well if an photon and electron are interchangeable and 1 photon can turn into 1 electron and vice versa, as quoted by quantum mechanics, then if you double the mass of an electron you increase its mass by the mass of an electron and then the same goes for the photon it increases mass by the mass of an electron and hence the rest mass orginially at 0 would be come the mass of an electron. Well that s a just a guessing idea im not saying its right atall.