  The effect of gravitational tidal forces on renormalized quantum fields
propagating in curved spacetime is investigated and a generalisation of the
optical theorem to curved spacetime is proved. In the case of QED, the
interaction of tidal forces with the vacuum polarization cloud of virtual e^+
e^- pairs dressing the renormalized photon has been shown to produce several
novel phenomena. In particular, the photon field amplitude can locally increase
as well as decrease, corresponding to a negative imaginary part of the
refractive index, in apparent violation of unitarity and the optical theorem.
Below threshold decays into e^+ e^- pairs may also occur. In this paper, these
issues are studied from the point of view of a non-equilibrium initial-value
problem, with the field evolution from an initial null surface being calculated
for physically distinct initial conditions and for both scalar field theories
and QED. It is shown how a generalised version of the optical theorem, valid in
curved spacetime, allows a local increase in amplitude while maintaining
consistency with unitarity. The picture emerges of the field being dressed and
undressed as it propagates through curved spacetime, with the local
gravitational tidal forces determining the degree of dressing and hence the
amplitude of the renormalized quantum field. These effects are illustrated with
many examples, including a description of the undressing of a photon in the
vicinity of a black hole singularity.
