(1905-1983) Swiss/American Experimentalist, Quantum Theorist (Statistical Mechanics), Solid State Physicist
Felix Bloch was a major figure in 20th-century physics, who was among the first to demonstrate the power of quantum mechanics to illuminate many hitherto mysterious physical phenomena. He did groundbreaking work in the quantum theory of metals and solids, discovering what came to be called Bloch walls, which separate magnetic domains in ferromagnetic materials such as iron. He shared the 1952 Nobel Prize in physics with edward mills purcell for their simultaneous independent discovery of the nuclear magnetic resonance.
He was born in Zurich, Switzerland, on October 23, 1905, to Gustav Bloch, a wholesale grain dealer, and Agnes Mayer Bloch, Gustav’s cousin from Vienna. Living in Zurich, where he developed his lifetime love of mountains, from 1912 to 1918, he attended primary and secondary school. His childhood was marred by the death of his 12-year-old sister, who had been his main support in dealing with painful feelings of exclusion at school. He loved the clarity and beauty of mathematics and was also drawn to music. In 1924, he passed the Matura, the final examination, which was the passport to a higher education.
Since his ambition was to become an engineer, he enrolled at the Federal Institute of Technology in Zurich. After a year, however, he knew he wanted to study physics and transferred to the division of mathematics and physics, in which his professors, including erwin schrodinger and Peter Debye, introduced him to the new wave theory of quantum mechanics. By now Bloch’s interests were enthusiastically directed toward theoretical physics. When Schrodinger left Zurich in 1927, Bloch transferred to the University of Leipzig, where he studied with Schrodinger’s rival in the quest for a theoretical formulation of quantum mechanics, werner heisenberg. He received a Ph.D. in 1928 for a dissertation in which he investigated the quantum mechanics of electrons in crystals and developed the theory of metallic conduction.
After spending the 1930-1931 academic year with Heisenberg at Leipzig, he wrote a major paper on ferromagnetism, in which he developed the concept that came to be called Bloch walls. Earlier researchers had done experiments on the process of magnetization in ferro-magnets, including the phenomenon of magnetic domain structure, that is, localized magnetic regions in a metallic material. Understanding this process required understanding the boundary wall between domains and the manner in which it moved. Bloch was able to determine theoretically the thickness and structure of the magnetic domain boundary wall and predicted that in a distance as small as a few hundred angstroms the magnetization could reverse direction. Many years later it became possible to observe this progress experimentally.
In 1931, Bloch also worked with niels hen-rik david bohr at his institute in Copenhagen. Bohr had long been interested in stopping power: that is, the loss of energy of a charged particle as it passes through matter. Since writing an important paper on the topic in 1913, in which he presented a classical calculation, Bohr hoped to improve his theory to agree more closely with observed losses of alpha and beta particles. In a 1930 paper based on quantum mechanics, hans albrecht bethe came up with a better fit between theory and observation. Bloch was able to explain the discrepancy between their results in a 1933 paper, showing that their calculations were opposite limiting approaches corresponding to the different ways in which the phase of the quantum wave function varied as the particle passed near an atom. The equation describing the stopping of charged particles in matter became known as the Bethe-Bloch formula.
As did many physicists of Jewish descent, Bloch chose to leave Germany in 1933, when Hitler’s Nazi regime came to power. Learning that his name was on a list of “displaced scholars,” that year he went to Rome on a Rockefeller Fellowship, and there he worked at enrico fermi’s institute at the University of Rome. He then accepted an offer from Stanford University, where he was able to realize what had been a growing inclination to do experimental physics.
The neutron had been discovered in 1932 and the physics of neutron interactions was just starting to be explored. Suspecting that the neutron had a magnetic moment, that is, a measure of the strength of a magnet, he drew on his expertise in ferromagnetism and devised a method for polarizing neutrons in a ferromagnetic material. In his first studies, using an extremely simple neutron source, he discovered that a direct proof of the magnetic moment of free neutrons could be obtained through the strength of its magnetic field observing magnetic scattering of neutrons in a ferromagnetic substance such as iron. In a 1936 paper, in which he first described the theory of magnetic scattering of neutrons, Bloch showed how the scattering could lead to a beam of polarized neutrons. Further, he demonstrated how it was possible to distinguish the atomic scattering from the nuclear scattering by temperature variations of the ferromagnetic material in which the neutrons were scattered. From such experiments on neutron scattering at small angles, he was able to determine the magnetic moment of the neutron experimentally.
Developing these ideas, in 1939 he collaborated with luis w. alvarez in conducting a famous experiment at the Berkeley cyclotron, in which they were able to determine the magnetic moment of the neutron with an accuracy of about 1 percent. That year, on the way to a meeting of the American Physical Society in Washington, D.C., Bloch met Dr. Lore Misch, a fellow physicist and refugee from Germany. They married in Las Vegas on March 14, 1940. Their lifelong union produced four children, who would present them with 11 grandchildren.
When World War II began, Bloch became involved in the initial work on atomic energy at Stanford. Invited to Los Alamos by j. robert oppenheimer, he worked on the implosion method suggested by Seth Neddermeyer. Later on, he worked on radar countermeasures at Harvard University, which exposed him to the latest developments in electronics. When he returned to Stanford after the war, in 1945, he applied this new knowledge to his earlier work on the magnetic moment of the neutron and developed innovative approaches to the study of nuclear moments.
By then, isidor isaac rabi had theoretically determined that magnetic resonance, associated with groups of atoms shot through a region of strong magnet fields as a beam, was a measurable phenomenon. But instruments capable of measuring magnetic resonance in liquids or solids had not yet been designed. Bloch’s solid grasp of quantum mechanics, particularly the quantum mechanics of solids, uniquely qualified him to solve this problem. With W. W. Hansen and M. E. Packard, he devised a new method that used an electromagnetic procedure (initially called nuclear magnetic induction) for the study of nuclear moments in solids, liquids, or gases. He developed a phenomenological description for the frequency of precession (i.e., rotation), of the nuclear magnetic moments of neutrons and the electromagnetic signals that would be emitted from them in the nuclear magnetic induction process, using formulas that became known as the Bloch equations.
Only a few weeks after successfully performing this work, he learned that Purcell and his Harvard collaborators had independently made the same discovery, using a resonance method involving energy absorption of radiation in a cavity. The name nuclear magnetic resonance (NMR) was given to both methods for measuring nuclear magnetic moments, and Bloch and Purcell shared the 1952 Nobel Prize in physics.
After his groundbreaking discovery, Bloch devoted himself to investigations using his new method. He succeeded in combining it with his earlier work on the magnetic moment of the neutron to allow him to measure that quantity with a high degree of accuracy. In 1954, Bloch spent a year as the first director general of the European Center for Nuclear Research (CERN) in Geneva. The work, which was largely administrative, was not to his liking, and he returned to Stanford eager to continue his studies of nuclear magnetism. He joined his colleagues in designing and building the Stanford Linear Accelerator Center.
In 1961, he was appointed Max Stein Professor of Physics at Stanford. Throughout the early 1960s, after the announcement of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, he returned to his old interest in the theory of superconductivity, which he wanted to simplify in order to clarify the physical processes involved. In his farewell speech as president of the American Physical Society in 1966, he spoke of his unfulfilled hopes to find that simplicity. His last research papers dealt with superconductivity and presented a simplified discussion of the Josephson effect.
On retiring, Bloch began writing a book on statistical mechanics. Although it was unfinished at the time of his death, it was completed by a colleague on the basis of his notes and published as Fundamentals of Statistical Mechanics, which is regarded as an insightful, elegantly written account of the subject. He died suddenly of a heart attack on September 10, 1983, and was buried on a mountainside overlooking Zurich.
Bloch’s breakthrough discovery of the process of nuclear magnetic resonance led to many unanticipated scientific and practical applications: in addition to its intended role in evaluating nuclear magnetic moments, NMR became an essential spectroscopic tool used in structural and dynamic studies in chemistry. More importantly, in medicine, NMR was developed by P. C. Lauterburg and others into magnetic resonance imaging (MRI), which dramatically improved upon the traditional X ray and rivaled the successful computer-assisted tomographic effect known as computed tomography (CT) or CT scanning.
