But, I propose you to read my letter about Photons, Supernovae, Black Holes and Quasars, send to some news groups.
Single photon is massless.
Pair of photons, moving in one direction, is massless. Pair of photons, moving in opposite directions, has the rest mass. One cubic meter of photon gas at the temperature 1000000000 K, enclosed in some reservoir, has the rest mass 8400 kg. Free radiation, moving in one direction, at any temperature is massless. This is not my idea.