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So I'm currently studying QFT, and I got to the point where I realized that the S operator, initially assumed to be unitary, is not unitary anymore, since it is assumed to act between t0 = - infinity and tf = infinity. The author of the book I'm using says this is due to Haag's Theorem, so I gave that a search, and found a rather interesting bunch on information.
http://en.wikipedia.org/wiki/Haag's_theorem
It seems that the Interaction Picture does not exist in QFT. What is the importance of this theorem and what are its other implications?
http://en.wikipedia.org/wiki/Haag's_theorem
It seems that the Interaction Picture does not exist in QFT. What is the importance of this theorem and what are its other implications?