College of Science Physics Dept Tatsu Takeuchi Special Relativity Lecture Notes 
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14. The Equivalence of All Inertial FramesWe have just argued that when observed from the ground frame, lengths and clocks on the moving frame underwent Lorentz contraction and time dilation. But from point of view of the observer in the moving frame, say on a moving train, it is the train that is at rest and the ground and the other observer on it moving relative to the train in the other direction. And since all inertial frames are equivalent, all that we concluded about observing lengths and clocks on the train from the ground frame, must apply to observing lengths and clocks on the ground from the train frame. So when observed from the train frame, lengths and clocks on the ground undergoes Lorentz contraction and time dilation. This may seem to be a peculiar conclusion, but there is no contradiction in it. It just means that the same reality viewed from different perspectives could look vastly different.
