The Invariant Nature of Light's Speed
The speed of light in a vacuum (c) is a fundamental physical constant, meaning it always measures the same value, approximately 299,792,458 meters per second, regardless of the motion of the light source or the observer. This principle is not derived from first principles but is an observed property of our universe, forming a core postulate of Albert Einstein's theory of special relativity.
Maxwell's Equations and Electromagnetism
Before Einstein, James Clerk Maxwell's equations, which describe the behavior of electric and magnetic fields, predicted the existence of electromagnetic waves (like light) and calculated their speed. Crucially, these equations showed that the speed of these waves depended only on fundamental constants of electromagnetism, not on the motion of the source. This implied light does not need a medium to propagate and its speed is fixed, contradicting classical Galilean relativity which suggested speeds should add or subtract.
Experimental Confirmation: The Michelson-Morley Experiment
The constancy of light's speed was famously confirmed by the Michelson-Morley experiment in 1887. This experiment attempted to detect a hypothetical 'luminiferous aether' that light was thought to travel through. However, it found no difference in the speed of light traveling in different directions, regardless of Earth's motion. This null result profoundly challenged classical physics and supported the idea that light's speed in a vacuum is absolute.
Implications for Special Relativity
Einstein embraced this experimental fact as a postulate in his theory of special relativity (1905). By accepting light's constant speed, independent of the observer, he derived radical consequences for space and time, including time dilation (time passing slower for moving objects) and length contraction (objects appearing shorter in the direction of motion). These effects are only noticeable at speeds approaching the speed of light, but they are crucial for understanding the universe at high velocities.