Einstein's theory of relativity, both special relativity and general relativity, does indeed maintain that the speed of light in a vacuum is a constant. This fundamental principle is a cornerstone of modern physics and has been extensively validated by numerous experiments and observations.
In special relativity, which deals with objects moving at constant velocities in the absence of gravitational effects, the constant speed of light (ccc) is a postulate. It means that the speed of light is the same for all observers, regardless of their relative motion. No matter how fast an observer is moving with respect to a light source, they will always measure the speed of light to be approximately 299,792,458299,792,458299,792,458 meters per second (ccc).
The constant speed of light in special relativity has several profound consequences, including time dilation, length contraction, and the equivalence of mass and energy (E=mc²).
In general relativity, which extends the principles of special relativity to include the effects of gravity, the constant speed of light still holds locally in small regions of spacetime. However, in the presence of massive objects like stars or black holes, the curvature of spacetime can affect the paths that light takes, giving rise to phenomena like gravitational lensing. But even in these cases, the speed of light remains constant locally.
The constant speed of light in relativity is not a limitation of the theory; rather, it is a fundamental property of spacetime. It means that the speed of light serves as a universal "speed limit" in the universe. Nothing with mass can reach or exceed the speed of light, as doing so would require infinite energy.
The constancy of the speed of light has been verified through countless experiments and observations, and it forms the foundation of our understanding of the behavior of the universe at both macroscopic and microscopic scales.