Black holes and other clues to the quantum structure of gravity

TL;DR

This paper discusses the quantum structure of gravity through black holes, exploring how unitarity and information transfer might be preserved, and suggests potential observational tests for new interactions outside black hole horizons.

Contribution

It proposes an effective framework for understanding subsystems in quantum gravity and explores how new interactions could resolve the unitarity crisis in black hole evolution.

Findings

Perturbative gravity indicates a different structure from quantum field theory.

Black holes may behave like familiar subsystems requiring new entanglement transfer interactions.

Potential observational signatures in electromagnetic or gravitational waves could constrain these interactions.

Abstract

Bringing gravity into a quantum-mechanical framework is likely the most profound remaining problem in fundamental physics. The "unitarity crisis" for black hole evolution appears to be a key facet of this problem, whose resolution will provide important clues. Investigating this raises the important structural question of how to think about subsystems and localization of information in quantum gravity. Paralleling field theory, the answer to this is expected to be an important ingredient in the mathematical structure of the theory. Perturbative gravity results indicate a structure different from that of quantum field theory, but suggest an avenue to defining subsystems. If black holes do behave similarly to familiar subsystems, unitarity demands new interactions that transfer entanglement from them. Such interactions can be parameterized in an effective approach, without directly addressing the question of the fundamental dynamics, whether that is associated with quantum spacetime, wormholes, or something else. Since such interactions need to extend outside the horizon, that raises the question of whether they can be constrained, or might be observed, by new electromagnetic or gravitational wave observations of strong gravity regions. This note overviews and provides connections between these developments.