  In quantum theory, the curved spacetime of Einstein's general theory of
relativity acts as a dispersive optical medium for the propagation of light.
Gravitational rainbows and birefringence replace the classical picture of light
rays mapping out the null geodesics of curved spacetime. Even more remarkably,
{\it superluminal} propagation becomes a real possibility, raising the question
of whether it is possible to send signals into the past. In this article, we
review recent developments in the quantum theory of light propagation in
general relativity and discuss whether superluminal light is compatible with
causality.
