SIDELIGHTS ON RELATIVITY

Back Forward Menu

in nature, and that therefore the properties predicated of rigid
bodies do not apply to physical reality,--this objection is by
no means so radical as might appear from a hasty examination. For
it is not a difficult task to determine the physical state of a
measuring-rod so accurately that its behaviour relatively to other
measuring-bodies shall be sufficiently free from ambiguity to allow
it to be substituted for the "rigid" body. It is to measuring-bodies
of this kind that statements as to rigid bodies must be referred.

All practical geometry is based upon a principle which is accessible
to experience, and which we will now try to realise. We will
call that which is enclosed between two boundaries, marked upon a
practically-rigid body, a tract. We imagine two practically-rigid
bodies, each with a tract marked out on it. These two tracts are
said to be "equal to one another" if the boundaries of the one tract
can be brought to coincide permanently with the boundaries of the
other. We now assume that: