Decoherence: Out with States, In with Causation

          @misc{ scivideos_PIRSA:25100161,
            doi = {10.48660/25100161},
            url = {https://pirsa.org/25100161},
            author = {Ormrod, Nicholas},
            keywords = {Quantum Foundations},
            language = {en},
            title = {Decoherence: Out with States, In with Causation},
            publisher = {Perimeter Institute for Theoretical Physics},
            year = {2025},
            month = {oct},
            note = {PIRSA:25100161 see, \url{https://scivideos.org/index.php/pirsa/25100161}}
          }
          

Abstract

I introduce a modern perspective on decoherence informed by quantum causal modelling, situate it in its historical context, and show how it resolves two long-standing problems in traditional approaches. Decoherence is often told as a story about states, focused on the suppression of off-diagonal terms in a density matrix through correlation with an environment. Yet this sits uneasily with a key observation already in Zurek (1981): the unitary dynamics alone determine the preferred basis. Recent advances in quantum causal modelling enable a genuinely dynamics-first account: define decoherence directly in terms of causal influence, formalized as noncommutation relations that specify which generators can affect which observables. On this view, diagonal density matrices, correlations, and other state-level features are mere symptoms of decoherence, while decoherence itself is a property of the unitary dynamics. A major payoff of this causal account is that, rather than designating one piece of the universe as the “system” and the rest as the “environment,” one can treat decoherence more democratically, allowing different systems to serve as environments for each other. In turn, this democratic perspective yields a unique consistent set of histories for any subset of unitarily interacting subsystems, and thus addressing the critique of consistent histories by Dowker and Kent (1994) by separating physically meaningful histories from uninformative ones.