The second question is much more difficult for two reasons:
Consider the following:
(Observer) O X M (Mirror) OM = c A v = .5c ----->I have an Observer at O, a mirror M at distance c, and an Astronaut A who will travel the same distance (at speed .5c). The point X see later.
The experiment involves the following
First observer will transmit a light pulse to M, the refection comes back after 2 seconds. Conclusion the speed of light is c.
In order to calculate the speed of the Astronaut by O you need two clocks. When the trip begins you again transmit a light pulse and you start two clocks. When the lightpuls comes back you stop clock 1 and when the Astronaut comes back you stop clock 2. When the number of counts on clock 2 (4sec) is twice of clock 1 (2sec) than the Astronaut has travelled at .5c
In order to measure the speed of A by the Astronaut you give A two
identical clocks. A will stop his first clock when he receives the light
pulse (reflected via the Mirror) at point X and the second clock after
returning back to base.
The Observer does the same (and after some simple calculations) and will
say:
I will not make any claim if the number of counts of clock 2 of both O and A are identical (both were stopped at the same moment.) When the Astronaut comes back he will compare his two clocks and the relation will be (in my opinion) 4/3 versus 4, (or 4 versus 12) which is in agreement with the predicted value of O. If that is the case the Astronaut will also conclude that during the time that he has travelled a distance "4" light has travelled a distance "8", or that his speed was 0.5c.
Personal I have no objections if one astronaut moves at 0.7c in one direction and a second astronaut at 0.7c in the opposite direction, to claim that, the relative speed of the other is 1.4c
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