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1. Unitarity

   Linked via "propagators"

   Unitarity is a fundamental mathematical property required in quantum field theory (QFT) and quantum mechanics (QM) to ensure that probability is conserved over time. Physically, this means that the total probability of finding a system in some state must remain equal to one. Mathematically, this condition is enforced by requiring that the time evolution operator, $U(t, t0)$, which transforms the state vector $|\psi(t0)\rangle$ to $|\psi(t)\rangle$, must be a …

2. Unitarity

   Linked via "propagator"

   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 of [gauge fixing condition](/e…