The reason for the difference in mass behavior between quarks and photons lies in the fundamental properties of these particles and the underlying theories that describe them. Let's break it down:
Quarks and Mass: Quarks, according to the Standard Model of particle physics, are always confined within composite particles called hadrons (such as protons and neutrons). Quarks are bound together by the strong nuclear force, which is carried by particles called gluons. The energy associated with the strong force is what contributes to the mass of hadrons, including the mass of the quarks within them. Quarks gain mass not directly from their kinetic energy but from the overall energy and interactions within the system.
Photons and Masslessness: Photons, on the other hand, are elementary particles that are carriers of electromagnetic radiation, including light. They do not possess mass in the traditional sense. According to the theory of special relativity, massless particles always travel at the speed of light in a vacuum. The speed of light in a vacuum is considered a fundamental constant of nature. Photons are massless because they are associated with the electromagnetic force, which is characterized by the symmetry of Maxwell's equations. This symmetry results in the photon having zero rest mass.
It's important to note that the concept of mass in particle physics can be different from everyday notions of mass. Mass in particle physics is intimately linked to energy and how particles interact with various fields. The Higgs mechanism, for example, is a key part of the Standard Model that explains how certain particles acquire mass. However, it does not apply to photons since they are massless and do not interact with the Higgs field.
In summary, the difference in mass behavior between quarks and photons arises from the nature of their interactions, the forces they are associated with, and the underlying theoretical frameworks that describe them. Quarks gain mass from their interactions and the strong nuclear force, while photons remain massless due to their association with the electromagnetic force and the fundamental properties of light and relativity.