One way to understand that Einstein was talking about invariance, rather than declaring everything to be relative, is to think about how far a light beam would travel in a given period of time. That distance would be the speed of light multiplied by the amount of time it traveled. If we were on a platform observing this happening on a train speeding by, the elapsed time would appear shorter {time seems to move more slowly on the moving train}, and the distance would appear shorter {rulers seem to be contracted on the moving train}. But there is a relationship between the two quantities-a relationship between the measurements of space and of time-that remains invariant, whatever your frame of reference.62 A
( Walter Isaacson )
[ Einstein: His Life and ]
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