In the theory of special relativity, the motion of the source of light does not affect the speed or direction of the light beam itself. According to the principle of relativity, the laws of physics, including the behavior of light, are the same in all inertial reference frames.
So, if the source of light moves laterally relative to an observer, the light beam will continue to propagate in a straight line at the same speed, independent of the source's motion. This principle is known as the constancy of the speed of light.
The constancy of the speed of light has important implications for Einstein's thought experiment involving light clocks. In this experiment, two synchronized light clocks, which consist of a light beam bouncing between two mirrors, are observed by an observer moving relative to the clocks.
When the observer moves, the path of the light beams within the clocks appears to change due to the observer's motion. However, the speed of light remains constant for all observers, regardless of their motion. As a result, to maintain the constancy of the speed of light, time itself must dilate for the moving observer relative to the observer at rest. This phenomenon is known as time dilation.
Therefore, in the context of Einstein's light clocks, the constancy of the speed of light implies that the passage of time is relative and depends on the relative motion between observers. The moving observer will measure a different elapsed time compared to the observer at rest, reflecting the effects of time dilation.
Overall, the constancy of the speed of light is a fundamental principle of special relativity that underlies various phenomena, including time dilation and the independence of the light beam's motion from the source's motion.