Theoretical Basis:
* Special Relativity: Einstein's theory of special relativity establishes a deep connection between energy (E), momentum (p), and mass (m) through the famous equation E² = (mc²)² + (pc)².
* Mass-Energy Equivalence: This equation implies that mass and energy are interchangeable. A massive object at rest has energy solely due to its mass (E=mc²). However, a massless object (like a photon) can still possess energy, but this energy comes entirely from its momentum.
* Photon Momentum: Photons have momentum (p = h/λ, where h is Planck's constant and λ is the wavelength). This momentum, coupled with the mass-energy equivalence, suggests that photons, though massless, carry energy.
Experimental Evidence:
* Speed of Light: Photons always travel at the speed of light (c) in a vacuum. If they had rest mass, they would require an infinite amount of energy to reach this speed, violating the principle of energy conservation.
* Particle Physics Experiments: Experiments in particle physics, such as those conducted at CERN, have consistently shown that photons do not exhibit the behavior of massive particles. For example, they can be produced and annihilated without any mass-energy violation.
Important Note:
* Effective Mass: While photons have zero rest mass, they can exhibit an "effective mass" in certain interactions. For instance, in a dense medium like water, photons interact with the medium and can appear to have a higher effective mass, resulting in a lower speed. This is not a true rest mass but rather an effect of the interaction with the medium.
In conclusion, photons are considered massless based on theoretical foundations and experimental observations. Their energy is solely derived from their momentum, and they always travel at the speed of light, further supporting this concept.
