Henri Bergson: Half-relativity – Michelson-Morley experiment (6)

More generally, let us still call S a system immobile in the ether, and S’ a double of this system, which first coincided with it and then detaches itself in a straight line with the speed v. While S’ contracts in the direction of its movement, its Time expands. A man attached to the system S, seeing S’ and fixing his attention on a second clock of S’ at the precise moment of the doubling, would see the second of S lengthening on S’ like an elastic thread that is pulled, like a trait that we look at with a magnifying glass. Let us be clear: no change has occurred in the mechanism of the clock, nor in its functioning. The phenomenon is in no way comparable to the lengthening of a pendulum. It is not because clocks go slower that Time has lengthened; it is because Time has lengthened that the clocks, remaining as they are, find themselves moving more slowly. By the effect of the movement, a longer Time, stretched, dilated, fills the interval between two positions of the pointers. Same slowdown, moreover, for all the movements and all the changes of the system, since each of them could also become representative of Time and set itself up as a clock.

We have just assumed, it is true, that the terrestrial observer followed the coming and going of the light ray from 0 to A and from A to 0, and measured the speed of light without having to consult any other clock than that from point 0. What would happen if we measured this speed on the outward journey only, by consulting two clocks (1) placed respectively at the points 0 and A? In fact, in all terrestrial measurements of the speed of light, it is the double path of the ray that is measured. The experiment we are talking about was therefore never carried out. But nothing proves that it is unachievable. We will show that it would still give the same number for the speed of light. But let us remember, for this, what the concordance of our clocks consists of.

(1) It goes without saying that we call a clock, in this paragraph, any device making it possible to measure an interval of time or to locate exactly two instants in relation to each other. In experiments relating to the speed of light, Fizeau’s cogwheel and Foucault’s rotating mirror are clocks. Even more general will be the meaning of the word throughout the present study. It will apply equally well to a natural process. Clock will be the Earth that turns. On the other hand, when we speak of the zero of a clock, and of the operation by which we will determine the place of the zero on another clock to obtain concordance between the two, it is only to fix the ideas that we involve dials and pointers. Given any two devices, natural or artificial, used for measuring time, and consequently given two movements, we can call zero any point, arbitrarily chosen as the origin, of the trajectory of the first mobile. Fixing the zero in the second device will simply consist of marking, on the path of the second mobile, the point which will be supposed to correspond to the same instant. In short, the “fixing of zero” must be understood in and which follows as the real or ideal operation, carried out or simply thought, by which two points denoting a first simultaneity will have been marked respectively, on the two devices.