- #26

Dale

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A photon has "relativistic mass", but not "invariant mass". Most physicists don't use relativistic mass any more, but it is still found in a lot of textbooks and many historical papers, so it is good to know what it is.What kind of mass would we say a photon has. Aren't there like 3 different types of masses.

I was actually refering to the invariant mass of the system. This can be much higher than the rest masses of the individual particles that make up the system, actually that is the whole purpose of a particle collider. As an example, a 1 MeV photon moving to the right it has 0 invariant mass, and a 1 MeV photon moving to the left also has 0 invariant mass, but a system of a 1 MeV photon moving to the right and a 1 MeV photon moving to the left has an invariant mass of 2 MeV/c². The invariant mass is given by m²c² = E²/c² - p². For a system with no overall momentum this reduces to the familiar E = mc².ok , I was just reading Dalespams post and he said the mass would be unchanged but we could transform energy into matter.