It is well known that if we attempt to apply Maxwell’s electrodynamics,
as conceived at the present time, to moving bodies, we are led to
asymmetry which does not agree with observed phenomena. Let us think of
the mutual action between a magnet and a conductor. The observed
phenomena in this case depend only on the relative motion of the
conductor and the magnet, while according to the usual conception, a
distinction must be made between the cases where the one or the other of
the bodies is in motion. If, for example, the magnet moves and the
conductor is at rest, then an electric field of certain energy-value is
produced in the neighbourhood of the magnet, which excites a current in
those parts of the field where a conductor exists. But if the magnet be
at rest and the conductor be set in motion, no electric field is
produced in the neighbourhood of the magnet, but an electromotive force
which corresponds to no energy in itself is produced in the conductor;
this causes an electric current of the same magnitude and the same
career as the electric force, it being of course assumed that the
relative motion in both of these cases is the same.
2. Examples of a similar kind such as the unsuccessful attempt to
substantiate the motion of the earth relative to the “Light-medium” lead
us to the supposition that not only in mechanics, but also in
electrodynamics, no properties of observed facts correspond to a concept
of absolute rest; but that for all coordinate systems for which the
mechanical equations hold, the equivalent electrodynamical and optical
equations hold also, as has already been shown for magnitudes of the
first order. In the following we make these assumptions (which we shall
subsequently call the Principle of Relativity) and introduce the further
assumption,—an assumption which is at the first sight quite
irreconcilable with the former one—that light is propagated in vacant
space, with a velocity _c_ which is independent of the nature of motion
of the emitting body. These two assumptions are quite sufficient to give
us a simple and consistent theory of electrodynamics of moving bodies on
the basis of the Maxwellian theory for bodies at rest. The introduction
of a “Lightäther” will be proved to be superfluous, for according to the
conceptions which will be developed, we shall introduce neither a space
absolutely at rest, and endowed with special properties, nor shall we
associate a velocity-vector with a point in which electro-magnetic
processes take place.
3. Like every other theory in electrodynamics, the theory is based on
the kinematics of rigid bodies; in the enunciation of every theory, we
have to do with relations between rigid bodies (co-ordinate system),
clocks, and electromagnetic processes. An insufficient consideration of
these circumstances is the cause of difficulties with which the
electrodynamics of moving bodies have to fight at present.