Introduction

Maxwell’s electrodynamics when applied to moving bodies, leads to asymmetries which are not inherent in the phenomena.

An example is the reciprocal electrodynamic action of a magnet and a conductor.

This depends only on the relative motion of the conductor and the magnet.

When the magnet is in motion and the conductor at rest, an electric field is created in the neighbourhood of the magnet with a certain definite energy.

This produces 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.

The conductor has an electromotive force that leads to electric currents of the same path and intensity as those produced by the electric forces in the former case.

This, together with the failure to detect any relative motion of the Earth to the Fresnel aether, suggest that electrodynamics and mechanics are constantly moving.

They suggest a “Principle of Relativity”.

This is made up of 2 postulates:

1. The same laws of electrodynamics and optics are valid for all frames of reference where mechanics are valid.
2. Light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body.

We will raise this conjecture (the purport of which will hereafter be called the ) to the status of a postulate, and also introduce another postulate, which is only apparently irreconcilable with the former, namely, that

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 1 The preceding memoir by Lorentz was not at this time known to the author. 1assign 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 kine- matics 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.