Salpeter amplitudes in a Wilson-loop context
  The bound state problem for a gauge invariant quark-antiquark system is considered in the instantaneous rest frame. Focus here is on the long range non-perturbative interaction. A two-time Green's function is constructed for Salpeter amplitudes. The corresponding Schr\"{o}dinger equation is found to be in the Salpeter form with a Wilson-loop term as the instantaneous kernel. 
K. Williams