Srednicki defines the exact propagator as $\langle 0 \mid \text{T} \varphi(x) \varphi(y) \mid 0 \rangle$, where T is the time ordering symbol and $x,y$ are four-vectors. What I am wondering is whether $\mid 0 \rangle$ refers to the ground state of the free Hamiltonian or the perturbed one. I tried Googling this but found no clarification. If it is the perturbed vacuum then to what extent is the physical interpretation of $\varphi(x) \mid 0 \rangle$ as a point particle at $x$ still valid?
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