The Library of Babel

TL;DR

This paper demonstrates that heavy pure states in gravity, such as certain black holes, can appear mixed to most probes, with microstates represented by a spacetime foam invisible to typical measurements.

Contribution

It introduces an information-theoretic approach to understanding how heavy pure states appear mixed in gravity, providing exact descriptions for some supersymmetric states.

Findings

Heavy pure states appear mixed to almost all probes.

Microstates are described by a spacetime foam invisible to most measurements.

Results are exact for certain supersymmetric states.

Abstract

We show that heavy pure states of gravity can appear to be mixed states to almost all probes. Our arguments are made for $\rm{AdS}_5$ Schwarzschild black holes using the field theory dual to string theory in such spacetimes. Our results follow from applying information theoretic notions to field theory operators capable of describing very heavy states in gravity. For certain supersymmetric states of the theory, our account is exact: the microstates are described in gravity by a spacetime ``foam'', the precise details of which are invisible to almost all probes.