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Unformatted text preview: ON THE ELECTRODYNAMICS OF MOVING BODIES By A. EINSTEIN June 30, 1905 It is known that Maxwells electrodynamicsas usually understood at the present timewhen applied to moving bodies, leads to asymmetries which do not appear to be inherent in the phenomena. Take, for example, the recipro- cal electrodynamic action of a magnet and a conductor. The observable phe- nomenon 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 neighbour- hood 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 electro- motive force, to which in itself there is no corresponding energy, but which gives riseassuming equality of relative motion in the two cases discussedto elec- tric 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. 1 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 Maxwells 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. 1 assign a velocity-vector to a point of the empty space in which electromagnetic processes take place....
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