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It is known that Maxwell's electrodynamics—as usually
understood at the present time—when applied to moving bodies,
leads to asymmetries which do not appear to be inherent in the
phenomena.

Take, for example, the reciprocal electrodynamic action
of a magnet and a conductor. The observable phenomenon here depends
only on the relative motion of the conductor and the magnet,
whereas the customary view draws a sharp distinction between the
two cases in which either the one or the other of these bodies is
in motion. For if the magnet is in motion and the conductor at
rest, there arises in the neighbourhood of the magnet an electric
field with a certain definite energy, producing a current at the
places where parts of the conductor are situated. But if the magnet
is stationary and the conductor in motion, no electric field arises
in the neighbourhood of the magnet. In the conductor, however, we
find an electromotive force, to which in itself there is no
corresponding energy, but which gives rise—assuming equality of
relative motion in the two cases discussed—to electric currents of
the same path and intensity as those produced by the electric
forces in the former case.

Examples of this sort, together with the unsuccessful attempts
to discover any motion of the earth relatively to the "light
medium," suggest that the phenomena of electrodynamics as well as
of mechanics possess no properties corresponding to the idea of
absolute rest. They suggest rather that, as has already been shown
to the first order of small quantities, the same laws of
electrodynamics and optics will be valid for all frames of
reference for which the equations of mechanics hold good. We will
raise this conjecture (the purport of which will hereafter be
called the "Principle of Relativity") to the status of a
postulate, and also introduce another postulate, which is only
apparently irreconcilable with the former, namely, that light is
always propagated in empty space with a definite velocity c
which is independent of the state of motion of the emitting body.
These two postulates suffice for the attainment of a simple and
consistent theory of the electrodynamics of moving bodies based on
Maxwell's theory for stationary bodies. The introduction of a
"luminiferous ether" will prove to be superfluous inasmuch as the
view here to be developed will not require an "absolutely
stationary space" provided with special properties, nor assign a
velocity-vector to a point of the empty space in which
electromagnetic processes take place.

The theory to be developed is based—like all
electrodynamics—on the kinematics of the rigid body, since the
assertions of any such theory have to do with the relationships
between rigid bodies (systems of co-ordinates), clocks, and
electromagnetic processes. Insufficient consideration of this
circumstance lies at the root of the difficulties which the electrodynamics of moving bodies at present encounters.