[Paper Review] Quantum Confusions, Cleared Up (or so I hope)

Sabine Hossenfelder|arXiv (Cornell University)|Sep 21, 2023

Philosophy and History of ScienceArts and Humanities3 citations

TL;DR

This paper uses an instrumentalist approach to clarify common misconceptions in quantum foundations, arguing that interpretations like Many Worlds and Bohmian Mechanics are non-local and only partially solve the measurement problem. It shows the Copenhagen Model’s collapse postulate is necessary, decoherence only partially resolves the measurement problem, and neither Bohmian Mechanics nor Many Worlds fully address the detector identification issue.

ABSTRACT

I use an instrumental approach to investigate some commonly made claims about interpretations of quantum mechanics, especially those that pertain questions of locality. The here presented investigation builds on a recently proposed taxonomy for quantum mechanics interpretations.

Motivation & Objective

• To resolve long-standing confusions in quantum foundations by adopting an instrumentalist framework that focuses on predictions rather than metaphysical interpretations.
• To clarify whether key claims—like non-locality in Many Worlds or the completeness of Bohmian Mechanics—are physically justified or terminological misrepresentations.
• To investigate whether decoherence, the collapse postulate, or alternative interpretations fully resolve the measurement problem.
• To demonstrate that interpretations such as Many Worlds and Bohmian Mechanics are non-local in Bell’s sense, despite common claims to the contrary.
• To argue that the measurement problem is not truly solved by any interpretation, but rather sidestepped or redefined depending on the formalism used.

Proposed method

• Adopting an instrumentalist interpretation of quantum mechanics, where the theory is treated as a predictive tool rather than a metaphysical description of reality.
• Using a taxonomy of quantum interpretations from prior work to classify and compare different approaches, including Copenhagen, Many Worlds, and Bohmian Mechanics.
• Applying axioms A1–A6 (Hilbert space states, Hermitian operators, Schrödinger evolution, collapse postulate, Born’s rule, tensor product structure) as a calculational model for predictions.
• Analyzing the role of decoherence and conditional wave-functions in simulating collapse-like behavior without postulating actual collapse.
• Evaluating whether detector identification is derivable from the initial state or requires additional assumptions, especially in Bohmian Mechanics and Many Worlds.
• Assessing local causality using Bell’s criterion: determining whether measurement outcomes at space-like separation can be explained by local hidden variables.

Experimental results

Research questions

• RQ1 Is the collapse postulate necessary for making predictions in quantum mechanics, or can it be derived from other principles?
• RQ2 To what extent does decoherence resolve the measurement problem, and what assumptions are required for it to do so?
• RQ3 Is the Many Worlds Interpretation locally causal in Bell’s sense, despite claims of locality in the literature?
• RQ4 Does Bohmian Mechanics solve the measurement problem, or does it merely avoid it by positing definite particle positions from the start?
• RQ5 Can the identification of a system as a 'measuring device' be derived from the dynamics, or does it require external input in interpretations like Bohmian Mechanics or Many Worlds?

Key findings

• The collapse postulate (Axiom A4) is necessary for the Copenhagen Model to make predictions, and cannot be derived from the other five axioms.
• Decoherence partially resolves the measurement problem by suppressing interference, but does not explain why a particular outcome is selected, leaving the preferred basis problem unresolved.
• The Many Worlds Interpretation is not locally causal in Bell’s sense, as measurement outcomes at space-like separation can reveal non-local correlations not contained in the backward lightcone.
• Bohmian Mechanics avoids the measurement problem by positing definite particle positions from the outset, but does not solve the problem of identifying which degrees of freedom constitute a measuring device.
• Conditional wave-functions in Bohmian Mechanics can mimic collapse, but only if one already knows which particles are part of the detector and environment—this knowledge is not derivable from the initial state.
• No interpretation, including Bohmian Mechanics or Many Worlds, fully resolves the measurement problem in a way that satisfies both physical and conceptual criteria without additional assumptions.