The fact that light has no rest mass is supported by various experimental observations and is a fundamental principle in modern physics. Here are a few key pieces of evidence:
Theory of Special Relativity: Albert Einstein's theory of special relativity, published in 1905, played a crucial role in establishing that light has no rest mass. According to this theory, the laws of physics should be the same for all observers in uniform motion relative to each other. One consequence of special relativity is the famous equation E = mc^2, where E represents energy, m represents mass, and c represents the speed of light. This equation implies that an object with rest mass cannot attain or exceed the speed of light. Since light travels at the speed of light in a vacuum, it must have zero rest mass.
Particle Accelerators: High-energy particle accelerators, such as the Large Hadron Collider (LHC), have been used to study the behavior of particles. These experiments have provided strong evidence that photons (particles of light) have no rest mass. By examining the behavior of particles in particle collisions and interactions, scientists have been able to verify the predictions of relativistic theories and confirm that photons are massless particles.
Experimental Limits: Experiments have been conducted to directly measure the rest mass of photons, setting upper limits on their mass. These experiments include studying the behavior of photons over long distances, precision measurements of the speed of light, and observations of phenomena like the cosmic microwave background radiation. These experiments have consistently confirmed that the rest mass of photons, if any, is extremely close to zero.
Electromagnetic Wave Properties: The behavior of light as an electromagnetic wave can be explained most consistently if it has no rest mass. Electromagnetic waves, such as light, are governed by Maxwell's equations, which describe the interaction between electric and magnetic fields. These equations predict that electromagnetic waves propagate at the speed of light, indicating that light has no rest mass.
Collectively, these experimental observations and theoretical frameworks strongly support the conclusion that light, as well as other electromagnetic waves, has no rest mass.