"The Principle of Relativity is the hypothesis that it is impossible by means of physical experiments to determine the absolute velocity of a body through space." (Cunningham "Relativity and the Electron Theory," p. 2).
We cannot, for example, determine the velocity of the earth relative to the ether.
This is of importance when we are dealing with the idea of "simultaneity"—an idea which, as we saw in Chapter IV. is closely associated with our notion of Time. For our criterion of simultaneity has in practice been based on optical communication. (Cp. Ibid, pp. 5 and 28). But it is easy to show that "the setting up of a standard of simultaneity by means of light signals is not possible until a definite velocity is assigned to the observer. Thus the hypothesis of relativity requires a reconsideration of the way in which we measure time." (Ibid, pp. 5, 28, 29).
"This again reacts on the measurement of the length of a material body, the 'distance between two points' being the distance between simultaneous positions of those points. Thus it becomes necessary also to examine the way in which we measure space. It becomes impossible to consider space and time separately; the two measures are interrelated to such an extent that Minkowski felt himself constrained to say that 'from henceforth time by itself and space by itself are mere shadows, that they are only two aspects of a single and indivisible manner of co-ordinating the facts of the physical world.'" (Ibid, pp. 5 and 6.)
When it is remembered that the Principle of Relativity is firmly established in scientific thought it will be realised that this conclusion arrived at as a result of purely physical considerations is of the very utmost importance as an independent confirmation of the general line of thought developed in the preceding pages.
I therefore feel it legitimate to claim that in so far as physical science throws any light on the subject at all its testimony is distinctly favourable.
CHAPTER VII
THE CONNECTING LINK