  Photons propagating in curved spacetime may, depending on their direction and
polarisation, have velocities exceeding the ``speed of light'' c. This
phenomenon arises through vacuum polarisation in QED and is a tidal
gravitational effect depending on the local curvature. It implies that the
Principle of Equivalence does not hold for interacting quantum field theories
in curved spacetime and reflects a quantum violation of local Lorentz
invariance. These results are illustrated for the propagation of photons in the
Reissner-Nordstr\"om spacetime characterising a charged black hole. A general
analysis of electromagnetic as well as gravitational birefringence is
presented.
