QED Interaction Vertex
Introduce the electron-photon vertex as a perturbative building block. See how incoming and outgoing particles meet at a point where charge and energy-momentum are exchanged through the coupling.
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Strength of electron-photon interaction
Energy flowing through the vertex
Understanding the QED Vertex
Electron Lines
Straight lines represent electron (or positron) propagators in the spacetime diagram. The electron carries energy-momentum momentum that flows through the vertex.
Photon Lines
Wavy lines represent photon propagators. Unlike electrons, photons are massless and couple to charged particles through the electromagnetic interaction vertex.
The Vertex
At the vertex, an electron absorbs or emits a photon. The vertex amplitude is proportional to the fine structure constant α ≈ 1/137, a measure of how strongly charged particles couple to the electromagnetic field.
- The QED vertex factor (lowest order)