SIDELIGHTS ON RELATIVITY

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ETHER AND THE THEORY OF RELATIVITY

An Address delivered on May 5th, 1920, in the University of Leyden



How does it come about that alongside of the idea of ponderable
matter, which is derived by abstraction from everyday life, the
physicists set the idea of the existence of another kind of matter,
the ether? The explanation is probably to be sought in those phenomena
which have given rise to the theory of action at a distance, and
in the properties of light which have led to the undulatory theory.
Let us devote a little while to the consideration of these two
subjects.

Outside of physics we know nothing of action at a distance. When
we try to connect cause and effect in the experiences which natural