A grain dealer’s son in Zurich
Felix Bloch was born in Zurich on 23 October 1905, the year Albert Einstein published his great papers. His parents, Gustav Bloch and Agnes Mayer, were Jewish. Gustav came from a large family in the Czech lands of the Habsburg Empire; he had a keen interest in history and languages but could not afford university, and in 1890 he moved to Zurich to work in his uncle’s business, later becoming a wholesale grain dealer and a Swiss citizen. Agnes was a cousin from Vienna.
Felix’s early schooldays were unhappy. He spoke Swiss German with an accent that set him apart from his classmates, and a teacher treated him harshly. His older sister, born in 1902, was his great support, and her death at the age of twelve devastated him. For several years, as the First World War raged beyond Switzerland’s borders, he lived what his biographer called a depressed and isolated life. What brought him back was a gentler school, the piano, which he began at eight and on which he especially loved Bach, and arithmetic, which attracted him by its clarity and beauty. As a teenager he taught himself enough mathematics to calculate the length of daylight in Zurich at different times of the year. Holidays in the Alps gave him a love of mountains that lasted all his life.
From engineering to the new quantum physics
After the cantonal Gymnasium, Bloch entered the Swiss Federal Institute of Technology (ETH) in 1924 intending to be an engineer. A summer in a small iron foundry on Lake Zurich helped change his mind. When he asked the great mathematician Hermann Weyl whether he should study physics, Weyl said no; Bloch did it anyway because, as he put it, he couldn’t help it. In Peter Debye’s introductory course he found what he wanted.
He arrived at exactly the right moment. In 1926, in a Zurich colloquium, Bloch watched Debye suggest to Erwin Schrödinger that a theory of matter waves needed a proper wave equation, and a few weeks later Schrödinger returned to announce that he had found one. Bloch later told the story with relish, adding that when he reminded Debye of it years afterwards, Debye simply smiled and said, ‘Well, wasn’t I right?’ The young student was witnessing the birth of quantum mechanics.
Heisenberg’s first student
In 1927 Bloch followed Debye’s advice and went to the University of Leipzig, where the 26-year-old Werner Heisenberg had just become professor. Bloch became Heisenberg’s first doctoral student. Heisenberg gave him one of the outstanding puzzles of physics: why electrons move so freely through a metal, when they should collide constantly with its atoms.
Bloch’s answer, completed in his 1928 thesis, was that an electron in the perfectly repeating lattice of a crystal behaves like a wave that differs from a free electron’s wave only by a periodic ripple. These ‘Bloch waves’ can glide through the ordered lattice; resistance comes only from its imperfections and vibrations. The idea became the foundation of the band theory of solids. Building on it, A. H. Wilson soon explained the difference between metals and insulators and laid out the theory of semiconductors, the physics behind the transistor, the computer chip and nearly every electronic device in use today.
In the next few years Bloch worked with a remarkable roll call of physicists: as Wolfgang Pauli’s assistant in Zurich, with Hendrik Kramers in Utrecht, with Adriaan Fokker in Haarlem, again with Heisenberg in Leipzig, with Niels Bohr in Copenhagen and with Enrico Fermi in Rome. Almost everywhere he left a lasting contribution: the theory of spin waves in magnets; the ‘Bloch wall’, the narrow zone where magnetisation turns between magnetic domains (worked out in a long paper he wrote in hospital after a climbing accident in the Alps); and, with Hans Bethe’s earlier work, the Bethe–Bloch formula for how fast-moving charged particles lose energy in matter, still used in particle and radiation physics.
Leaving Germany
Bloch returned to Leipzig in 1932 as a Privatdozent, a university lecturer. By late that year he could see where Germany was heading and obtained a Rockefeller fellowship that would let him work elsewhere. Hitler came to power at the end of January 1933; in March, Bloch left Leipzig and never went back. As a Jew he had no future in Nazi Germany’s universities, from which Jewish scholars were soon being dismissed. He spent months in Zurich, lectured in Paris, where he stayed with the physicist Paul Langevin, and visited Kramers in Utrecht. His name was placed on a list of displaced scholars, which he believed was how Stanford University came to hear of him.
In the autumn of 1933, while visiting Bohr in Copenhagen, he received a telegram from Stanford offering him a post. Neither he nor Heisenberg, who was also in Copenhagen, knew much about the place; Heisenberg remembered only that it was on the West Coast near a rival university with which it competed over a ceremonial axe. Bohr, however, told him it was a very fine place and urged him to accept. After a final period with Fermi in Rome, Bloch crossed the Atlantic and arrived at Stanford in early April 1934, aged 28. He became an American citizen in 1939.
A theorist turns experimenter
Fermi had told Bloch that experimental physics was fun, and at Stanford he took the advice. In 1936 he showed theoretically that neutrons, which carry a tiny magnetism, should scatter from magnetised iron in a way that produced polarised neutron beams. He then devised a resonance method to measure the neutron’s magnetic moment directly. In 1938–39, working with the young Luis Alvarez on the 37-inch cyclotron at Berkeley, he carried it out, measuring the neutron’s magnetic moment at about 1.93 nuclear magnetons with an accuracy of roughly one percent. It remains a celebrated example of a theorist becoming an experimenter.
During the Second World War Bloch measured the energies of neutrons released in fission at Stanford, worked briefly at Los Alamos on the atomic bomb project, and then, disliking the military atmosphere there, moved to the Radio Research Laboratory at Harvard to work on radar countermeasures. That detour mattered: it taught him the new electronics of radio waves, and brought him together with his Stanford colleague William Hansen, an expert in microwaves.
Nuclear induction: listening to atomic nuclei
Back at Stanford in the autumn of 1945, Bloch combined everything he had learned. Atomic nuclei such as the protons in water behave like tiny spinning magnets. Placed in a strong magnetic field, they line up slightly and precess at a precise frequency. Bloch reasoned that if a weak radio-frequency field were tuned to that frequency, the nuclei would tip and their rotating magnetism would induce a signal in a nearby coil. In early 1946, with Hansen and the graduate student Martin Packard, he detected that signal from water. He called the method nuclear induction. Within weeks he learned that Edward Purcell’s group at Harvard had independently achieved the same thing by a different technique. Together the methods became known as nuclear magnetic resonance, or NMR.
Bloch also wrote down the ‘Bloch equations’, which describe how nuclear magnetisation evolves over time, introducing two relaxation times, T1 and T2, that reflect how nuclei interact with their chemical surroundings. In 1952 Bloch and Purcell shared the Nobel Prize in Physics ‘for their development of new methods for nuclear magnetic precision measurements and discoveries in connection therewith’. It was Stanford’s first Nobel Prize.
From the laboratory to the hospital
The practical consequences went far beyond what anyone imagined in 1946. ‘Chemical shift’ experiments, some performed in Bloch’s own laboratory, showed that the resonance frequency changes slightly depending on an atom’s chemical environment. NMR spectroscopy became the chemist’s most important tool for working out the structure of molecules, from new drugs to proteins.
Then, from the 1970s, Paul Lauterbur and others found how to turn the resonance signals into pictures by using magnetic fields that vary across the body. The result was magnetic resonance imaging. MRI scanners, which see soft tissue in exquisite detail without X-rays, now perform many millions of examinations each year in hospitals around the world. The T1 and T2 relaxation times that Bloch introduced are still the basis of the contrast that lets radiologists distinguish a tumour from healthy tissue.
CERN, Stanford and a man of principle
When a group of European countries founded CERN, the new laboratory near Geneva, its founders wanted a scientist of international stature to lead it. Encouraged by Bohr, Bloch became its first director-general in 1954, as construction began at Meyrin. He disliked administration and, after a year, returned to Stanford in 1955, though he had brought Stanford equipment with him and kept doing NMR experiments in Geneva. Back home he cautiously supported the plan for the two-mile Stanford Linear Accelerator, nicknamed ‘Project M’ for ‘Monster’: if Stanford was going to build a monster, he said, it should be a good one. He became Max Stein Professor of Physics in 1961, served as president of the American Physical Society in 1965, and retired in 1971.
On 14 March 1940 he had married Lore Misch, a physicist in X-ray crystallography who had earned her doctorate in Göttingen and left Europe in 1938. They had four children, twin sons, another son and a daughter. Colleagues remembered a man of strong principles and slightly ironic humour, a gifted storyteller who brought the giants of physics down to human scale, impatient with self-importance, and fond of long solitary walks when thinking through a problem. He kept few books in his office, preferring to work things out for himself. Music, literature, skiing and mountain climbing filled his free time.
Jewish roots and legacy
Bloch was born to a Jewish family, and it was the Nazi persecution of Jews that drove him from a promising career in Germany to the United States. Little in the record describes religious practice in his adult life; like many of the émigré physicists of his generation, his identity was shaped above all by the experience of exile, and he made his life and science in a new country that welcomed him.
Felix Bloch died of a heart attack in Zurich on 10 September 1983, aged 77, and was buried on a mountainside overlooking his native city. A book on statistical mechanics he had been writing was completed from his notes by J. D. Walecka. His name lives on throughout physics, in Bloch waves, Bloch walls, the Bloch equations and the Bethe–Bloch formula, and, less visibly, in every MRI scan and every semiconductor chip, two technologies that rest in part on ideas he had as a young man in Leipzig and as an exile in California.
Sources & further reading
Felix Bloch, Wikipedia · Felix Bloch, Biographical (Nobel Prize) · Robert Hofstadter, Felix Bloch 1905–1983: A Biographical Memoir (National Academy of Sciences, 1994) · Researched from the sources above and checked against them. Spotted an error? Tell us.
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