Thought experiment: Our sun burns hydrogen at a prodigious rate, billions of kilograms per second. Let m = the mass of the sun at t=0. Inside the imaginary sphere expanding exactly with the velocity of light emanating from the sun, the total mass cannot change. After sixty seconds, the amount of mass lost by the sun is 60 x 6.3x10^11th kilograms, or 3.78 x 10^13th kilograms. This is the precise mass of the sunlight contained in the sphere surrounding the sun which is 60 light seconds larger than the sun, but excludes the sun itself. It is often claimed with a presumption of scientific authority that light has "no rest mass," but light is never "at rest." You may slow it down, but stopping light converts it into something that is no longer light. Anyone wish to claim that 3.78 x 10^13 kilograms of mass has vanished into photons, and is now weightless? I don't see this happening. This thought experiment is quite similar to the compelling argument that when you burn combustible materials inside a sealed chamber, the total mass of the chamber does not change, i.e. it doesn't become "lighter" because the wood, paper, or other material consumed has oxidized into ash.