According to Einstein's theory of relativity, the speed of light in a vacuum is constant and independent of the motion of the source or the observer. This principle is known as the constant speed of light or "c."
Einstein's theory of relativity consists of two main parts: the special theory of relativity and the general theory of relativity. The special theory of relativity, proposed in 1905, deals with the behavior of objects moving at constant speeds in the absence of gravitational forces. It introduces the concept of "c" as the maximum speed in the universe.
In the special theory of relativity, Einstein postulated that the laws of physics should be the same for all observers moving at constant speeds relative to each other. This means that the measured speed of light should be the same for all observers, regardless of their relative motion.
To reconcile this postulate with the observation that light appears to have the same speed for all observers, regardless of their relative motion, Einstein introduced the concept of time dilation and length contraction. According to these concepts, as an observer's relative velocity approaches the speed of light, time slows down and lengths appear to contract in the direction of motion. These effects ensure that the measured speed of light remains constant for all observers.
In other words, if an observer is moving relative to a light source, the observer will measure the same speed of light as an observer who is at rest relative to the light source. This is because time dilation and length contraction compensate for the relative motion, ensuring that the speed of light remains constant.
It's important to note that the constant speed of light applies specifically to light traveling in a vacuum. When light passes through a medium such as air, water, or glass, its speed can be slower due to interactions with the atoms or molecules in the medium. However, in a vacuum, the speed of light is always constant and is approximately 299,792,458 meters per second (or about 186,282 miles per second).
The constancy of the speed of light is a fundamental principle of the theory of relativity and has been extensively confirmed by experimental observations and measurements.