Light travels at a constant speed in a vacuum, which is approximately 299,792,458 meters per second, often approximated as 3 x 10^8 meters per second.
The constancy of the speed of light, denoted by the symbol "c," is a fundamental principle in physics and is a cornerstone of Einstein's theory of relativity. The speed of light being constant means that it does not depend on the motion of the source or the observer; it remains the same for all observers regardless of their relative velocities.
There are several ways we know that the speed of light is constant:
Michelson-Morley Experiment: In the late 19th century, Albert A. Michelson and Edward W. Morley conducted an experiment to detect the hypothetical "ether" through which light was thought to propagate. The experiment aimed to measure any change in the speed of light due to the Earth's motion through the ether. However, the results consistently showed that the measured speed of light was the same in all directions, regardless of Earth's motion.
Theory of Special Relativity: Albert Einstein's theory of special relativity, published in 1905, proposed that the laws of physics are the same for all observers in uniform motion. It introduced the postulate that the speed of light is constant in all inertial frames of reference. This theory has been extensively tested and confirmed by numerous experiments and observations.
Experimental Confirmations: Over the years, various experiments have been conducted to measure the speed of light using different methods. These experiments, including timing the travel of light over known distances and interferometry techniques, consistently yield results that confirm the constancy of the speed of light.
Electromagnetic Theory: The speed of light is intimately connected with the fundamental properties of electromagnetic waves. Maxwell's equations, which describe the behavior of electric and magnetic fields, predict that the speed of electromagnetic waves in a vacuum should be precisely equal to the speed of light. Subsequent experiments have validated this prediction.
Combining these experimental and theoretical pieces of evidence, scientists have established that the speed of light is a fundamental constant of nature and does not change in a vacuum.