Sidelights on Relativity | Page 2

in the first half of the nineteenth century the far-reaching
similarity was revealed which subsists between the properties of light
and those of elastic waves in ponderable bodies, the ether hypothesis
found fresh support. It appeared beyond question that light must be
interpreted as a vibratory process in an elastic, inert medium filling up
universal space. It also seemed to be a necessary consequence of the
fact that light is capable of polarisation that this medium, the ether,
must be of the nature of a solid body, because transverse waves are not
possible in a fluid, but only in a solid. Thus the physicists were bound
to arrive at the theory of the "quasi-rigid" luminiferous ether, the parts
of which can carry out no movements relatively to one another except
the small movements of deformation which correspond to light-waves.
This theory--also called the theory of the stationary luminiferous
ether--moreover found a strong support in an experiment which is also
of fundamental importance in the special theory of relativity, the
experiment of Fizeau, from which one was obliged to infer that the
luminiferous ether does not take part in the movements of bodies. The
phenomenon of aberration also favoured the theory of the quasi-rigid
ether.
The development of the theory of electricity along the path opened up
by Maxwell and Lorentz gave the development of our ideas concerning
the ether quite a peculiar and unexpected turn. For Maxwell himself the
ether indeed still had properties which were purely mechanical,
although of a much more complicated kind than the mechanical
properties of tangible solid bodies. But neither Maxwell nor his
followers succeeded in elaborating a mechanical model for the ether
which might furnish a satisfactory mechanical interpretation of
Maxwell's laws of the electro-magnetic field. The laws were clear and
simple, the mechanical interpretations clumsy and contradictory.
Almost imperceptibly the theoretical physicists adapted themselves to a
situation which, from the standpoint of their mechanical programme,
was very depressing. They were particularly influenced by the
electro-dynamical investigations of Heinrich Hertz. For whereas they
previously had required of a conclusive theory that it should content
itself with the fundamental concepts which belong exclusively to
mechanics (e.g. densities, velocities, deformations, stresses) they

gradually accustomed themselves to admitting electric and magnetic
force as fundamental concepts side by side with those of mechanics,
without requiring a mechanical interpretation for them. Thus the purely
mechanical view of nature was gradually abandoned. But this change
led to a fundamental dualism which in the long-run was insupportable.
A way of escape was now sought in the reverse direction, by reducing
the principles of mechanics to those of electricity, and this especially as
confidence in the strict validity of the equations of Newton's mechanics
was shaken by the experiments with beta-rays and rapid kathode rays.
This dualism still confronts us in unextenuated form in the theory of
Hertz, where matter appears not only as the bearer of velocities, kinetic
energy, and mechanical pressures, but also as the bearer of
electromagnetic fields. Since such fields also occur _in vacuo_--i.e. in
free ether--the ether also appears as bearer of electromagnetic fields.
The ether appears indistinguishable in its functions from ordinary
matter. Within matter it takes part in the motion of matter and in empty
space it has everywhere a velocity; so that the ether has a definitely
assigned velocity throughout the whole of space. There is no
fundamental difference between Hertz's ether and ponderable matter
(which in part subsists in the ether).
The Hertz theory suffered not only from the defect of ascribing to
matter and ether, on the one hand mechanical states, and on the other
hand electrical states, which do not stand in any conceivable relation to
each other; it was also at variance with the result of Fizeau's important
experiment on the velocity of the propagation of light in moving fluids,
and with other established experimental results.
Such was the state of things when H. A. Lorentz entered upon the scene.
He brought theory into harmony with experience by means of a
wonderful simplification of theoretical principles. He achieved this, the
most important advance in the theory of electricity since Maxwell, by
taking from ether its mechanical, and from matter its electromagnetic
qualities. As in empty space, so too in the interior of material bodies,
the ether, and not matter viewed atomistically, was exclusively the seat
of electromagnetic fields. According to Lorentz the elementary
particles of matter alone are capable of carrying out movements; their
electromagnetic activity is entirely confined to the carrying of electric
charges. Thus Lorentz succeeded in reducing all electromagnetic

happenings to Maxwell's equations for free space.
As to the mechanical nature of the Lorentzian ether, it may be said of it,
in a somewhat playful spirit, that immobility is the only mechanical
property of which it has not been