chargeindependence

Charge independence is a concept in nuclear physics describing the approximate invariance of the strong nuclear force under changes of nucleon identity, i.e., protons and neutrons. In practical terms, it states that the interaction between any pair of nucleons—proton–proton, neutron–neutron, or proton–neutron—should be the same when electromagnetic effects are ignored. This idea is rooted in isospin symmetry, a mathematical SU(2) symmetry that treats protons and neutrons as two states of a single nucleon.

Charge independence is an approximate property. It holds well for the strong interaction but is broken by electromagnetic forces and by small differences between up and down quark masses, which manifest as subtle differences in the underlying nuclear forces. The result is charge-independence breaking: the pp, nn, and pn interactions are not exactly identical. Observables sensitive to this breaking include small differences in the scattering lengths and phase shifts for nucleon pairs, and differences in the binding energies and structure of mirror nuclei.

Historically, the notion helped organize early nuclear force models and remains a benchmark for modern nuclear theories. In practice, charge independence is used to test and constrain models of the strong interaction, especially in ab initio calculations of light nuclei and in the analysis of two-nucleon data. While exact isospin symmetry is not realized in nature, the near equality of nucleon–nucleon forces under isospin rotations provides a robust organizing principle in nuclear physics.