A physical protocol for observers near the boundary to obtain bulk information in quantum gravity

TL;DR

This paper presents a protocol enabling boundary observers in AdS to reconstruct bulk information using gravitational back-reaction and entanglement, highlighting holography's boundary-bulk information link.

Contribution

It introduces a physical protocol for boundary observers to access bulk data in AdS, leveraging gravitational effects and entanglement, with implications for understanding holography.

Findings

Boundary observers can fully identify low-energy bulk states.

The protocol relies on gravitational back-reaction and entanglement.

It fails in non-gravitational theories, emphasizing gravity's role.

Abstract

We consider a set of observers who live near the boundary of global AdS, and are allowed to act only with simple low-energy unitaries and make measurements in a small interval of time. The observers are not allowed to leave the near-boundary region. We describe a physical protocol that nevertheless allows these observers to obtain detailed information about the bulk state. This protocol utilizes the leading gravitational back-reaction of a bulk excitation on the metric, and also relies on the entanglement-structure of the vacuum. For low-energy states, we show how the near-boundary observers can use this protocol to completely identify the bulk state. We explain why the protocol fails completely in theories without gravity, including non-gravitational gauge theories. This provides perturbative evidence for the claim that one of the signatures of holography -- the fact that information about the bulk is also available near the boundary -- is already visible in the low-energy theory of gravity.