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Page 1
Outside of physics we know nothing of action at a distance. When
we try to connect cause and effect in the experiences which natural
objects afford us, it seems at first as if there were no other mutual
actions than those of immediate contact, e.g. the communication of
motion by impact, push and pull, heating or inducing combustion by
means of a flame, etc. It is true that even in everyday experience
weight, which is in a sense action at a distance, plays a very
important part. But since in daily experience the weight of bodies
meets us as something constant, something not linked to any cause
which is variable in time or place, we do not in everyday life
speculate as to the cause of gravity, and therefore do not become
conscious of its character as action at a distance. It was Newton's
theory of gravitation that first assigned a cause for gravity by
interpreting it as action at a distance, proceeding from masses.
Newton's theory is probably the greatest stride ever made in
the effort towards the causal nexus of natural phenomena. And yet
this theory evoked a lively sense of discomfort among Newton's
contemporaries, because it seemed to be in conflict with the
principle springing from the rest of experience, that there can be
reciprocal action only through contact, and not through immediate
action at a distance. It is only with reluctance that man's desire
for knowledge endures a dualism of this kind. How was unity to
be preserved in his comprehension of the forces of nature? Either
by trying to look upon contact forces as being themselves distant
forces which admittedly are observable only at a very small
distance--and this was the road which Newton's followers, who were
entirely under the spell of his doctrine, mostly preferred to
take; or by assuming that the Newtonian action at a distance is
only _apparently_ immediate action at a distance, but in truth is
conveyed by a medium permeating space, whether by movements or by
elastic deformation of this medium. Thus the endeavour toward a
unified view of the nature of forces leads to the hypothesis of an
ether. This hypothesis, to be sure, did not at first bring with it
any advance in the theory of gravitation or in physics generally,
so that it became customary to treat Newton's law of force as an
axiom not further reducible. But the ether hypothesis was bound
always to play some part in physical science, even if at first only
a latent part.
When 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.
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