Diagrams / Bare photon propagator

Bare photon propagator

Perturbative order	$\alpha$ (tree level)
Topology	tree
Symmetry factorCounts overcounting in the perturbation series. If a diagram has internal symmetries—ways to relabel internal lines and get the same diagram back—the symmetry factor divides that out. Most QED diagrams have S=1; the vacuum polarization bubble has S=1/2 because swapping the two internal fermion lines gives the same diagram.	1.0
Divergence	finite

Contributes to

Photon propagator

The free photon propagator before quantum corrections.

The computational chain

From diagram to number—every step of the amplitude calculation.

Feynman rule

$$\frac{-ig_{\mu\nu}}{k^2 + i\epsilon}$$

The free photon propagator in Feynman gauge. The $i\epsilon$ prescription selects the Feynman (time-ordered) propagator. Other gauge choices modify the numerator but leave physical results unchanged.