The existence of quarks with fractional charges might seem to contradict the idea of charge quantization, which states that electric charge is always observed in discrete multiples of the elementary charge carried by the electron. However, it's important to understand that quarks are never observed in isolation, but always exist in bound states within particles.
The concept of fractional charges arises due to a phenomenon called color confinement. Quarks carry an additional property known as color charge, which is related to the strong nuclear force. The strong force, which binds quarks together to form composite particles called hadrons (such as protons and neutrons), behaves differently from the electromagnetic force.
The strong force is mediated by particles called gluons, which also carry color charge. Unlike photons, which mediate the electromagnetic force and have no electric charge themselves, gluons carry color charge and can interact with other gluons. This interaction leads to a unique property of the strong force called asymptotic freedom.
Asymptotic freedom means that at very high energies or short distances, the strong force weakens. This allows quarks to behave as approximately free particles, and their fractional charges become meaningful. However, as quarks move apart, the energy stored in the gluon field between them increases. Eventually, it becomes energetically favorable to create new quark-antiquark pairs from the vacuum, forming new bound states rather than observing isolated quarks.
This phenomenon is known as color confinement because it confines quarks within composite particles, rendering them unobservable as free particles. As a result, we only observe hadrons with integer charges, even though they are composed of quarks with fractional charges.
In summary, the fractional charges of quarks arise due to the complex nature of the strong force and the phenomenon of color confinement. While quarks themselves have fractional charges, they are always observed in bound states, which appear to have integer charges, thus maintaining the quantization of charge.