Einstein's theory of relativity, specifically special relativity, explains that the speed of light in a vacuum is constant and is denoted by the symbol "c." According to this theory, the speed of light in a vacuum is the same for all observers, regardless of their relative motion. This fundamental principle is known as the "constancy of the speed of light."
Special relativity is based on two postulates:
The Principle of Relativity: The laws of physics are the same in all inertial reference frames. An inertial reference frame is a frame of reference that is not accelerating.
The Constancy of the Speed of Light: The speed of light in a vacuum is the same for all observers, regardless of the motion of the source of light or the observer.
These postulates have far-reaching consequences. They lead to phenomena such as time dilation, length contraction, and the equivalence of mass and energy (expressed by the famous equation E = mc²).
The constancy of the speed of light arises from the mathematical structure of spacetime in special relativity. In this theory, space and time are woven together into a four-dimensional continuum called spacetime. The speed of light represents the maximum speed at which information or causal influence can travel through this spacetime fabric.
As a consequence of the constancy of the speed of light, other physical quantities, such as time and space intervals, become relative and depend on the observer's relative motion. This gives rise to effects like time dilation, where time appears to pass slower for objects in motion relative to an observer at rest.
It's important to note that the constancy of the speed of light applies specifically to light in a vacuum. When light travels through a medium such as air or water, its speed is slower due to interactions with the atoms or molecules in that medium. However, the speed of light in a vacuum remains the same regardless of the observer's motion.