I had a discussion with my physics teacher. He claimed that photons have mass (although no rest mass), since they have energy and E=mc². I argued that having energy isn't the same as having mass, but that energy could be converted into mass which is a different thing (e.g. a photon can disintegrate into a positron-electron pair which have mass, but that doesn't mean the photon itself had mass). My second argument against the claim that photons have mass, is that (if v=c): m = m0 / sqrt(1-c²/c²) = 0 / 0 which is undefined. Who is right?