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# A THOUGHT EXPERIMENT IN SPACE BELOW IS A BIG SPACESHIP...

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 Question by marlos jacob de melo Submitted on 3/11/2007 Related FAQ: sci.physics.relativity Welcome - read this first! Rating: Not yet rated Rate this question: N/A Worst Weak OK Good Great A THOUGHT EXPERIMENT IN SPACE BELOW IS A BIG SPACESHIP (ABC). Its movement, if any, is supposed to be uniform (v=constant),and only along the X axis (to the left or to the right). The SPACESHIP is supposed to be far distant of  any matter concentration.    A--------------B---------------C -------- X axis    A sends pulse to B             C sends pulse to B A and C are some kind of apparatus which can send (emit) a pulse of light to B at the same instant t=0. B is some kind of receiver. It can register the instants t=Ta, and t=Tc when it receives the pulses that come from A and C respectively. Distance AB = Distance CB. There is an observer, who is travelling in the spaceship. He had got some knowledge on kinematics, but almost none on Special Relativity. At some moment he felt  that he could not be sure about the movement of the spaceship (To the left? To the right? Or was it not moving?). To discover it, he commands emitters A and C to emit a pulse of light in the direction of receiver B, both departing at the same instant t=0. After that he reads the registered times Ta and Tc in the receiver B. Clearly, there will be only three possibilities: 1:Ta>Tc;         2: TaTc       Because Ta>Tc, he will know that the spaceship is moving to the right. POSSIBILITY 2: He finds Ta Tc, and that, too, was not the case; c- Consequently, if the spaceship is not moving to the right, neither to the left, he only may conclude that the spaceship is not moving at all, in space. In other words, he concludes that the spaceship is in absolute rest. Final Conclusion: IT SEEMS THAT THE ABOVE IS A PHYSICAL EXPERIMENT THAT ENABLES ONE TO DECIDE IF A BODY IS UNIFORMLY MOVING, OR IF IT IS AT REST (in the case,absolute rest!). MY QUESTION IS: Is this reasoning correct? If not, why? OBS. The reasoning above was influenced by the old story of Einsteinīs long train, whose front and back ends were striken by two lightnings. Marlos Jacob de Melo, february 2007.        email>> marlosjacob@hotmail.com

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