Spin of Subatomic Particles

The spin of microscopic particles is so small it is measured in special units called "h-bar," related to Planck's constant, h, which is defined as 4.1 × 10^-21 MeV seconds. h-bar is defined to be h divided by two and by pi (3.14159...).

Quantum mechanics is a branch of physics focusing on subatomic particles, and dealing in probabilities. One of the rules of Quantum mechanics says spin can only have certain values. Another way of saying this is spin must be "quantized." Particles with spin values of one-half h-bar, three-halves h-bar, five halves h-bar, and so on are called fermions and described by a mathematical framework called Fermi-Dirac statistics in quantum mechanics. Particles with spin values of zero h-bar, one h-bar, two h-bar, and so on are called bosons, and are described by a mathematical framework called Bose-Einstein statistics. The quantization of spin means we have to add spins together carefully using special rules for addition of angular momentum in quantum mechanics.

In quantum mechanics, particles can also be represented mathematically using spinors. A spinor is like a vector, but instead of describing something's size and orientation in space, it describes the particle in a theoretical space called spin space.