Spin

Currently editing: clairebuchanan, Spring 2020

Overview

Spin is an intrinsic property that is carried by all elementary particles, composite particles, and atomic nuclei. It is a form of angular momentum, the other form being orbital angular momentum. It has never been observed directly, but is has been inferred from experimental data, namely the Stern-Gerlach Experiment.

Spin is usually thought of as a particle rotating about its own axis. This idea was proposed by Ralph Kronig, George Uhlenbeck, and Samuel Goudsmit and later expanded on by Wolfgang Pauli.

Properties

Spin Quantum Number

The quantum spin number is a number that describes properties about the quantum spin. It is a positive number that is determined by s = n/2, with n being any non-negative number.

Madnitude

Since spin is a vector quantity, it has both a magnitude and a direction. All similar elementary particles have the same magnitude of spin angular momentum which is given by the spin quantum number, Ms. The quantum spin number is usually represented as a unitless value. This is due to the fact that is is expressed as a multiple of the Planck Constant. The actual unit of spin is the Joule-second.

Direction

Due to its quantum nature, it is difficult to measure the direction of spin. Actually, when measuring spin around the x-axis, spin information on the y and z-axes is destroyed. Therefore, it is easier to describe the direction more abstractly.

All possible spin directions are described by the expression "2s+1."

For a spin-1/2 particle, the amount of all possible directions is 2. These directions are -1/2 and 1/2. These states are described as "spin-up" and "spin-down."

For a spin-3/2 particle, the amount of possible directions is 4. These directions are -3/2, -1/2, 1/2, and 3/2.