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Probing the Nucleus

Peter Westwick

The Fly in the Cathedral: How a Group of Cambridge Scientists Won the International Race to Split the Atom. Brian Cathcart. xii + 308 pp. Farrar, Straus and Giroux, 2004. $25.

Popular histories of 20th-century physics tend to present the revolution of quantum mechanics as having been more or less completed by the late 1920s. This portrayal obscures the problems that still plagued several areas of physics at the time, including what would come to be known as nuclear physics. The Fly in the Cathedral, by reporter-turned-science-writer Brian Cathcart, highlights these difficulties, some of which were solved by experiment in 1932, a celebrated year in physics.

Cathcart begins with the 1909 experiment that led Ernest Rutherford to the nuclear model of the atom, in which almost all of the atom's mass is concentrated in the middle; the nucleus is the tiny "fly" in the cavernous "cathedral" of the atom. Cathcart then jumps ahead to the problems besetting the nuclear model in the late 1920s: reconciling atomic structure with quantum mechanics and figuring out how protons and electrons, which at the time were both presumed to be constituents of the nucleus, cohabited. Cathcart focuses on the Cavendish Lab at the University of Cambridge and in particular on John Cockcroft and Ernest Walton.

The acceleration tube...Click to Enlarge Image

Cockcroft and Walton built a high-voltage particle accelerator to give protons enough energy to penetrate the electrical barrier of the nucleus and serve as a nuclear probe. In 1932 they found that lithium nuclei bombarded with protons were splitting into two helium nuclei, with an energy release equal to that calculated by Einstein's famous equation, E=mc2. In addition to producing the first artificial disintegration of an atom, the Cockcroft-Walton experiment demonstrated the scientific value of the particle accelerator, which went on to become the primary tool (along with detectors) for nuclear and then particle physics. It also won the two men the Nobel Prize for physics in 1951.

Cathcart is very good at sketching personalities. However, like many writers of popular science, he does not always make clear where the historical record leaves off and speculation begins, and he skirts technical details, relying instead on an array of metaphors and analogies. But he also highlights the driving force of scientific curiosity, a feature often neglected in more scholarly work, and he captures well the vexations of lab research—the tedious breaking down and reassembling of balky apparatus, the patient pursuit of vacuum leaks and background effects. He relies heavily on the abundant reminiscences of the historical actors, although he does take advantage of the recent opening of Walton's personal papers too. Cathcart also highlights the role of George Gamow, whose theory of quantum tunneling underpinned the experimental efforts to sneak protons through the energy barrier. Finally, as the book's subtitle indicates, Cathcart captures the sense of international competition, in this case the race to high energy against Merle Tuve at the Carnegie Institution, Charles Lauritsen at Caltech and especially Ernest Lawrence at the University of California, Berkeley. Why did the Cavendish get there first? Cathcart's answer: the lab's preoccupation with the nucleus, starting with Rutherford and expressed also in James Chadwick's discovery of the neutron.

But there is another reason the Cavendish won this race. A mythology has grown up around the Cavendish approach to experimentation—namely, that with hands-on inventiveness and economy its scientists jury-rigged experiments out of string and sealing wax, which then produced groundbreaking research. Cathcart's narrative, if not the book's flyleaf, makes clear that Cavendish scientists were far from "the last true gentlemen scientists" tinkering on tabletops. On the contrary, their equipment filled entire custom-built labs and depended on industrial engineering expertise, and they tapped government funding and developed organizational skills. As Cathcart notes, the Metropolitan-Vickers firm (known as Metro-Vick) supplied transformers, vacuum pumps and a new type of plasticine, and Cockcroft himself had previously worked for Metro-Vick, as had other Cavendish physicists; Cathcart could also have observed that the BTH Company provided thyratrons for electronic particle counters. The string-and-sealing-wax mythology has obscured the fact that the Cavendish outpaced its American counterparts, including Berkeley, in the initial integration of academic physics and industrial engineering, or what came to be known as big science.

If this book is not about jury-rigging gentleman scientists, neither is it exactly about the "greatest scientific discovery of the age," as newspapers called it. The discovery of the neutron, announced by Chadwick at the Cavendish scant months before the Cockcroft-Walton results and also described briefly by Cathcart, was probably more important to nuclear science: It made possible a quantum field theory of a nucleus composed of protons and neutrons, and it provided experimentalists with a new tool for provoking nuclear disintegrations, because the neutron is not repelled by the positive charge of the nucleus.

Cathcart rightly notes that it was not scientists but rather journalists who seized on the Cockcroft-Walton results as the "splitting of the atom" and hence as the first step toward limitless energy, whether peaceful or not. Never mind that their experiment had produced a number of discrete events, not a chain reaction, and moreover that it consumed more energy than it released. Cathcart has previously written on the British atomic-bomb program, and he notes here that "Modern perceptions of what was achieved in the Cavendish laboratory in 1932 are inevitably coloured by the atomic bombs and the Cold War that followed." He denies that the Cavendish physicists were driven by dreams of bombs or reactors—and anyway, he notes, the neutron likewise proved more crucial to the eventual detection and development of atomic fission. But if this book is not about the last gentleman scientists, or the central experiment of the age, or even the initial step on the path to Hiroshima, it does show how intellectual curiosity led physicists simultaneously into the nucleus, into new partnerships with industry and into new modes of research.

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