The General Theory of Relativity

Abstract

In the previous chapter on Einstein’s special theory of relativity, we saw that this special theory is necessary because the relativity of Galileo and Newton places the Newtonian laws of motion in a preferred position with respect to the laws of optics. As we have seen, only the Newtonian laws of mechanics remain the same in all inertial frames if we adopt the Galilean-Newtonian concepts of absolute space and absolute time; the laws of optics (in particular, the law concerning the speed of light in a vacuum as deduced from Maxwell’s electromagnetic theory of light) take on different forms in different inertial frames. To obtain a description of nature which treats all the laws on the same footing so that neither mechanical nor optical phenomena can be used to differentiate between any two inertial observers, we must use Einstein’s four-dimensional space-time concept. Thus, the special theory of relativity is required if we are to consider all inertial frames of reference to be equal or equivalent in the eyes of nature. If the special theory were not true, the laws (in particular the laws of optics) would have to be so formulated that they are valid in only one frame of reference—the frame which is at absolute rest in the universe.

A theory is the more impressive the greater the simplicity of its principles is, the more different kinds of things it relates, and the more extended is its area of applicability.

—albert einstein, Autobiographical Notes