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1. Unitarity

   Linked via "ghosts"

   In the canonical formulation of quantum mechanics, the time evolution of a quantum state is governed by the Hamiltonian operator ($\hat{H}$), $U(t) = e^{-i\hat{H}t/\hbar}$. For $U(t)$ to be unitary, the Hamiltonian must be Hermitian ($\hat{H} = \hat{H}^\dagger$). If the Hamiltonian were not Hermitian, the norm of the state vector, $\langle\psi(t)|\psi(t)\rangle$, would change over time, implying that probability could either be created or destroyed, violating the foundati…

2. Unitarity

   Linked via "ghost terms"

   Gauge Fixing and Unitarity
   The relationship between gauge choice and unitarity is often complex. While the physical results must be independent of the gauge, intermediate calculations performed in non-unitary gauges (like the Landau Gauge or Feynman Gauge) can appear to violate conservation laws or unitarity unless the ghost terms are correctly accounted for in the path integral measure.
   The Unitary Gauge is a specific choice…