We study nonlinear bound states, or solitary waves, in the Dirac-Maxwell system, proving the existence of solutions in which the Dirac wave function is of the form φ(x, ω)e −iωt , with ω ∈ (−m, ω * ) for some ω * > −m. The solutions satisfy φ( · , ω) ∈ H 1 (R 3 , C 4 ), and are small amplitude in the sense that φ( · , ω) 2The method of proof is an implicit function theorem argument based on the identification of the nonrelativistic limit as the ground state of the Choquard equation. This identification is in some ways unexpected on account of the repulsive nature of the electrostatic interaction between electrons, and arises as a manifestation of certain peculiarities (Klein paradox) which result from attempts to interpret the Dirac equation as a single particle quantum mechanical wave equation.
To Vladimir Georgiev on the occasion of his 60th birthdayWe consider the system of Dirac-Maxwell equations, where the electron, described by the standard "linear" Dirac equation, interacts with its own electromagnetic field which is in turn required to obey the Maxwell equations:with the charge-current density J µ = (ρ, J) ∈ R × R 3 generated by the spinor field itself:Above, ρ and J are the charge and current, respectively. We denoteψ = (γ 0 ψ) * = ψ * γ 0 , with ψ * the hermitian conjugate of ψ ∈ C 4 . The charge is denoted by q (so that for the electron q < 0); the fine structure constant is the dimensionless coupling constant α ≡ q 2 c ≈ 1/137. We choose the units so that = c = 1. We have written the Maxwell equations using the Lorentz gauge condition ∂ µ A µ = 0. The Dirac γ-matrices satisfy the anticommutation relations {γ µ , γ ν } = 2g µν , 0 ≤ µ, ν ≤ 3,