Felix Bloch (1905-1983) is best known for his development of nuclear magnetic resonance techniques, which allowed highly precise measurements of the magnetism of atomic nuclei and became a powerful tool in both physics and chemistry to analyze large molecules.

Felix Bloch made many important contributions to twentieth-century solid-state physics, including several theorems and laws named for him. He is best known for his development of nuclear magnetic resonance techniques, which allowed highly precise measurements of the magnetism of atomic nuclei and became a powerful tool in both physics and chemistry to analyze large molecules. Bloch was awarded a share of the 1952 Nobel Prize for his work in this field. Bloch was born in Zurich, Switzerland, on October 23, 1905, the son of Agnes Mayer Bloch and Gustav Bloch, a wholesale grain dealer. Bloch's early interest in mathematics and astronomy prompted his family to enroll the boy in an engineering course at the Federal Institute of Technology in Zurich in 1924. His first year's introductory course in physics revealed to Bloch what his true career would be. After completing his studies in the Division of Mathematics and Physics at the Institute in 1927, Bloch studied at the University of Leipzig in Germany under Professor Werner Karl Heisenberg, who was engaged in ground-breaking research in quantum mechanics. Bloch earned his Ph.D. in physics from Leipzig in 1928 with a dissertation on the quantum mechanics of electronics in crystals.

Returning to Zurich, Bloch worked as a research assistant from 1928 to 1929. A Lorentz Fund fellowship allowed him to do research in 1930 at the University of Utrecht in the Netherlands, and later that year he returned to Leipzig to do more work with Heisenberg. An Oersted Fund fellowship took him to the University of Copenhagen in 1931, where he worked with Niels Bohr, director of the university's Institute for Theoretical Physics. From 1932 to 1933 Bloch once again returned to the University of Leipzig, where he was a lecturer in theoretical physics. After Adolf Hitler came to power, Bloch, who was Jewish, left Germany, lecturing at Paris's Institut Henri Poincaré and working with Enrico Fermi in Rome on a Rockefeller Fellowship. In 1934, Bloch accepted an invitation to join the faculty of Stanford University in the United States as an assistant professor of physics. He became a full professor in 1936 and remained at Stanford in that capacity, with a few leaves of absence, until his retirement in 1971, when he became professor emeritus.

European refugees like Bloch were a boon to physics in the United States, as many of them—again, like Bloch— were theorists who added valuable insight to the discoveries of U.S. experimental physicists. Practicing physics in the United States, in turn, was advantageous to Bloch and his fellow refugees because they could attain professorship, accumulate graduate students, and secure research money and facilities with much greater ease in the U.S. than they could in Europe.

Even before he came to the United States at the age of twenty-eight, Bloch had made significant contributions to theoretical physics. His concept of the conduction of electrons in metals, presented in his Ph.D. thesis, became the foundation of the theory of solids. In 1928 he developed the Bloch-Fouquet theorem, which specifies the form of wave functions for electrons in a crystal. (Fouquet was a mathematician who solved an identical abstract math problem many years earlier.) Functions that satisfy the conditions of the theorem are called Bloch functions by physicists, who use them in theoretically probing the nature of metals. Bloch also derived the Bloch-Grüneisen relationship in 1928, which gives a theoretical explanation for Eduard Grüneisen's law about the temperature dependence of the electric conductivity of metals. The Bloch T 3/2 law describes how magnetization in ferromagnetic material is dependent upon temperature, Bloch walls are the transition region between parts of a ferromagnetic crystal that are magnetized with different orientations, and the Bloch theorem eliminates some of the possible explanations for super-conductivity. In 1932 Bloch developed the Bethe-Bloch expression, extending the work of Bohr and Hans Bethe on the slowing down of charged particles in matter. He also advanced the quantum theory of the electromagnetic field and, once in the United States, worked with Nordsieck to resolve the infrared problem in quantum electrodynamics. Bloch began contributing to scientific publications in 1927, while still a student.

Soon after arriving at Stanford, Bloch's interest was drawn to the neutron, a nuclear particle that had been discovered in 1932 by James Chadwick. Otto Stern's experiments in 1933 suggested that the neutron had a magnetic moment (magnetic strength). As he explained in his Nobel Prize address, Bloch was fascinated by the idea that an elementary particle with no electrical charge could have a magnetic moment. Paul Dirac had explained that the electron's magnetic moment resulted from its charge. Clearly, Bloch explained in his Nobel address, "the magnetic moment of the neutron would have an entirely different origin," and he set out to discover it. First, he needed direct experimental proof that the neutron's magnetic moment actually existed. He predicted in 1936 that the proof could be obtained by observing the scattering of slow neutrons in iron and that magnetic scattering of the neutrons would produce polarized neutron beams. These predictions were confirmed in 1937 by experimenters at Columbia University.

The next step was to measure the neutron's magnetic moment accurately. In 1939—the same year he became a naturalized American citizen—Bloch moved from theoretical to experimental physics and achieved that goal, working with Luis Alvarez and the cyclotron at the University of California at Berkeley. As Bloch described in his Nobel address, the two physicists passed a polarized neutron beam through an area with a weak, oscillating magnetic field superimposed on a strong, constant magnetic field. Bloch's experiments were halted by World War II, when he took a leave of absence from Stanford. He joined the Manhattan Project in 1941, whose goal was to produce an atomic bomb, and he worked on that goal at Los Alamos in New Mexico from 1942 to 1944, studying uranium isotopes. In 1944 he joined the Harvard University Radio Research Laboratory, where he was an associate group leader in counter-radar research.

The knowledge Bloch acquired of radio techniques at Harvard proved invaluable when he returned to his nuclear magnetic moment research at Stanford in 1945. I. I. Rabi had developed a technique in the 1930s for measuring nuclear magnetic moments through resonance, that is, by exciting atomic nuclei with electromagnetic waves and then measuring the frequencies of the signals the vibrating nuclei emit. Rabi's technique, however, worked only with rays of molecules, was not particularly precise, and vaporized the sample being studied. Working with William W. Hansen and Martin Packard, Bloch used the basic principle of magnetic resonance —the reorientation of nuclei after being excited—to develop a new method of "nuclear induction." In Bloch's technique, small containers of the material being studied (for Bloch, it was the hydrogen nuclei in water solutions) are placed in a strong electromagnetic field. A much weaker electromagnetic field controlled by radio frequencies then excites the nuclei. The nuclei, induced to spin by the electromagnetism, act like tiny radio transmitters, giving off signals detected by a receiver. These signals make it possible to measure the nuclear magnetic moment of an individual nucleus very precisely and provide a great deal of very accurate and valuable information about the nuclear particles emitting them. Precise measurements of magnetic moment and angle of momentum of individual nuclei made possible by Bloch's nuclear induction technique provided new knowledge about nuclear structure and behavior. Observations of changes in the frequency of the nuclear signals depending on the strength of the magnetic field aided the design of much improved magnetometers, especially useful in measuring the earth's magnetic field. Nuclear induction also provided new knowledge about the interaction of nuclear particles and about isotopes. Because magnetic moment is affected by surrounding charged electrons, and because each atom has a characteristic nuclear frequency, nuclear induction also yielded information about the atomic and molecular structure of solids, gases, and liquids—all without destroying the subject material, as Rabi's method had.

Bloch announced his discovery in two papers published in Physical Review in 1946. The first, a paper titled "Nuclear Induction," described the theory of his technique and the second, written with Hansen and Packard and titled "The Nuclear Induction Experiment," described the mechanics of the experiment itself. At about the same time, Edward Mills Purcell of Harvard University and his colleagues H. C. Torrey and Robert Pound published the nearly identical results of their totally independent work with protons in paraffin. Purcell and his group called their technique "nuclear magnetic resonance absorption." Bloch and Purcell soon saw that their work, although it initially appeared different, was based on the same principle. The two men shared the 1952 Nobel Prize in physics for, in the words of the Nobel committee, their "development of high precision methods in the field of nuclear magnetism and the discoveries which were made through the use of these methods." Although the two had not worked together, Bloch described Purcell at the time as his "good friend" and a "distinguished scientist" and commented in the New York Times that he was very happy to be sharing the award with his colleague. "NMR," as Bloch's and Purcell's method came to be known, has become an invaluable tool of physics and analytic chemistry, revealing information about the molecular structure of complex compounds. The fact that NMR is nondestructive later led to its use as a sophisticated diagnostic tool in medicine. NMR scanners were developed that could produce images of human tissue that were both safer (because they did not use X rays) and more advanced that those produced by CAT scanners.

Bloch's prominence as a physicist was recognized by his election to the National Academy of Sciences in 1948. In April 1954 he was unanimously chosen to serve as the first director-general of CERN, the Conseil Européendela Recherche Nucléaire (European Council of Nuclear Research) in Geneva, a twelve-nation project for research into peacetime uses of atomic energy. Again he left Stanford on a leave of absence, returning after 1955 to continue his research on nuclear and molecular structure and uses of NMR. He also worked with the theory of superconductivity.

Bloch married Lore C. Misch in Las Vegas in 1940. His wife was a professor's daughter and fellow German-born physicist who had immigrated to the United States a few years after Bloch. She had been working as a research associate at the Massachusetts Institute of Technology when the two met in New York at a professional society function. They had three sons, George, Daniel, and Frank, and a daughter, Ruth. In addition to his research, Bloch published many articles in professional journals, especially Physical Review, and he enjoyed piano playing, skiing, and mountain climbing. He held an endowed chair as Max H. Stein Professor of Physics at Stanford from 1961 until his retirement in 1971. He was also a fellow of the American Academy of Arts and Sciences and the American Physical Society. After retiring, Bloch returned to his birthplace of Zurich, where he died of a heart attack on September 10, 1983, at the age of seventy-seven.

Further Reading on Felix Bloch

Chodorow, Marvin, editor, Felix Bloch and Twentieth-Century Physics, William Marsh Rice University Press, 1980.

Kevles, Daniel J., The Physicists: The History of a Scientific Community in Modern America, Harvard University Press, 1987.

Magill, Frank N., The Nobel Prize Winners: Physics, Volume 1, 1901-1937, Salem Press, 1989.

Walecka, John Dirk, Fundamentals of Statistical Mechanics, Manuscript and Notes of Felix Bloch, Stanford University Press, 1989.

New York Times, November 7, 1952, pp. 1, 21; September 12, 1983, p. D13.