Sidelights on Relativity | Page 3

deprived by H. A. Lorentz. It may be
added that the whole change in the conception of the ether which the
special theory of relativity brought about, consisted in taking away
from the ether its last mechanical quality, namely, its immobility. How
this is to be understood will forthwith be expounded.
The space-time theory and the kinematics of the special theory of
relativity were modelled on the Maxwell-Lorentz theory of the
electromagnetic field. This theory therefore satisfies the conditions of
the special theory of relativity, but when viewed from the latter it
acquires a novel aspect. For if K be a system of co-ordinates relatively
to which the Lorentzian ether is at rest, the Maxwell-Lorentz equations
are valid primarily with reference to K. But by the special theory of
relativity the same equations without any change of meaning also hold
in relation to any new system of co-ordinates K' which is moving in
uniform translation relatively to K. Now comes the anxious
question:--Why must I in the theory distinguish the K system above all
K' systems, which are physically equivalent to it in all respects, by
assuming that the ether is at rest relatively to the K system? For the
theoretician such an asymmetry in the theoretical structure, with no
corresponding asymmetry in the system of experience, is intolerable. If
we assume the ether to be at rest relatively to K, but in motion
relatively to K', the physical equivalence of K and K' seems to me from
the logical standpoint, not indeed downright incorrect, but nevertheless
inacceptable.
The next position which it was possible to take up in face of this state
of things appeared to be the following. The ether does not exist at all.
The electromagnetic fields are not states of a medium, and are not
bound down to any bearer, but they are independent realities which are
not reducible to anything else, exactly like the atoms of ponderable
matter. This conception suggests itself the more readily as, according to
Lorentz's theory, electromagnetic radiation, like ponderable matter,
brings impulse and energy with it, and as, according to the special
theory of relativity, both matter and radiation are but special forms of
distributed energy, ponderable mass losing its isolation and appearing

as a special form of energy.
More careful reflection teaches us, however, that the special theory of
relativity does not compel us to deny ether. We may assume the
existence of an ether; only we must give up ascribing a definite state of
motion to it, i.e. we must by abstraction take from it the last mechanical
characteristic which Lorentz had still left it. We shall see later that this
point of view, the conceivability of which I shall at once endeavour to
make more intelligible by a somewhat halting comparison, is justified
by the results of the general theory of relativity.
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