Page:Relativity (1931).djvu/82

 systems one ($$K$$) corresponding to a particular state of motion is physically unique. This result was interpreted physically by regarding $$K$$ as at rest with respect to a hypothetical æther of space. On the other hand, all co-ordinate systems $$K'$$ moving relatively to $$K$$ were to be regarded as in motion with respect to the æther. To this motion of $$K'$$ against the æther (“æther-drift” relative to $$K'$$) were assigned the more complicated laws which were supposed to hold relative to $$K'$$. 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 $$T$$ 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 $$T'$$ 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 $$v$$ with reference to the æther, this time $$T'$$ is different