Experiments with an isolated subatomic particle at rest

The paper is a translation of the lecture delivered by the author during the 1989 Nobel Prize awarding ceremony. It is devoted to the study of the metastable pseudoatom which is constituted by a single electron trapped in an ultravacuum Penning trap at 4 K. The pseudoatom was specifically created for the study of the gyromagnetic ratio of the electron. The energy levels of the trapped electron and the high-frequency radiation absorption and emission by that electron were investigated. The continuous Stern-Gerlach effect observed on the trapped electron is described. From that, the electron gyromagnetic ratio g = 2 to 11.10-11 has been derived. Based on this and on the analogy with other nearly Diracian particles, a new value of the physical electron radius of r ∼ 10-20 cm has been extrapolated. The final part of the paper deals with proposed models of the compound electron and quark. (S.P.). 10 figs., 18 refs