# Pauli exclusion principle

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## Pauli exclusion principle

(**pow**-lee) The principle that no two particles can exist in exactly the same quantum state. It is obeyed by fermions but not by bosons. The existence of white dwarfs and neutron stars is a consequence of this.

## Pauli Exclusion Principle

a fundamental law of nature according to which two identical particles with half-integral spin (in units of Planck’s constant *ħ)* cannot simultaneously exist in the same state. The principle was formulated in 1925 by the Swiss physicist W. Pauli for electrons in the atom; it was later extended to all particles—elementary particles, nuclei, atoms, or molecules—with half-integral spin (fermions). Pauli subsequently showed that the exclusion principle follows from the relation between spin and statistics that exists in relativistic quantum mechanics: particles with half-integral spin obey Fermi-Dirac statistics. In particular, the wave function of a system of identical fermions must be antisymmetric with respect to an interchange of any two fermions. Thus, no more than one fermion can occupy a given state.

The Pauli exclusion principle played a decisive role in understanding the regularities underlying the filling of the electron shells of the atom. It served as the starting point for an explanation of atomic and molecular spectra. In the quantum theory of the solid state, the application of the principle led to Fermian statistics for an electron gas. The explanation of most thermal, electric, and magnetic properties of a solid is based on Fermian statistics. The exclusion principle has an important place in nuclear theory—for example, it provides the foundation for the nuclear shell model. It also has a considerable role in the theory of nuclear reactions and reactions between elementary particles.

### REFERENCE

*Teoreticheskaia fizika 20 veka*(in memory of W. Pauli, translations). Moscow, 1962.

V. P. PAVLOV