The Meaning of Relativity / Four lectures delivered at Princeton University, May, 1921

by Albert Einstein

, its radius

in motion, might be different from

, since the theory of relativity does not make the assumption that the shape of bodies with respect to a space of reference is independent of their motion relatively to this space of reference. But all directions in space must be equivalent to each other.

may therefore depend upon the magnitude

of the velocity, but not upon its direction;