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Apr27-11, 05:25 PM
So, imagine I am stood on earth holding a clock. My friend Jack passes me in his super fast space ship travelling at say 0.7c He also has a clock and as he passes me his clock is synchronised with mine. He then continues his journey.
A second friend Jill is on a return journey to earth in her super fast space ship coming from the opposite direction to Jack.
Jill also has a clock and as Jack and Jill pass each other, Jillís clock is synchronised to Jackís.
Jill then continues her journey and as she passes me on earth, I take a reading of her clock.
As there has been no acceleration between the synchronisation of the clocks, does Jillís clock read the same as mine as she passes me?
No, you would get a different time, but this time the situation is symmetric. In principle it's not distinguishable which of you was moving.