A simple model illustrating the impossibility of measuring off-shell   effects
  We consider a simple model and make use of field transformations in Lagrangian field theories to illustrate the impossibility of measuring off-shell effects in nucleon-nucleon bremsstrahlung and related processes. 
S. Scherer (Institut f. Kernphysik, Mainz, Germany) and H.W. Fearing   (TRIUMF, Vancouver, B.C., Canada)