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 ReissnerNordström spacetime characterising a charged black hole. A general analysis of electromagnetic as well as gravitational birefringence is presented.SWAT 93/9

arXiv:gr-qc/9508048v1 22 Aug 1995 SWAT 95/71 Abstract The effective action for QED in curved spacetime includes equivalence principle violating interactions between the electromagnetic field and the spacetime curvature. These interactions admit the possibility of superluminal yet causal photon propagation in gravitational fields. In this paper, we extend our analysis of photon propagation in gravitational backgrounds to the Kerr spacetime describing a rotating black hole. The results support two general theorems -a polarisation sum rule and a 'horizon theorem'. The implications for the stationary limit surface bounding the ergosphere are also discussed.SWAT 95/71 CERN-TH/95-229 August 1995 * See ref. [12] for an extended version of this section.

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