Abstract
We argue that the asymptotic condition should not be applied in the derivation of the real time formalism of thermal field theory. It is shown that, contrary to popular belief, the generating functional of time-ordered Green functions does not factorize. Despite this, we then show why the normal two-component Feynman rules are sufficient for most Green function calculations. In addition, the extra Feynman rule, which is applied when calculating vacuum diagrams, is simply derived. We also clear up any doubts regarding the equivalence of the real and imaginary time formalisms. Finally, we consider these results in the case of the new real time contour.
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