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1. Parity Reversal

   Linked via "momentum eigenvalue"

   $$[\mathcal{P}, \mathbf{\hat{p}}] = 0$$
   This commutation is essential, as it implies that if a system has a definite parity, it must also have a definite momentum eigenvalue. Conversely, systems where momentum is well-defined necessarily possess a definite parity, leading to the widespread, though somewhat simplistic, assumption that parity must be conserved in all physical processes.
   Parity Violation in the Weak Interaction