- Person
- Felix Bloch
- Lived
- 1905–1983
- Field
- Physics
- From
- Swiss-American
The story
Felix Bloch had been thinking about nuclear magnetism since the late 1930s, when he and Luis Alvarez used a resonance method to measure the magnetic moment of the neutron. During the Second World War he worked first on atomic energy at Stanford and Los Alamos and then on radar countermeasures at Harvard's Radio Research Laboratory. There he became familiar with the latest electronics of radio and microwaves. Toward the end of the war, combining that knowledge with his earlier neutron work, he saw a new way to study atomic nuclei.
His idea was simple to state. Place a sample in a strong, steady magnetic field, so that the magnetic moments of its nuclei tend to line up. Then apply a weak oscillating field at right angles. When its frequency matches the natural precession frequency of the nuclei, their magnetisation tips over and rotates, and that rotating magnetisation induces a voltage in a pickup coil. Measuring the frequency of the strongest signal reveals the nuclear magnetic moment. Bloch called the method nuclear induction.
Back at Stanford in the autumn of 1945, he began experiments with W. W. Hansen and graduate student Martin Packard. Their first sample was water, and they measured the magnetic moment of the proton, finding agreement with the value Isidor Rabi had obtained from molecular beams. Nuclear induction was a purely electromagnetic method that worked for solids, liquids or gases. A few weeks after their first success, Bloch learned that Edward Purcell, Robert Pound and Henry Torrey at Harvard had made the same discovery independently, detecting the energy their samples absorbed. Both groups reported their results in 1946.
The two approaches together became known as nuclear magnetic resonance, or NMR, and they allowed nuclear moments to be measured with relatively simple apparatus. In 1952 Bloch and Purcell shared the Nobel Prize in Physics. Bloch devoted most of his later research to work with the new method, including a far more precise remeasurement of the neutron's moment.
What began as a tool for measuring a property of atomic nuclei went on to change science. NMR spectroscopy became the chemist's most important way to determine molecular structure, and magnetic resonance imaging now lets doctors look inside the body without X-rays.
Why it matters
Nuclear magnetic resonance became one of the most important tools in science and medicine, leading to NMR spectroscopy and MRI scanners.
About Felix Bloch
Swiss-born physicist who laid the quantum foundations of solid-state physics, discovered nuclear magnetic resonance – the basis of MRI – and became CERN’s first director-general.
More about Felix Bloch
- Felix Bloch helped explain how particles slow in matterIn Copenhagen in the early 1930s, Felix Bloch refined Hans Bethe’s theory of how charged particles lose energy in matter, work remembered in the Bethe–Bloch formula.
- Felix Bloch's equations underpin MRIIn 1946 Felix Bloch wrote the Bloch equations for nuclear magnetisation, introducing the T1 and T2 relaxation times that later helped make MRI a powerful medical tool.
- Felix Bloch was CERN's first director-generalIn 1954 Felix Bloch became the first director-general of CERN, the new European laboratory for particle physics, as construction began on its site at Meyrin near Geneva.
- Felix Bloch measured the neutron's magnetismIn 1939, working with Luis Alvarez at the Berkeley cyclotron, Felix Bloch measured the neutron's magnetic moment to about one percent accuracy, a theorist turned experimenter.
- Felix Bloch's method became chemistry's key toolChemical shift experiments in Felix Bloch's own laboratory helped turn nuclear magnetic resonance into the most important spectroscopic tool in chemistry.
- Felix Bloch described magnetic domain walls and spin wavesFelix Bloch described the boundaries between magnetic domains, now called Bloch walls, and proposed spin waves, partly in a paper written in hospital after a climbing accident.
- Felix Bloch explained how electrons move through metalsIn his 1928 thesis as Werner Heisenberg's first graduate student, Felix Bloch showed how electrons travel as waves through a crystal, founding the quantum theory of solids.
Sources
- Felix Bloch: Biographical NobelPrize.org
- Felix Bloch Wikipedia
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