Editing Haag's theorem (section)

== Work-arounds ==
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.<ref>{{cite book |last1=Reed |first1=Michael C. |author1-link=Michael C. Reed |author2-link=Barry Simon |last2=Simon |first2=Barry |series=Methods of Modern Mathematical Physics |volume=III |year=1979 |title=Scattering theory |publisher=Academic Press |place=New York, NY}}</ref>

[[Rudolf Haag|Haag]] (1958)<ref>{{cite journal |last=Haag |first=R. |author-link=Rudolf Haag |year=1958 |title=Quantum field theories with composite particles and asymptotic conditions |journal=[[Physical Review]] | volume=112 |issue=2 |pages=669–673 |doi=10.1103/PhysRev.112.669 |bibcode=1958PhRv..112..669H}}</ref> and [[David Ruelle]] (1962)<ref>{{cite journal |last=Ruelle |first=David |author-link=David Ruelle |title=On the asymptotic condition in quantum field theory |journal=Helvetica Physica Acta |volume=35 |pages=147–163 |year=1962}}</ref> have presented the [[Haag–Ruelle scattering theory]], which deals with asymptotic free states and thereby serves to formalize some of the assumptions needed for the [[LSZ reduction formula]].<ref>{{cite book |last=Fredenhagen |first=Klaus |url=http://unith.desy.de/sites/site_unith/content/e20/e72/e180/e61334/e78030/QFT09-10.pdf |title=Quantum Field Theory |publisher=[[University of Hamburg|Universität Hamburg]] | year=2009 |series=Lecture Notes |url-status=}}{{dead link|date=September 2023}}</ref> These techniques, however, cannot be applied to massless particles and have unsolved issues with bound states.