SIDELIGHTS ON RELATIVITY

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but the hypothesis of ether in itself is not in conflict with the
special theory of relativity. Only we must be on our guard against
ascribing a state of motion to the ether.

Certainly, from the standpoint of the special theory of relativity,
the ether hypothesis appears at first to be an empty hypothesis. In
the equations of the electromagnetic field there occur, in addition
to the densities of the electric charge, _only_ the intensities
of the field. The career of electromagnetic processes _in vacuo_
appears to be completely determined by these equations, uninfluenced
by other physical quantities. The electromagnetic fields appear as
ultimate, irreducible realities, and at first it seems superfluous
to postulate a homogeneous, isotropic ether-medium, and to envisage
electromagnetic fields as states of this medium.

But on the other hand there is a weighty argument to be adduced
in favour of the ether hypothesis. To deny the ether is ultimately