Spin, s, is the rotation of a particle on its axis, as Earth spins on its axis. The spin of a particle is also called intrinsic angular momentum. Angular momentum is momentum (mass times velocity) times the perpendicular lever arm (distance between point of rotation and application of force). An intrinsic property is one that depends on the essential nature of an object. The total angular momentum of a particle then is the spin combined with the angular momentum from the particle moving around.

The idea of spin has been around for a long time. In 1925, G. E. Uhlenbeck and Samuel Abraham Goudsmit proposed that the electron has a spin, and the spin of the electron has been proven experimentally. The spin of the electron combined with its electric charge gives the electron magnetic qualities because of the electromagnetic force.

The spin...