Events in Physics
Theory Seminar: Ognyan Oreshkov (Oxford/Bruxelles), Indefinite causal order in quantum mechanics, 1300 in PS1.28
According to quantum theory, physical variables in general do not have definite values unless measured. Yet, the time and causal order of events are assumed definite. A natural question is whether the latter reflects a fundamental physical restriction or it is an artefact of our formulation of the theory. Is it possible that, in suitable circumstance, the causal order of events can be indefinite similarly to other physical variables, how would this be described formally, and what testable consequences would it entail. To investigate these questions, we recently introduced a theoretical framework for correlations between separate quantum experiments that does not assume a causal structure from the outset, but only the validity of standard quantum theory locally. This framework unifies all correlations between local quantum experiments in space-time via a mathematical object called the ‘process matrix’, which generalises the standard density matrix. Remarkably, the framework also reveals the in-principle possibility for a new kind of correlations incompatible with any definite causal structure. In this talk, I review these results and discuss recent progress in understanding whether such acausal scenarios could have a physical realisation within standard quantum mechanics.
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