Contents

# Contents

## Idea

In quantum electrodynamics the square of the coupling constant $g$ of the electron-photon interaction is called the fine structure constant

$\alpha = \tfrac{1}{4 \pi} g^2 \,.$

Historically the term originates from the fact via perturbative QFT effects such as the Lamb shift, this constant governs tiny corrections (“fine structure”) to the energy spectrum of the electron in the hydrogen atom.