Sidelights on Relativity | Page 4

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 to
assume that empty space has no physical qualities whatever. The
fundamental facts of mechanics do not harmonize with this view. For
the mechanical behaviour of a corporeal system hovering freely in
empty space depends not only on relative positions (distances) and
relative velocities, but also on its state of rotation, which physically
may be taken as a characteristic not appertaining to the system in itself.
In order to be able to look upon the rotation of the system, at least
formally, as something real, Newton objectivises space.
Since he classes his absolute space together with real things, for him
rotation relative to an absolute space is also something real. Newton
might no less well have called his absolute space "Ether"; what is
essential is merely that besides observable objects, another thing, which
is not perceptible, must be looked upon as real, to enable acceleration
or rotation to be looked upon as something real.
It is true that Mach tried to avoid having to accept as real something
which is not observable by endeavouring to substitute in mechanics a
mean acceleration with reference to the totality of the masses in the
universe in place of an acceleration with reference to absolute space.
But inertial resistance opposed to relative acceleration of distant masses
presupposes action at a distance; and as the modern physicist does not
believe that he may accept this action at a distance, he comes back once
more, if he follows Mach, to the ether, which has to serve as medium
for the effects of inertia. But this conception of the ether to which we
are led by Mach's way of thinking differs essentially from the ether as

conceived by Newton, by Fresnel, and by Lorentz. Mach's ether not
only conditions the behaviour of inert masses, but is also conditioned in
its state by them.
Mach's idea finds its full development in the ether of the general theory
of relativity. According to this theory the metrical qualities of the
continuum of space-time differ in the environment of different points of
space-time, and are partly conditioned by the matter existing outside of
the territory under consideration. This space-time variability of the
reciprocal relations of the standards of space and time, or, perhaps, the
recognition of the fact that "empty space" in its physical relation is
neither homogeneous nor isotropic, compelling us to describe its state
by ten functions (the gravitation potentials g_(mn)), has, I think, finally
disposed of the view that space is physically empty. But therewith the
conception of the ether has again acquired an intelligible content,
although this content differs widely from that of the ether of the
mechanical undulatory theory of light. The ether of the general theory
of relativity is a medium which is itself devoid of all mechanical and
kinematical qualities, but helps to determine mechanical (and
electromagnetic) events.
What is fundamentally new in the ether of the general theory of
relativity as opposed to the ether of Lorentz consists in this, that the
state of the former is at every place determined by connections with the
matter and the state of the ether in neighbouring places, which are
amenable to law in the form of differential equations; whereas the state
of the Lorentzian ether in the absence of electromagnetic fields is
conditioned by nothing outside itself, and is everywhere the same. The
ether of the general theory of relativity is transmuted conceptually into
the ether of Lorentz if we substitute constants for the functions of space
which describe the former, disregarding the causes which condition its
state. Thus we may also say, I think, that the ether of the general theory
of relativity is the outcome of the Lorentzian ether, through
relativation.
As to the part which the new ether is to play in the physics of the future
we are not yet clear. We know that it determines the metrical relations
in the space-time continuum, e.g. the configurative possibilities of solid
bodies as well as the gravitational fields; but we do not know whether it
has an essential share