I'm not sure that i'm going to understand relativity someday. Well, I can comprehend that each frame of reference has its own time. But for me time is not something physical, I believe that time is simply the velocity with which physical phenomena occurs. If i am travelling with a clock, then the hands of the clock will rotate more slow than if I am at rest. Then I imagine two points A and B, and a beam of photons that made his way crossing first point A and then point B. I am at certain distance, for example point C. When a photon pass through point A, I click my chronometer, and stop the chronometer when the photon pass through point B. Perfect, 5 seconds for example. Now, imagine that I'm going to repeat the experiment. I'm in point C. But when the photon cross through point A,I click the chronometer and then I start a little trip, and return at point B just in the moment that the photon pass through point B, and I stop the chronometer. During the trip, the chronometer must have been going slowest than in rest, then now I've measured another time for the voyage of the photon from A to B I've measured 2 differents velocities for the light! I'm misconcepting something? I would appreciate any help.