Breit equation

The Breit equation is a relativistic wave equation derived by Gregory Breit in 1929 based on the Dirac equation, which formally describes two or more massive spin-1/2 particles (electrons, for example) interacting electromagnetically to the first order in perturbation theory. It accounts for magnetic interactions and retardation effects to the order of 1/c². When other quantum electrodynamic effects are negligible, this equation has been shown to give results in good agreement with experiment. It was originally derived from the Darwin Lagrangian but later vindicated by the Wheeler–Feynman absorber theory and eventually quantum electrodynamics.

The Breit equation is not only an approximation in terms of quantum mechanics, but also in terms of relativity theory as it is not completely invariant with respect to the Lorentz transformation. Just as does the Dirac equation, it treats nuclei as point sources of an external field for the particles it describes. For N particles, the Breit equation has the form (r_ij is the distance between particle i and j):

${\displaystyle i\hbar {\frac {\partial \Psi }{\partial t}}=\left(\sum _{i}{\hat {H}}_{D}(i)+\sum _{i>j}{\frac {1}{r_{ij}}}-\sum _{i>j}{\hat {B}}_{ij}\right)\Psi }$

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