Hello P.F. I have previously raised the idea of a light clock that ticked each time the light travelled one metre. Dale has said if I could explain how it could be done, I could open a new thread. Here goes. A normal light clock ticks each time the light goes mirror to mirror. I propose a light clock that ticks each time the light goes one metre.With the mirrors one metre apart in the stationary frame, the two are the same. What happens when the train is moving? The mirrors will still be one metre apart in both frames. The diagonal light path will still be the same diagonal light path. The sensors locations however have to change. Remove the existing sensors and replace with sensors at one metre intervals along the light's path. So if the perpendicular path is one metre, the clock ticks once. If the diagonal path is two metres the clock ticks twice. Both clocks tick at the same rate, once per metre, irrespective of the light paths length. This conflicts with S.R. can you explain. Thanks.