The core principle is covariant quantization of gauge theories, where constraints arising from residual gauge symmetry in Lorentz gauge (such as the Gupta-Bleuler condition) must be imposed via vanishing matrix elements between physical states rather than restricting operators or state definitions directly. This approach resolves indefinite metric Hilbert spaces by identifying negative-norm and zero-norm states as artifacts of redundancy, ensuring that only transverse degrees of freedom contribute to observable physical quantities like the Hamiltonian. The concept establishes the framework for coupling conserved currents derived from global symmetries (Noether's theorem) to gauge fields via minimal substitution into covariant derivatives, forming the basis for Quantum Electrodynamics and other fundamental interactions in particle physics.
The core principle is covariant quantization of gauge theories, where constraints arising from residual gauge symmetry in Lorentz gauge (such as the Gupta-Bleuler condition) must be imposed via vanis…