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Assume an alternate universe with same physical laws as here. In this universe nothing exists except 3 observers, each in a transparent box with a clock. Observer A is travelling at .5c. Observer C is travelling in the opposite direction at .5c. Observer B is at rest.

In the absence of outside landmarks each would feel they were at rest and their clocks were functioning normally. Suppose they pass each other. To A it would appear B had zoomed by at .5c and C at c. To B it would seem A and C had zoomed by at .5c in opposite directions.

Suppose they noted the functioning of each others' clocks as compared to their own in passing, and they met later to compare notes on their observations. What observations would they have concerning clock function? If C shone a light beam ahead of himself, how would it appear to A?

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"To A it would appear B had zoomed by at .5c and C at c" This is wrong. Relativistic velocities don't just add together. A would observe C travelling at 0.8c –  Jimdalf the Grey Jul 8 '13 at 22:56
Assuming the three observers synchronise their clocks as they pass each other, they cannot then have met later to compare notes on their observations unless at least two of them accelerated to change their inertial frame to match the third observer. This means you have to calculate the effect of the acceleration as well as the effect of the relative speed. The situation is a minor varient of the twin paradox. –  John Rennie Jul 9 '13 at 8:59
Assuming as you say they accelerate to match third observer in order to meet how does this affect whatever observations were made in passing? –  victorhiggins Jul 10 '13 at 0:41

1 Answer 1

To A it would appear B had zoomed by at .5c and C at c. To B it would seem A and C had zoomed by at .5c in opposite directions.

According to accepted physical laws in this universe, if something travels at c according to one observer, it travels at c according to all observers.

So, it would be impossible for A to observe C with speed c and B to observe C with any other speed but c.

This is well known in this universe but, evidently, not so much in your alternate universe.

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Actually, pretty much all standard cosmological theories involving multiple universes hold the speed of light and the fundamentals of relativity constant across all universes. So that's true in his alternate universe too. –  Jimdalf the Grey Jul 9 '13 at 23:48
@Jim, yes it's true but, evidently, not known. :) –  Alfred Centauri Jul 9 '13 at 23:50
If B is at rest and C is travelling at .5c how then will it appear that C is travelling at C to B? Jim has corrected my notion that A believing himself at rest would observe C at .8c and not c however C's actual velocity is .5c. How will this not be apparent to B? –  victorhiggins Jul 10 '13 at 1:39
What I am trying to find out here is whether A and C passing each other each believing themselves at rest and noting the apparent velocity of the other would see the other's time dilating or whether their actual velocities were the same would see both clocks operating the same and be able to deduce from that their own motion –  victorhiggins Jul 10 '13 at 2:08
It would be easier to deal with a condescending attitude had, at least been given a correct answer –  victorhiggins Jul 11 '13 at 21:34

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