  Light in a dielectric medium moves slower than in vacuum. The corresponding
electromagnetic field equations are then no longer invariant under ordinary
Lorentz transformations, but only under such transformations corresponding to
this reduced velocity. Based on this physical symmetry, an effective theory for
low-energy electromagnetic phenomena in dielectrics is constructed. It has none
of the problems of the old formulations of Minkowski and Abraham. Dispersion in
the optical regime and the Kerr effect arise in a natural way from higher order
interaction terms. The effective field theory is quantized by standard methods
and quantum corrections can be calculated in a systematic way. Thus it relates
many different classical and quantum optical phenomena into a unified
description.
