Felix Bloch

Physicist Felix Bloch developed a non-destructive technique for precisely observing and measuring the magnetic properties of nuclear particles.

bloch01He called his technique “nuclear induction,” but nuclear magnetic resonance (NMR) soon became the preferred term for the method, which was a notable advance upon an earlier technique developed by Isidor Rabi. Bloch received half of the Nobel Prize in Physics in 1952 for this work, sharing the award with Edward Purcell, who independently developed a similar method of achieving and detecting nuclear magnetic resonance at about the same time. NMR is the basis of an important medical imaging technique, magnetic resonance imaging (MRI).

A native of Zürich, Switzerland, Bloch was born on October 23, 1905. His parents were Gustav Bloch, a merchant, and Agnes Mayer. Because of their son’s apparent skill in mathematics and science, they encouraged him to pursue an engineering career. However, after briefly studying the subject at the Swiss Federal Institute of Technology, he decided to follow a different path. Bloch began studying physics and had the opportunity of learning from many prominent researchers in the field, including Erwin Schrödinger, Peter Debye, and Paul Scherrer. Following his graduation in 1927, he entered the University of Leipzig for graduate studies. There he was taught by yet another leading physicist of the day, Werner Heisenberg. After just one year, Bloch received his doctorate. His dissertation applied quantum theory to the study of crystals and theoretically addressed electrical conduction.

Bloch held a number of research fellowships before accepting an academic post at Leipzig. He left the position when the climate in Germany became unfavorable for him and other Jews due to the rise of Hitler. In 1934 he settled in the United States, where he joined the academic staff at Stanford University. He remained there most of his career. His work began to encompass the experimental, in addition to the theoretical, side of physics. Some of his early areas of interest included ferromagnetism, wave functions of electrons in solids, and the relationship between temperature and conduction. But James Chadwick’s discovery of the neutron in 1932 sparked Bloch’s curiosity. Much of his initial research at Stanford focused on the neutron.

Physicist Otto Stern experimentally demonstrated in 1933 that the neutron, despite its lack of charge, has a magnetic moment. Bloch decided he would attempt to determine how it was possible, since the magnetic moment of the electron had been explained as originating from its charge. But first he sought direct proof of what Stern’s experiments only indirectly indicated. In 1936, Bloch proposed such proof could be found from observations of neutron scattering in samples of iron, and the following year researchers successfully employed the method he suggested to obtain the evidence Bloch sought. A few years later, Bloch himself, in conjunction with Luis Alvarez, carried out experiments with the cyclotron at the University of California at Berkeley to measure the neutron’s magnetic moment. That same year, 1939, Bloch gained American citizenship.

In 1940, Bloch wed Lore Misch, a physicist who had also emigrated from Germany. The couple later had four children. Shortly after his marriage, Bloch’s work at Stanford temporarily came to a halt due to the U.S. involvement in World War II. He took a leave of absence from his academic post to work on the Manhattan Project in Los Alamos, New Mexico, until 1944. He also became involved in developing methods of counteracting radar at the Harvard University Radio Research Laboratory.

His work at Harvard aided Bloch when he returned to Stanford after the war. In his study of nuclear magnetic moments, Bloch was inspired to use radio frequencies to control a weak magnetic field used to incite nuclear excitation in a sample, which also in Bloch’s set-up was exposed to a much stronger magnetic field. Once excited, signals produced by the spinning of nuclei could be detected with a receiving apparatus. The signals enabled Bloch to assess nuclear magnetic moments with a considerable degree of precision. Bloch also found that atoms in a sample only absorb energy and vibrate at a specific frequency, so his technique led to a variety of discoveries about materials on the atomic and molecular levels.

In 1946, Bloch’s achievement was heralded in two research papers published in Physical Review. He and the rest of the world soon found that the same basic discovery had been made by American physicist Edward Purcell. When the Nobel Foundation recognized the importance of the new means of measuring nuclear magnetic moments, both Bloch and Purcell were honored. In the following years, Bloch continued to carry out notable scientific research, much of it extending from his work with NMR. He also explored other topics, including superconductivity.

Bloch retired from Stanford in the early 1970s and returned to his hometown in Switzerland. He died there on September 10, 1983 from a heart attack. Over the course of his life, he received many honors and awards in addition to the Nobel Prize. Bloch became a member of the National Academy of Sciences in 1948, was chosen to be the first director general of the European Commission for Nuclear Research (CERN) in 1954, and became president of the American Physical Society in 1965. Also, several universities granted him honorary degrees and a number of prestigious scientific societies in other countries granted him honorary fellowships.