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Page 3
Think of waves on the surface of water. Here we can describe two
entirely different things. Either we may observe how the undulatory
surface forming the boundary between water and air alters in the course
of time; or else--with the help of small floats, for instance--we
can observe how the position of the separate particles of water
alters in the course of time. If the existence of such floats for
tracking the motion of the particles of a fluid were a fundamental
impossibility in physics--if, in fact, nothing else whatever were
observable than the shape of the space occupied by the water as it
varies in time, we should have no ground for the assumption that
water consists of movable particles. But all the same we could
characterise it as a medium.
We have something like this in the electromagnetic field. For we may
picture the field to ourselves as consisting of lines of force. If
we wish to interpret these lines of force to ourselves as something
material in the ordinary sense, we are tempted to interpret the
dynamic processes as motions of these lines of force, such that each
separate line of force is tracked through the course of time. It is
well known, however, that this way of regarding the electromagnetic
field leads to contradictions.
Generalising we must say this:--There may be supposed to be extended
physical objects to which the idea of motion cannot be applied.
They may not be thought of as consisting of particles which allow
themselves to be separately tracked through time. In Minkowski's
idiom this is expressed as follows:--Not every extended conformation
in the four-dimensional world can be regarded as composed
of world-threads. The special theory of relativity forbids us to
assume the ether to consist of particles observable through time,
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
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.
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