I have been searching the net for measurements and how they are done of the speed of light. For some reason, I don't find much on them. I see sites promoting various theories whether sped up or down or is not constant. Claiming prehistorical differences in the speed of light is unverifiable I presume. I am looking for negative proofs of the variability of light speed in a vacuum, i.e. it is constant. SR states that speed of light is the same in all frames from all sources. Is that a definition or verified fact? If it is a definition (rather than fact) then of course the Lorentz transformations have to be made. If it is fact, the same is true. Have any measurements actually been done on the "instanteous" speed of light as opposed to simply dividing the distance by time to arrive at the speed? I see claims of faster than light group velocity. Is that really any different from simply picking an underlying lower amplitude signal out of a higher frequency signal superimposed upon it? Also, light is deflected by stars due to gravity "wells." Have there been exercises done that assign some mass to a photon that would cause the same deflection as in gtr and then prove it doesn't weigh that much? I see it argued that the law of causality requires light to be constant. I don't see that. "Causality" is broken all the time. Just go a baseball game. We see a ball being hit before we hear it. Of course there is an unlying principle of the speed of sound being less than that of light. Arguments are put out that variability of the speed of light could allow a situation where you could arrive before you left. Seeing something from a distant point after seeing the same object nearby proves nothing, since seeing is responding not to reality but light which travels at a finite speed. I see a site stating that the periods of Io's eclipses varies (from an Earth observation) according to whether or not we are approaching or receding from it, thus claiming the speed of light changes (as received, not emitted). Given that the distances are changing, the times should change as well, so does not prove anything since the math is missing.