Question

Some literature simply says that light has no mass, others say it has a very tiny bit of mass, others go as far as saying that light has exactly 0.0 mass.

So... What is the true statement?

Answer 1

When they talk about mass of a photon they are talking about relativistic mass. The concept of relativistic mass is not used now a days. A photon has zero and exactly zero mass. Does the (relativistic) mass change? Why? Why does an object with higher speed gain more (relativistic) mass? there are many experimental evidences. Experimental checks on photon mass

Current commonly accepted physical theories imply or assume the photon to be strictly massless, but this should be also checked experimentally. If the photon is not a strictly massless particle, it would not move at the exact speed of light in vacuum,c. Its speed would be lower and depend on its frequency. Relativity would be unaffected by this; the so-called speed of light,c, would then not be the actual speed at which light moves, but a constant of nature which is the maximum speed that any object could theoretically attain in space-time.Thus, it would still be the speed of space-time ripples ( gravitational wavesand gravitons), but it would not be the speed of photons.

A massive photon would have other effects as well. Coulomb's law would be modified and the electromagnetic field would have an extra physical degree of freedom. These effects yield more sensitive experimental probes of the photon mass than the frequency dependence of the speed of light. If Coulomb's law is not exactly valid, then that would cause the presence of an electric fieldinside a hollow conductor when it is subjected to an external electric field. This thus allows one to testCoulomb's law to very high precision. A null result of such an experiment has set a limit ofm? 10?14 eV/c2. Sharper upper limits have been obtained in experiments designed to detect effects caused by the galactic vector potential. Although the galactic vector potential is very large because the galactic magnetic field exists on very long length scales, only the magnetic field is observable if the photon is massless. In case of a massive photon, the mass termwould affect the galactic plasma. The fact that no such effects are seen implies an upper bound on the photon mass ofm<3