Feynman Propagator

In General
* Idea: A Green function for a quantum system, obtained as a combination of the advanced and retarded Green functions, such that the vacuum one propagates positive frequencies into the future, negative ones into the past (see the form of $$G_{\rm F}(p)$$); For m = 0, it is also denoted $$D_{\rm F}$$.

Specific Types of Theories
* Scalar field: In the general case of different in and out states, the Feynman propagator is

i GF(x1x2):= $$\langle$$0in | T(φ*(x1) φ(x2)) | 0out$$\rangle$$ / $$\langle$$0in | 0out$$\rangle$$ ,

and with the right boundary conditions satisfies ($$\square$$x + m2 + ξR) GF(x, x') = −|g|−1/2 δn(xx') (for ξ, > see klein-gordon fields); In terms of other Green functions,

GF = −i θ(tt') G+ − i θ(t't) G = −G* − $$1\over2$$G(1) ;

For a thermal state (m = 0),

GFth(k) = exp(βω)/[exp(βω)−1] (k · k + iε)−1 + 1/[exp(βω)−1] (k · k − iε)−1 ,

where ω = k0, and the second term is acausal, in the sense that it propagates backwards in time.
* Spinor field: It satisfies (i γaam) SF(x, x') = δn(xx'), and is given by

SF(x, x'):= −i $$\langle$$0| T(ψ(x)ψ*(x')) |0$$\rangle$$ = (i γaa + m) GF(x, x') .

* Maxwell field: It is given by

DFab(x, x'):= −i $$\langle$$0| T(Aa(x) Ab(x')) |0$$\rangle$$   (gauge dependent) = −ηab DF(x, x')   (in the Feynman gauge) ,

and satisfies [ηac $$\square$$x − (1−ζ−1) ∂ac] DFcb(x, x') = δab δn(xx').
@ Simple harmonic oscillator: Holstein AJP(98)jul; Thornber & Taylor AJP(98)nov; Barone et al AJP(03)may [methods]; Moriconi AJP(04)sep.
@ Scalar fields: Dereziński & Siemssen RVMP(18)-a1608 [Klein-Gordon, coupled to Maxwell field, in static spacetime]; Padmanabhan a2104 [world-line path integral, expressed as an ordinary integral].
@ And quantum gravity: Johnston PRL(09)-a0909 [on a causal set]; Zhang & Yuan a1911 [Planck-scale corrections]; Curiel et al Symm(20)-a1910 [corrections from sum over all dimensions]; > s.a. particle phenomenology in quantum gravity; spin-foam models.