systems one () corresponding to a particular state of motion is physically unique. This result was interpreted physically by regarding as at rest with respect to a hypothetical æther of space. On the other hand, all co-ordinate systems moving relatively to were to be regarded as in motion with respect to the æther. To this motion of against the æther (“æther-drift” relative to ) were assigned the more complicated laws which were supposed to hold relative to . Strictly speaking, such an æther-drift ought also to be assumed relative to the earth, and for a long time the efforts of physicists were devoted to attempts to detect the existence of an æther-drift at the earth’s surface.
In one of the most notable of these attempts Michelson devised a method which appears as though it must be decisive. Imagine two mirrors so arranged on a rigid body that the reflecting surfaces face each other. A ray of light requires a perfectly definite time to pass from one mirror to the other and back again, if the whole system be at rest with respect to the æther. It is found by calculation, however, that a slightly different time is required for this process, if the body, together with the mirrors, be moving relatively to the æther. And yet another point: it is shown by calculation that for a given velocity with reference to the æther, this time is different
