According to Einstein's theory of special relativity, the speed of light is constant and appears the same to all observers, regardless of their relative motion. This principle is known as the "constancy of the speed of light."
However, what changes for observers in motion is their perception of time and space due to a phenomenon called "time dilation" and "length contraction." These effects arise from the relative motion between the observer and the source of light.
Time dilation refers to the slowing down of time for an object in motion relative to a stationary observer. As an observer moves faster relative to another observer, time appears to pass more slowly for the moving observer compared to the stationary observer. This means that clocks on the moving object would run slower from the perspective of the stationary observer.
Length contraction, also known as Lorentz contraction, refers to the shortening of an object's length in the direction of its motion as perceived by a stationary observer. Objects in motion appear compressed along their direction of travel when observed from a stationary reference frame.
Importantly, although time dilation and length contraction occur for moving observers, the speed of light itself remains the same for all observers, regardless of their relative motion. No matter how fast an observer is moving or in which direction, the speed of light will always be measured as approximately 299,792,458 meters per second in a vacuum.
These concepts challenge our intuitive understanding of space and time, but they have been experimentally verified and are fundamental principles in the theory of relativity.