The Second Postulate of Relativity

Abstract

In a recent article by Lewis and Tolman [1] a non-analytical method I was developed for obtaining the more important conclusions which can be drawn from the principle of relativity. Our reasoning was based only upon the first and second postulates of relativity, and those fundamental conservation laws of mass, energy and momentum which science has never in a single instance been forced to abandon. Since the method of attack avoided any use of involved mathematical analysis, restricting itself to the simplest processes of logical reasoning, and, further, made no use of the assumptions of electromagnetic theory, it may be concluded that the unexpected nature of the results of the theory of relativity is due to something unusual in the two postulates of relativity themselves. No objections have ever been made to the first postulate of relativity, as stated in its original form by Newton, that it is impossible to measure or detect absolute translatory motion through space. In the development of the theory of relativity, this postulate has been modified to include the impossibility of detecting translatory motion through any ether or medium which might be assumed to pervade space. In support of this principle is the general fact that no "ether drift" has ever been detected, but, especially, the conclusive experiments of Michelson and Morley, and Trouton and Noble, in which, a motion through the ether, of the earth in its path around the sun would certainly have been detected. For the purposes of this article we shall consider that the first postulate of relativity needs no further proof. It is Einstein (to whom, indeed we owe the development of relativity along its present broad lines) who first stated the second postulate of relativity in a general form, namely, that the velocity of light in free space appears the same to all observers, regardless of the relative motion of the source of light and the observer. This is the assumption which has forced the theory of relativity to its strange conclusions, and it is for its further consideration that this paper is designed. A simple example will make the extraordinary nature of the second postulate evident.

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