>>10853992every test of the photon's mass has an upper limit that has been gradually decreasing
we need light to travel at c in order for relativity to work
since photons are what makes up light, we model them as massless particles in accordance with relativity and what we observe
if photons had a positive mass, then all it would mean is that they wouldn't travel at c, and the electromagnetic force would lose strength over large distances. maxwell's equations would need to be adjusted slightly. it wouldn't change the universal speed limit of c, though.
