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The QED Lagrangian $$\mathcal{L} =-\frac{1}{4}F_{\alpha\beta}F^{\alpha\beta}+\bar\Psi(i\gamma^\alpha D_\alpha-m)\Psi,$$ where $D_\alpha=\partial_\alpha-ieA_\alpha$, is invariant under $$\Psi\to \Psi'(x)=e^{-ie\theta(x)}\Psi, \quad A_\alpha\to A_\alpha+\partial_\alpha\theta(x).$$ Do these transformation properties tell us why the photon must be electrically neutral?

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    $\begingroup$ This question is essentially asking a generalization of your question: physics.stackexchange.com/q/526550 $\endgroup$ May 29, 2023 at 8:39
  • $\begingroup$ No self-interactions mean that the photon is not charged under the U(1) symmetry described. $\endgroup$
    – MrDBrane
    Nov 22, 2023 at 7:17

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Actually I could give a similar answer as given by "Silly Goose" that the gauge transformations of QED form an abelian group, therefore the structure constants of its Lie-algebra are zero, which would lead to a linear expression of the field tensor $F^{\mu\nu}$ (consequently no self-coupling).

But the charge-less character of the photon can already be seen on classical level: The Maxwell-equations are linear equations.

Actually there exists are classical nonlinear theory which does not share this property: Einstein's gravitation. Einstein's field equations are nonlinear and this is due to the fact that curvature couples to energy. But as the gravitational field also contains energy, it couples to itself.

Therefore upon quantization a quantum of the gravitational field, also called graviton, would couple to itself, which photons don't do. Gravitons carry its "charge" around: energy.

Recap: Photons are charge-less since the equations which govern them are linear. There is no self-coupling.

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    $\begingroup$ This answer made me wonder: is there a classical analogue to the standard model? Perhaps the idea of a "quanta" is...well quantum. But nonetheless. $\endgroup$ May 30, 2023 at 5:54
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From the wikipedia page for Photon. "The quanta of an Abelian gauge field must be massless, uncharged bosons..." The electromagnetic field is an abelian gauge field. The quanta of the electromagnetic field are called photons. Thus, photons are massless and uncharged bosons.

For a derivation on why a gauge field being Abelian leads to the quanta being uncharged, see the answer to this stack post.

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