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Interactions incorporating the vacuum polarization effects in curved
backgrounds modify the null cone structure in such a way that the photon
trajectories would not be the spacetime geodesics anymore. The gravitational
birefringence introduced as a direct consequence of these effects, will
allow shifts in the photon velocities leading to polarization dependent
superluminal propagation. Taking these effects into account we study Fermat's
principle in the context of the 1+3 (threading) formulation of the spacetime decomposition. Then, as an application of the above ideas, photon velocity shifts in the case of the NUT space as the background gravitational field are calculated.
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