  Gravitational tidal forces acting on the virtual e+ e- cloud surrounding a
photon endow spacetime with a non-trivial refractive index. This has remarkable
properties unique to gravitational theories including superluminal
low-frequency propagation, in apparent violation of causality, and
amplification of the renormalized photon field, in apparent violation of
unitarity. Using the geometry of null congruences and the Penrose limit, we
illustrate these phenomena and their resolution by tracing the history of a
photon as it falls into the near-singularity region of a black hole.
