2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/207901Photons 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-Nordström spacetime characterising a charged black hole. A general analysis of electromagnetic as well as gravitational birefringence is presented.17pages, SWAT 93-9, UGVA-DPT 1993/08-830 (includes three macros undefined in originally submitted version)High Energy Physics - TheoryGeneral Relativity and Quantum Cosmologytext