Constraints on the Photon Charge from Observations of Extragalactic Sources

Having analyzed high-resolution observations of extragalactic compact radio sources with modern systems of radio telescopes, we obtained an estimate of the upper limit for the photon electric charge, e γ ≲ 3×10-33 of the elementary charge (assuming the photon charge to be energy independent). This is three orders of magnitude better than the limit obtained from radio pulsar timing. We also set a limit on the charge of a gamma-ray (energy ̃0.1 MeV) photon. In the future, the estimate based on extragalactic sources can be improved significantly.