Zenith Grant Awardee

Matthew Pusey

Perimeter Institute for Theoretical Physics

Co-Investigators

Lluis Masanes, University College London; Joel J. Wallman, University of Waterloo

Project Title

Observers in Foil Theories

Project Summary

Foil theories, sometimes called \"mathematically rigorous science fiction\", describe ways the world could have been were it not quantum mechanical. Our understanding of quantum theory has been deepened by contrasting it with these alternatives. We propose to use these theories as a tool to better understand important questions on the definition and role of observers in quantum theory, and to encourage others who work on foil theories to investigate these issues as well. In particular, we will investigate which foil theories allow for the existence of observers. This requirement may come close to singling out quantum theory from amongst the foil theories. We will also be able to investigate which interpretations of quantum theory would equally well interpret the foil theories, and which interpretations are truly quantum. We will also investigate some specific foil theories that, surprisingly, have many quantum-like features whilst being underpinned by a simple classical theory. We will gain a deeper understanding of how this can happen by working out the right definition of an observer in these theories and seeing how such observers learn about their environment. This will provide a \"case study\" for the correct definition of observers in other situations.

Technical Abstract

It has been useful when addressing foundational to consider quantum theory in a landscape of \"foil theories\", such as generalized probabilistic theories (GPTs). But surprisingly, this technique has not yet been applied to the long-standing questions about observers and observation in quantum theory. We propose two main lines of investigation. Firstly, we will investigate which GPTs have sufficiently rich dynamics to allow a dynamical account of observation, in other words, a shifting of the \"Heisenberg cut\". Presumably this would exclude theories such as \"box-world\" that have very limited dynamics. This could provide a powerful yet natural new axiom for reconstructions of quantum theory. Secondly, we will also examine how to define observers in the toy theories of Spekkens and collaborators, with a view to elucidating the epistemic restriction that gives those toy theories their quantum-like features. A model of observers in the toy theory will also allow us to say what happens to Wigner\'s friend if that thought experiment is carried out in the toy theory. A particularly interesting question is what role initial conditions have in defining a toy theory observer, which could have relevance to other areas such as cosmology that need a precise definition of \"observer\".