Questions About Quarks
Two questions remained for physicists to answer. The first was: If quarks had spin −1/2, and were therefore subject to the Pauli exclusion principle (which says that two identical spin −1/2 particles cannot coexist in a quantum system), how then could two up quarks be inside the proton at the same time? Physicists solved this problem with the introduction of the property of "color"(again, the term is fanciful, not literal). Each quark could also have one of three colors, which were given the names red, green, and blue. If the two up quarks in a proton were different colors, Pauli's exclusion principle would be satisfied the quark model could go on, with the rule that all combinations of quarks had to be overall colorless: for example, red + green + blue (which, in actual color, combine to make up white light).
The second question was perhaps more puzzling. Physicists found that no matter how much energy they used—no matter how hard they smashed things into protons or protons into things—they never found a quark on its own, isolated. Quarks seemed to travel only in well-hidden packs, only inside baryons and mesons.
This problem was solved with the introduction of a sophisticated mathematical treatment of quarks termed quantum chromodynamics (QCD). QCD gave reality to the idea of color, considering it akin to electric charge: quarks were attracted to one another through their color "charges." Whereas electrons attract and repel other electric charges by exchanging a photon, quarks do so by a new particle called the gluon (which acts like glue). Unlike photons, which have no electric charge, gluons have a color charge, or rather, combinations of color charges. A red quark can emit a gluon and turn into a green quark. The emitted gluon will have the color combination red + anti-green. There are eight different gluons.
Putting this all together in the QCD theory, physicists found that because gluons have color, they will attract one another. The result is that quarks do not like to be separated—they prefer to remain near one another, that is, within the diameter of the proton (about 10–15 m). But if one tries to separate them, they emit more and more gluons until everything breaks apart into new combinations of quarks, anti-quarks, and gluons—not individual quarks or gluons. This strange property is called asymptotic freedom. Unlike the force between electric charges, which decreases with distance, the force between color charges increases (sharply) with distance.
What are the masses of the quarks? This is a difficult question, since because of asymptotic freedom they've never been seen alone. The up and down quarks appear to have masses of about one-third that of the proton, and the strange quark about one-half that of the proton. Quantum chromodynamics and its predictions has been well established by many different experiments, and it is the accepted theory of the nuclear force.