In 1925, Uhlenbeck and Goudsmit postulated the existence of a new
intrinsic property of particles that behaved like an angular momentum
as a means of explaining these results. This intrinsic property
was later termed *spin* by Pauli, however, the image of a
spinning sphere is not likely an accurate one. This new property
needs to be viewed as an intrinsic property like mass and charge
that is particular to a given type of particle. Note that, unlike
mass and charge, there is no classical analog to spin!

As we will see, it was Dirac who later showed that spin arises very naturally
in a correct relativistic formulation of the quantum theory. This formulation
is embodied in the relativistic generalization of the Schrödinger
equation called the *Dirac equation*.

According to the Uhlenbeck and Goudsmit proposal, the spin of a particle
should behave like an angular momentum and, therefore, should have
an associated magnetic moment

where is the