  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.
