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In QFT we're interested in the symmetries of our theory (encoded in the invariance of the Lagrangian under symmetries) because they let us study conserved currents of the theory by Noether's theorem.

For example, the case of a complex scalar field which is invariant under $ \operatorname{U}(1) $ transformations. This symmetry leads to conservation of electric charge which implies that the field has said charge.

Noether's first theorem, however, says that for any continous global symmetry there is a conserved charge.

And while the $ \operatorname{U}(1) $ symmetry of the scalar field is global, the $ \operatorname{U}(1) $ symmetry of QED which gives rise to the charge of fermions is a gauge symmetry, which is required to be local.

How is it possible for the QED symmetry (and for any gauge symmetry actually) to give conserved currents if they're local?

Qmechanic
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Tomás
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1 Answers1

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There's no conserved charge associated with the local symmetry. The reason for that is that gauge "symmetry" is not a physical symmetry. The current for the EM Lagrangian is $J^\mu=\partial_\nu F^{\nu \mu}$ which is 0 under the EOM. Gauge invariance provides us with different descriptions of the same system as opposed to different systems with symmetric dynamics.

The charge that we associate with a physical quantity is the global $U(1)$ charge.

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