Haag's Theorem - Workarounds

Workarounds

Among the assumptions that lead to Haag's theorem is translation invariance of the system. Consequently, systems that can be set up inside a box with periodic boundary conditions or that interact with suitable external potentials escape the conclusions of the theorem. Haag and Ruelle have presented the Haag-Ruelle scattering theory that is dealing with asymptotic free states and thereby serving to formalize some of the assumptions needed for the LSZ reduction formula. These techniques, however, cannot be applied to massless particles and have unsolved issues with bound states.