According to the theory of special relativity, the speed of light in a vacuum, denoted by "c," is constant and the same for 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 how they perceive events and measurements of time and space. Two key concepts in special relativity are time dilation and length contraction.
Time dilation: According to special relativity, time appears to pass more slowly for an observer in motion relative to a stationary observer. This means that if an observer is moving relative to another observer, the moving observer will measure time passing more slowly compared to the stationary observer. This effect becomes more pronounced as the relative velocity between the observers approaches the speed of light. The speed of light itself remains constant for both observers.
Length contraction: Similarly, observers in relative motion will perceive a contraction in the length of objects along the direction of motion. This means that an object in motion will appear shorter when observed by a stationary observer. Again, this effect becomes more significant as the relative velocity increases, but the speed of light remains the same for both observers.
These effects arise from the fundamental postulates of special relativity, which challenge our intuitive notions of space and time. They have been confirmed by numerous experiments and are crucial for understanding the behavior of objects moving at relativistic speeds.