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## Main Question or Discussion Point

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.

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.

I was asking if the light clock mind experiment was not contradicting the reference frame principle.

I added that to my drawings:Raymond Potvin said:Here are three diagrams showing the relativity of motion between a light source at A and an observer at B. Fig. 1 and 2 show how aberration and doppler effect occur for two bodies moving in different reference frames. Fig. 3 shows how aberration and doppler effect could actually be occurring at the source but would later be nullified at the observer, thus would always be unobservable, for bodies moving in the same reference frame.

Fig. 1 and 2 show how aberration and doppler effect occur for two bodies moving in different reference frames. Fig. 3 shows how aberration and doppler effect could actually be occurring at the source but would later be nullified at the observer, thus would always be unobservable, for bodies moving in the same reference frame.

At fig. 1, the observer is considered moving while the source is at rest. While the observer at B travels to B', light travels from A to B', so for the observer at B', the light ray suffers the aberration angle α and has the apparent direction of the dotted red arrow, but it also suffers doppler effect because the observer is moving at an angle to the incoming ray.

At fig. 2, the source is considered moving while the observer is at rest. While the source at A travels to A', light travels from A to B, so for the observer at B, the light ray does not suffer aberration but has the same apparent direction as in fig. 1. For us, it makes the same angle α with A'B, and it does not suffer doppler effect because the ray was emitted at a normal to the motion of the source, but it suffers relativistic doppler effect, which gives exactly the same number as in fig. 1 where the observer's speed was producing doppler effect while moving at an angle to the ray.

At fig. 3, both source and observer are considered moving at the same speed and in the same direction, so the only difference with fig. 1 is that the source is also traveling. For the observer at B', the light ray still has the same apparent direction as in fig. 1 since it suffers the same aberration angle, but this time, its direction points to the actual position of the light source because it has traveled the same distance as the observer, there is no measurable doppler effect because the one produced at the source nullifies the one produced at the observer, and there is no measurable relativistic doppler effect either because the two sources travel at the same speed. Of course, it would give the same result if the observer was at A and the source at B.

To me, fig. 3 means that if the source was a laser beam aimed perpendicularly to its motion, this beam would never hit the observer, which seems to contradict the reference frame principle. Does it?

And DaleSpam answered this:Elapsed time is not measured with light paths, but with frequencies, thus with light waves if the clock is a light clock. The light clock mind experiment shows a longer path for the light ray, but it doesn't show how this longer distance would be accounted for by the clock. If we replace the two mirrors by a light source and an observer, there would be no way for the observer to measure that distance, because even if, to travel in the direction of the future position of the observer, the ray was emitted at an angle to the direction of motion, thus producing doppler effect at the source, this effect would be nullified at the observer because he would be meeting the same ray at the same angle and at the same speed but in the opposite direction. Moreover, he would have no way to measure the real direction of the ray either because aberration at the observer would indicate that the source was not in motion.

So here is my answer Dale:DaleSpam said:In a light clock the time is not measured using the frequency of the light, the Doppler shift is 0 and the actual frequency is not relevant. The time is measured by measuring the "echo time" from a target at a known range.

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.