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

• Main Author: Chandramouli Chowdhury, Olga Papadoulaki, and Suvrat Raju
• Format: Article
• Language: English
• Published: SciPost 2021-05-01
• Series: SciPost Physics
• Online Access: https://scipost.org/SciPostPhys.10.5.106

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.