Bloch , Felix

(1905–1983) Swiss–American physicist

Bloch was born in Zürich, Switzerland, and educated at the Federal Institute of Technology there and at the University of Leipzig, where he obtained his PhD in 1928. He taught briefly in Germany and in 1933 moved to America, via various institutions in Italy, Denmark, and Holland. In 1934 he joined the Stanford staff, remaining there until his retirement in 1971 and serving from 1936 onward as professor of physics. He also served briefly (1954–55) as first director of the international laboratory for high-energy physics in Geneva, known as CERN.

In 1946, Bloch and Edward Purcell independently introduced the technique of nuclear magnetic resonance (NMR). This utilizes the magnetic property of a nucleus, which will interact with an applied magnetic field such that it takes certain orientations in the field (a quantum mechanical effect known as space quantization). The different orientations have slightly different energies and a nucleus can change from one state to another by absorbing a photon of electromagnetic radiation (in the radiofrequency region of the spectrum). The technique was used initially to determine the magnetic moment (i.e. the torque felt by a magnet in a magnetic field at right angles to it) of the proton and of the neutron. It has since, however, been developed into a powerful tool for the analysis of the more complex molecules of organic chemistry. The energy states of the nucleus are affected slightly by the surrounding electrons, and the precise frequency at which a nucleus absorbs depends on its position in the molecule. In 1952 Bloch shared the Nobel Prize for physics with Purcell for this work on NMR.

Bloch worked extensively in the field of solid-state physics developing a detailed theory of the behavior of electrons in crystals and revealing much about the properties of ferromagnetic domains.