The concept of mass can be a bit nuanced when it comes to particles in the realm of quantum mechanics. While it's true that photons, as elementary particles, have no rest mass, they do possess energy and momentum. According to the theory of special relativity, the energy and momentum of a particle are related through the famous equation E = mc², where E represents energy, m represents mass, and c represents the speed of light.
For photons, their energy E is related to their momentum p by the equation E = pc, where c is the speed of light in a vacuum. Since photons always travel at the speed of light, their energy is directly proportional to their momentum. While photons lack rest mass, they still possess energy and momentum, allowing them to exhibit particle-like behavior.
In the realm of quantum mechanics, particles can exhibit both wave-like and particle-like characteristics. Photons, for example, exhibit wave-particle duality, meaning they can behave as both waves and particles depending on how they are observed or measured. This dual nature is a fundamental aspect of quantum mechanics and has been experimentally verified through various phenomena such as the double-slit experiment.
In summary, while photons have no rest mass, they possess energy and momentum, allowing them to behave as particles. The understanding of particles in quantum mechanics extends beyond the classical notion of mass and encompasses their energy, momentum, and wave-particle duality.