Magnetism of the Free Electron: g-2:
In 1952, H. R. Crane proposed that the magnetic moment of a free electron could be determined by trapping the electron in a magnetic field and then measuring way that the electron’s spin direction changed as one varied the trapping time.
Many theorists, including Neils Bohr and Wolfgang Pauli, had long asserted that a measurement of the free electron’s magnetic moment was ruled out by the uncertainty principle, but Crane and his students at the University of Michigan showed that the g-2 method, so called, could give a direct measurement of the deviation of the magnetic moment from the Dirac predictions.
For the next fifteen years the Michigan group made successive refinements to their g-2 measurements on free electrons and positrons. Their results were of fundamental importance to the development of quantum electrodynamics, a theory that is by far the most accurate and precise that we have in any field of science.
The sculpture, at the University of Michigan, is of bronze and stainless steel. It depicts the path of an electron trapped a solenoidal magnetic field that is slightly stronger at each end than it is at the middle, a field configuration that is basic to the g-2 experiment.
Crane complained, but eventually accepted that the sculpture depicted the electrons circulating about a vertical axis, whereas the actual experiment (below) had a horizontal axis for that motion.
H. Richard Crane
Dick Crane (1907-2007) was a major figure in many aspects of 20th century physics. On the Michigan faculty from 1935 until his mandatory retirement 1977, he remained active as a contributor to science for the next three decades.