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This chapter explores the heuristic fruitfulness of the exclusion principle in opening up new avenues of research: namely the idea of ‘coloured’ quarks and the development of quantum chromodynamics (QCD) in the 1960s to 1970s. Sections 5.1 and 5.2 reconstruct the origin of the quark theory from Gell-Mann's so-called ‘eightfold way’ for elementary particles. The experimental discovery of the Ω− particle confirmed the validity of Gell-Mann's model, but it also provided negative evidence against quarks obeying the exclusion principle. Two alternative research programmes emerged in the 1960s to deal with this piece of negative evidence (Section 5.3): the first programme (Section 5.3.1) rejected the strict validity of the exclusion principle and explored the possibility that quarks obeyed parastatistics; the second (Section 5.3.2) retained the exclusion principle and reconciled it with negative evidence by introducing a further degree of freedom for quarks (‘colour’). The Duhem–Quine thesis seems to loom on the horizon (Section 5.4): the choice as to whether questioning the principle or introducing an auxiliary assumption to reconcile it with negative evidence seems to be underdetermined by evidence. However, I shall argue that it was exactly via the development of these two rival research programmes that the exclusion principle came to be validated, and that there was a rationale for retaining the principle despite prima facie recalcitrant evidence. Introduction As we have seen in Chapter 4, soon after 1924 Pauli's exclusion rule was incorporated into the growing quantum mechanical framework, where its role came to be redefined and its nomological scope extended. [ABSTRACT FROM AUTHOR]

This panel was intended to function as a discussion, but instead it emerged as a series of short presentations by the participants Robert Brout, Tian Yu Cao, and Peter Higgs, with an introductory discussion by the chair. The present chapter consists of a revised and edited version of those reports and also includes a later submission by Yoichiro Nambu, who was scheduled to be on the panel originally but was unable to attend. Introduction The two sectors of the current Standard Model of particle physics, the strong color and the electroweak sectors, are distinct and are tied together only by ontology. Together, they describe the interactions, other than gravitation, of the three generations of quarks and leptons. The dream of representing the strong and weak “nuclear” interactions (as they were known before the acceptance of the quarks) as quantum field theories (QFT) goes back to the 1930s. The first such QFT, other than quantum electrodynamics, was Enrico Fermi's weak-interaction theory of 1934. This theory was almost immediately extended by Werner Heisenberg in 1935 to include the strong interactions (thus making it the first unified QFT) whose exchanged “quanta” were those of the electron-neutrino “Fermi-field.” In 1935, Hideki Yukawa invented “U-quanta,” now called pions, to represent the field of strong interactions, adjusting their mass to fit the range of nuclear forces. This was again a unified QFT, as the U-quanta were also intended to serve as intermediate bosons of the weak interaction. [ABSTRACT FROM AUTHOR]

The Standard Model is surely one of the major intellectual achievements of the twentieth century. In the late 1960s and early 1970s, decades of path-breaking experiments culminated in the emergence of a comprehensive theory of particle physics. This theory identifies the basic fundamental constituents of matter and describes all the forces of nature relevant at accessible energies – the strong, weak, and electromagnetic interactions. Science progresses in a much more muddled fashion than is often pictured in history books. This is especially true of theoretical physics, partly because history is written by the victorious. Consequently, historians of science often ignore the many alternate paths that people wandered down, the many false clues they followed, the many misconceptions they had. These alternate points of view are less clearly developed than the final theories, harder to understand and easier to forget, especially as these are viewed years later, when it all really does make sense. Thus reading history one rarely gets the feeling of the true nature of scientific development, in which the element of farce is as great as the element of triumph. The emergence of quantum chromodynamics, or QCD, is a wonderful example of the evolution from farce to triumph. During a very short period, a transition occurred from experimental discovery and theoretical confusion to theoretical triumph and experimental confirmation. We were lucky to have been young then, when we could stroll along the newly opened beaches and pick up the many beautiful shells that experiment had revealed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The history of science is usually told in terms of experiments and theories and their interaction. But there is a deeper level to the story – a slow change in the attitudes that define what we take as plausible and implausible in scientific theories. Just as our theories are the product of experience with many experiments, our attitudes are the product of experience with many theories. It is these attitudes that one usually finds at the root of the explanation for the curious delays that often occur in the history of science, as for instance, the interval of 15 years between the theoretical work of Alpher and Herman and the experimental search for the cosmic microwave radiation background. The history of science in general and this conference in particular naturally deal with things that happened, with successful theories and experiments, but I think that the most interesting part of the history of science deals with things that did not happen, or at least not when they might have happened. To understand this sort of history, one must understand the slow changes in the attitudes by which we are governed. But it is not easy. Experimental discoveries are reported in The New York Times, and new theories are at least reported in physics journals, but the change in our attitudes goes on quietly and anonymously, somewhere behind the blackboard. The rise of the Standard Model was accompanied by profound changes in our attitudes toward symmetries and toward field theory. [ABSTRACT FROM AUTHOR]