#Sidewalker

Got to time stamp 18:40 to begin --the gist of it--   to understand in clear, intuitive way, why photons do not have mass, yet they do have mass, if they travel even minutely less than the speed-of-radiation.

This Brilliant vid, explains the above in three, clearly and intuitive ways, mass { rest }, speed-of-radiation { c = motion }, speed of Causality { C }. If you have the time just go to the time stamp. We presume photons move at speed-of-Causality, because that is simpler, than the alternative reality that--- Einstein, Feynman and others believes exists---, as it is more complex, so why bother, if not necessary to deal with more complexity of space-time.