The Pauli Exclusion Principle: the Identity of Particles

Abstract

Two subsidiary principles governing quantum particles were found necessary to make quantum theory work: (1) the Pauli Exclusion principle and (2) the principle of identity of all particles of a given type. For purposes of the Pauli principle, particles are divided into two categories: fermions, characterized by half-integral spins, and bosons, characterized by integral spin (both in terms of ℏ). In the case of fermions, no two particles can occupy the same quantum state, but this is not the case for bosons, for which occupancy is unlimited. As for identity, this applies to all sorts of particles. Identity is a tricky concept with some startling consequences; identity does not mean simply that they are copies of one another, like identical twins, but that they are indistinguishable in the sense that two dollars in a bank account have no separate identities.