(b. Zürich, Switzerland, 23 October 1905; d. Zürich, 10 September 1983), theoretical and experimental physics, solid-state physics, superconductivity.
Bloch is considered one of the founders of solid-state physics. He made particularly significant contributions to the quantum theory of metals and solids, he worked on the magnetic scattering of neutrons and, together with Luis Alvarez, he experimentally measured the magnetic moment of the neutron. His discovery of nuclear magnetic resonance won him the Nobel Prize in Physics for 1952, which he shared with Edward Mills Purcell.
Early Years and Education. Felix Bloch was born in Zürich on 23 October 1905. His father Gustav Bloch was a wholesale grain dealer in Zürich. His mother Agnes Meyer was from Vienna. Both parents were Jews. His father moved to Zürich in 1890 to take a position in his uncle’s business and became a Swiss citizen. Gustav and Agnes had a daughter in 1902.
In his primary school years Felix had difficulties relating to other children, and the teachers were not particularly encouraging. But this changed when he moved to another school where they used the Pestalozzi method. In the spring of 1918, he entered the gymnasium run by the Canton of Zürich, which had a six-year curriculum preparing students for the university. He seemed to like mathematics; he applied elementary mathematics to astronomy and could successfully calculate the length of daylight in Zürich at various times of the year. In the fall of 1924, he entered the Federal Institute of Technology (ETH) in Zürich, planning to study engineering. The next year he decided to study physics, despite the initial discouragement by Hermann Weyl. One of the persons who had made a lasting impression on him was his teacher at the ETH, Peter Debye. Years later, in 1976, Felix Bloch still remembered the acute manner by which Debye was assessing the early developments in quantum mechanics and how his comments were rather catalytic in the development of the wave equation by Schrodinger:
Once at the end of a colloquium I heard Debye saying something like: “Schrödinger, you are not working right now on very important problems anyway. Why don't you tell us some time about that thesis of de Broglie, which seems to have attracted some attention?” … Schrödinger gave a beautifully clear account of how de Broglie associated a wave with a particle and how he could obtain the quantization rules of Niels Bohr and Sommerfeld by demanding that an integer number of waves should be fitted along a stationary orbit. When he had finished, Debye casually remarked that this way of talking was rather childish. As a student of Sommerfeld he had learned that, to deal properly with waves, one had to have a wave equation. It sounded quite trivial and did not seem to make a great impression, but Schrödinger evidently thought a bit more about the idea afterwards. Just a few weeks later he gave another talk in the colloquium which he started by saying: “My colleague Debye suggested that one should have a wave equation; well I have found one!” (Bloch, 1976, pp. 23–24)
In 1927, Debye accepted an appointment at the University of Leipzig. Bloch followed him there and started to work for his doctorate with the twenty-six-year-old Werner Heisenberg, who had just been appointed professor of theoretical physics at the university. Heisenberg asked his first graduate student to examine the problems related to the conductivity of metals with the new quantum mechanics. There had been some semiclassical treatments which gave satisfactory agreements with the experimental results. Wolfgang Pauli had assumed that the conduction electrons behaved as if they were an ideal gas, obeying Fermi statistics, and was able to derive the temperature independence of the paramagnetism of metals.With Arnold Sommerfeld, Pauli derived the relationship between electrical and thermal conductivity. Still, physicists could not comprehend why the conduction electrons should be treated as an ideal gas of free electrons.
Research on Theory of Metals and Collective Phenomena. Felix Bloch proposed a satisfactory electron theory of conduction on the basis of quantum mechanics in his doctoral thesis, “Über die Quantenmechanik der Elektronen in Kristallgittern’’ (The quantum mechanics of electrons in crystal lattices), which was published in the Zeitschrift für Physik (1928). The electrons in a metal were considered to be uncoupled, though the field in which any one electron moved was found by an averaging process over the other electrons. If the metal was at absolute zero, its lattice determined a periodic potential field for the electronic motions, and the electrical resistance by the immobile lattice was zero. An electron could move freely through a perfect crystal and a finite free path could only be due to the imperfections in the lattice. In general the imperfections were caused predominantly by the thermal motion of the atoms and were strongly temperature dependent, increasing with increasing temperature. Impurities, however, also scattered the electrons, but in this case the free path would not vary appreciably with temperature. The resistance, therefore, consisted of the “impurity resistance’’ and the resistance due to the thermal motion of the atoms. According to Bloch’s analysis of the motion of an electron in a perfect lattice, all the electrons in a metal could be considered to be “free,” but it did not necessarily follow that they were all conduction electrons. This theory accounted for metals, semiconductors, and insulators but not for superconductors.
But even though in 1928 there was a successful theory of electrical conductivity, superconductivity regarded as a phenomenon of infinite conductivity was still not understood. Bloch, after a suggestion by Wolfgang Pauli, started working on superconductivity while he spent the year 1928–1929 as Pauli’s assistant in Zürich. Though the phenomenon had been discovered by Heike Kamerlingh Onnes in 1911, there was no satisfactory theory. Such a theory could not be derived in any straightforward manner from the calculations of Bloch’s thesis, since his approach using single electrons in order to derive the resistance of metals in low temperatures could not give rise to superconductivity. It appeared that something radically new was needed for the theoretical explanation of the phenomenon. His interpretation was suggested through analogy with ferromagnetism. He showed that the most stable state of a conductor, in the absence of an external magnetic field, was a state with no currents. And since superconductivity was a stable state displaying persistent currents without external fields, it was difficult to see how a theory for superconductivity could be constructed. “This brought me to the facetious statement that all theories of superconductivity can be disproved, later quoted in the more radical form of ‘Bloch’s theorem’: ‘Superconductivity is impossible’” (Bloch, 1966, p. 27).
At the beginning of November 1933, there appeared a short letter in Naturwissenschaften by Meissner and Ochsenfeld that presented strong evidence that, contrary to every expectation and belief of the past twenty years, a superconductor expelled the magnetic field. Superconductors were found to be diamagnetic. The assumption that there was a perfect shielding of the superconductors by their persistent currents ceased to be valid. In the experiment by Meissner and Ochsenfeld, it appeared that the magnetic field was pushed out after the transition to the superconducting state and the magnetic flux became zero. The phenomenon of transition to the superconducting state turned out to be a reversible phenomenon.
By the end of September 1934, Fritz and Heinz London had formulated the phenomenological theory of the electrodynamics of a superconductor. They had assumed that the diamagnetism must be taken to be an intrinsic property of an ideal superconductor, and not merely a consequence of perfect conductivity. They proposed that superconductivity demanded an entirely new relation in which the current was connected not with the electric, but with the magnetic field.
In a letter, London had stated that
The progress I claim is mainly a logical one: by a new and more cautious interpretation of the facts I tried to avoid a fundamental difficulty (the so called theorem of Bloch) which stood in the way of explaining superconductivity by the customary theory of electrons in metals and which could not be overcome as long as one has considered this phenomenon as a limiting case of ordinary conductivity. (London to McLennan 21 June 1935, in Gavroglu, 1995, p. 129)
Another significant contribution Bloch made while he was in Zürich was his improvement of Heisenberg’s theory of ferromagnetism, where the exchange interaction of electrons played a dominant role. Bloch was able to show that the zero- point energy of the electrons figured importantly in determining whether a metal would be ferromagnetic. He spent the academic year 1929–1930 in Utrecht, where Hendrik Anthony Kramers was. There he developed the concept of spin-waves, examining their connection with ferromagnetism and derived the dependence of the magnetic moment on the absolute temperature in the low-temperature region.
After spending the academic year 1930–1931 with Heisenberg in Leipzig, Bloch wrote his Leipziger Habilitationsschrift. In this work, he systematically studied exchange-interaction problems and residual magnetization in ferromagnets and, at the same time, developed much of the formalism which has been used ever since in condensed-matter theory and problems of collective phenomena. Beyond its contribution to the theory of domain walls, this work serves as a bridge between the quantum theory of ferromagnetism in the 1930s and present theories of many-particle systems (see Hoddeson et al.). Bloch was also able to work out the thickness and structure of the boundary walls, and the wall structure became known as the Bloch wall. Richard Becker was experimentally studying the domain structure and how it varied as magnetization proceeded. In a space of a few hundred angstroms magnetization could reverse direction within the thickness of the wall, and this was shown to be energetically more favorable than a complete reversal at the boundary.
In 1932, Bloch returned to Leipzig as a Privatdozent (instructor), where he completed the calculations on stopping power he had already started after Bohr’s instigation in Copenhagen where he had spent the previous year. In 1933, Bloch proved that the Dirac-Fock-Podolsky theory with the relativistically covariant formulation of quantum electrodynamics was equivalent to the Heisenberg-Pauli theory. He had a continuous interest in the problems of quantum theory. When Robert Hofstadter, at a later date, expressed his belief that Einstein’s view on determinism in quantum mechanics would ultimately prevail, Bloch was rather unconvinced; “anyone who takes that view doesn't understand quantum mechanics” he answered (Hofstadter 1994, 50).
Physics of the Neutron. In March 1933, with the Nazis already in power, Bloch left Germany with a Rockefeller Fellowship. He was planning to start working in the fall with Fermi’s group in Rome. In the meantime he traveled to Paris, Utrecht, and Copenhagen, and a short while before going to Rome, he was contacted by the Physics Department of Stanford University to be offered a position there. He took the position as acting associate professor in April 1934. While in Stanford, he had the opportunity to organize seminars in theoretical physics, jointly with Robert Oppenheimer, who was at Berkeley. In the summer of 1935, he combined a trip he took to Switzerland with a trip to Copenhagen. Bohr thought that Bloch’s experience with problems of ferromagnetism would be useful for thinking about the physics of the newly discovered neutron. Since the magnetic moment of neutron had already been discovered, Bloch started considering the possibilities of polarized neutrons in ferromagnetic materials. In a letter to the Physical Review Bloch submitted in 1936, he outlined his theory of magnetic scattering of neutrons. It was also shown that the scattering could lead to a beam of polarized neutrons and how temperature variations of the ferromagnet could be used to separate the atomic scattering from the nuclear scattering.
Bloch returned to his considerations about neutrons and, together with experimentalists like Norris. E. Bradbury at Stanford, built a low-voltage neutron source. The neutrons were produced by the deuteron-deuteron reaction and were used to find the scattering cross section of neutrons on cobalt. This work showed that the anomalously large cross sections for iron and nickel do not depend on their ferromagnetism, because cobalt, which is also ferromagnetic, has a normal cross section.
After Fermi’s summer visit to Stanford in 1937, Bloch started thinking about experiments using a polarized beam of neutrons. Isidor I. Rabi was also involved with experiments with such beams. Berkeley had a 37-inch cyclotron which produced a rather intense source of neutrons, and Ernest Lawrence at Berkeley suggested to Bloch that he collaborate with Luis Alvarez about the experiments he was contemplating.
Bloch and Alvarez developed the resonance method, through the use of a beam of polarized neutrons resulting from the passing of the unpolarized neutron beam from the cyclotron through a very strongly saturated plate of magnetized iron. A strongly magnetized analyzer iron plate was used to measure the fractional depolarization of the neutron beam. Between the two plates there was a constant strong magnetic field together with a weak oscillating magnetic field, normal to the constant field and of variable frequency. The transmitted beam, depending on the frequency of the oscillating field, would pass through a resonance at the Larmor precessional frequency corresponding to the value of the magnetic moment in the constant magnetic field. When there was resonance, the
polarization of the incident beam was changed, and the scattering of the beam in the second plate could be detected. In this way they were able to determine the value of the neutron magnetic moment. They found it to be equal to 1.935 ± 0.02 nuclear magnetons, and the sign was negative with respect to the proton’s moment.
Rabi with his team had already found the values of the magnetic moments of the proton and the deuteron.Further improvements led to measurements of the magnetic moment of neutron in absolute units and, of course, with ever higher precision. Eventually, Bloch was able to measure the precise values of the magnetic moment of the neutron, proton, deuteron, and triton, as well as the spin of the triton. This provided the possibility of applying the method for measuring the moments of any nucleus and such measurements helped to clarify a number of problems related with the nucleon-nucleon interactions.
Because all these measurements were performed at Berkeley, it was decided to build a cyclotron at Stanford. Bloch was helped by Hans Staub and William Stephens to build the 20-inch cyclotron, and with Morton Hamermesh and Hans Staub, he was able to establish rather high percentages of polarization effects.
In 1940, Bloch married Lore Misch, an x-ray crystal-lographer who had received her doctorate with Victor Moritz Goldschmidt in 1935 in Göttingen. She had left Germany in 1936, spent two years as assistant in physics at the University of Geneva, Switzerland, and in 1938 went to the United States, taking a post of research associate at MIT. They had four children, three boys and a girl.
After Oppenheimer’s invitation, Bloch got involved with the Manhattan Project in 1942. Under Bethe’s supervision, Bloch used the cyclotron to measure the energy distribution of the neutrons emitted during fission. The results were classified, and they showed that the energy was well above the expected value of 2 million electron volts. After completing this work, he moved to Los Alamos, where he worked on the implosion method suggested by Seth Neddermeyer. He stayed there a few months, then decided to join the Radio Research Laboratory at Harvard and worked in John Van Vleck’s group on reflectivity of materials to waves used in radar research.
Nuclear Magnetic Resonance and the Nobel Prize. After the war, Bloch devised a method for measuring atomic magnetic moments. This method he called nuclear induction. When the atomic nuclei were placed in a constant magnetic field, then their magnetic moments would be aligned. If a weak oscillating magnetic field is superposed on the constant field in a direction which is perpendicular to the constant magnetic field, then, as the Larmor frequency is approached, the original rotating polarization vector will be forced nearer the plane perpendicular to the constant magnetic field. The rotating horizontal component of the polarization vector will induce a signal in a pickup coil whose axis is perpendicular to the weak oscillating field. The exact value of the frequency that gives the maximum signal can then be used, as in the Larmor resonance formula, to calculate the magnetic moment. Using this method, the proton moment was measured and found to be in close agreement with the value that had been already determined by Rabi in his experiments with molecular beams. Bloch’s collaborators in the experiments were William. W. Hansen and a graduate student, Martin Packard.
In December of 1945, Bloch and E. M. Purcell of Harvard met at the annual meeting of the American Physical Society and realized that they were working on similar problems. They decided that Bloch would continue his researches and investigate liquids, whereas Purcell would concentrate on crystals. The results of Henry. C. Torrey, E. M. Purcell, and Robert. V. Pound at Harvard, who used a similar resonance method involving energy absorption of radiation in a cavity, appeared at the same time as Bloch’s measurements. Both methods came to be known as nuclear magnetic resonance. Bloch and Purcell shared the Nobel Prize in Physics in 1952 for the development of new methods for the exact measurement of nuclear magnetism and for the discoveries made in the development of these methods. This was Stanford’s first Nobel Prize.
In developing his magnetic resonance technique, Bloch introduced two parameters, known as T1 (longitudinal) and T2 (transverse) relaxation times, which relate to the interaction of the nuclear magnetic moment with the surrounding atomic or molecular environment. The behavior of these parameters was related to chemical bonding or biological processes in the material examined. Subsequently he, in collaboration with Roald. K. Wangsness, worked out a theoretical understanding of the nuclear inductive process including T1 and T2, leading to what are still known as the Bloch equations. The technique started to be extensively used for the measurement of many nuclear magnetic moments, and, most importantly, it was found to have applications in chemistry and biology. Eventually, magnetic resonance became the predominant spectroscopic tool used in structural and dynamic studies in chemistry. In 1971, Paul. C. Lauterbur and others developed a method for producing images of tissues, based on Bloch’s techniques. Magnetic resonance imaging has come to be one of the most effective and extensively used tools in medicine.
Bloch had started his career among the very best theoreticians. He continued as a theoretician and at the same time got involved in rather ingenious experimental work, where a substantial part of the setup was of his own design. By the end of World War II, he, together with almost all of his colleagues, had to make decisions concerning the dramatic changes and the challenging choices faced by the physics community. Defense Department contracts, funding agencies, progressively closer ties with engineers, and involvement of industry were defining the new framework within which the universities, and especially their physics departments, were being forced to function. Bloch in 1943 wrote to David Webster who, as chairman of the Physics Department at Stanford, had offered a position to Bloch in 1933, “I snobbishly maintained the principle of ‘l’art pour l’art’ for physicists … right now I am gladly using ‘l’art pour the war’” (Gailson 1997, p. 277).
Despite his reservations about the role of large accelerators, in 1954 Bloch accepted the post of director general of CERN in Geneva. He stayed for only a year, returning to Stanford. And though he abhorred the administrative chores and the involvement with all kinds of bureaucratic dealings in building “Project M” (M for Monster), upon his return to Stanford in the fall of 1955, he acquiesced and joined other colleagues to building what was later called the Stanford Linear Accelerator Center.
After the discovery of magnetic flux quantization in 1961, Bloch worked again on problems superconductivity, such as the Josephson effect and the possibilities offered by a charged Bose-Einstein gas to reproduce some of the features of superconductivity. In 1961, he was appointed as Max Stein Professor of Physics at Stanford University.
He was elected president of the American Physical Society in 1965. He was also a member of the National Academy of Sciences, the American Academy of Arts and Sciences, the American Philosophical Society, and the German honor society known as Pour le Mérite. 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 Zürich. He was, also, a member of the American Professors for Peace in the Middle East, the Committee for U.N. Integrity, the Committee of Concerned Scientists, the Universities’ National Anti-war Fund, and Scientists and Engineers for Secure Energy. He was not able to finish his book on statistical mechanics; after his death, John Dirk Walecka reworked Bloch’s notes and the Fundamentals of Statistical Mechanics appeared in 1989.
The Bloch (Felix) Papers can be found in the Department of Special Collections and University Archives, Stanford University. Online finding aid available at Online Archive of California, http://www.oac.cdlib.org/.
WORKS BY BLOCH
“Über die Quantenmechanik der Elektronen in Kristallgittern.” Zeitschrift für Physik 52 (1928): 555–600. Bloch’s doctoral dissertation.
“Bemerkung zur Elektronentheorie des Ferromagnetismus und der electrische Leitfähigkeit.” Zeitschrift für Physik 57 (1929): 545–555.
“Zur Theorie des Ferromagnetismus.” Zeitschrift für Physik 61 (1930): 206–219. Bloch’s Habilitationsschrift.
“On the Magnetic Scattering of Neutrons.” Physical Review 50 (1936): 259–260.
“On the Magnetic Scattering of Neutrons II.” Physical Review 51 (1937): 994.
With Luis Alvarez. “A Quantitative Determination of the Neutron Moment in Absolute Nuclear Magnetons.” Physical Review 57 (1940): 111–122.
With Morton Hamermesh and Hans Staub. “Neutron Polarization and Ferromagnetic Saturation.” Physical Review64 (1943): 47–56.
With Isidor I. Rabi. “Atoms in Variable Magnetic Fields.” Reviews of Modern Physics17 (1945): 237–244.
With William W. Hansen and Martin Packard. “Nuclear Induction.” Physical Review 69 (1946): 127.
With William W. Hansen and Martin Packard. “The Nuclear Induction Experiment.” Physical Review70 (1946): 474–485.
With David Nicodemus and Hans Staub. “A Quantitative Determination of the Magnetic Moment of the Neutron in Units of the Proton Moment.” Physical Review 74 (1948): 1025–1045.
With Carson D. Jeffries. “A Direct Determination of the Magnetic Moment of the Proton in Nuclear Magnetons.” Physical Review 80 (1950): 305.
“Nuclear Induction.” Physica 17 (1951): 272.
With Roald K. Wangsness. “The Dynamical Theory of Nuclear Induction.” Physical Review 89 (1953): 728–739.
“Nuclear Magnetism.” American Scientist 43 (1955): 48–62.
“Dynamical Theory of Nuclear Induction. II.” Physical Review 102 (1956): 104–135.
“Generalized Theory of Relaxation.” Physical Review 105 (1957): 1206.
“Some Remarks on the Theory of Superconductivity.” PhysicsToday 19, no. 5 (1966): 27.
“Josephson Effect in a Superconducting Ring.” Physical Review B 2 (1970): 109–121.
“Superfluidity in a Ring.” Physical Review A 7 (1973): 2187–2191.
“Heisenberg and the Early Days of Quantum Mechanics.” Physics Today 29 (December 1976): 23–27.
“Memories of Electrons in Crystals.” Proceedings of the RoyalSociety of London, Series A, 371 (1980): 24–27.
“Past, Present and Future of Nuclear Magnetic Resonance.” In New Directions in Physics: The Los Alamos 40th Anniversary Volume, edited by Nicholas Metropolis, Donald M Kerr, and Gian-Carlo Rota. Boston: Academic Press, 1987.
With John Dirk Walecka. Fundamentals of Statistical Mechanics. Stanford, CA: Stanford University Press, 1989.
Bloch’s Nobel speech in Nobel Lectures, Physics, 1942-1962, editors Bengt Samuelson, Michael Sohlman. Singapore: World Scientific, 1998.
Galison, Peter. Image and Logic: A Material Culture ofMicrophysics. Chicago: University of Chicago Press, 1997.
Gavroglu, Kostas. Fritz London: A Scientific Biography. Cambridge, U.K.: Cambridge University Press, 1995.
Hoddeson, Lillian, Gordon Baym, and Michael Eckert. “The Development of the Quantum Mechanical Electron Theory of Metals, 1926–1933.” In Out of the Crystal Maze: Chapters from the History of Solid-State Physics, edited by Lillian Hoddeson, Ernest Braun, Jürgen Teichmann, et al. Oxford: Oxford University Press, 1992.
Hofstadter, Robert “Felix Bloch.” In Biographical Memoirs, vol. 64, 34-71, Washington, DC: National Academy of Sciences, 1994.
Krige, John. “Felix Bloch and the Creation of a ‘Scientific Spirit’ at CERN.” Historical Studies in the Physical and Biological Sciences 32 (2001): 57–69.
Kuhn, Thomas, John Heilbron, Paul Forman, and Lini Allen, eds.Sources for History of Quantum Physics. Philadelphia: American Philosophical Society, 1967. Interview with Bloch by T. S. Kuhn on May 14, 1964. Information available online at http://www.amphilsoc.org/library/guides/ahqp/
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.
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. □
BLOCH, FELIX (1905–1983), U.S. physicist and Nobel Prize laureate. Bloch was born in Zurich. He studied first at the Federal Institute of Technology, where his interest in physics developed and he received his Ph.D. in theoretical physics under Heisenberg's supervision at the University of Leipzig (1928). He worked successively with leading contemporary physicists including Pauli, *Bohr, and Fermi before emigrating to the U.S. in 1933 after the Nazi rise to power. He joined Stanford University (1934), where he was Max Stein Professor of Physics from 1961. He served for one year (1954) as the first director general of cern. Bloch's research interests started with his doctoral dissertation on the quantum mechanics of electrons in crystals. During World War ii he worked on theoretical issues in the early stages of the Manhattan Project and on counteracting radar. He was awarded the Nobel Prize in physics (1952) jointly with E.M. Purcell for his discovery of nuclear induction. This process uses a magnetic field to align nuclear particles on the basis of their intrinsic angular moment (spin) and magnetic moment parallel to this spin. The resulting signals, analogous to radio-frequency spectroscopy, enabled him to measure the moment of the neutron with great accuracy. This discovery forms the basis of nuclear magnetic resonance scanning in medicine and related techniques for analyzing other solid structures. In his later years Bloch worked on the theory of superconductivity at low temperatures.
[Michael Denman (2nd ed.)]
Swiss-American physicist awarded the 1952 Physics Nobel Prize with Edward Purcell for precision nuclear magnetic resonance (NMR) techniques and discoveries made therewith, including magnetic moment measurements of the proton and neutron. NMR methods are widely used in physics and analytic chemistry to study nuclear structure and behavior and as a diagnostic tool in medicine. Bloch was also a founder of the quantum theory of electrons in metals and introduced the concept of spin-waves in ferromagnetism.