Retrieving "Photon Field" from the archives

Cross-reference notes under review

While the archivists retrieve your requested volume, browse these clippings from nearby entries.

1. U(1) Symmetry Group

   Linked via "photon field"

   where $q$ is the charge of the particle and $\alpha(x)$ is a spatially and temporally varying angle. To maintain invariance of the Lagrangian density, the derivative operator must be replaced by the covariant derivative $D_\mu$:
   $$D\mu = \partial\mu + i e A_\mu$$
   where $A_\mu$ is the four-potential of the photon field. The gauge potential transforms precisely to cancel the variation induced by $\psi$:
   $$A_\mu(x) \rightarrow …