RELATIVITY, The Principle of. The principle of relativity consists essentially in the hypothesis that it is impossible by physical ex periment to measure or detect the absolute mo tion of a body through space. As a conse quence the expressions "absolute velocity" and "body at rest" have no significance. Only rela tive motion can be detected or even defined. If this were all the subject would be of interest to the philosopher rather than the physicist. But it can be shown as a consequence that space and time have no absolute significance. They depend on the motion of the observer. As stated by Minkowski, "From now on, space by itself and time by itself, are mere shadows, and only a kind of blend of the two exists in its own right." Absolute Motion.— It has been known for a long time that the dynamical properties of a system in uniform motion are identical with those of the same system at rest. A person shut in a box moving with constant velocity could not determine by mechanical means whether be was moving or not. Certain optical experi ments, however, suggested that optical methods might determine absolute motion. Thus the index of refraction of a piece of glass is the ratio of the velocity of the incident light to that of the refracted light. Arago, in 1818,, suggested that this might be used to determine, absolute velocity. The experiment did not sue-, ceed. Fresnel explained this by supposing the light to be partially convected, or dragged, by the moving medium. Fizeau tested this notion by sending a beam of light along the direction. of motion through water flowing in a tube and • found that the velocity of light in the moving water was slightly different from that in water at rest. The experiment, however, only indi
cated the difference in velocity of the water in two tubes and not the absolute velocity.
The most famous experiment for detecting absolute motion was devised by A. A. Michel-, son in 1:•:1, repeated with greater refinement by E. W. Morley in 1887, and again repeated with greater care by E. W. Morley and D. B. Miller in 1905. It is generally called the 'Michelson-Morley Experiment." The fundamental idea is as follows: A beam of light from a source S is divided by partial reflection at a plate of glass A into two portions traveling along the paths AM, and AM,. These portions are reflected back by mirrors at M1, M3, and, on striking the plate again, a part of the beam from M1 is trans witted and a part of that from if, is reflected, the two being brought to interference at C.
Suppose the whole apparatus moving with velocity v in the direction AM,. Let c be the velocity of light. As the light goes from A to .M, its velocity relative to the apparatus is c—v, while on the return journey it is The time to pass from A to M, and back again is then 13 213c c v — vs Since the apparatus moves the distance v while light moves the distance c, the beam passing from A to M and back describes a path ABA in space. By similar triangles The time required to describe the path' ABA' is then