Because photons mediate all electromagnetic interactions, limits on the mass of the photon are extremely strin-gent. Even a tiny mass would produce easily measurable effects. Some of the best mass limits come from studies of Earth’s magnetic field, but there are many other independent measurements that are nearly as good. All of them indicate the photon’s mass must be less than about 10-50 kilograms. Compared to the limit on the neutrino mass, which is about an electron volt, or 10-36 kg, the photon’s mass is at least 14 orders of magnitude smaller.
The photon density left over from the Big Bang — the cosmic microwave background radiation — is comparable to the neutrino density (a few hundred per cubic centimeter). The contribution of neutrinos to the universe’s mass is at most about 1 percent. Because the photon is at least 1014 times lighter than the neutrino, photons cannot contribute significantly to the universe’s mass. — DAVID CASPER, UNIVERSITY OF CALIFORNIA, IRVINE