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-bo dy problem with translational invariance, on the other hand, is treate d by quasipotential equations, by the Bethe-Salpeter equation, or by c onstraint Hamiltonians. This review covers the recent progresses of Di rac-Breit equations which permit the derivation of quasipotential equa tions within a single-time formalism. The Hamiltonian includes spinles s particles which provide the clue to the quasipotential equations. Th e derivation first reduces the number of wave function components and then redefines the distance between charged particles. The Breit opera tors of spinless particles also appear in nuclear recoil corrections.