The Life and work of Felix Bloch

Felix Bloch was born in Zurich, Switzerland on October 23, 1905, the son of Jewish parents Gustav and Agnes Bloch. From 1912 to 1918, Bloch attended the public primary school. After attending the Gymnasium of the Canton of Zurich from 1918 to 1924, Bloch passed his Matura examination, enabling him to study at an institution of higher learning. Interested in science from an early age, he had studied astronomy and was fascinated by mathematics. But his father was a wholesale grain dealer and Bloch recognized the practical necessity of earning a living.

When Bloch enrolled at the Federal Institute of Technology in Zurich in the fall of 1924, he began studying engineering. He soon found, however, that his interests really lay in theoretical directions; after a year, he decided, putting all practicality aside, to switch to the study of physics, considered at that time an “entirely useless” field since it led to no material achievements. His instructors included several eminent physicists: Peter Debye, Hermann Weyl, and Erwin Schroedinger. Debye, from whom Felix took his first introductory course in physics, left in 1927 to take a position at the University of Leipzig in Germany and urged his young student to join him. Bloch went to Leipzig and there began graduate work under the newly appointed professor of theoretical physics, twenty-six year old Werner Heisenberg.

Working with Heisenberg was a wonderful opportunity for Bloch. Heisenberg, one of the founders of the young quantum mechanical theory of the atom, conveyed a thorough understanding of this “new” physics, a foundation upon which Bloch's later work would be based. As a first step, Heisenberg encouraged Bloch to complete some calculations on the radiation damping of wave packets, work which Bloch had begun while still a student in Zurich. After publication of this research, he turned to the problem of determining the quantum mechanical structure of electrons in crystal lattices, again at Heisenberg's urging. Bloch's thesis, published in 1928, established him as a brilliant young theorist and still serves as the basis for the study of electron conduction in metals.

After completing his thesis, Bloch began the customary years of visiting and studying at various centers of physics research in Europe. In the summer of 1928, he moved back to Zurich to work as an assistant to Wolfgang Pauli. Pauli, considering superconductivity to be the only interesting topic left in the theory of metals, set Bloch to work on this problem. Unsuccessful in his attempts to formulate a theory of superconductivity, Bloch recognized and clarified the fundamental theoretical difficulties involved.

From Zurich, Bloch traveled to Utrecht where he worked under Henrik Kramers as a Fellow of the Lorentz Foundation. There he studied the theories of electric conductivity of metals and of ferromagnetism at low temperatures. The following summer, Bloch assisted Adriaan Fokker at the Teyler's Stichting in Haarlem in his studies of the spinning electron.

During the academic year beginning in the fall of 1930, Bloch returned to Germany to work on the theory of ferromagnetism and remanent magnetization and to serve as Heisenberg's assistant at the University of Leipzig. This research served as the basis for his Habilitationsschrift when he became Privatdozent (lecturer) at Leipzig in 1932. In the winter of 1931-32, as a Fellow of the Oersted Foundation, he traveled to Copenhagen to work with Niels Bohr. Interested in the stopping power of charged particles in matter, Bloch wrote a paper which bridged the gap between the classical and the quantum theory of stopping power.

Upon his return to Leipzig in the spring of 1932, Bloch became increasingly aware of the emerging tensions in Germany. The students at the University of Leipzig were among Hitler's most fervent supporters and anti-Semitism, though still a predominantly theoretical attitude, was becoming more and more prevelant. When the first Jewish professors were dismissed in 1933, Bloch, at Heisenberg's insistence, applied for a Rockefeller Fellowship for the fall. Political events moved even more quickly than Bloch had anticipated; though his Swiss citizenship prevented his dismissal for the time being, he resigned and left Leipzig in March of 1933.

For the next few months, Bloch stayed mostly at his home in Zurich, but he also traveled to France, Holland, and Denmark. During his summer visit to Copenhagen to see Niels Bohr, he received his first offer from the chairman of the Stanford University physics department, David Locke Webster. Originally, Bloch later confessed, he knew nothing about Stanford so he mentioned the offer to Bohr and Heisenberg and asked for their advice. Heisenberg knew only that Stanford was in California and that the students from Stanford and another school nearby stole each other's axes. Bohr's opinion was definitive: Stanford was a good school; he should go.

Since he had received the Rockefeller Fellowship, however, Bloch decided to postpone going to Stanford until the spring of 1934. First he wanted to go to Rome to work with Enrico Fermi. In Rome, Fermi, one of the few famous European physicists involved with both experiment and theory, emphasized the need for Bloch to conduct experiments as a supplement to his theoretical work.

Despite the additional provision in the Rockefeller Fellowship for six months of study with Ernest Rutherford in England, Bloch left for Stanford in the spring of 1934. He knew little about the University and the members of its physics department, though he had already met several American physicists including Eugene Wigner, Johann Von Neumann, John Van Vleck, William Houston, George Breit, and J. Robert Oppenheimer. After visiting Breit and Van Vleck in New York, Bloch took a train across the country to Palo Alto where he was met by physics chairman Webster.

Though the physics department at that time consisted of nine members, including P. A. Ross, Paul Kirkpatrick, and William Hansen, Bloch was the only theorist. He found himself teaching graduate courses on a variety of subjects: electrodynamics, mechanics, thermodynamics, and quantum mechanics. Very soon after his arrival, Bloch established contact with Oppenheimer, who was teaching at the University of California at Berkeley. Together they quickly organized weekly theoretical seminars attended primarily by Berkeley physicists and graduate students. Bloch's students from Stanford were also often in attendance. Though originally quite small and informal, these seminars soon became very popular.

In the fall of 1935, Bloch returned to Europe to visit his family and friends. Bloch had long suspected that the neutron might have a magnetic moment; what fascinated him about this idea was that a chargeless particle could have magnetic properties. While still a student with Heisenberg, he had stressed the importance both of demonstrating the neutron's magnetic moment and of determining its magnitude. His trip to Europe reaffirmed his convictions; he decided to begin research on neutron physics upon his return to Stanford. In early 1936, heeding Fermi's advice about experimentation, he set out to build a neutron source. Using mostly X-ray and microwave equipment from the physics labs, he and Norris Bradbury, Oppenheimer's successor as director of the Los Alamos National Laboratory, built the source themselves. (Bloch later pointed out that this equipment was more important as a source of inspiration than of neutrons.) Eventually, he extended his use of neutron sources to studies of neutron polarisation, a link to his earlier research in ferromagnetism.

These studies served as a basis for a collaborative effort with Luis Alvarez (then a graduate student at UC Berkeley, later a Nobel laureate). In the fall of 1938, Alvarez and Bloch began working with Berkeley's 37" cyclotron to determine the magnetic moment of the neutron. Because the machine at that time operated very sporadically, Bloch spent many days simply waiting for news that the accelerator was functioning. By the summer of 1939, though, they were able to publish very precise results. This experiment was, in fact, one of the first important uses of Ernest Lawrence's cyclotron.

After publication of their measurements, Bloch felt that there was still neutron research to be done with cyclotrons. Rather than continuing to commute to Berkeley, however, he and some colleagues decided to build a cyclotron at Stanford. Support from both Lawrence and Isidor Rabi proved crucial in obtaining funding for the project. The cyclotron, begun in the fall of 1939, was built from scratch by Bloch and a few of his Stanford colleagues, notably Hans Staub.

Soon after the work on the cyclotron began, Bloch met Lore Misch through mutual friends in New York. Lore, also a physicist, had done graduate work at Goettingen under the supervision of V. M. Goldschmidt, an eminent geophysicist. In 1935, she received her Ph.D. in the field of crystallography. After leaving Germany in 1936, she served for two years as assistant in physics at the University of Geneva in Switzerland. She came to the United States in 1938 and was appointed research associate at the Massachusetts Institute of Technology. Felix and Lore were married in March 1940. Twins George Jacob and Daniel Arthur were born the following year. Frank Samuel was born in 1945, and Ruth Hedy in 1949.

During these early years at Stanford, Bloch instituted a summer program for visiting professors. Through this program, many eminent physicists, including Fermi, Rabi, George Gamow, Willis Lamb, Viktor Weisskopf, Hans Bethe, and Edward Teller, visited the physics department and helped to establish its reputation as a center of physics research on the west coast.

In 1942, Oppenheimer asked Bloch to work on the Manhattan Project. Bloch, collaborating with Bethe, Teller, and Staub, used the homemade Stanford cyclotron for the first experimental determination of the energy distribution of neutrons from fission. After this was completed, Bloch left for Los Alamos, New Mexico where he worked on special theoretical problems under Bethe's supervision. Quickly bored with this, however, he joined Seth Neddermeyer's implosion group.

After only a few months at Los Alamos, unhappy with the military atmosphere and feeling that he was no longer useful, Bloch left to work at the Harvard Radio Research Laboratory under the direction of Stanford engineer Frederick E. Terman. Bloch did both experimental and theoretical work: although primarily interested in theoretical problems with radar, he also relied on earlier Stanford experiences with microwave research to conduct some experiments on the reflectivity of certain materials. Bloch found the civilian life in Boston much more pleasant than the military one in Los Alamos.

By early 1945, with the end of the war in sight, Bloch's thoughts turned to post-war research. He spent a lot of time with both William Hansen, who had developed the klystron and had done a lot of work with microwaves, and Rabi, who had received the 1944 Nobel Physics Prize for his use of molecular beams in the determination of nuclear moments. Bloch's experiences with radio techniques at the Harvard Laboratory and with the measurement of the magnetic moment of the neutron convinced him that there might be a simpler way of making Rabi's measurements.

When Bloch returned to Stanford in September 1945, he began work on this new project immediately. Martin Packard built the original radio equipment using Hansen's design specifications; Bloch worked primarily with the magnet and its properties. At a meeting of the American Physical Society in December, Bloch met Edmund Purcell of Harvard University and the two discussed their recent research. Both recognized that the theoretical basis of their respective projects was the same, although they had been using slightly different techniques to achieve experimental results. So they decided to split up the field: Bloch would use the effect in the study of liquids; Purcell would examine crystals. The Stanford group gathered its first positive results in January 1946 (see Physical Review 69, 127(L) (1946)).

Because of its simplicity and accuracy, the nuclear induction technique, or nuclear magnetic resonance (NMR) as it has come to be known, could be used in a variety of ways in many different fields. It allowed physicists, for example, to measure the magnetic moments of nuclei, important for the development of the shell model of the nucleus. And it served as the basis for many modern developments in chemistry, biology, and medicine. (For a clear description of nuclear induction, see Bloch's article “Nuclear Magnetism”, American Scientist, 43 (1955): 48-62.)

Recognition for the achievements of both Bloch and Purcell came in 1952 when they were jointly awarded the Nobel Prize for Physics for “the development of new methods for the exact measurement of nuclear magnetism and for the discoveries made in the development of these methods”. Felix Bloch thus became Stanford University's first Nobel Prize winner.

Further recognition for Bloch came in 1954 when he was asked to serve as the first Director-General of the European Center for Nuclear Research (CERN). He originally heard that he was being considered for the position in the fall of 1953 when he received letters in quick succession from both Bohr and Heisenberg. The selection committee settled on Bloch because he was “a man dominating in an exceptional way both the fields of theory and experiment and who could have a deep understanding of the theoretical deductions on which the whole of the big synchrotron project (was) based”.

From the beginning, Bloch was not enthusiastic about the offer. Though he considered it a great honor, he felt that he was not well-suited for the sort of administrative responsibilities which would be so much a part of the first Director's job. But he was under great pressure from his friends in Europe and received their repeated assurances that he would merely oversee the administrative tasks while remaining primarily responsible for scientific programs at CERN. In the spring of 1954, after the unanimous approval of the international governing Council, Bloch accepted the offer for a period of two years. Appointed with him were Edoardo Amaldi as Deputy Director and Cornelis J. Bakker as representative of the scientific groups in charge of the construction of the machines.

Upon his arrival in Geneva in October of 1954, though, Bloch found the situation quite different from what he had expected. Amaldi, whom Bloch thought would be handling most of the administrative matters, had been very active in the preliminary stages of the organization but planned to spend the majority of his time working elsewhere once Bloch arrived. Left with the responsibility of managing the day to day administrative work, Bloch found it almost impossible to conduct his own scientific research, the possibility of which had been one of the primary reasons for his acceptance.

Within a few months of his arrival, Bloch realized that he didn't want to stay at CERN for two years. Citing a passage in a letter to Bohr which gave him the option of leaving after only a year, Bloch asked the Council in the spring of 1955 to accept his resignation. With much regret, the request was granted; Bakker was appointed in his stead.

Bloch returned to Stanford and began teaching again the following fall. During the next few years, he and Leonard Schiff, department chairman from 1954 through 1966, built one of the leading physics teaching and research facilities in the world. They and the other senior members of the department persuaded the Atomic Energy Commission to construct the huge linear accelerator (SIAC) on the Stanford campus. They appointed outstanding faculty members and continued to invite distinguished physicists to visit the department. They oversaw the development of the undergraduate curriculum through their emphasis on the importance of introductory courses. And they set consistently high standards for both their students and their colleagues.

Though Bloch was considered by most a theoretical physicist, as Anatole Abragam said in his memorial:

[he] was apt to describe himself as an experimental physicist. If his own attempts to get actively engaged in laboratory experiments were not, in the opinions of his students, always crowned with success, he was an experimenter in a deeper sense, aware of the possibilities and limitations of experimental methods and apparatus and designing the experiments that his colleagues carried out.

The numerous formulae and effects associated with Bloch's name serve as evidence of the extent and importance of his scientific contributions, both theoretical and expeirmental. Bloch wave functions, Bloch spin waves, Bloch walls, the Bethe-Bloch formula, Bloch's theorem, the Bloch-Nordsieck theory and Bloch's equations have all become part of twentieth-century physics.

As such a well-known scientist, Bloch was involved with many professional organizations and received numerous distinctions. The extensive list of organizations to which he belonged includes the American Academy of Arts and Sciences, the American Philosophical Society, the National Academy of Sciences, the Weizmann Institute of Science, and the American Physical Society. He was appointed an honorary member of the Swiss Physical Society and received honorary degrees from Grenoble University, Oxford University, the University of Jerusalem, and the University of Zurich. In 1965, he served as president of the American Physical Society, one of the largest scientific organizations in the country.

But Bloch was also active in many humanitarian causes. He was a member of the American Professors for Peace in the Middle East, the Committee for UN Integrity, the Committee of Concerned Scientists, the Universities' National Anti-War Fund, and Scientists and Engineers for Secure Energy. He was especially concerned with the fate of his colleagues trapped in Soviet-bloc countries.

Bloch's Stanford colleagues Robert Hofstadter, Marvin Chodorow, Arthur Schawlow, and J. Dirk Walecka describe him this way:

Felix loved nature, particularly mountains, and was a mountain climber in his younger days. He was very fond of skiing and even in his later years could be seen enjoying the slopes in his old ski suit and his old-fashioned and battered skis. He was also a lover of music and art and played the piano well and with tremendous satisfaction. Despite the extraordinary gifts that Felix gave to the world, he remained a basically modest person all his life. We do not mean quietly modest, for he held strong opinions and was usually outspoken in expressing them. No one had any doubt about what Felix was saying or where he stood on any issue. He enjoyed a good intellectual fight, and together with his colleagues at Stanford there were often many sparks flying. He had tremendous self-confidence and his firm and steadfast character opposed anything mean, petty, or base and he spoke out fearlessly against anything he thought politically or morally bad.

After his retirement as Max H. Stein Professor of Physics in 1971, Bloch continued to travel, teach, and publish research papers. He died in Zurich on September 10, 1983.