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arXiv:2007.14909

Epistemic Horizons: This Sentence is $\frac{1}{\sqrt{2}}(|True\rangle + |False\rangle)$

Jochen Szangolies

Published Jul 29, 2020• 0 citations
arXivHTMLPDF

Abstract

In [Found. Phys. 48.12 (2018): 1669], the notion of 'epistemic horizon' was introduced as an explanation for many of the puzzling features of quantum mechanics. There, it was shown that Lawvere's theorem, which forms the categorical backdrop to phenomena such as Gödelian incompleteness, Turing undecidability, Russell's paradox and others, applied to a measurement context, yields bounds on the maximum knowledge that can be obtained about a system, which produces many paradigmatically quantum phenomena. We give a brief presentation of the framework, and then demonstrate how it naturally yields Bell inequality violations. We then study the argument due to Einstein, Podolsky, and Rosen, and show how the counterfactual inference needed to conclude the incompleteness of the quantum formalism is barred by the epistemic horizon. Similarly, the paradoxes due to Hardy and Frauchiger-Renner are discussed, and found to turn on an inconsistent combination of information from incompatible contexts.

DOI
10.48550/arxiv.2007.14909

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Epistemic Horizons: This Sentence is 12(∣True⟩+∣False⟩)\frac{1}{\sqrt{2}}(|True\rangle + |False\rangle)2​1​(∣True⟩+∣False⟩)
In [Found. Phys. 48.12 (2018): 1669], the notion of 'epistemic horizon' was introduced as an explanation for many of the puzzling features of quantum mechanics. There, it was shown that Lawvere's theorem, which forms the categorical backdrop to phenomena such as Gödelian incompleteness, Turing undecidability, Russell's paradox and others, applied to a measurement context, yields bounds on the maximum knowledge that can be obtained about a system, which produces many paradigmatically quantum phenomena. We give a brief presentation of the framework, and then demonstrate how it naturally yields Bell inequality violations. We then study the argument due to Einstein, Podolsky, and Rosen, and show how the counterfactual inference needed to conclude the incompleteness of the quantum formalism is barred by the epistemic horizon. Similarly, the paradoxes due to Hardy and Frauchiger-Renner are discussed, and found to turn on an inconsistent combination of information from incompatible contexts.
Why ψψψ is incomplete indeed: a simple illustration
With the Nobel Prize attributed to Aspect, Clauser, and Zeilinger, the international scientific community acknowledged the fundamental importance of the experimental violation of Bell's inequalities. It is however still debated what fails in Bell's hypotheses, leading to these inequalities, and usually summarized as "local realism", or maybe more appropriately "classical local realism". The most common explanation is "quantum non-locality", that remains however fully compatible with relativistic causality; this makes wondering whether any non-local phenomenon is really involved in these experiments. Here we want to recapitulate another option, sometimes called "predictive incompleteness", closely related to the idea that the usual state vector ψψψ is incomplete indeed, as it was claimed by Einstein, Podolsky and Rosen. However, the right way to complete ψψψ has nothing to do with hidden variables, but requires to specify the measurement context, as it was claimed by Bohr. Here we will consider the simple case of two spin 1/2, or two qubits, in order to keep the argument simple, but it does apply generally in quantum mechanics.
How can we consistently model the knowledge of the natural world provided by physical theories? Philosophers frequently use epistemic logic to model reasoning and knowledge abstractly, and to formally study the ramifications of epistemic assumptions. One famous example is Fitch's paradox, which begins with minimal knowledge axioms and derives the counter-intuitive result that agent knows every true statement. Accounting for knowledge that arises from physical theories complicates matters further. For example, quantum mechanics allows observers to model other agents as quantum systems themselves, and to make predictions about measurements performed on each others' memories. Moreover, complex thought experiments in which agents' memories are modelled as quantum systems show that multi-agent reasoning chains can yield paradoxical results. Here, we bridge the gap between quantum paradoxes and foundational problems in epistemic logic, by relating the assumptions behind the recent Frauchiger-Renner quantum thought experiment and the axioms for knowledge used in Fitch's knowability paradox. Our results indicate that agents' knowledge of quantum systems must violate at least one of the following assumptions: it cannot be distributive over conjunction, have a kind of internal continuity, and yield a constructive interpretation all at once. Indeed, knowledge provided by quantum mechanics apparently contradicts traditional notions of how knowledge behaves; for instance, it may not be possible to universally assign objective truth values to claims about agent knowledge. We discuss the relations of this work to results in quantum contextuality and explore possible modifications to standard epistemic logic that could make it consistent with quantum theory.
When it was first put forward, special relativity struck many people as counterintuitive and possibly inconsistent, as we saw when we considered the twin paradox in Chapter 17. Quantum mechanics had a similar effect, only to a much greater and longer lasting degree, from the time of its inception in the mid 1920s until the present. This reaction to quantum theory was not confined to an initial confusion that often accompanies a new subject, but remained a life-long puzzle for Einstein, to mention only the most prominent opponent of what he took to be the reigning orthodoxy. In this chapter we study a famous attempt by Einstein and two of his colleagues to show that quantum mechanics was, if not logically inconsistent, then at least an incomplete theory. This charge of incompleteness is just that raised by Schrödinger's cat paradox (Section 21.4). THE EPR PARADOX In 1935 Einstein, Boris Podolsky (1896–1966) and Nathan Rosen (1909–1995) (hereafter referred to as EPR) published a paper titled ‘Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?’. They take as their criterion for the completeness of a theory that it contains terms corresponding to every relevant entity found in reality. For example, in the classical description of a system, such as a planet going around the sun, there are symbols representing the position r , the momentum p and so forth for the various parts of the system.
What if the paradoxical nature of quantum theory could find its source in some undecidability analog to that of Gödel's incompleteness theorem ? This essay aims at arguing for such Gödelian hunch via two case studies. Firstly, using a narrative based on the Newcomb problem, the theological motivational origin of quantum contextuality is introduced in order to show how this result might be related to a Liar-like undecidability. A topological generalization of contextuality by Abramsky et al. in which the logical structure of quantum contextuality is compared with "Liar cycles" is also presented. Secondly, the measurement problem is analyzed as emerging from a logical error. A personal analysis of the related Wigner's friend thought experiment and and a recent paradox by Frauchiger and Renner is presented, by introducing the notion of "meta-contextuality" as a Liar-like feature underlying the neo-Copenhagen interpretations of quantum theory. Finally, this quantum Gödelian hunch opens a discussion of the paradoxical nature of quantum physics and the emergence of time itself from self-contradiction.