A Reverse Black Hole Information Problem

TL;DR

This paper explores the formation, detection, and coarse-graining of black holes in AdS/CFT, addressing the tension between unitarity and Hawking radiation, and proposing methods to distinguish black hole states and understand their semiclassical behavior.

Contribution

It introduces boundary probes to identify small AdS black holes and analyzes coarse-graining methods to reconcile semiclassical black hole behavior with unitary dynamics.

Findings

Boundary probes can distinguish small black holes from other states.

Coarse-graining procedures affect the purity of the CFT state.

Semiclassical behavior emerges from ensemble averaging of unitary dynamics.

Abstract

We study the formation, detection and coarse-graining of black holes in AdS/CFT, with an emphasis on the tension between boundary unitarity and the production of mixed state Hawking radiation in the bulk. We construct CFT states dual to black hole formation and evaporation by colliding bulk particle wavepackets at trans-Planckian energy. We propose boundary probes which are able to distinguish small AdS black holes from other states within the microcanonical ensemble. We investigate different coarse-graining prescriptions acting on the evolving CFT state, including averaging over CFT data, Hamiltonians and time windows, and compare their purities to those expected from the bulk semiclassical description. Our results clarify how semiclassical black hole behaviour can arise from an ensemble-averaging of the exact unitary dynamics, and take a step towards a better understanding of coarse-graining in the single-sided black hole information problem.