SIDELIGHTS ON RELATIVITY

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exist at all. The electromagnetic fields are not states of a medium,
and are not bound down to any bearer, but they are independent
realities which are not reducible to anything else, exactly like
the atoms of ponderable matter. This conception suggests itself
the more readily as, according to Lorentz's theory, electromagnetic
radiation, like ponderable matter, brings impulse and energy with
it, and as, according to the special theory of relativity, both
matter and radiation are but special forms of distributed energy,
ponderable mass losing its isolation and appearing as a special
form of energy.

More careful reflection teaches us, however, that the special theory
of relativity does not compel us to deny ether. We may assume the
existence of an ether; only we must give up ascribing a definite
state of motion to it, i.e. we must by abstraction take from it the
last mechanical characteristic which Lorentz had still left it. We
shall see later that this point of view, the conceivability of which