In the late 1970s elementary particle physicists began speaking of the “Standard Model” as the basic theory of matter. This theory is based on sets of fundamental spin-½ particles called “quarks” and “leptons,” which interact by exchanging generalized quanta, particles of spin 1. The model is referred to as “standard,” because it provides a theory of fundamental constituents – an ontological basis for describing the structure and behavior of all forms of matter (gravitation excepted), including atoms, nuclei, strange particles, and so on. In situations where appropriate mathematical techniques are available, it can be used to make quantitative predictions that are completely in accord with experiment. There are no well-established results in particle physics that clearly disagree with this theory.

This pleasing state of affairs is quite new in particle physics. It contrasts markedly with the theoretical situation in the early 1960s, when there were a variety of different ideas about the subatomic realm. For example, in 1964 most particle physicists considered protons, neutrons, pions, kaons, and a host of other strongly interacting particles (i.e., hadrons) to be in a certain sense “elementary.” By 1979 the consensus had emerged that the hadrons were not elementary after all but are composed of more basic building blocks called quarks, held together by the exchange of another kind of particle called the gluon. Or consider the particle interactions. In 1964 almost all physicists thought the strong, weak, and electromagnetic interactions were independent phenomena, perhaps requiring different types of theories for their description.