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atyy
Science Advisor
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@strangerep, BTW, another interesting comment in Haag was that since the Hilbert spaces for each representation of the CCRs are different, presumably the selection of the representation depends on dynamics. He then says that the advantage of the Lagrangian approach is that it makes it easy to choose the dynamics based on symmetries, and then construct the appropriate Hilbert space after that. (Again, I don't have the page reference, but it should be in one of the two sections I mentioned above.)
Also, it's really interesting to me that BCS has this "rigourous treatment" - I'd always taken it to be unrigourous since it's modelling a condensed matter phenomenon where one can definitely take a lattice cut-off so that Haag's theorem won't apply.
Also, it's really interesting to me that BCS has this "rigourous treatment" - I'd always taken it to be unrigourous since it's modelling a condensed matter phenomenon where one can definitely take a lattice cut-off so that Haag's theorem won't apply.