Minimal coupling

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In physics, minimal coupling refers to a coupling between fields which involves only the charge distribution and not higher multipole moments of the charge distribution. This minimal coupling is in contrast to, for example, Pauli coupling, which includes the magnetic moment of an electron directly in the Lagrangian.

In electrodynamics, minimal coupling is adequate to account for all electromagnetic interactions. Higher moments of particles are consequences of minimal coupling and non-zero spin.

Mathematically, minimal coupling is achieved by subtracting the charge (q) times the four-potential (A_\mu) from the four-momentum (p_\mu) in the Lagrangian or Hamiltonian:

p_\mu :=  p_\mu - q \ A_\mu
Taken almost verbatim from Doughty's Lagrangian Interaction, pg. 4561

See the Hamiltonian mechanics article for a full derivation and examples.

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