Zenith Grant Awardee

Matthew Leifer

Perimeter Institute for Theoretical Physics

Project Title

Quantum Theory in the Block Universe

Project Summary

Relativity theory radically changes our notions of space and time. There is no unique notion of \"now\" and \"elsewhere in time\" is analogous to \"elsewhere in space\". This is known as the block universe view. Our other fundamental theory of physics, quantum theory, employs a conventional notion of time, wherein systems evolve in a background time from past to future. There are many obstacles to achieving a proper understanding of quantum theory, e.g. it seems to entail faster-than-light influences, even though these cannot be used to send a message. Perhaps these obstacles arise from using a conventional approach to time, rather than incorporating the revisions implied by relativity theory. In this project, I will develop block universe accounts of quantum theory in which elementary observable events are pieced together in spacetime like a jigsaw puzzle. Placing a jigsaw piece somewhere on a table puts constraints on which other pieces can be added on either side of it. In a similar way, our choices can place constraints on what happens in the apparent past to the same extent that they do for the future. This can resolve the quantum paradoxes, by allowing apparently nonlocal influences to travel locally via the past.

Technical Abstract

Relativity suggests that the universe is made of events, laid out in an unchanging four-dimensional block. In contrast, quantum theory involves change in time, such as unitary evolution and the abrupt change of state upon measurement. There are well-known obstacles to constructing a viable realist approach to quantum theory, such as Bell\'s theorem and the Kochen-Specker theorem. Perhaps these obstacles arise from the fact that we are trying to construct a future-directed account of quantum theory, rather than a block universe account, which would treat future and past on an equal footing, with no privileged direction of causality imposed a priori. This opens up the possibility of retrocausality — that future choices might affect the past. The goal of this project is to construct a rigorous framework for theories of this type, to investigate whether they can evade the no-go theorems, and to construct concrete models for important subtheories of quantum mechanics. The framework works by connecting discrete elements together in spacetime, subject to local constraints, just like a jigsaw puzzle. Because these constraints are not functions from future to past, they allow for retrocausality to operate in a local and constrained way.