Einstein's second postulate, also known as the "postulate of the constancy of the speed of light," is a fundamental principle in the theory of special relativity. It states that the speed of light in a vacuum is constant and is the same for all observers, regardless of their relative motion.
This postulate implies that the speed of light, denoted by the symbol 'c', has a fixed value of approximately 299,792,458 meters per second (or about 186,282 miles per second) and does not depend on the motion of the source of light or the observer measuring it.
Einstein's second postulate was a departure from classical physics, where it was believed that the speed of light would change depending on the motion of the source or the observer. By postulating the constancy of the speed of light, Einstein introduced a radical new framework for understanding the nature of space, time, and the laws of physics.
The constancy of the speed of light has been confirmed by numerous experiments and observations. It has profound implications, such as time dilation, length contraction, and the equivalence of mass and energy (as expressed by the famous equation E=mc²). These ideas form the basis of special relativity and have revolutionized our understanding of the physical world.