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Relativity paradox in exponentially accelerating train 
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#1
Mar1610, 04:22 PM

P: 26

Using Einstein's train thought experiment, suppose a train is accellerating exponetially so that the engine remains forever ahead of a beam of light without it ever reaching the speed of light.
V = c[ 1  e^(t)] for the train The light would approach the engine but never quite reach it. So it would pass the second car in a finite time. If you are sitting in the second car, you could wait til the light beam passes you outside, then get up and walk to the front of the engine. Your speed would be very slow relative to the train and fast but less than the speed of light relative to the ground. But in doing this walk, you will have caught up to and passed a beam of light. This should be impossible! So what prevents this from happening? Is it impossible to walk to the front of the train? If so, why? It is a short finite distance and the acceleration of the train is never infinite. Jeff kochanskij@yahoo.com 


#2
Mar1610, 05:01 PM

Sci Advisor
P: 8,470

By the way, if someone wants to do a detailed mathematical analysis, the problem may be easier if instead of "exponential" acceleration you assume every part of the train is accelerating with the same constant proper acceleration (constant Gforce as measured by anyone on the train, and constant coordinate acceleration in the instantaneous inertial rest frame of any point on the train at each moment). An observer with constant proper acceleration will still have a Rindler horizon where light emitted from points at or beyond that horizon will never catch up with them. And this observer's velocity as a function of time as seen in the inertial frame where they had a velocity of 0 at t=0 is given on the relativistic rocket page as v = at / sqrt(1 + (at/c)^{2}), where a is the proper acceleration. 


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