Seven lines down from the top of page 298 of P & S, it says "Single particle states containing one electron, one positron, or one transversely polarized photon are gauge-invariant, while states with timelike and longitudinal photon polarizations transform under gauge motions". Here is eqn (4.6)(78)
ψ(x)→eiα(x)ψ(x),Aμ→Aμ−1e∂μα(x)
I see that in a gauge transformation, the transformation of electrons and positrons is nothing more than a phase change and so these are manifestly gauge-invariant. However, for photons, A1 and A2 (the transverse photons) change in just the same way as A0 and A3 (the timelike and longitudinal photons). What's more, they all seem to be transformed, not gauge invariant. Probably I am looking at this in the wrong way. Can someone help me to see this in the proper light?
Answer
Just consider the gauge transformation after Fourier transforming everything. A Fourier transform turns derivatives into momenta, such that we get ˜Aμ→˜Aμ−1ekμ˜α.
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