulk detector setup to analyze black hole information and examines the implications for the Ryu-Takayanagi prescription and unitarity.

Findings

Bulk detectors can respond as if radiation is pure despite semiclassical predictions.

Resolving unitarity may require nonlocal effects or a firewall at the horizon.

Ensemble interpretations can reconcile boundary entropy with bulk unitarity.

Abstract

Under semiclassical evolution, black holes retain a smooth horizon but fail to return information. Yet, the Ryu-Takayanagi prescription computes the boundary entropy expected from unitary CFT evolution. We demonstrate this in a novel setting with an asymptotic bulk detector, eliminating an assumption about the entanglement wedge of auxiliary systems. We consider three interpretations of this result. (i) At face value, information is lost in the bulk but not in the CFT. This conflicts with the AdS/CFT dictionary. (ii) No unique QFT state (pure or mixed) governs all detector responses to the bulk Hawking radiation. This conflicts with the existence of an S-matrix. (iii) Nonlocal couplings to the black hole interior cause asymptotic detectors to respond as though the radiation was pure, even though it is naively thermal. This invalidates the standard interpretation of the semiclassical state, including its smoothness at the horizon. We conclude that unitary boundary evolution requires asymptotic bulk detectors to become unambiguously pure at late times. We ask whether the RT prescription can still reproduce the boundary entropy in this bulk scenario. We find that this requires a substantial failure of semiclassical gravity in a low-curvature region, such as a firewall that purifies the Hawking radiation. Finally, we allow that the dual to semiclassical gravity may be an ensemble of unitary theories. This appears to relax the tensions we found: the ensemble average of out-states would be mixed, but the ensemble average of final entropies would vanish.