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Tiny Particles, Big Questions

Kate Scholberg

Understanding the Universe: From Quarks to the Cosmos. Don Lincoln. xxiv + 567 pp. World Scientific Publishing, 2004. Cloth, $88; paper, $28.

In his new book, Don Lincoln takes us on a rollicking tour of the universe: The reader finds out what we particle physicists understand about it, how we arrived at that understanding and where we think we're going next with our research (we may, of course, end up somewhere altogether different). Although some of the territory will be familiar to many readers, Lincoln enlivens the landscape with fresh details, irreverent (yet never unkind) remarks on the cast of characters, and explanations that are homey, humorous and often completely original.

Understanding the Universe: From Quarks to the Cosmosreveals that the story of particle physics has been marked by a series of tensions and resolutions, of proliferations followed by unifications. Lincoln underscores this point with a quotation from William James: "Any one will renovate his science who will steadily look after the irregular phenomena. And when the science is renewed, its new formulas often have more of the voice of the exceptions in them than of what were supposed to be the rules."

In the early 20th century, physics seemed to be tidily wrapped up, until some quiet but persistent voices demanded attention. Among the first discrepancies were these discoveries: cathode rays (William Crookes), x rays (Wilhelm Roentgen) and radioactivity (Marie and Pierre Curie). By mid-century, the particle "zoo" had proliferated. The picture came into focus only gradually, but by the end of the century physicists were able to tease order from the apparent chaos.

The hunt for this underlying order led to increasingly sophisticated experimental techniques, culminating in today's town-sized particle accelerators and building-sized detectors. These colossal and complex machines, attended by village-sized collaborations of experimentalists, are designed to observe colliding particles traveling at tremendous velocities in order to reveal the essential elements of their composition and interactions.

Lincoln does a first-class job of explaining just how these fundamental properties emerge from the experimental results. Our current best description of the basic constituents of the universe and their interactions is quite simple: Fundamental matter is made up of only two main types of particles, quarks and leptons, which come in three successively heavier generations. These constituents interact with one another in only four ways, determined by the four forces: gravity, electromagnetism, the strong (or nuclear) force and the weak force. For the latter three, we now know what the associated force transfer particles are: the photon (electromagnetism), the gluon (the strong force) and the W and Z bosons (the weak force).

Lincoln, an experimental physicist at Fermilab, fondly describes his experiences as a member of the D0 (pronounced D-zero) experiment, one of two large international research collaborations involved in the search for the heaviest of the quark flavors, the top quark. It was finally discovered simultaneously by both sets of researchers in 1995. In a chapter on accelerators and detectors, Lincoln explains in some detail the workings of Fermilab's Tevatron, which is four miles in circumference.

Our current picture, the Standard Model of particle physics, may be the most concise and elegant yet, but it's still unsatisfying in many ways. The known particles and forces have been enumerated and described, but no underlying explanation for the observed patterns has been found. Why do quarks and leptons come in three generations of increasing mass? Nobody knows. Why are the masses of the lightest particles, the neutrinos, minuscule compared with those of the quarks? Again, nobody knows. Another yawning gap in our understanding concerns the nature of the forces: We believe that at high energy, electromagnetic, weak and strong forces have the same strength: they unify. But how exactly does the unification happen? Even murkier are questions of whether and how gravity, the weakest force, merges with the other three.

Ask a particle experimentalist to list the field's most compelling questions and the list will certainly include the existence and nature of the Higgs boson (referred to by Leon Lederman, with tongue in cheek, as the "God particle"), which is intimately connected with electroweak unification. Why the Higgs matters is notoriously difficult to explain to the uninitiated: The arguments requiring its existence are quite abstract, and the Higgs seems even to some physicists to be born of "just a math trick." Fortunately, Lincoln is up to the task of making the Higgs concept accessible. There's a chance that the Higgs will be detected in the near term at the Tevatron, but more likely we'll have to wait for the newest, shiniest accelerator on the world scene to be turned on: the Large Hadron Collider (LHC) at CERN on the Swiss-French border. The LHC will collide protons and antiprotons. We expect results around the end of the decade. If the telltale signatures of the Higgs fail to appear, we may have an exception raising its voice to herald a new round of glorious confusion.

Lincoln explores in detail many of the questions accessible to physicists in the relatively near term. For example, the reader learns about slippery, faintly interacting neutrinos, which for many years were suspected to have exactly zero mass. However, measurements made recently in giant underground experiments reveal neutrinos to have tiny masses. A new generation of experiments is determining the properties of neutrinos and attempting to understand how they fit into the big picture.

One of the most looming questions of all: Where is all the antimatter? Anti-matter has properties very similar to those of ordinary matter, but matter and antimatter annihilate each other to liberate energy. One would naively think that the universe had been created with equal quantities of matter and antimatter, but the latter exists in our universe only in very tiny quantities. Why the universe we live in is vastly asymmetric in this regard is completely baffling. Tiny differences between the properties of matter and antimatter may give us clues to an explanation for this asymmetry.

Exotic phenomena that may be explored soon include the postulates of supersymmetry, which imply that every known particle has a massive partner with different spin properties. If those postulates hold, theoretical discomforts are eased and force unification is smoothly achieved. Another idea that may be tested in the short term in collider experiments is the possibility that spatial dimensions beyond our familiar three may exist, allowing gravity to partially "leak away," which would account for its extreme feebleness compared with the other forces. Tests of concepts that are even more exotic, such as the idea that the universe consists of tiny wriggling strings, are as yet out of reach but may become possible in the distant future.

Lincoln ends the book with a discussion of cosmology, the study of the history and evolution of the universe. In the last decade, cosmology has become a precision experimental science, thanks to recent tour de force measurements of the cosmic microwave background, galaxy distributions and distant supernovae. The biggest questions on astrophysical scales now overlap those on microscopic scales, and recently the distinction between cosmology and particle physics has become increasingly blurred. We cannot understand the evolution of the universe without understanding the microscopic processes that took place at the earliest times. Astronomers have long puzzled over the nature of the dark matter that makes up a significant fraction of the universe. The hypothesis that this substance is supersymmetric matter will be tested at the LHC. Even more mysterious is the dark energy responsible for the observed accelerating expansion of the universe. That too may have a deep connection with particle physics. Future experiments, both on the scale of the quark and on the scale of the cosmos, will bring new understanding.

In his epilogue Lincoln addresses explicitly the question of why particle physicists ask why. That why is what really animates this book, just as it animates those engaged in research. One can make compelling arguments for the long-term benefits to society of gaining fundamental knowledge, and certainly few particle physicists would deny feeling pleased about applications for their work. But the real reason we do research is simply this: It's tremendously fun to figure the universe out.—Kate Scholberg, Physics, Duke University

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