The Library of Babel: On the origin of gravitational thermodynamics

TL;DR

This paper demonstrates how heavy pure states in gravity, especially black holes, appear mixed to most probes, and proposes a universal mechanism where effective thermodynamics emerges from singular spacetime geometries representing microstates.

Contribution

It introduces a framework using information theory to explain the thermodynamic behavior of gravity through microstates represented by spacetime foam and singular geometries.

Findings

Heavy pure states appear mixed to most probes.

Microstates of black holes are described by spacetime foam.

Effective thermodynamics arises from singular geometries like the superstar.

Abstract

We show that heavy pure states of gravity can appear to be mixed states to almost all probes. For AdS_5 Schwarzschild black holes, our arguments are made 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 half-BPS states of the theory which are incipient black holes, our account is exact: typical microstates are described in gravity by a spacetime ``foam'', the precise details of which are almost invisible to almost all probes. We show that universal low-energy effective description of a foam of given global charges is via certain singular spacetime geometries. When one of the specified charges is the number of D-branes, the effective singular geometry is the half-BPS ``superstar''. We propose this as the general mechanism by which the effective thermodynamic character of gravity emerges.