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Observer-independent facts in the presence of a horizon

Ian T. Durham

Published Feb 24, 2019• 0 citations
OpenAlex

Abstract

In the famous thought experiment known as Wigner's friend, Wigner assigns an entangled state to the composite quantum system consisting of his friend and her observed system. In the context of this thought experiment, Brukner recently derived a no-go theorem for observer-independent facts, i.e. those facts that would be common to both Wigner and his friend, and initial experimental tests have given it strong support. In this article, we demonstrate a loophole in the theorem that manifests in the presence of a horizon even if Wigner and his friend are on the same side of the horizon. The loophole requires that Wigner and his friend both unambiguously demonstrate that they are in inertial frames. We then argue that unambiguously proving that they are both in inertial frames requires unambiguously proving the existence of Unruh radiation. However, we also argue that if they cannot prove this, then there are still limitations on observer-independent facts that require that some of those facts remain unknowable. As such, observer-independent facts remain illusory.

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Observer-independence in the presence of a horizon

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In the famous thought experiment known as Wigner's friend, Wigner assigns an entangled state to the composite quantum system consisting of his friend and her observed system. In the context of this thought experiment, Brukner recently derived a no-go theorem for observer-independent facts, i.e. those facts that would be common to both Wigner and his friend, and initial experimental tests have given it strong support. In this article, we demonstrate a loophole in the theorem that manifests in the presence of a horizon even if Wigner and his friend are on the same side of the horizon. The loophole requires that Wigner and his friend both unambiguously demonstrate that they are in inertial frames. We then argue that unambiguously proving that they are both in inertial frames requires unambiguously proving the existence of Unruh radiation. However, we also argue that if they cannot prove this, then there are still limitations on observer-independent facts that require that some of those facts remain unknowable. As such, observer-independent facts remain illusory.

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In the famous thought experiment known as Wigner's friend, Wigner assigns an entangled state to the composite quantum system consisting of his friend and her observed system. In the context of this thought experiment, Brukner recently derived a no-go theorem for observer-independent facts, i.e. those facts that would be common to both Wigner and his friend, and initial experimental tests have given it strong support. In this article, we demonstrate a loophole in the theorem that manifests in the presence of a horizon even if Wigner and his friend are on the same side of the horizon. The loophole requires that Wigner and his friend both unambiguously demonstrate that they are in inertial frames. We then argue that unambiguously proving that they are both in inertial frames requires unambiguously proving the existence of Unruh radiation. However, we also argue that if they cannot prove this, then there are still limitations on observer-independent facts that require that some of those facts remain unknowable. As such, observer-independent facts remain illusory.
In the famous thought experiment known as Wigner's friend, Wigner assigns an entangled state to the composite quantum system consisting of his friend and her observed system. In the context of this thought experiment, Brukner recently derived a no-go theorem for observer-independent facts, i.e. those facts that would be common to both Wigner and his friend, and initial experimental tests have given it strong support. In this article, we demonstrate a loophole in the theorem that manifests in the presence of a horizon even if Wigner and his friend are on the same side of the horizon. The loophole requires that Wigner and his friend both unambiguously demonstrate that they are in inertial frames. We then argue that unambiguously proving that they are both in inertial frames requires unambiguously proving the existence of Unruh radiation. However, we also argue that if they cannot prove this, then there are still limitations on observer-independent facts that require that some of those facts remain unknowable. As such, observer-independent facts remain illusory.
In his famous thought experiment, Wigner assigns an entangled state to the composite quantum system made up of Wigner's friend and her observed system. While the two of them have different accounts of the process, each Wigner and his friend can in principle verify his/her respective state assignments by performing an appropriate measurement. As manifested through a click in a detector or a specific position of the pointer, the outcomes of these measurements can be regarded as reflecting directly observable "facts". Reviewing arXiv:1507.05255, I will derive a no-go theorem for observer-independent facts, which would be common both for Wigner and the friend. I will then analyze this result in the context of a newly-derived theorem arXiv:1604.07422, where Frauchiger and Renner prove that "single-world interpretations of quantum theory cannot be self-consistent". It is argued that "self-consistency" has the same implications as the assumption that observational statements of different observers can be compared in a single (and hence an observer-independent) theoretical framework. The latter, however, may not be possible, if the statements are to be understood as relational in the sense that their determinacy is relative to an observer.
Does quantum theory apply at all scales, including that of observers? New light on this fundamental question has recently been shed through a resurgence of interest in the long-standing Wigner’s friend paradox. This is a thought experiment addressing the quantum measurement problem—the difficulty of reconciling the (unitary, deterministic) evolution of isolated systems and the (non-unitary, probabilistic) state update after a measurement. Here, by building on a scenario with two separated but entangled friends introduced by Brukner, we prove that if quantum evolution is controllable on the scale of an observer, then one of ‘No-Superdeterminism’, ‘Locality’ or ‘Absoluteness of Observed Events’—that every observed event exists absolutely, not relatively—must be false. We show that although the violation of Bell-type inequalities in such scenarios is not in general sufficient to demonstrate the contradiction between those three assumptions, new inequalities can be derived, in a theory-independent manner, that are violated by quantum correlations. This is demonstrated in a proof-of-principle experiment where a photon’s path is deemed an observer. We discuss how this new theorem places strictly stronger constraints on physical reality than Bell’s theorem. For a scenario of two separated but entangled observers, inequalities are derived from three fundamental assumptions. An experiment shows that these inequalities can be violated if quantum evolution is controllable on the scale of an observer.
The precise interpretation of quantum mechanics is still open to discussion. A number of non-intercompatible interpretations, with a not unsignificant respective following, are at the time of writing still viable. When Eugene Wigner stated his titular thought experiment he managed to highlight how varying assumptions and accompanying mechanisms can have implications even on a macro-scale. Caslav Brukner expands the thought experiment of Wigner's Friend in his recent paper and derives a theorem using the setup. In our paper, we empirically validate the theorem derived in Brukner's paper, employing a setup based on his extended Wigner's Friend experiment. More precisely, we realise this setup on a 5-qubit Quantum Computer and achieve a violation of a Bell-type inequality, close to what is theoretically possible. This seems to imply that local observer-independent facts are not possible within the current framework of quantum mechanics.
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.
The scientific method relies on facts, established through repeated measurements and agreed upon universally, independently of who observed them. In quantum mechanics the objectivity of observations is not so clear, most markedly exposed in Wigner's eponymous thought experiment where two observers can experience seemingly different realities. The question whether the observers' narratives can be reconciled has only recently been made accessible to empirical investigation, through recent no-go theorems that construct an extended Wigner's friend scenario with four observers. In a state-of-the-art six-photon experiment, we realize this extended Wigner's friend scenario, experimentally violating the associated Bell-type inequality by five standard deviations. If one holds fast to the assumptions of locality and free choice, this result implies that quantum theory should be interpreted in an observer-dependent way.