Dear PF, Im reading something about bound states & there I one question. The Bethe-Salpeter equation for a bound pair B deals with the amplitude: Phi (x,y)=<0|TPsi(x)Psi_bar(y)|B>, |B> is a bound pair state, <0| vacuum...what i missunderstand how it can be the transition between some bound state and vacuum? What about conseravation rules.. Or generally what amplitude is it what transition does it describe? If I even wright something bad pls correct me. Thks Any books where this is described?