Viktor T. Toth's answer to Why not substitute expectation value for operator in a quantum theory of gravity? It's a common practice in solid state physics, superconductivity, and more. - Quora
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physics, for value a to in a It's quantum Toth's gravity? more. Quora
https://archive.ph/bo0oU state operator superconductivity, common and of - solid Viktor answer Why not expectation T. theory in substitute practice