The distances between these objects and the passenger observer do not change. Therefore, this observer measures all of the seats to be at rest, since he is stationary from his own perspective.
TheAn observer standing on the platform would see exactly the same objects but interpret them very differently. The distancesdistance between themselfthe platform observer and the seats on the train car areis changing, and so theythe concludeplatform observer concludes that they (the seats) are moving forward, as is the whole train. Thus for one observer the seats are at rest, while for the otherothers the seats are moving, and both are correct, since they are using different definitions of "at rest" and "moving". Each observer has a distinct "frame of reference" in which velocities are measured, ''the rest frame of the platform'' and ''the rest frame of the train'' – or simply ''the platform frame'' and ''the train frame''.
Why can't we select one of these frames to be the "correct" one? Or more generally, why is there not a frame we can select to be the basis for all measurements, an "absolutely stationary" frame?
