Bound on the photon charge from the phase coherence of extragalactic radiation.

If the photon possessed a nonzero charge, then electromagnetic waves traveling along different paths would acquire Aharonov-Bohm phase differences. The fact that such an effect has not hindered interferometric astronomy places a bound on the photon charge estimated to be at the 10(-32)e level if all photons have the same charge and 10(-46)e if different photons can carry different charges.