The effect of gravitational tidal forces on vacuum polarization: How to undress a photon

The effect of gravitational tidal forces on photon propagation in curved spacetime is investigated. It is found that the imaginary part of the local refractive index Imn(u;ω) may be negative as well as positive, corresponding to a local amplification as well as attenuation of the amplitude of the renormalized photon field. This is interpreted in terms of the effect of tidal forces on the virtual e+e− cloud surrounding the bare photon field-a positive/negative Imn(u;ω) corresponds to an increased dressing/undressing of the bare photon. Below threshold decays of the photon to e+e− pairs can occur. Photon undressing in the vicinity of a black hole singularity is described as an example. These results are shown to be consistent with unitarity and the optical theorem in curved spacetime, which is derived here both in a local form and integrated over the photon trajectory.