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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.

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