Singularities of thermal correlators at strong coupling

Understand

We analyze the singularities of the two-point function in a conformal field theory at finite temperature.

• In a free theory, the only singularity is along the boundary light cone.
• In the holographic limit, a new class of singularities emerges since two boundary points can be connected by a nontrivial null geodesic in the bulk, encircling the photon sphere of the black hole.
• We show that these new singularities are resolved by tidal effects due to the black hole curvature, by solving the string worldsheet theory in the Penrose limit.