Nuclear Forces: The Making of the Physicist Hans Bethe

By Silvan S. Schweber, Cambridge, MA: Harvard University Press, 579 pages, illustrated, $35.00

Reviewed by Michael Nauenberg

Hans Bethe was among the most versatile physicists of the 20th century, contributing to virtually every branch of the discipline, writing superb review articles, and teaching and mentoring students. In addition, in World War II he directed the Theoretical Division at Los Alamos during the development of the atomic bomb. Later in his life, he called upon all scientists to “cease and desist” from working on any aspect of nuclear weaponry.

When I was an MIT undergraduate in the early 1950s, I asked Victor Weisskopf for his suggestions about graduate schools, and he recommended without hesitation that I should go to Cornell to study physics with his friend Bethe. I followed Vicki’s advice. It was my good fortune to attend Bethe’s lectures and seminars, and later to become one of his Ph.D. students. By then he was the chief scientific US adviser during the initial negotiations with the Soviet Union toward nuclear disarmament. This activity often kept him in Geneva, Switzerland, so he had less time for his Cornell activities in this period.

Nuclear Forces is an excellent personal and scientific biography of Bethe covering the first third of his life. Schweber also provides biographical sketches of prominent physicists who interacted with Bethe and played major roles in the early development of quantum mechanics, plus chapters on two women in his life: his life-long wife and partner, Rose Ewald, and an earlier love, Hilde Levi. The book also contains mathematical discussions of some of Bethe’s major achievements, such as his well-known “ansatz,” which can be understood, however, only by readers with an advanced background in physics or mathematics.

Bethe was born in Strasbourg in 1906. His father Albrecht Bethe, a distinguished physiologist, nurtured his precocious talent and scientific interests. His mother Anna, however, appears to have crimped his emotional development, particularly with respect to female companions. Later, after he married Rose, his mother lived for some time with them and “nearly wrecked their marriage,” according to Schweber.

In 1926 Bethe began to study physics in Munich under Arnold Sommerfeld, who taught him to apply advanced mathematical methods to physical problems. When he later visited Enrico Fermi in Rome in the early 1930s, he learned Fermi’s back-of-the-envelope approach to obtain physical insights and semi-quantitative answers to physical problems more rapidly. During these visits Bethe also learned quantum electrodynamics in the form Fermi originally developed. In his own words, “Fermi changed my whole style of doing physics and weaned me from the formal structure of most European universities.”(quoted from Physics Today, June 2002).

Early on, Bethe also exhibited his sense of humor when he and two collaborators published a hoax in 1931, in the first issue of Naturwissenschaften. Following Eddington’s numerology, they claimed to obtain a relation between the value of the inverse finestructure constant, 137, and the lowest possible temperature, T₀ ≈ −237°C. This hoax should have been obvious to the editor, as the value of T₀ depends on the arbitrarily chosen temperature units, but he and the reviewers were fooled.

Bethe had a lifelong close friendship with Rudolph Peierls, his fellow classmate studying under Sommerfeld. Peierls introduced him to the problem of nuclear forces, whose study had been initiated by Werner Heisenberg after Chadwick’s 1932 discovery of the neutron. Before that, the constituents of the nucleus were believed to be protons, with embedded electrons accounting for the nuclear charge and beta decay. Although Schweber used the phrase “nuclear forces” as the title, only a fraction of Bethe’s diverse achievements described in the book occurred in this area.

Bethe’s early fame and growing recognition stemmed from his masterful review articles, starting with a Handbuch der Physik article on the one and two electron problem. Later his reviews of nuclear physics, written in collaboration with M. Stanley Livingston and Robert Bacher in Reviews of Modern Physics became known as “Bethe’s Bible.” In 1939 Bethe was asked to write a review article on stellar energy generation for Reports on Progress in Physics. In this case he uncharacteristically asked his student Robert Marshak to draft it, with the proviso that it would be published under both names and that Marshak would receive any earnings.

When Hitler was appointed German Chancellor in 1933 and persecution of Jews began in earnest, Bethe (whose mother was Jewish) lost his appointment as a lecturer in Tübingen. It rankled him that he received only a “cold note” confirming his dismissal from Hans Geiger, who initially had welcomed him warmly. Fortunately, Sommerfeld helped Bethe obtain a year-long position in Manchester. Afterwards, he received a visiting position at Cornell, and soon he became an assistant professor in its physics department. He liked Cornell and his colleagues, and he remained there his entire career, building the Cornell physics department into a world-class institution.

In addition to his mastery of physics, one of Bethe’s advantages was the speed with which he could carry out calculations. In 1934 Weisskopf, at the time an assistant to Wolfgang Pauli, asked him how long it would take to do a certain calculation; Bethe replied “Me, it will take three days. You, it will take three weeks.” But he wasn’t just bragging. After the 1948 Shelter Island Conference, in which Willis Lamb announced his experimental discovery of the splitting of the 2S_1/2 and 2P_1/2 levels in hydrogen, predicted to be degenerate in Dirac’s theory, Bethe applied Hendrik Kramers’s seminal idea of mass renormalization, which he and Weisskopf had learned at this conference, to calculate this Lamb shift, finishing it during the short train ride back to Cornell. Bethe’s non-relativistic calculation demonstrated for the first time that renormalization made higher-order calculations with quantum electrodynamics (QED) possible.

At the conference, Julian Schwinger, Robert Oppenheimer and Weisskopf had suggested that the Lamb shift was due to the electron’s interaction with its radiation field, but they thought that calculations of this shift would turn out infinite. According to Freeman Dyson, Bethe’s calculation “broke through a thicket of skepticism and open the modern era of particle physics. It showed us all how to connect QED with the real world.” (quoted from Physics Today, October 2005).

Bethe attended the 1938 Washington Conference organized by George Gamow, at which the constitution of stars and nucleosynthesis were discussed. With his knowledge of nuclear physics and stellar structure, Bethe concluded that he could solve the problem of stellar energy generation. Collaborating with Charles Critchfield, he calculated that the p-p chain reaction of hydrogen into helium is the primary source of nuclear energy in the sun, and he reported their results before the end of the conference. For this achievement, Bethe won the 1967 Nobel Prize in physics. He deserved this prize for many of his other major contributions, the most important being his calculation of the Lamb shift. But to this reviewer, the 1967 prize, awarded for energy production in stars, should have been shared with Gamow and Critchfield.

As Schweber points out, Bethe on several occasions failed to give appropriate credit for ideas that influenced his work. For example, his Lamb shift calculation was based on Kramers’s idea of mass renormalization, but he did not acknowledge Kramers in his paper on the subject.

Successful physicists often tend to be or become arrogant, but Bethe never revealed this trait, in spite of his numerous accomplishments. His students, including myself, can testify to this. An oft-repeated story recounts how Bethe skipped testifying at a delayed Senate hearing because of a prior luncheon appointment with one of his students.

In 1947 Sommerfeld invited Bethe to succeed him at Munich, but Bethe declined the offer, responding:

I am very gratified and very honored that you have thought of me as your successor. If everything since 1933 could be undone, I would be very happy to accept this offer…unfortunately it is not possible to extinguish the last 14 years.… I am much more home in America than I ever was in Germany.

Like Fermi and Peierls, Bethe was among the most versatile physicists of the 20th century, able to contribute significantly to many subfields. General readers, and physicists in particular, will enjoy Schweber’s masterful biography and benefit from learning not only about Bethe’s life and work, but also about the history of physics in the first third of the century.

During a walk with Bethe at a University of Washingon workshop in 1990, he asked me whether I knew the status of this biography. I think he would have been very pleased with the long-awaited result.

Michael Nauenberg is Professor of Physics Emeritus at UC Santa Cruz. He just received the University of California’s 2013 Panunzio Award for his research and writings on the history of 17th-century physics.

---

The articles in this issue represent the views of their authors and are not necessarily those of the Forum or APS.