The speed of light being accepted as a universal constant is supported by several lines of evidence from experimental observations and theoretical frameworks. Here are some key pieces of evidence:
Maxwell's Equations: In the 19th century, James Clerk Maxwell formulated a set of equations that unified electricity and magnetism, known as Maxwell's equations. These equations predicted the existence of electromagnetic waves and provided a theoretical framework for the propagation of light. The equations revealed that the speed of these waves is determined by the fundamental constants of the electromagnetic field, namely the permittivity of free space (ε₀) and the permeability of free space (μ₀). By solving these equations, Maxwell derived a speed that matched the known speed of light, suggesting that light is an electromagnetic wave.
Michelson-Morley Experiment: In 1887, Albert A. Michelson and Edward W. Morley conducted an experiment to detect the hypothetical "ether" medium through which light waves were thought to propagate. Their experiment aimed to measure the difference in the speed of light in different directions as the Earth moved through the ether. Surprisingly, they found no significant difference in the measured speed of light in different directions, suggesting that the speed of light is constant and does not depend on the motion of the observer. This experiment provided strong evidence against the existence of the ether and supported the constancy of the speed of light.
Time Dilation and Length Contraction: The constancy of the speed of light is a fundamental postulate of Einstein's theory of special relativity. According to this theory, the laws of physics are the same for all observers in uniform motion relative to each other, and the speed of light is invariant in all inertial reference frames. As a consequence of this postulate, special relativity predicts phenomena like time dilation and length contraction, which have been experimentally confirmed in numerous high-precision experiments, such as the measurements of particle accelerators and cosmic ray observations.
Implications for physics: The constancy of the speed of light has profound implications for physics, especially when compared with other fundamental constants like Planck's constant. Here are a few implications:
Lorentz Invariance: The constancy of the speed of light is intimately connected to the principle of Lorentz invariance, which is a cornerstone of special relativity. Lorentz invariance means that the laws of physics are invariant under Lorentz transformations, which describe the relationship between space and time coordinates between different observers in relative motion. This has far-reaching consequences for our understanding of causality, simultaneity, and the interplay between space and time.
Mass-Energy Equivalence: Einstein's famous equation, E=mc², demonstrates the equivalence between mass and energy. The constancy of the speed of light is crucial in this equation, as it implies that the energy content of an object with mass is inherently connected to its mass itself. This principle underlies the understanding of nuclear energy, the conversion of mass into energy in nuclear reactions, and the concept of rest mass as a form of stored energy.
Fundamental Constants: The constancy of the speed of light and other fundamental constants, such as Planck's constant, play significant roles in determining the nature of the physical laws and the fundamental units of measurement. These constants are used to define units like the meter, second, kilogram, and ampere, allowing for accurate and consistent measurements across different experimental setups. Furthermore, they are crucial in formulating theories and calculations involving quantum mechanics and the behavior of particles at the microscopic scale.
In summary, the evidence for the constancy of the speed of light comes from experimental observations, theoretical frameworks like Maxwell's equations and special relativity, and their successful predictions. The constancy of the speed of light has far-reaching implications for our understanding of space, time, energy, and the fundamental constants that govern the laws of physics.