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.