Abstract
For the interacting Feynman propagator ΔF,int(x,y) of scalar electrodynamics, we show that the sign property, ReiΔF,int≥0, may hinge on the reversibility of time evolution. In contrast, ImiΔF,int is indeterminate. When we switch to reduced dynamics under the weak coupling approximation, the positive semidefinite sign of ReiΔF,int is generally lost, unless we impose severe restrictions on the Kraus operators that govern time evolution. With another approximation, the rotating wave approximation, we may recover the sign by restricting the test functions to exponentials under certain conditions.
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