Genius_ The Life and Science of Richard Feynman - James Gleick [182]
As the particle era unfolded, however, it made other demands of top theorists—whose ranks, meanwhile, were expanding. They had to demonstrate new kinds of flair in sorting through the relations between particles. They competed to invent abstract concepts to help organize the information arriving from accelerators. A new quantum number like isotopic spin—a quantity that seemed to be conserved through many kinds of interactions—implied new incarnations of symmetry. This notion increasingly dominated the physicists’ discourse. Symmetry for physicists was not far removed from symmetry for children with paper and scissors: the idea that something remains the same when something else changes. Mirror symmetry is the sameness that remains after a reflection of left and right. Rotational symmetry is the sameness that remains when a system turns on an axis. Isotopic spin symmetry, as it happened, was the sameness that existed between the two components of the nucleus, the proton and the neutron, two particles whose relationship had been oddly close, one carrying charge and the other neutral, their masses nearly but not exactly identical. The new way to understand these particles was this: They were two states of a single entity, now called a nucleon. They differed only in their isotopic spin. One was “up,” the other “down.”
Theorists of the new generation had not only to master the quantum electrodynamics set forth by Feynman and Dyson. They also had to arm themselves with a rococo repertoire of methods suited to the new territory. Physicists had long utilized exotic variations of the idea of space—imaginary spaces in which the axes might represent quantities other than physical distance. “Momentum space,” for example, allowed them to plot and to visualize a particle’s momentum as though it were merely another spatial variable. One grew comfortable with such spaces, and now they were multiplying. Isotopic spin space became essential to understanding the strong forces acting on nucleons.
Other concepts, too, had to become second nature. Symmetries suggested that various particles must come in families: pairs, or triplets, or (as physicists now said) multiplets. Physicists experimented with what they called “selection rules”—rules about what must or must not happen in particle collisions on account of the conservation of quantities like charge. A physicist Feynman’s age, Abraham Pais, guessed at a rule called “associated production”—certain collisions must produce new particles in groups, preserving some putative new quantum number, the nature of which was unknown. Feynman had had a similar idea in Brazil but had not liked it enough to pursue it diligently. For a few years associated production became an important catchphrase. Experimenters looked for examples or counterexamples. In the longer term its main contribution to physics was that its popularity rankled a younger theorist, Murray Gell-Mann. He thought Pais was wrong, and he was jealous.
Murray
At fourteen he had been declared “Most Studious” and “wonder boy” by his classmates at Columbia Grammar, a private school on the Upper West Side of New York, and that was the last they saw of him, for he was already a senior, and he started at Yale that fall. Gell-Mann’s surname was subtly difficult to pronounce. It was wrong to unstress the second syllable, as if the name were Gelman, although Murray’s older brother,