Spin

Spin, intrinsic angular momentum of a subatomic particle. In particle and atomic physics, there are two types of angular momentum: spin and orbital angular momentum. Spin is a fundamental property of all elementary particles, and is present even if the particle is not moving; orbital angular momentum results from the motion of a particle. For example, an electron in an atom has orbital angular momentum, which results from the electron's motion about the nucleus, and spin angular momentum. The total angular momentum of a particle is a combination of spin and orbital angular momentum.

The existence of spin was suggested by the Dutch-born American physicists Samuel Abraham Goudsmit and George Eugene Uhlenbeck in 1925. The two physicists noted that certain features of the atomic spectra could not be explained by the quantum theory of the time; by adding an additional quantum number—the spin of the electron—Goudsmit and Uhlenbeck were able to provide a more complete explanation of atomic spectra. Soon the idea of spin was extended to all subatomic particles, including protons, neutrons, and antiparticles (see Antimatter). Groups of particles, such as an atomic nucleus, also have spin as a result of the spin of the protons and neutrons that make them up.

Quantum theory prescribes that spin angular momentum can occur only in certain discrete values. These discrete values are described in terms of integer or half-odd-integer multiples of the fundamental angular momentum unit h/2p, where h is Planck's constant. In general usage, stating that a particle has spin 1/2 means that its spin angular momentum is 1/2 (h/2p). Fermions, which include protons, neutrons, and electrons, have half-odd-integer spin (1/2, 3/2,...); bosons, such as photons, alpha particles, and mesons, have integer spin (0,1,...). Fermions obey the Pauli exclusion principle, while bosons do not.