and
in their respectively moving frames. Each observer holds a watch in his hand, and these watches, when at rest with respect to each other, beat the same time; they define congruent stretches of time. The classical idea of practical time-congruence, founded on approximate observations, led us to maintain that these conditions would still endure regardless of the positions of the clocks in space and regardless of their relative motions.
But it now appears that the time-stretches defined by the ticking of a clock at rest in the frame of reference
would appear to an observer
in relative motion to be longer than they would appear to