The discovery that light has no mass is rooted in a combination of experimental evidence and theoretical developments in the late 19th and early 20th centuries. Here's a brief overview of the key steps that led to this realization:
Electromagnetic Theory: In the 19th century, James Clerk Maxwell formulated a set of equations known as Maxwell's equations, which successfully described the behavior of electric and magnetic fields. These equations predicted the existence of electromagnetic waves, with light being one such wave.
Speed of Light: The speed of light was measured by a series of experiments, most notably the work of Albert Michelson and Edward Morley in 1887. Their experiments aimed to detect the hypothetical "ether" through which light was thought to propagate. The experiments yielded null results, indicating that the speed of light was constant and independent of the motion of the observer.
Wave Theory of Light: The experimental findings on the speed of light, along with the success of Maxwell's equations, led to the acceptance of the wave theory of light. According to this theory, light is an electromagnetic wave propagating through space.
Einstein's Theory of Special Relativity: In 1905, Albert Einstein published his special theory of relativity. One of the key insights of this theory was the equivalence of mass and energy, expressed by the famous equation E=mc². This equation showed that energy and mass are interchangeable, and any form of energy contributes to an object's mass.
Photon Theory: In 1905, Einstein also proposed the concept of quanta of light, now known as photons. He suggested that light consists of discrete packets of energy, each carrying a specific amount of energy proportional to its frequency. The energy of a photon is given by E = hf, where h is Planck's constant and f is the frequency of light.
Photon Mass: The mass of a particle is a measure of its inertia, and it affects its behavior under the influence of forces. Experiments and theoretical considerations showed that light's behavior did not match that of massive particles. In particular, the behavior of light in gravitational fields, as predicted by general relativity, confirmed that light does not possess mass.
Through a combination of experimental measurements, theoretical developments, and the conceptual framework of special relativity and quantum mechanics, scientists came to the understanding that light consists of massless particles called photons. This understanding is now a cornerstone of modern physics and has been extensively verified through numerous experiments and observations.