Quantum gravity on the bulk cone

Abstract

Bulk-cone singularities in holographic correlators encode propagation along bulk null geodesics. The prototypical example is an AdS black hole, where incoming null geodesics can wind around the photon sphere and return to the boundary. These singularities appear in real time in the thermal two-point function at finite volume. We show that conformal field theories dual to gravitational theories at finite $G_N$ cannot have such singularities. We rule out these singularities both for individual microstates and for the thermal state. The main thrust of the argument is to invoke an analytic continuation in the temperature. As a microscopic example, we construct an AdS$_3$ geometry with two colliding shocks which exhibits a bulk cone singularity. We show by an explicit calculation in the CFT that this singularity is smoothed out at finite $G_N$. Qualitatively, the singularity resolution is a consequence of the fuzziness of the bulk solution in quantum gravity.