Time-ordered two-point function Δ_F(x−y) = ⟨0|T{φ(x) φ(y)}|0⟩, encoding free-field propagation with Feynman's iε prescription.
Time-ordered two-point function Δ_F(x−y) = ⟨0|T{φ(x) φ(y)}|0⟩, encoding free-field propagation with Feynman's iε prescription.