  We consider a modification of electrodynamics in which right- and
left-circularly polarized photons are coupled to charged sources differently.
Even though photon helicity is a Lorentz invariant quantity, such a
modification breaks Lorentz symmetry, as well as locality. The modified theory
includes novel magnetic forces between perpendicular currents. Existing data
can be used to constrain the modification at approximately a 2 x 10^(-3) level.
