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Fowler, William A.

FOWLER, WILLIAM A.

(b. Pittsburgh, Pennsylvania, 9 August 1911; d. Pasadena, California, 14 March 1995),

physics, nuclear astrophysics.

Willy Fowler, as he was universally known, shared the 1983 Nobel Prize in Physics, along with Subrahmanyan Chandrasekhar, for his role in showing that all the elements from carbon to uranium could be produced by nuclear processes in stars, starting only with the primordial hydrogen and helium produced in the Big Bang. His citation reads, “for his theoretical and experimental studies of the nuclear reactions of importance in the formation of the chemical elements in the universe.” Eager to make the public feel they were a part of the enterprise, Fowler never tired of telling people, as he did at the close of his Nobel lecture, “your bodies consist for the most part of these heavy elements.… Thus it is possible to say that you and your neighbor and I, each one of us and all of us, are truly and literally a little bit of stardust.”

Early Years . Fowler was one of three children of John MacLeod Fowler and Jennie Summers Watson; the family moved to Lima, Ohio, in 1913, when Willy’s father, an accountant, was transferred there from Pittsburgh. The grandson of Scottish and Irish immigrants, Fowler attributed his lifelong interest in railways and steam engines to the many hours he spent as a boy hanging around the switchyards of the Pennsylvania Railroad and sheds of the Lima Locomotive Works in his hometown. He attended the public schools of Lima and after graduating from high school in 1929 entered Ohio State University, in Columbus, where he studied physics, electrical engineering, chemistry, and mathematics; spent his Sundays in the engineering laboratories; and joined a campus fraternity. In his autobiography, “From Steam to Stars to the Early Universe” (1992), Fowler later recalled waiting tables, washing dishes, stoking furnaces, and cutting and selling meat and cheese at the central market in Columbus to pay for his education.

Having set his sights on doing pure science, Fowler elected to do his bachelor’s thesis in physics “Low Voltage Electron Streams” under the supervision of the physicist Willard H. Bennett, a pioneering plasma physicist and former National Research Council fellow at the California Institute of Technology (Caltech). Not only did Bennett tutor Fowler in experimental techniques in the laboratory, but he also inspired him to become a nuclear physicist, citing a long list of front-page stories about developments in physics in 1932: the discovery of the positron, the neutron, and the deuteron, as well as the discovery by the British physicists John D. Cockcroft and Ernest T. S. Walton at Cambridge University that it was possible to split atomic nuclei with artificially accelerated particles. More importantly, he pointed Fowler in the direction of Caltech, where Charles C. Lauritsen, the versatile experimental physicist and director of the W. K. Kellogg Radiation Laboratory at Caltech, had wasted little time following up on what Cockcroft and Walton had done. Lauritsen modified one of the laboratory’s x-ray tubes. He had been using the tube to accelerate electrons in order to produce a beam of high-energy x-rays. Save for using an ion source instead of an electron source, Lauritsen had basically the same equipment as the English researchers. By the time Fowler finished up his bachelor’s degree in engineering physics in 1933 and entered Caltech as a graduate student that fall, Lauritsen had converted one of Kellogg’s high-voltage x-ray tubes into an instrument with which to accelerate ions and to split nuclei and was sending the paper announcing his discovery of artificial neutrons to Physical Review.

Move to Caltech . Then in the throes of the Great Depression, Caltech offered Fowler an assistantship consisting of tuition plus lodging and board in the Athenaeum, the school’s faculty club, but no cash. Fowler started working in the Kellogg laboratory during his first quarter at the institute and remained associated with Kellogg until the end of his life. To earn spending money, Fowler built equipment for handling radium for one of the doctors involved in the cancer research that was also carried out in the Kellogg laboratory during the 1930s. Indeed, the laboratory led a double life for much of the 1930s. By day, Lauritsen’s students, including Fowler, operated and maintained the high-potential x-ray tube used to treat cancer patients. By night, they did nuclear physics research. Hailed by Fowler as “the greatest influence in my life,” Lauritsen supervised Fowler’s doctoral thesis on the production of radioactive elements of carbon, nitrogen, oxygen, and other chemical elements of low atomic number. As Fowler later told an interviewer, “We were the first to come to the conclusion that the nuclear forces were charge symmetrical on the basis of experiment,” adding, “That was very nice and very fundamental.” Fowler received his doctorate in 1936, and for the next three years he and Lauritsen studied the excitation curves of carbon and nitrogen isotopes bombarded with protons. Fowler became a research fellow in nuclear physics in 1936, advancing to assistant professor in 1939, associate professor in 1942, professor of physics in 1946, and institute professor of physics in 1970, a chair he held until his retirement in 1982. On 24 August 1940 Fowler married Ardiane Foy Olmsted, who died in May 1988. They had two daughters. In December 1989 he married Mary Dutcher.

Research on Nuclear Resonances . The discovery of radiative capture of protons by carbon, a discovery made by Lauritsen and his graduate student Richard Crane, in 1934, became the focus of Kellogg’s nuclear physics research for the rest of the decade. In radiative capture, carbon plus a proton produces nitrogen 13, plus a powerful gamma ray to take off the excess energy. The reaction is resonant in the sense that the carbon and proton must combine with exactly the right energy for it to occur. “So by studying resonances,” Fowler later recalled in an interview, “you can study the excited states of nuclei and all their properties.” Convinced that the excitation levels in the light nuclei were the key to understanding the structure of the nucleus, Lauritsen and his students undertook detailed measurements of nuclear reaction rates of all the light nuclei.

At the end of this decade, the theoretical physicist Hans Bethe made the seminal suggestion that the conversion of hydrogen into helium depended on the catalytic carbon-nitrogen reaction—a chain reaction involving the

isotopes of nitrogen and carbon. This was just the first of six nuclear reactions involved in the transformation process. When Fowler read Bethe’s paper on the energy production of stars in Physical Review in March 1939, he realized that the quantitative measurements of the group of carbon-nitrogen reactions that he had been working on in Kellogg had something to do with the operation of nuclear reactions in the Sun and other stars. Ultimately, to explain how the Sun works, the energies and capture probabilities for all these reactions would have to be measured in detail. “When Bethe came out with the carbon-nitrogen cycle, we kind of felt a proprietary interest in this group of reactions,” Fowler later recalled in an interview, “because we had been working on them.…[It] all tied very closely together.” By 1939 the Kellogg researchers had switched from an alternating-current high-voltage tube to a 2-million-electron-volts direct-current Van de Graaff electrostatic accelerator, capable of high-resolution work. With the new machine, Lauritsen and Fowler began to measure carefully all the effects associated with the resonance phenomena. They measured reaction rates at resonance, the width of resonances, and the resulting gamma-ray spectra. In fact, Bethe had proposed two possible solutions, the carbon-nitrogen cycle and the proton-proton chain, because not enough was known about nuclear reactions to choose between them; only by accurately measuring nuclear reaction rates could problems such as Bethe’s application of nuclear physics to astronomy be solved.

Wartime Research . During World War II, Fowler shelved plans to follow up Bethe’s theoretical ideas. The electro-static accelerator was moved into a corner of the lab on the second floor of Kellogg, now retooled to work exclusively on rocket development for the Naval Bureau of Ordnance, with Lauritsen as scientific director. In January 1941, Fowler left for Washington, D.C., where he started working on photoelectric proximity fuses for bombs at the Carnegie Institution’s Department of Terrestrial Magnetism. Nine months later, he returned to Caltech to serve as the rocket project’s assistant director, and to carry out research and development on defense projects, particularly the design of proximity fuses for artillery shells, the type that detonates when it is near the target. In spring 1944, Fowler spent three months in the South Pacific, talking to sailors and marines who were already familiar with rocket weapons. There he learned a lot about what they liked and did not like about them. He also helped set up for the navy the Naval Ordnance Test Station at Inyokern, and was acting director of research there until 1944. Then he helped produce components of the atom bomb.

Nuclear Astrophysics . After the war, Fowler and Lauritsen resumed their work on the carbon-nitrogen cycle, and this led, in the late 1940s, to a vigorous nuclear astrophysics program at Caltech. Postwar strategies for studying thermonuclear processes in the stars included a short series of informal, weekly seminars with Mount Wilson astronomers at the director Ira Bowen’s house. As a consequence of those seminars, Lauritsen and Fowler made the deliberate decision to stay in low-energy nuclear physics and focus on those nuclear reactions that take place in stars. Fowler also began a collaboration with a diverse group of scientists ranging from the cosmologist Fred Hoyle to the astronomers Margaret and Geoffrey Burbidge. In 1949 Jesse Greenstein came to Caltech to organize work in astronomy, and his interests, particularly in the abundance of the elements in stars, stimulated Caltech’s nuclear physicists to pay more attention to the astronomical side of nuclear astrophysics. However, the most important step was to initiate an experimental program that would strike at the heart of Bethe’s theory. In 1946 Fowler’s graduate student Robert N. Hall took as his topic for a PhD thesis the determination of the rates of the reactions in the carbon-nitrogen cycle at stellar conditions. Four years later, Fowler and Hall published their first paper on the problem. Ironically, in the end Fowler and his students concluded that the carbon cycle is not the primary source of energy in the sun. However, Bethe had also suggested another process, the proton-proton chain, and measurements made in the Kellogg lab supported the latter process, in which protons combine to form helium, with the emission of large amounts of energy. To the question “What does the sun shine on?” Fowler’s group answered, “It starts with the proton-proton chain.” That answer marked the start of Fowler’s second career, as an experimental nuclear astrophysicist.

In the early 1950s, Fowler turned his attention to the great complex of nuclear reactions that mark the later stages of stellar evolution. In 1954 he spent a sabbatical year at the Cavendish Laboratory in Cambridge, England, as a Fulbright scholar, working with Hoyle, whom Fowler once hailed as “the second great influence in my life.” The prime mover behind the grand concept of nucleosynthesis in stars, in 1946 Hoyle published his first paper on the synthesis of the chemical elements from hydrogen by nuclear processes inside stars. By the time he visited Caltech in 1953, he had expanded somewhat his work on the origin of the elements from carbon to nitrogen, including the prediction that an excited state in carbon-12 existed and that a jump from helium to carbon-12 could happen in real stars. In an oral history memoir, Fowler recalls Hoyle saying that “there has to be a resonance in the reaction between beryllium-8 and the alpha particle.” Moreover, Hoyle predicted where the resonance would be. Still, Fowler brushed Hoyle aside, reportedly telling him, “Go away, Hoyle, don't bother me.” But, when Ward Whaling, a recent faculty addition to Kellogg, went looking for— and found—the predicted state almost at once, a light went off inside Fowler’s head. Not only did he then read Hoyle’s 1946 paper, but also he “saw that by studying charged-particle reactions, which we could do in Kellogg, you could go all the way up to iron. So we had a program. We had a future.”

While on sabbatical in Cambridge, Fowler worked mainly with Geoffrey and Margaret Burbidge; they spent much of their time together trying to find a source of neutrons that would produce the anomalous abundances of various elements that Margaret had been finding in various types of stars. In 1956 the Burbidges and Hoyle came to Caltech and, while they were there, wrote with Fowler the classic paper “Synthesis of the Elements in Stars,” published in the Reviews of Modern Physics (1957), in which they showed that all of the elements from carbon to uranium could be produced by nuclear processes in stars starting with the hydrogen and helium produced in the big bang. Their paper came to be known as “B2 FH” after the names of its authors.

After the theory of the origin of the chemical elements, Fowler turned his attention to the field of relativistic astrophysics, solar neutrinos, the dynamics of the expansion of the universe, supernovas, nuclear chronology, and thermonuclear reactions in stars. Fowler continued to collaborate with Hoyle; in 1965 they coauthored a book, Nucleosynthesis in Massive Stars and Supernovae; in 1967 Fowler gave a series of lectures on nuclear astrophysics at the American Philosophical Society in Philadelphia, which the society subsequently published. Fowler, who succeeded Lauritsen as director of the Kellogg laboratory, continued to attend Caltech’s weekly astronomy seminar on Wednesdays, the physics seminar on Thursdays, and the nuclear physics seminar on Fridays until he died.

Outside the laboratory, Fowler’s life included a stint in 1951 as director of Project Vista, a study project organized at Caltech to study the defense of Western Europe, including the possible use of tactical atomic weapons on the battlefield; a member of the National Science Board (1968–1974) and of the Space Science Board (1970–1973 and 1977–1980); and president of the American Physical Society (1976). Among his many other honors, Fowler received the National Medal of Science from President Gerald Ford in 1974, and the Legion d’Honneur from President François Mitterand of France in 1989. Elected to the National Academy of Sciences in 1956, he also relished his membership in the Los Angeles Live Steamers and the National Association of Railroad Passengers, a reflection of his lifelong passion for steam locomotives and steam traction engines.

BIBLIOGRAPHY

Fowler’s papers occupy 94 shelf feet in the California Institute of Technology Archives in Pasadena, California. The archives also contain a 1983–1984 oral history interview in eight sessions with John Greenberg (and a supplement with Carol Bugé in 1986), and a transcript of an interview with Albert B. Christman (1969) about the history of the China Lake Naval Weapons Center.

WORKS BY FOWLER

With E. Margaret Burbidge, Geoffrey R. Burbidge, and Fred Hoyle. “Synthesis of the Elements in Stars.” Reviews of Modern Physics 29 (1957): 547–650.

With Fred Hoyle. Nucleosynthesis in Massive Stars and Supernovae. Chicago: University of Chicago Press, 1965.

Nuclear Astrophysics. Philadelphia: American Philosophical Society, 1967.

“From Steam to Stars to the Early Universe.” Annual Review of Astronomy and Astrophysics 30 (1992): 1–9.

“Experimental and Theoretical Nuclear Astrophysics: The Quest for the Origin of the Elements.” In Nobel Lectures in Physics, 1981–1990, edited by Gösta Ekspong. Singapore: World Scientific, 1993. Text also available on the official Web site of the Nobel Foundation.

Greenberg, John. “A Conversation with William A. Fowler, Part II.” Physics in Perspective 7, no. 1 (2005): 66–106.

———.“A Conversation with William A. Fowler, Part II.” Physics in Perspective 7, no. 2 (2005):165–203.

OTHER SOURCES

Barnes, Charles A. “William A. Fowler.” Physics Today 48 (1995): 116–118.

Barnes, Charles A., Donald D. Clayton, and David N. Schramm, eds. Essays in Nuclear Astrophysics: Presented to William A. Fowler, on the Occasion of His Seventieth Birthday. Cambridge, U.K.: Cambridge University Press, 1982. Contains a bibliography of Fowler’s publications and research papers as well as twenty-three invited papers relating to Fowler’s career in nuclear astrophysics.

Goodstein, Judith R. “Nuclear Reactions.” In Millikan’s School: A History of the California Institute of Technology. New York: Norton, 1991.

Hoyle, Fred. “William Alfred Fowler (1911–95).” Nature 374 (1995): 406.

“William A. Fowler, 83, Astrophysicist, Dies.” New York Times, 16 March 1995.

“William A. Fowler, Nobel Laureate 1983.” Special insert in Engineering & Science 47, no. 2 (November 1983): unpaginated.

Judith Goodstein

David Goodstein

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