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 Im n(u; omega) 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 Im n(u; omega) 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. (C) 2010 Elsevier B.V. All rights reserved.