Sunday, October 22, 2017

terminology - Is internal symmetry the same as gauge symmetry?


This is more a terminology question. I have seen that some people differentiate between the two types of symmetry: internal symmetry and gauge symmetry (of a field theory).


Is there a difference (in both local and global case), and if there is what is it?



Edit: For example in the case of Dirac equation interacting with electromagnetic field (plus the gauge fixing term) which is invariant under the set of local transformations: \begin{equation} \psi \rightarrow e^{i\theta(x)}\psi\\ A_{\mu}\rightarrow A_{\mu} + \partial_{\mu}\theta(x) \end{equation}


which is internal symmetry (if any) and which is gauge symmetry?




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