I had this questioning about the light clock on another thread, and DaleSpam suggested that I open my own thread, so here it is. I was asking if the light clock mind experiment was not contradicting the reference frame principle. I added that to my drawings: And DaleSpam answered this: So here is my answer Dale: It seems to me that the relativity principle would prevent an observer on one of the two mirrors from being able to measure that longer echo time because the clock used to measure it would also be slowed by the motion, and in the same proportion as the echo time. So how would it be measured? Another question: for the same observer to measure a longer echo time, the light ray has to travel at an angle to the motion, in such a way that if it was a laser beam, it would have to be aimed at the future position of the mirror, which seems to contradict the reference frame principle, because by definition, a reference frame is considered at rest.