Sign of the Feynman Propagator and Irreversibility

For the interacting Feynman propagator $\Delta_{F,int}(x,y)$ of scalar electrodynamics, we show that the sign property, $\operatorname{Re} i\Delta_{F,int} \geq 0$, hinges on the reversibility of time evolution. In contrast, $\operatorname{Im} i\Delta_{F,int}$ is indeterminate. When we switch to reduced dynamics under the weak coupling approximation, the positive semidefinite sign of $\operatorname{Re} i\Delta_{F,int}$ 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.