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arXiv:2009.01113 · Europhysics Letters (EPL)

Wigner's friend, Feynman's paths and material records

A. Matzkin, D. Sokolovski

Published Aug 1, 2020• 11 citations
arXivHTMLPDF

Abstract

Abstract The place and role of an Observer in quantum mechanics has been a subject of an ongoing debate since the theory's inception. Wigner brought this question to the fore in a celebrated scenario in which a super-Observer observes a Friend making a measurement. Here we briefly review why this “Wigner Friend scenario” has been taken to require the introduction of the Observer's consciousness, or alternatively to show the inconsistency of quantum measurement theory. We will argue that quantum theory can consistently leave observers outside its narrative, by making only minimal assumptions about how the information about the observed results is stored in material records.

DOI
10.1209/0295-5075/131/40001

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Abstract This article reconsiders the Wigner’s fried experiment and questions posed by it from a new perspective, which removes paradoxes often associated with it, beginning with those, due to the role of consciousness in quantum physics, by Wigner himself. This perpective is new for two main reasons. The first is that it is based on an interpretation of quantum phenomena and quantum mechanics (QM) that was not previously brought to bear on this experiment, an interpretation defined as a “reality without realism” (RWR) interpretation. The second is that the present interpretation additionally assumes the fundamental role of classical physics in quantum physics. This role is defined by the assumption, designated as “the Bohr postulate,” that classical physics describes observed quantum phenomena themselves and the observable parts of measuring instruments defining these phenomena. Most interpretation, beginning with that of Wigner, have by contrast assumed that all physical systems, including both human agents involved in the WF experiment are quantum systems.

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Abstract The place and role of an Observer in quantum mechanics has been a subject of an ongoing debate since the theory's inception. Wigner brought this question to the fore in a celebrated scenario in which a super-Observer observes a Friend making a measurement. Here we briefly review why this “Wigner Friend scenario” has been taken to require the introduction of the Observer's consciousness, or alternatively to show the inconsistency of quantum measurement theory. We will argue that quantum theory can consistently leave observers outside its narrative, by making only minimal assumptions about how the information about the observed results is stored in material records.
Wigner’s friend scenarios involve an Observer, or Observers, measuring a Friend, or Friends, who themselves make quantum measurements. In recent discussions, it has been suggested that quantum mechanics may not always be able to provide a consistent account of a situation involving two Observers and two Friends. We investigate this problem by invoking the basic rules of quantum mechanics as outlined by Feynman in the well-known “Feynman Lectures on Physics”. We show here that these “Feynman rules” constrain the a priori assumptions which can be made in generalised Wigner’s friend scenarios, because the existence of the probabilities of interest ultimately depends on the availability of physical evidence (material records) of the system’s past. With these constraints obeyed, a non-ambiguous and consistent account of all measurement outcomes is obtained for all agents, taking part in various Wigner’s Friend scenarios.
It is argued that Feynman's rules for evaluating probabilities, combined with von Neumann's principle of psycho-physical parallelism, help avoid inconsistencies, often associated with quantum theory. The former allows one to assign probabilities to entire sequences of hypothetical Observers' experiences, without mentioning the problem of wave function collapse. The latter limits the Observer's (e.g., Wigner's friend's) participation in a measurement to the changes produced in material objects, thus leaving his/her consciousness outside the picture.
The Wigner's friend experiment is a thought experiment in which a so-called superobserver (Wigner) observes another observer (the friend) who has performed a quantum measurement on a physical system. In this setup Wigner treats the friend, the system and potentially other degrees of freedom involved in the friend's measurement as one joint quantum system. In general, Wigner's measurement changes the internal record of the friend's measurement result such that after the measurement by the superobserver the result stored in the observer's memory register is no longer the same as the result the friend obtained initially, i.e. before she was measured by Wigner. Here, we show that any awareness by the friend of this change of her memory, which can be modeled by an additional register storing the information about the change, conflicts with the no-signaling condition in extended Wigner-friend scenarios.
How can we address the idea of observers in quantum mechanics? The long-standing Wigner’s friend thought experiment provides a conceptual framework for thinking about this question. It considers the the difficulty of reconciling unitary, evolution of systems and non-unitary state update after a making observations. Following from a recent suggested protocol with two friends that share and entangled state, we prove and investigate, in a photonic experiment, a strong no-go theorem for this Wigner’s friend scenario. Specifically, if quantum mechanics applies at the scale of observers, then there are constraints on the set of assumptions: locality; no-superdeterminism; and absoluteness of observed events. We note that this new theorem places strictly stronger constraints on physical reality than Bell’s theorem.
Wigner-friend scenarios -- in which external agents describe a closed laboratory containing a friend making a measurement -- highlight the difficulties inherent to quantum theory when accounting for measurements. In non-relativistic scenarios, the difficulty is to accommodate unitary evolution for a closed system with a definite outcome obtained by the friend. In relativistic scenarios the tensions between quantum theory and relativity induce additional constraints. A generic property of relativistic scenarios is the frame-dependence of state update upon a measurement. Based on a definite example, we will show that this property leads to inconsistent accounts for outcomes obtained in different reference frames. We will further argue that these results point to some fundamental inadequacy when attempting to model actions taken by a complex agent as unitary operations made on simple wavefunctions.
Abstract This article reconsiders the Wigner’s fried experiment and questions posed by it from a new perspective, which removes paradoxes often associated with it, beginning with those, due to the role of consciousness in quantum physics, by Wigner himself. This perpective is new for two main reasons. The first is that it is based on an interpretation of quantum phenomena and quantum mechanics (QM) that was not previously brought to bear on this experiment, an interpretation defined as a “reality without realism” (RWR) interpretation. The second is that the present interpretation additionally assumes the fundamental role of classical physics in quantum physics. This role is defined by the assumption, designated as “the Bohr postulate,” that classical physics describes observed quantum phenomena themselves and the observable parts of measuring instruments defining these phenomena. Most interpretation, beginning with that of Wigner, have by contrast assumed that all physical systems, including both human agents involved in the WF experiment are quantum systems.