2005

Einstein believed that Galileo's principle of relativity, which states that the laws of mechanics are expressed in the same form in all inertial frames, holds true not only in mechanics but also in optics and electromagnetism. This is called the special theory of relativity. If the laws of electromagnetism are expressed in the same form in any inertial frame, then the speed c of light, which is an electromagnetic wave, must be the same speed in any inertial frame. This is called the "principle of the constant speed of light". In 1905, exactly 100 years before him, Einstein succeeded in constructing the special theory of relativity using the special principle of relativity and the principle of the constant speed of light.

1.1 As shown in figure, let us consider setting (synchronizing) stationary clocks at two points A and B that are separated by a distance l in the same inertial frame (coordinate frame in which the law of inertia holds). When a clock placed at point A is displaying the time t_A , light is emitted from A. When this light arrives at point B, which is a distance l, the time t_B displayed by the clock placed at point B is expressed as t_B = t_A + l/c. In the same way, when the light emitted from point B arrives at point A, the displays on both clocks are adjusted in the same way.