Do photons have mass?

2 hours ago, King E said:

How do we know that?

While we don't know it to 100% absolute certainty, ( experiments have only been able to set an upper limit for photon mass), It is consistent with our understanding of the universe.

Relativity, which has passed every test thrown at it so far, Says that anything that has zero rest mass must travel at c and only c (in a vacuum).  If something has even the slightest bit of rest mass, it can travel at any speed from 0 up to, but not including, c.

So, if a photon had any rest mass at all, we should be able to find at least some photons traveling at speeds well below c.  So far this has not been the case.

So both theory and observation seem to indicate that photons are indeed massless, and unless something crops up to cast doubt on this, it's the way to bet.