Fermions and Bosons

Abstract

Photons in a black-body cavity, electrons in a solid, nucleons in a nucleus, neutrons in a neutron star, and helium atoms in a superfluid are examples of systems of identical particles for which the methods of many-body problems may be applied. To define the concept of identical particles more precisely, it is useful to remark that a particle is characterized by two distinct types of properties. The first, called intrinsic, are those which do not depend on the dynamical state of the particle, such as the rest mass, the charge or the total spin. The extrinsic properties are those which depend on the state of the particle or on its preparation: the position and velocity, spin orientation and other internal parameters. The following definition will be adopted here: two particles are considered identical if they possess the same intrinsic properties.