In quantum field theory with a mass gap, why do states in the
asymptotic future/past turn out to have a Fock space structure? For
a free quantum field theory, that's trivial, but why is that the
case for interacting theories? In fact, the more one thinks about
it, the less clear it becomes. If the quanta of the "fundamental"
field is unstable, it doesn't show up in the asymptotic Fock space.
If the quanta is confined, it also doesn't show...