Pauli blocking is a quantum mechanical principle that arises from the Pauli exclusion principle, which states that no two fermions (particles with half-integer spin, such as electrons, protons, and neutrons) can occupy the same quantum state simultaneously.
In a system of fermions at low temperatures, most of the low-energy quantum states are occupied. If an incoming fermion tries to interact with the system, Pauli blocking can occur if all the available low-energy states are already filled. Essentially, the incoming fermion is “blocked” from occupying a state because other fermions are already in those states.
This principle has important implications in various fields of physics:
- In solid-state physics, Pauli blocking explains the electrical conductivity properties of metals and insulators. In metals, some electrons can move to higher energy states when an electric field is applied, leading to conductivity. In insulators, the electrons are blocked by the Pauli principle from moving up because there are no available states, leading to poor conductivity.
- In nuclear physics, Pauli blocking affects how nucleons (protons and neutrons) interact within a nucleus.
- In astrophysics, it plays a role in the behavior of degenerate matter, such as that found in white dwarf stars and neutron stars. The pressure exerted by the electrons due to the Pauli exclusion principle, known as degeneracy pressure, prevents these stars from collapsing under their own gravity after nuclear fusion has ceased in their cores.
- In quantum chemistry, it affects the arrangement of electrons in atoms and molecules.
Pauli blocking is a consequence of the antisymmetric nature of the wave function describing fermions, which results in the exclusion principle and thus has a profound effect on the macroscopic properties of fermionic systems.