The speed of light in a vacuum, denoted by c, is a fundamental constant of nature. It equals exactly 299,792,458 meters per second. According to Einstein's theory of special relativity, this speed is the same for all observers, regardless of their motion or the motion of the light source. This constancy of the speed of light is one of the most important principles in modern physics.
In 1905, Albert Einstein revolutionized physics with his theory of special relativity. A fundamental postulate of this theory is that the speed of light in a vacuum is constant for all observers, regardless of their relative motion or the motion of the light source. This was a radical departure from classical physics, which assumed that speeds add together. In Einstein's framework, light always travels at the same speed, which leads to fascinating consequences like time dilation and length contraction.
The constancy of the speed of light has been confirmed by numerous experiments over the past century. The famous Michelson-Morley experiment in 1887 failed to detect any difference in the speed of light in different directions, contradicting the prevailing ether theory. Later, the Kennedy-Thorndike experiment in 1932 further confirmed this result. Today, modern atomic clocks can measure time with incredible precision, allowing scientists to verify that light speed remains constant. Even our GPS satellite system requires corrections based on relativity to function accurately, providing daily confirmation of Einstein's theory.
The constancy of the speed of light leads to several profound consequences that challenge our intuitive understanding of space and time. First, time dilation occurs when objects move relative to an observer - moving clocks run slower. Second, length contraction means that moving objects appear shorter in the direction of motion. Third, Einstein's famous equation E equals mc squared shows that mass and energy are equivalent and interconvertible. Finally, objects gain relativistic mass as they approach the speed of light, making it impossible to accelerate any object with mass to light speed, as it would require infinite energy.
To summarize what we've learned: First, the speed of light in a vacuum is exactly 299,792,458 meters per second. Second, this speed is constant for all observers, regardless of their relative motion or the motion of the light source. Third, this constancy is a fundamental postulate of Einstein's special theory of relativity. Fourth, it leads to remarkable consequences including time dilation, length contraction, and the equivalence of mass and energy. Finally, numerous experiments over the past century have confirmed this principle, making it one of the most well-tested concepts in physics.