Translation
On the Electrodynamics of Moving Bodies
Zur Elektrodynamik bewegter Körper
It is known that Maxwell’s electrodynamics — as it is customarily understood at present — leads in its application to moving bodies to asymmetries which do not seem to be inherent in the phenomena. Think, for example, of the electrodynamic interaction between a magnet and a conductor. The observable phenomenon here depends only on the relative motion of conductor and magnet, whereas according to the customary view the two cases, in which one or the other of these bodies is the one in motion, are to be strictly separated from each other. For if the magnet moves and the conductor is at rest, there arises in the surroundings of the magnet an electric field with a certain energy value, which produces a current at the places where parts of the conductor are situated. But if the magnet is at rest and the conductor moves, no electric field arises in the surroundings of the magnet, but instead there arises in the conductor an electromotive force, to which no energy corresponds in itself, but which — assuming equality of the relative motion in the two cases considered — gives rise to electric currents of the same magnitude and the same course as those produced by the electric forces in the first case.
Examples of a similar kind, together with the unsuccessful attempts to detect a motion of the earth relative to the “light medium”, lead to the conjecture that the concept of absolute rest corresponds to no properties of the phenomena, not only in mechanics but also in electrodynamics, but rather that for all coordinate systems for which the equations of mechanics hold, the same electrodynamic and optical laws also hold, as has already been shown for the quantities of the first order. We shall raise this conjecture (whose content will be called the “principle of relativity” in what follows) to the status of a presupposition, and in addition introduce the presupposition, only apparently incompatible with it, that light is always propagated in empty space with a definite velocity which is independent of the state of motion of the emitting body. These two presuppositions suffice to arrive at a simple and consistent electrodynamics of moving bodies, taking as a basis Maxwell’s theory for bodies at rest. The introduction of a “light ether” will prove to be superfluous inasmuch as, according to the view to be developed, neither will an “absolutely resting space” endowed with special properties be introduced, nor will a velocity vector be assigned to a point of empty space in which electromagnetic processes take place.
The theory to be developed is based — like every other electrodynamics — on the kinematics of the rigid body, since the assertions of any theory concern relations between rigid bodies (coordinate systems), clocks and electromagnetic processes. Insufficient consideration of this circumstance is the root of the difficulties with which the electrodynamics of moving bodies currently has to contend.
I. Kinematical Part.
II. Electrodynamical Part.
In conclusion I remark that in working on the problem treated here my friend and colleague M. Besso stood loyally by my side, and that I owe him several valuable suggestions.
Bern, June 1905.
(Received 30 June 1905.)