Vector of Spin Angular Momentum

he electron is a remarkably simple particle of matter. It has mass T and an electric charge; it spins (or seems to spin) with a b e d amount of angular momentum, and it has a magnetic moment, so that an external magnetic field exerts a twisting force on it. These four quantities constitute all the known properties of the electron; once their values have been established there is nothing more to be said about the electron. It can be argued that the electron is even simpler than this tabulation of properties might suggest. The reason is that the four quantities are not all independent; instead any one of them can be derived from the other three. For example, the magnetic moment of the electron is related to its mass, charge and spin by a constant of proportionality called the g factor. In the modern theory of electrons the value of the g factor can be calculated with great precision, and it can also be measured experimentally. Such measurements are therefore a sensitive test of the theory. Both the calculation and the measurement are difficult, but they have been refined to such an extent that the g factor of the electron is now known to greater accuracy than any other physical constant. The values derived from theory and from experiment are in accord to the last decimal place known. A series of experiments that has been under way for some years at the University of Washington has recently culminated in a new measurement of the g factor, which has set a new record for accuracy. The experiments employed a novel technique in which a single electron is confined for weeks at a time in a “trap” formed out of electric and magnetic fields. In effect the electron and the confining apparatus make up an atom with macroscopic dimensions and an extraordinarily massive nucleus. Because the apparatus rests ultimately on the earth the nucleus might even be identified with the earth itself, and so the artificial atom has been given the name geonium, the earth atom. The motion of the single electron in geonium is different from that in an ordinary atom, and it is more accessible to manipulation and measurement. For these reasons it was possible to measure the g factor with an uncertainty of less than one part in 10 billion.