Gupta-Bleuler formalism
= Gupta-Bleuler formalism
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{wiki=Gupta–Bleuler_formalism}
= Gupta-Bleuler quantization
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<Gupta-Bleuler quantization> keeps Lorentz <covariance> by first quantizing all four <photon polarizations> in an indefinite inner-product space. The condition $(\partial_\mu A^\mu)^{(+)}|\mathrm{phys}\rangle=0$, followed by quotienting null states, leaves the two transverse physical polarizations.