SIDELIGHTS ON RELATIVITY

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geometry," and shall distinguish it in what follows from "purely
axiomatic geometry." The question whether the practical geometry
of the universe is Euclidean or not has a clear meaning, and its
answer can only be furnished by experience. All linear measurement
in physics is practical geometry in this sense, so too is geodetic
and astronomical linear measurement, if we call to our help the
law of experience that light is propagated in a straight line, and
indeed in a straight line in the sense of practical geometry.

I attach special importance to the view of geometry which I
have just set forth, because without it I should have been unable
to formulate the theory of relativity. Without it the following
reflection would have been impossible:--In a system of reference
rotating relatively to an inert system, the laws of disposition of
rigid bodies do not correspond to the rules of Euclidean geometry
on account of the Lorentz contraction; thus if we admit non-inert
systems we must abandon Euclidean geometry. The decisive step in