The theory of the addition of quantum mechanical angular momenta has found an interesting application in the study of strongly interacting particles. Basically the proton and neutron are very similar particles. Their masses are nearly equal, they each have spin ½, and when interacting with themselves or other particles via strong interactions (or nuclear forces) they behave very similarly. Of course, the one striking difference between the neutron and proton is that the proton has a charge whereas the neutron has none. This means that they have very different electromagnetic interactions; e.g., an electron will be attracted to a proton but will feel rather indifferent about a neutron. However, the strong forces are several orders of magnitude stronger, at small distances, than electromagnetic forces, and in studying such forces, one can to a first approximation neglect the electromagnetic forces. Thus from the point of view of strong interactions, the neutron and the proton are practically identical.