In the first place then, how has it come about that our conceptions of the fundamental notions of mechanics have been proved wanting? This crime like many another may safely be laid at the door of the physicists, those restless beings who, with their eternal experimenting, are continually raising disturbing ghosts, and then frantically imploring the aid of the mathematicians in order to exorcise them. Let us briefly consider the experiment which led us into those difficulties from which the principle of relativity alone apparently can extricate us.
Consider a source of sound A at rest (Fig. 1), and surrounded by air, in which sound is propagated, also at rest. Now, as every schoolboy knows, the time taken for sound to go to B is the same as that taken to go to C, if B and C are at the same distance from A. The same is true
| Fig. 1. | Fig. 2. |
also if A, B and C are all moving with uniform velocity in any direction, carrying the air with them. This may be realized by a closed railway car or a boat. But if the points A, B, and C are moving with uniform velocity, and the air is at rest relative to them, or what is the same thing, if they are at rest and the air is moving past them with uniform velocity, the state of affairs is very different. If the three points are moving in the direction indicated by the arrow (Fig. 2), and if the air is at rest, and if a sound wave is sent out from A, then the time required for this sound wave to go from A to C is not the same as that required to go from A to B. Now as sound is propagated in air, so is light in an imaginary medium, the ether. Moreover, this ether is stationary, as many experiments show, and the earth is moving through it, in its path around the sun with a considerable velocity. Therefore we have exactly the same case as before, and it should be very easy to show that the velocity of light in a direction perpendicular to the earth's direction of motion is different from that in a direction which coincides with it. But a famous experiment of Michelson and Morley, carried out with the utmost precision, showed not the slightest difference in these velocities. So fundamental are these two simple experimental facts, that it will be worth while to repeat them in slightly different form. If the three
