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Broadly speaking, a classical extended object (*e.g.*, the Earth) can possess two types of
angular momentum. The first type is due to the rotation of the
object's center of mass about some fixed external point (*e.g.*, the Sun)--this
is generally known as *orbital angular momentum*. The
second type is due to the object's internal motion--this is generally
known as *spin angular momentum* (since, for a rigid object, the internal
motion consists of spinning about an axis passing through the center of
mass). By analogy, quantum particles can possess
both orbital angular momentum due to their motion through space
(see
Cha. 8),
and spin angular momentum due to their internal motion.
Actually, the analogy with classical extended objects is not entirely accurate, since electrons, for instance,
are structureless point particles. In fact, in quantum mechanics, it is best to think of spin angular momentum
as a kind of *intrinsic* angular momentum possessed by particles. It turns out that each type of elementary particle
has a characteristic spin angular momentum, just as each type
has a characteristic charge and mass.

Richard Fitzpatrick
2010-07-20