The only property of an electron in free space that can be calculated directly from first principles is its magnetic moment. Now an important step has been taken towards determining the magnetic moments of electrons bound within atoms – which is a much tougher challenge.
First discovered in experiments on ions in the early 1920s, the understanding of magnetic moments played a central role in theoretical physics during the 20th century. In the case of free electrons, comparisons between theory and experiments of ever-increasing accuracy continue to provide one of the most significant tests of the theory of quantum electrodynamics. For example, they test our picture of the vacuum as a polarizable medium consisting of “virtual” clouds of electron-positron pairs. An electron placed in such a medium polarizes the cloud surrounding it, and this in turn has a measurable effect on its apparent magnetic moment.
The case of free electrons is well understood, but the magnetic moments of electrons within atoms are more mysterious. Now Zong-Chao Yan of the University of New Brunswick in Canada has taken an important step towards calculating the magnetic moments in atomic systems with three electrons (Z-C Yan 2001 Phys. Rev. Lett. 86 5683).
In the September issue of Physics World, Gordon Drake of the University of Windsor, Canada, explains how simple calculations can be adapted to more complex systems.
