  We calculate the rate of precession of the direction of polarization of a
photon traversing a sea of plane-polarized photons moving in the opposed
direction, where the interaction is the one-loop "vacuum" Heisenberg-Euler
coupling of four fields. Substantial precession can take place in a distance
many orders of magnitude shorter than the free path for photon-photon
scattering, mediated by the same interaction. We consider briefly the
possibility of some interesting collective effects in the case in which instead
of a particle and a sea, two seas are caused to collide.
