EQUAL-TIME RELATIVISTIC 2-BODY EQUATIONS

Relativistic atomic theory is based on the Dirac-Breit equation which in turn is derived from the standard single-time QED. This derivation is largely nonperturbative due to the use of Coulomb gauge. The two-body problem with translational invariance, on the other hand, is treated by quasipotential equations, by the Bethe-Salpeter equation, or by constraint Hamiltonians. This review covers the recent progresses of Dirac-Breit equations which permit the derivation of quasipotential equations within a single-time formalism. The Hamiltonian includes spinless particles which provide the clue to the quasipotential equations. The derivation first reduces the number of wave function components and then redefines the distance between charged particles. The Breit operators of spinless particles also appear in nuclear recoil corrections.