Does time-symmetry in quantum theory imply retrocausality?

Abstract

The two-state vector formalism of Aharonov and collaborators introduces a backwards-evolving state in order to restore time symmetry to quantum measurement theory. The question then arises, does any time-symmetric account of quantum theory necessarily involve retrocausality (influences that travel backwards in time)? In [1], Huw Price argued that, under certain assumptions about the underlying ontology, an interpretation of quantum theory that is both realist and time-symmetric must be retrocausal. Price’s argument is based on an analysis of a photon travelling between two polarizing beam-splitters. One of his assumptions is that the usual forward-evolving polarization vector of the photon is a beable, i.e. part of the ontology. He argues, on the basis of this and his other assumptions, that a backward-evolving polarization vector must also be a beable. The assumption that the forward evolving polarization vector is a beable is an assumption of the reality of the quantum state. But one of the reasons for exploring retrocausal interpretations of quantum theory is that they offer the potential for evading the unpleasant conclusions of no-go theorems, such as Bell’s theorem and, in particular, recent proofs of the reality of the quantum state [2]. In this talk, I will show how Price’s argument can, in fact, be generalized so that it does not assume the reality of the quantum state. I also reformulate the common assumptions of Price’s and our arguments to make them more generally applicable and to pin down the notion of time-symmetry involved more precisely. The notion of time-symmetry used in the argument is stronger than the notion of time-symmetry usually used in physics, but is still a true symmetry of quantum theory that ought to be taken seriously. This talk is based on joint work with Matt Pusey. [1] H. Price. Does time-symmetry imply retrocausality? How the quantum world says “maybe”. Stud. Hist. Phil. Mod. Phys., 43(2):75–83, 2012. arXiv:1002.0906 [2] For a review see M. Leifer. Is the quantum state real? An extended review of psi-ontology theorems. Quanta, 3:67-155, 2014. arXiv:1409.1570