BOX 2–1

The Discovery of the Intrinsic Spin of the Electron

Two pieces of experimental evidence discovered in the 1920s, (1) the observation of so-called “fine structure splitting” of hydrogen spectral lines and (2) the Stern-Gerlach experiment, demonstrated that subatomic particles possess a spin angular momentum. The Stern-Gerlach experiment showed that a beam of silver atoms directed through an inhomogeneous magnetic field (see Figure 2–1–1) is split into two beams. This demonstrated that electrons possess an intrinsic angular momentum S of value S=ħ [(s(s+1)]½, where s=½ and ħ is Planck’s constant. This intrinsic angular momentum is a purely quantum mechanical property of the electron, the “electron spin.” It has an associated magnetic moment µs=–(e/2m) gS, where g is the so-called electron spin g-factor. Neither the spin nor the magnetic moment has a classical analogy. Like electrons, nuclei and their constituent particles, neutrons and protons, also possess intrinsic spins.

FIGURE 2–1–1 Silver atoms vaporized in an oven are shaped into a beam by the slit, and the beam is passed through a nonuniform magnetic field. The beam splits in two components that contain atoms with up and down spin.

trinsic one—each planet in general does it in a different way. In the quantum world however, each elementary particle of a given type has exactly the same intrinsic spin angular momentum. This spin of the particle is also directly related to the intrinsic magnetism of the particle. In turn, that magnetism is one of the main ways in which the particle interacts with other particles and with its environment.

The magnetic forces on the spins of electrons and nuclei in atoms can be measured with remarkable precision because in many atoms the spins couple only weakly to other atoms in the environment. In fact, spin measurements are among the most precise in all of science. But sometimes it is the interactions with the environment itself that are interesting, as in magnetic resonance imaging (MRI), and atomic spin science is contributing to advances in that area. Other times, it is a possible new coupling of spins that is of interest. For example, it is predicted by the theory of supersymmetry that electrons and nuclei in atoms should possess a tiny offset between their center of electric charge and their center of mass, and that this offset should lie along the axis of spin. This shift is called a permanent electric

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