FASTER THAN LIGHT PHOTONS AND CHARGED BLACK-HOLES

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. These results are illustrated for the propagation of photons in the Reissner-Nordstrom spacetime characterising a charged black hole. A general analysis of electromagnetic as well as gravitational birefringence is presented.