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I have a question regarding accelerated motion in special relativity. Suppose an observer has a constant proper acceleration α, his velocity in an inertial frame of a distant observer will then be given [itex]v = c tanh(ατ/c)[/itex], where τ is the proper time of the accelerated observer. Since every quantity in the argument of the function is lorentzinvariant, v also should be an invariant quantity. But that would imply, that every observer agrees on the velocity of the accelerated observer at a given moment, which doesn't fit in my knowledge of special relativity so far... is there a fault in my reasoning or am I missing something else?

Thanks in advance!

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# Velocity of accelerated observer

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