Information and Coarse-Graining in Eternal Black Holes

TL;DR

This paper clarifies how coarse-graining in classical gravity within AdS/CFT leads to information loss in bulk fluctuations, despite the overall unitarity of the total quantum system.

Contribution

It elucidates the nature of coarse-graining in gravity and field theory, showing how information is transferred to nonclassical degrees, making recovery nearly impossible.

Findings

Bulk fluctuations transfer information to nonclassical degrees

Coarse-graining causes effective information loss

Total system remains unitarily evolving with preserved information

Abstract

Recently it was shown in the context of the AdS/CFT correspondence that the classical gravity description inevitably involves coarse-graining of some degrees. In this note we clarify the nature of this coarse-graining both in gravity and field theory sides. We show that the information carried by the bulk classical gravity fluctuation is transferred to the coarse-grained nonclassical degrees through tiny interactions, which makes the recovery of information almost impossible. Of course, the total system is evolving unitarily fully preserving its information as dictated by the basic construct of the AdS/CFT correspondence.