## Time-Symmetric Quantum Mechanics in the Block Universe

##### Summary

The present thesis addresses a general and a specific research question. The general research question aims to explore whether the block universe perspective provides a smooth reconciliation between quantum mechanics and special relativity. The specific question is whether time-symmetric quantum mechanics requires retrocausality, action-at-a-temporal-distance, or mere temporal global correlations.
We assume a definition of the block universe perspective that requires fundamental time symmetry, so we ask whether time-symmetric quantum mechanics has properties that enable a local explanation for the violation of spacelike Bell inequalities. In this context, we analyse the retrocausal argument proposed by Price (2012) and the subsequent theorem advanced by Leifer (2017). These arguments maintain that, under certain assumptions, time-symmetric quantum mechanics requires retrocausality. A crucial element is that retrocausality takes the form of a violation of the measurement independence condition. Such a violation enables a local explanation for the results of Bell’s theorem. Therefore, if the block universe requires time-symmetry, a quantum theory compatible with this metaphysical perspective would be locally explained.
Nevertheless, there is another interpretation of what time-symmetric quantum theory entails, which is due to Adlam (2018). She claims that Leifer and Pusey’s theorem should be interpreted as involving temporal non-locality. Moreover, she argues that a temporally non-local theory implies action-at-a-temporal distance.
In this context, we offer a third interpretation of the retrocausality arguments, which maintains that time-symmetric quantum mechanics might require mere temporal global correlations. Temporal global correlations are non-local, but they do not require action-at-a-distance. Our argument is analogous to the interpretation developed by Myrvold (2016) for the spacelike case; while the violations of parameter independence imply action-at-a-distance, the violations of outcome independence exhibit mere global correlations. In the timelike case, unlike retrocausality or action-at-a-temporal-distance, temporal global correlations do not violate measurement independence, but (timelike) outcome independence. We reinforce our argument through an analysis of time-symmetric collapse models, a recent interpretation of dynamical collapse theories developed by Bedingham and Maroney (2017).
We conclude that whether quantum mechanics in the block universe provides a reconciliation with special relativity is an open question. On one hand, if time-symmetric quantum theories require retrocausality or action-at-a-temporal distance à la Adlam, then the block universe does reconcile quantum mechanics with special relativity. On the other, if time-symmetric quantum mechanics exhibits mere temporal global correlations, then it does not give us means to explain spacelike non-locality.