A View of the Particle World
Chris Quigg

Conceptual Foundations of Modern Particle Physics. By Robert E. Marshak. xxvii+673 pp. Singapore: World Scientific.

Robert E. Marshak was a distinguished theoretical physicist who grew up with the field of particle physics. The author, with Hans Bethe, of the two-meson hypothesis that disentangled Yukawa’s nuclear-force carrier, the π-meson, from the unexpected muon, and, with E. C. G. Sudarshan, of the V–A hypothesis that brought Fermi’s theory of beta decay into accord with the evidence for parity violation, Marshak was for a half-century one of the great enthusiasts of particle physics. He was also an ardent internationalist, animator of the “Rochester Conferences” that grew into the biennial International Conference on High Energy Physics.

Marshak saw particle physics emerge from nuclear physics, pass through several periods of splendid confusion, and grow to maturity, nourished by a host of experimental results from cosmic rays and accelerators. Conceptual Foundations of Modern Particle Physics, which was completed just before his death at the end of 1992, is his intellectual summation, his analysis of the ideas that have brought the theoretical understanding of fundamental processes to its present highly successful, yet incomplete and tantalizing, state. It is the record of a life in particle physics, less a textbook than a statement of how Marshak thought, what problems captured his fancy, and what issues preoccupied him.

Conceptual Foundations opens with a rapid-fire survey of the intellectual history of particle physics, organized into three fifteen-year eras. During the “Startup Period,” 1945-1960, the idea of gauge invariance took root in the formulation of quantum electrodynamics and non-Abelian gauge theories. Spontaneous symmetry breaking became a familiar notion, while the phenomenology of pion physics, strange particles, and the universality of weak interactions led to a growing appreciation of the power of symmetry arguments. In the “Heroic Period” of 1960-1975, quarks and leptons were recognized as the basic constituents of matter and gauge theories emerged as the correct descriptions of the strong, weak, and electromagnetic interactions. Marshak calls the era 1975-1990 a “Period of Consolidation and Speculation” in which the electroweak theory and quantum chromodynamics survived increasingly rigorous experimental tests, the theoretical underpinnings of gauge theories were buttressed, and audacious speculations opened the possibility of a more comprehensive understanding. Such speculations—on the unification of the strong, weak, and electromagnetic interactions, on the origin of the three generations of quarks and leptons, and on the application of topological conservation laws—were the stuff of Marshak’s late work, and take up the last third of the book.

In the six chapters that make up the heart of Conceptual Foundations, Marshak reviews the key ideas that come together to define modern particle physics. He lays great stress on the gauge principle—the idea that symmetries determine interactions—and on the role of anomalies—quantum fluctuations that do not respect the symmetries of a classical field theory—in fixing the structure of the standard model. More idiosyncratic is the focus on issues that shaped his own thinking at crucial moments: chirality invariance, the Landau singularity in the running coupling constant of quantum electrodynamics, and no-go theorems that show the impossibility of combining Poincaré invariance with global internal symmetries.

Conceptual Foundations will be most useful to the reader who already knows the ideas treated. It is more an exegesis delivered to a knowledgeable colleague than a systematic development for students. There are no exercises to fix new concepts in a reader’s mind. Few will want to read this book from cover to cover, but it would be interesting to pull Conceptual Foundations from the library shelf, to see what Marshak has to say about a particular subject. There are some nice passages, including an unusually thorough and thoughtful treatment of the analogy between the Ginzburg-Landau picture of the superconducting phase transition and the Higgs mechanism for spontaneous breaking of electroweak symmetry.

A better subject index, as well as an author index, would have made dipping into Conceptual Foundations more rewarding. One of Marshak’s last influential pieces of research was his exploration of neutron-antineutron oscillations in SO(10) unified theories. Unfortunately, the first index entry leads, in error, to a section on neutrino oscillations. The reader who perseveres to the final entry will be rewarded by a clear, orderly, but swift presentation of the conditions that can give rise to neutron oscillations. In the matter of indexes and misprints, Marshak could have been better served by his publisher. A thorough copy-editing would have made the book much more appealing to read and use.

The strength of Conceptual Foundations is that Marshak is alive on the page, a man of many words, fully engaged, always with a definite point of view. His joy in understanding—in striving to understand—is a constant companion.

Published in Science 264, 1952 (1994).