Light Propagation in Non-Trivial QED Vacua

Within the geometric optics approximation, we derive the light cone condition for a class of homogeneous non-trivial QED vacua using the effective action approach. Our result generalizes the ``unified formula'' suggested by Latorre, Pascual and Tarrach which turns out to be a low-energy limit. We calculate the velocity shifts induced by electromagnetic fields and finite temperature and especially analyze the high-energy limit consistent with the loop approximation.