Einstein’s Special Theory

Abstract

Einstein’s paper “The Electrodynamics of Moving Bodies” (1905), in which he broached his Special Theory of Relativity, is a model of clarity, concision, and boldness. Einstein opens his paper with a brief statement of the difficulty which prompts his new theory. It is interesting that his point of departure is not the failure to detect the earth’s motion relative to the luminiferous aether but rather certain asymmetries in classical physics which he found troublesome. He gives the example, derived from Faraday’s experiments in 1831, of a magnet and an electric conductor which are in relative motion to each other. Faraday had shown that whether the magnet was at rest and the conductor moving or the conductor at rest and the magnet moving, their proximate, relative motion to each other induced an electric current in the conductor. But in classical physics, the theoretical explanation for this phenomenon was quite different in the one case than in the other. If the magnet is moving past a conductor at rest, then the current is conceived to be caused by an electric field associated with the moving magnet. But if the magnet is at rest and the conductor moves past it, the current is said to be induced by the force exerted by the magnet on the electric charges in the moving conductor, and no electric field arises in the vicinity of the magnet. Now Einstein found these sorts of asymmetrical explanations for identical phenomena simply “unbearable.” As he later explained, The thought that one is dealing here with two fundamentally different cases was for me unbearable. The difference between these two cases could be not a real difference but rather, in my conviction, only a difference in the choice of the reference point. Judged from the magnet, there were certainly no electric fields, [whereas] judged from the conducting circuit there certainly was one. The existence of an electric field was therefore a relative one, depending on the state of motion of the coordinate system being used, and a kind of objective reality could be granted only to the electric and magnetic field together, quite apart from the state of relative motion of the observer or the coordinate system. The phenomenon of the electromagnetic induction forced me to postulate the (special) relativity principle.1