No, it is not the ether theory of light, and that is a magnificent question: This idea was settled by Mickelson/Morley, who not only measured the round trip in

No, it is not the ether theory of light, and that is a magnificent question: This idea was settled by Mickelson/Morley, who not only measured the round trip in one direction, but measured it in several directions, and found the speed of light to be invariant. For which Albert Einstein received the Nobel Prize for his theory of light. Later, Richard Feynman used first principals to both confirm this for Enistienien physics, and break it for quantum physics. The best reference for this work is not the classic experiment, but on Henry Cavindish's balance, which led to the calculation of G, the gravitation constant to 7 digits of accuracy, based upon the speed of light calculated to 9+ digits of accuracy. The speed of light is invariant: What you the observer actually see, is a frame of reference in space-time, which transforms the space, so that light still travels as fast as it always does, but the space around it is transformed. There have been a few theories of exceptional note: Sir Fred Hoyle solved Einsteins equations for an invariant size of the universe based on a shrinking frame of reference, and found no contradictions. Hence the wimper theory of cosmogony. I count myself as pretty bright, on this subject, able to argue the point rather succinctly, but I never claim to hold a candle and a mirror ( Cavendish ) to Henry Cavendish, nor Sir Fredric Hoyle: You want to get the real brilliance of this total failure: "The Michelson-Morley Experiment (MMX) tried to prove the existence of ether, but they did not observe the movement of interference fringes, which led to the assumption that the speed of light is constant in the inertial reference frame, which is also the theoretical basis of Einstein's special relativity (SR)." It failed to prove the existence of ether. Failed. Richard Feynman also found that for Eisensteinian physics, this was also true from first principals. This is really one of the most brilliant failures in the history of Physics. “Success is the ability to go from failure to failure without losing your enthusiasm” ― Winston Churchill