According to the special theory of relativity, the speed of light in a vacuum is constant and does not change based on the motion of the observer. This fundamental principle is known as the constancy of the speed of light.
The special theory of relativity, developed by Albert Einstein, postulates that the laws of physics are the same for all observers in uniform motion relative to each other. In this framework, the speed of light in a vacuum is always measured to be approximately 299,792,458 meters per second (or about 186,282 miles per second) by all observers, regardless of their relative motion.
This constancy of the speed of light has profound implications, such as time dilation and length contraction, which are well-established effects of special relativity. These effects arise from the fact that the measured speed of light is constant for all observers, regardless of their relative motion.