(To rephrase your answer to see if I understand) so if we considered it an isolated system, it is because of extra energy inserted into the system that creates the difference in the speed of the clocks? It's still hard for me to even say, "The clock is ticking slower,".... I understand that a photon (of a moving atomic clock with mirrors) has to cover more distance, but I imagine the photon to move faster rather than time moving slower (time... moving slower...? Relatively...?). But then, you can't travel faster than the speed of light so the photon moving faster isn't possible? x_X
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