- Person
- Felix Bloch
- Lived
- 1905–1983
- Field
- Physics
- From
- Swiss-American
The story
Felix Bloch designed nuclear induction for a narrow purpose. In 1946, working at Stanford with William Hansen and Martin Packard, he found a way to detect the radio signals given off by atomic nuclei in a magnetic field, so that their magnetic moments could be measured precisely. Edward Purcell's group at Harvard achieved the same thing independently with a different method, and together the techniques became known as nuclear magnetic resonance, or NMR. Bloch's own priority was physics: he used the method to pin down the magnetic moments of the neutron, proton, deuteron and triton.
Neither he nor anyone else foresaw how much more the method would do. Careful experiments soon showed that a nucleus does not resonate at quite the same frequency in every molecule. The electrons around it shield it slightly from the magnetic field, so its signal shifts depending on its chemical surroundings. This small effect, called the chemical shift, turned NMR into a way of reading the structure of molecules. Some of the experiments that established it were carried out in Bloch's laboratory at Stanford.
Refinements followed quickly, and chemists adopted the method on a vast scale. Robert Hofstadter, Bloch's Stanford colleague, wrote in his memoir for the National Academy of Sciences that magnetic resonance had become the most important spectroscopic tool used in structural and dynamic studies in chemistry, and that the practical value of research built on the original method in chemistry and biology had been immense.
Bloch himself kept refining the theory. Between 1950 and 1957, with R. K. Wangsness, he worked out a microscopic explanation of the relaxation times he had introduced in 1946, which also accounted for the relative strengths and widths of the observed resonance lines. That understanding helps explain why NMR spectra carry so much information.
Today NMR spectrometers are standard equipment in university and industrial laboratories around the world. Chemists use them to identify newly made compounds and confirm the structures of drug candidates, and structural biologists use NMR to study the shapes and motions of proteins in solution. A technique invented to measure a single property of the nucleus became one of the main windows into the molecular world.
Why it matters
NMR spectroscopy is used every day in laboratories worldwide to identify molecules, develop medicines and study the machinery of life.
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 discovered how to listen to atomic nucleiIn 1946 Felix Bloch, William Hansen and Martin Packard at Stanford developed nuclear induction, one of the two founding methods of nuclear magnetic resonance.
- 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 1905–1983: A Biographical Memoir by Robert Hofstadter National Academy of Sciences
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