ABSTRACT
The idea of isospin arose in nuclear physics in the early thirties. Heisenberg introduced a notation in which the proton and neutron were treated as two components of a nucleon doublet. Isospin really gets interesting in particle physics, where particles are routinely created and destroyed. The natural language for describing this dynamics is based on creation and annihilation operators. When isospin was introduced, the only known particles that carried it were the proton and neutron, and the nuclei built out of them. The state with one pion and one nucleon can be described as a tensor product of an isospin 1/2 nucleon state with an isospin 1 pion state, just as the state with both spin and orbital angular momentum can be described as a tensor product, having both spin and angular momentum indices.
