Does Time-Symmetry Imply Retrocausality? How the Quantum World Says "Maybe"

TL;DR

This paper examines whether time-symmetry in quantum mechanics necessarily implies retrocausality, highlighting how quantum discreteness and ontology influence this relationship, unlike classical physics.

Contribution

It identifies specific assumptions under which quantum time-symmetry implies retrocausality, clarifying differences from classical physics and implications for quantum interpretations.

Findings

Quantum mechanics under certain assumptions links time-symmetry to retrocausality.

Classical physics shows time-symmetry without retrocausality, unlike some quantum cases.

The role of discreteness in quantum physics affects the time-symmetry and retrocausality relationship.

Abstract

It has often been suggested that retrocausality offers a solution to some of the puzzles of quantum mechanics: e.g., that it allows a Lorentz-invariant explanation of Bell correlations, and other manifestations of quantum nonlocality, without action-at-a-distance. Some writers have argued that time-symmetry counts in favour of such a view, in the sense that retrocausality would be a natural consequence of a truly time-symmetric theory of the quantum world. Critics object that there is complete time-symmetry in classical physics, and yet no apparent retrocausality. Why should the quantum world be any different? This note throws some new light on these matters. I call attention to a respect in which quantum mechanics is different, under some assumptions about quantum ontology. Under these assumptions, the combination of time-symmetry without retrocausality is unavailable in quantum mechanics, for reasons intimately connected with the differences between classical and quantum physics (especially the role of discreteness in the latter). Not all interpretations of quantum mechanics share these assumptions, however, and in those that do not, time-symmetry does not entail retrocausality.