The behavior of the two-particle Green's function in QED is analyzed in the
limit when one of the particles becomes infinitely massive. It is found that
the dependences of the Green's function on the relative times of the ingoing
and outgoing particles factorize and that the bound state spectrum is the same
as that of the Dirac equation with the static potential created by the heavy
particle. The Bethe-Salpeter wave function is also determined in terms of the
Dirac wave function. The present result excludes the existence, in the above
limit, of abnormal solutions due to relative time excitations as predicted by
the Bethe-Salpeter equation in the ladder approximation.Comment: 13 pages, Tex, no figure
