We have already seen that gauge symmetry that characterizes the electroweak force-the freedom to interchange electrons and neturinos-dictates the existence of the messenger electroweak fields {photon, W, and Z}. Similarly, the gauge color symmetry requires the presence of eight gluon fields. The gluons are the messengers of the strong force that binds quarks together to form composite particles such as the proton. Incidentally, the color "charges" of the three quarks that make up a proton or a neutron are all different {red, blue, green}, and they add up to give zero color charge or "white" {equivalent to being electrically neutral in electromagnetism}. Since color symmetry is at the base of the gluon-mediated force between quarks, the theory of these forces has become known as quantum chromodynamics. The marriage of the electroweak theory {which describes the electromagnetic and weak forces} with quantum chromodynamics {which describes the strong force} produced the standard model-the basic theory of elementary particles and the physical laws that govern them.
( Mario Livio )
[ The Equation That Couldn't Be ]
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