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arXiv:2008.01444 · Studies in History and Philosophy of Science Part B Studies in History and Philosophy of Modern Physics

Completely real? A critical note on the claims by Colbeck and Renner

Ronnie Hermens

Published Nov 1, 2020• 3 citations
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Abstract

In a series of papers Colbeck and Renner (2011, 2015a, 2015b) claim to have shown that the quantum state provides a complete description for the prediction of future measurement outcomes. In this paper I argue that thus far no solid satisfactory proof has been presented to support this claim. Building on the earlier work of Leifer (2014), Landsman (2015) and Leegwater (2016), I present and prove two results that only partially support this claim. I then discuss the arguments by Colbeck, Renner and Leegwater concerning how these results are to generalize to the full claim. This argument turns out to hinge on the implicit use of an assumption concerning the way unitary evolution is to be represented in any possible completion of quantum mechanics. I argue that this assumption is unsatisfactory and that possible attempts to validate it based on measurement theory also do not succeed.

DOI
10.1016/j.shpsb.2020.08.005

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•Jan 1, 2020•0 citations
In a series of papers Colbeck and Renner (2011, 2015a, 2015b) claim to have shown that the quantum state provides a complete description for the prediction of future measurement outcomes. In this paper I argue that thus far no solid satisfactory proof has been presented to support this claim. Building on the earlier work of Leifer (2014), Landsman (2015) and Leegwater (2016), I present and prove two results that only partially support this claim. I then discuss the arguments by Colbeck, Renner and Leegwater concerning how these results are to generalize to the full claim. This argument turns out to hinge on the implicit use of an assumption concerning the way unitary evolution is to be represented in any possible completion of quantum mechanics. I argue that this assumption is unsatisfactory and that possible attempts to validate it based on measurement theory also do not succeed.
In a series of papers Colbeck and Renner claim to have shown that the quantum state provides a complete description for the prediction of future measurement outcomes. In this paper I argue that thus far no solid satisfactory proof has been presented to support this claim. Building on the earlier work of Leifer, Landsman and Leegwater, I present and prove two results that only partially support this claim. I then discuss the arguments by Colbeck, Renner and Leegwater concerning how these results are to generalize to the full claim. This argument turns out to hinge on the implicit use of an assumption concerning the way unitary evolution is to be represented in any possible completion of quantum mechanics. I argue that this assumption is unsatisfactory and that possible attempts to validate it based on measurement theory also do not succeed.
In a series of papers Colbeck and Renner (2011, 2015a,b) claim to have shown that the quantum state provides a complete description for the prediction of future measurement outcomes. In this paper I argue that thus far no solid satisfactory proof has been presented to support this claim. Building on the earlier work of Landsman (2015) and Leegwater (2016) I highlight the implicit use of an assumption concerning the way unitary evolution is to be represented in any possible completion of quantum mechanics. I show that the assumption is quite crucial to the proof of the claim and argue that it is unwarranted. I further discuss a possible validation for a restricted version of this assumption that is based on considerations of measurement processes. I argue that also this restricted version is unsatisfactory.
In three papers, Colbeck and Renner [Nat. Commun. 2, 411 (2011); Phys. Rev. Lett. 108, 150402 (2012); e-print arXiv:1208.4123] argued that “no alternative theory compatible with quantum theory and satisfying the freedom of choice assumption can give improved predictions.” We give a more precise version of the formulation and proof of this remarkable claim. Our proof broadly follows theirs, which relies on physically well motivated axioms, but to fill in some crucial details, certain technical assumptions have had to be added, whose physical status seems somewhat obscure.
It was argued [1] that there can be no extension of quantum mechanics with improved predictive power on a measurement freely chosen, independently of any event that is not in its future light cone. The assumption of measurement choice was criticized [2] to be too strong to be physically necessary and extensions of quantum mechanics were shown [3] to be possible under a more relaxed measurement assumption. Here I point out an error in the criticism and observe that the actual mistake of the no-go theorem lies in an unwarranted assumption implicitly made in the proof of [1]. Hence, quantum mechanics is guaranteed to have the maximal predictive power only in situations of complete certainty and complete uncertainty about measurement outcomes. I then show that the measurement assumption can be further relaxed without affecting the conclusion on the predictive power of quantum mechanics versus alternative theories. I further study the optimal predicative improvement over quantum mechanics of local spin measurements on a pair of entangled qubits by any alternative theory and conjecture a strict upper bound.
Problem- Many attempts have been made, starting from Quantum mechanics (QM), to prove the non-viability of hidden-variable (HV) solutions, and to justify the unnaturalness of QM explanations over physical realism. In particular, Colbeck & Renner (C&R) (2011) claimed to prove that no extension of quantum theory can exist with better predictive power than quantum mechanics itself. This implies that QM is an ideal theory, and that physical realism should be abandoned. Purpose- This paper critiques this proof. Approach- Logical considerations are used to examine the premises in the proof. Findings- The C&R proof is show to be capable of entirely the opposite interpretation that its authors intended, namely that a new deeper physics may exist based on physical realism, but that it will definitely not be quantum theory or even an extension thereof. Also, the proof is falsified by presenting a viable theory of internal structures (hidden-variable solution) with demonstrably better ontological explanatory power than QM. Implications- QM is interpreted as an approximate stochastic representation of a deeper dynamic behaviour at the finer scale. Originality- This work predicts that physical realism still applies, and that hidden-variable solutions still have vitality as the way forward for fundamental physics.
There has recently been a renewed focus on ‘perspectival’ quantum theories. which simultaneously maintain the existence of single measurement outcomes and the universality of unitary evolution. At the same time, these theories have come under attack with results by Frauchiger and Renner, Baumann and Wolf, and others. This dissertation aims to respond to a number of these attacks by providing a groundwork for these types of theories. To lay this groundwork I focus on encapsulated measurements, which involve an isolated observer and a superobserver (who measures the observer). I first distinguish between invasive and non-invasive measurements. Each leads to a possible inconsistency: In non-invasive measurements, the observer is certain of the superobserver’s measurement outcome while the superobserver’s physics predicts multiple possible outcomes. In invasive measurements the superobserver can be certain of his measurement outcome while the observer predicts non-zero probabilities for all possible outcomes. I argue that in the case of non-invasive measurements, the perspectivalist avoids diffculty by denying that the observer’s result has any impact on the physics experienced by the superobserver. Consistency is then maintained between them by looking to the unitary evolution of the superobserver’s measurement. This response leads to a detailed discussion about the metaphysical commitments of the perspectival approach. Here I argue the perspectivalist must accept one surprising result – there is a significant divorce of fundamental ontological states from physical dynamics. Turning to invasive measurements, I argue that the concern here is entirely misplaced. Arguments that raise worries about invasive measurements assume the observer should describe herself to be in a quantum state of having observed her measurement outcome when predicting the superobserver’s measurement results. I argue that this is incorrect. Rather, I explain that it is impossible for any observer to know her quantum state and so she should never describe herself as being in any quantum state at all, let alone use such a description to make predictions about a superobserver’s measurement. To conclude, I explain how the perspectivalist responds to concerns raised about entanglement and the possibility of action at a distance. Combining this with the results above brings into focus how the perspectivalist may develop a consistent, single-world picture of quantum mechanics.