On the Electrodynamics
of Moving Bodies
By A. EINSTEIN
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 neighborhood 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 neighborhood 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 a 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.
Translated
from "Zur Elektrodynamik
bewegter Kӧrper," Annalen der Physik, 17, 1905.