The concept of a particle is not solely dependent on mass. In physics, particles are often defined by their properties and behavior, rather than just their mass. While it is true that photons have zero rest mass, they still exhibit particle-like behavior and are considered elementary particles.
In the framework of quantum mechanics, particles are described by wave-particle duality. This means that particles, including photons, can exhibit both wave-like and particle-like properties. Photons, as particles, are described by quantized packets of energy called quanta, or photons. They have discrete energy levels and can interact with matter as discrete entities.
Experimental evidence supports the particle nature of photons. For example, the photoelectric effect demonstrates that photons can transfer discrete amounts of energy to electrons, causing them to be ejected from a material. This phenomenon was explained by Albert Einstein, who proposed that light is composed of photons that act as individual particles.
It's important to note that the concept of massless particles extends beyond just photons. Other particles, such as gluons and gravitons, are also believed to be massless. These particles play crucial roles in the fundamental forces of nature.
In summary, while photons have no rest mass, they exhibit particle-like behavior and are considered fundamental particles due to their quantized energy and the experimental evidence supporting their discrete nature. The particle-wave duality inherent in quantum mechanics allows us to describe and understand the behavior of particles like photons, even in the absence of mass.