Abstract

Quantum chromodynamics, familiarly called QCD, is the modern theory of the strong interaction. Historically its roots are in nuclear physics and the description of ordinary matter--understanding what protons and neutrons are and how they interact. In the same pictorial language, QCD appears as an expanded version of QED. Whereas in QED there is just one kind of charge, QCD has three different kinds of charge, labeled by "color." Avoiding chauvinism, we might choose red, green, and blue. But, of course, the color charges of QCD have nothing to do with physical colors. Rather, they have properties analogous to electric charge. In particular, the color charges are conserved in all physical processes, and there are photon-like massless particles, called color gluons, that respond in appropriate ways to the presence or motion of color charge, very similar to the way photons respond to electric charge.