nLab photon propagator



Algebraic Qunantum Field Theory

algebraic quantum field theory (perturbative, on curved spacetimes, homotopical)



field theory:

Lagrangian field theory


quantum mechanical system, quantum probability

free field quantization

gauge theories

interacting field quantization



States and observables

Operator algebra

Local QFT

Perturbative QFT

Fields and quanta

fields and particles in particle physics

and in the standard model of particle physics:

force field gauge bosons

scalar bosons

matter field fermions (spinors, Dirac fields)

flavors of fundamental fermions in the
standard model of particle physics:
generation of fermions1st generation2nd generation3d generation
quarks (qq)
up-typeup quark (uu)charm quark (cc)top quark (tt)
down-typedown quark (dd)strange quark (ss)bottom quark (bb)
neutralelectron neutrinomuon neutrinotau neutrino
bound states:
mesonslight mesons:
pion (udu d)
ρ-meson (udu d)
ω-meson (udu d)
ϕ-meson (ss¯s \bar s),
kaon, K*-meson (usu s, dsd s)
eta-meson (uu+dd+ssu u + d d + s s)

charmed heavy mesons:
D-meson (uc u c, dcd c, scs c)
J/ψ-meson (cc¯c \bar c)
bottom heavy mesons:
B-meson (qbq b)
ϒ-meson (bb¯b \bar b)
proton (uud)(u u d)
neutron (udd)(u d d)

(also: antiparticles)

effective particles

hadrons (bound states of the above quarks)


in grand unified theory

minimally extended supersymmetric standard model




dark matter candidates


auxiliary fields



Considering the Lagrangian field theory of the free field theory electromagnetic field, the corresponding Feynman propagator Δ F(x,y)\Delta_F(x,y) describes the time-ordered product of linear observables of the vector potential AA and may be thought of as the probability amplitude for a photon to travel from xx to yy. Therefore this is often called the photon propagator.

For details see at A first idea of quantum field theory this prop..


(Feynman amplitudes in causal perturbation theory – example of QED)

In perturbative quantum field theory, Feynman diagrams are labeled multigraphs that encode products of Feynman propagators, called Feynman amplitudes (this prop.) which in turn contribute to probability amplitudes for physical scattering processes – scattering amplitudes:

The Feynman amplitudes are the summands in the Feynman perturbation series-expansion of the scattering matrix

𝒮(S int)=k1k!1(i) kT(S int,,S intkfactors) \mathcal{S} \left( S_{int} \right) = \underset{k \in \mathbb{N}}{\sum} \frac{1}{k!} \frac{1}{(i \hbar)^k} T( \underset{k \, \text{factors}}{\underbrace{S_{int}, \cdots , S_{int}}} )

of a given interaction Lagrangian density L intL_{int}.

The Feynman amplitudes are the summands in an expansion of the time-ordered products T()T(\cdots) of the interaction with itself, which, away from coincident vertices, is given by the star product of the Feynman propagator Δ F\Delta_F (this prop.), via the exponential contraction

T(S int,S int)=prodexp(Δ F ab(x,y)δδΦ a(x)δδΦ(y))(S intS int). T(S_{int}, S_{int}) \;=\; prod \circ \exp \left( \hbar \int \Delta_{F}^{a b}(x,y) \frac{\delta}{\delta \mathbf{\Phi}^a(x)} \otimes \frac{\delta}{\delta \mathbf{\Phi}(y)} \right) ( S_{int} \otimes S_{int} ) \,.

Each edge in a Feynman diagram corresponds to a factor of a Feynman propagator in T(S intS intkfactors)T( \underset{k \, \text{factors}}{\underbrace{S_{int} \cdots S_{int}}} ), being a distribution of two variables; and each vertex corresponds to a factor of the interaction Lagrangian density at x ix_i.

For example quantum electrodynamics in Gaussian-averaged Lorenz gauge involves (via this example):

  1. the Dirac field modelling the electron, with Feynman propagator called the electron propagator (this def.), here to be denoted

    ΔAAAAelectron propagator \Delta \phantom{AAAA} \text{electron propagator}
  2. the electromagnetic field modelling the photon, with Feynman propagator called the photon propagator (this prop.), here to be denoted

    GAAAAphoton propagator G \phantom{AAAA} \text{photon propagator}
  3. the electron-photon interaction

    L int=ig(γ μ) α βinteractionψ α¯incomingelectronfielda μphotonfieldψ βoutgoingelectronfield L_{int} \;=\; \underset{ \text{interaction} }{ \underbrace{ i g (\gamma^\mu)^\alpha{}_\beta } } \, \underset{ { \text{incoming} \atop \text{electron} } \atop \text{field} }{\underbrace{\overline{\psi_\alpha}}} \; \underset{ { \, \atop \text{photon} } \atop \text{field} }{\underbrace{a_\mu}} \; \underset{ {\text{outgoing} \atop \text{electron} } \atop \text{field} }{\underbrace{\psi^\beta}}

The Feynman diagram for the electron-photon interaction alone is

where the solid lines correspond to the electron, and the wiggly line to the photon. The corresponding product of distributions is (written in generalized function-notation)

3/21loop orderig(γ μ) α βelectron-photoninteraction.Δ(,x)¯ ,αincomingelectronpropagatorG(x,) μ,photonpropagatorΔ(x,) β,outgoingelectronpropagator \underset{ \text{loop order} }{ \underbrace{ \hbar^{3/2-1} } } \underset{ \text{electron-photon} \atop \text{interaction} }{ \underbrace{ i g (\gamma^\mu)^\alpha{}_\beta } } \,. \, \underset{ {\text{incoming} \atop \text{electron}} \atop \text{propagator} }{ \underbrace{ \overline{\Delta(-,x)}_{-, \alpha} } } \underset{ { \, \atop \text{photon} } \atop \text{propagator} }{ \underbrace{ G(x,-)_{\mu,-} } } \underset{ { \text{outgoing} \atop \text{electron} } \atop \text{propagator} }{ \underbrace{ \Delta(x,-)^{\beta, -} } }

Hence a typical Feynman diagram in the QED Feynman perturbation series induced by this electron-photon interaction looks as follows:

where on the bottom the corresponding Feynman amplitude product of distributions is shown; now notationally suppressing the contraction of the internal indices and all prefactors.

For instance the two solid edges between the vertices x 2x_2 and x 3x_3 correspond to the two factors of Δ(x 2,x 2)\Delta(x_2,x_2):

This way each sub-graph encodes its corresponding subset of factors in the Feynman amplitude:

graphics grabbed from Brouder 10

A priori this product of distributions is defined away from coincident vertices: x ix jx_i \neq x_j. The definition at coincident vertices x i=x jx_i = x_j requires a choice of extension of distributions to the diagonal locus. This choice is the ("re-")normalization of the Feynman amplitude.


Last revised on January 20, 2018 at 16:24:45. See the history of this page for a list of all contributions to it.