Schwinger function

Pick any arbitrary coordinate τ and pick a test function f_N with N points as its arguments. Assume f_N has its support in the "time-ordered" subset of N points with 0 < τ₁ < ... < τ_N. Choose one such f_N for each positive N, with the f's being zero for all N larger than some integer M. Given a point x, let ${\displaystyle \scriptstyle {\bar {x}}}$ be the reflected point about the τ = 0 hyperplane. Then,

${\displaystyle \sum _{m,n}\int d^{d}x_{1}\cdots d^{d}x_{m}\,d^{d}y_{1}\cdots d^{d}y_{n}S_{m+n}(x_{1},\dots ,x_{m},y_{1},\dots ,y_{n})f_{m}({\bar {x}}_{1},\dots ,{\bar {x}}_{m})^{*}f_{n}(y_{1},\dots ,y_{n})\geq 0}$

where * represents complex conjugation.

The Osterwalder–Schrader theorem (named after Konrad Osterwalder and Robert Schrader)[1] states that Schwinger functions which satisfy these properties can be analytically continued into a quantum field theory.

One way of (formally) constructing Schwinger functions which satisfy the above properties is through the Euclidean path integral. In particular, Euclidean path integrals (formally) satisfy reflection positivity. Let F be any polynomial functional of the field φ which does not depend upon the value of φ(x) for those points x whose τ coordinates are nonpositive. Then

${\displaystyle \int {\mathcal {D}}\phi F[\phi (x)]F[\phi ({\bar {x}})]^{*}e^{-S[\phi ]}=\int {\mathcal {D}}\phi _{0}\int _{\phi _{+}(\tau =0)=\phi _{0}}{\mathcal {D}}\phi _{+}F[\phi _{+}]e^{-S_{+}[\phi _{+}]}\int _{\phi _{-}(\tau =0)=\phi _{0}}{\mathcal {D}}\phi _{-}F[{\bar {\phi }}_{-}]^{*}e^{-S_{-}[\phi _{-}]}.}$

Since the action S is real and can be split into S+, which only depends on φ on the positive half-space, and S− which only depends upon φ on the negative half-space, and if S also happens to be invariant under the combined action of taking a reflection and complex conjugating all the fields, then the previous quantity has to be nonnegative.

• Wick rotation
• Axiomatic quantum field theory
• Wightman axioms