[Paper Review] Notwithstanding Bohr, the Reasons for QBism

Christopher A. Fuchs|arXiv (Cornell University)|May 9, 2017

Quantum Mechanics and Applications68 references53 citations

TL;DR

The paper argues QBism is a distinct interpretation of quantum mechanics, grounded in three tenets (Born rule as normative, all probabilities subjective, measurement outcomes as personal experiences) and developed through Bayesian and information-theoretic ideas, diverging from Bohr while reinterpreting Bohr’s notions.

ABSTRACT

Without Niels Bohr, QBism would be nothing. But QBism is not Bohr. This paper attempts to show that, despite a popular misconception, QBism is no minor tweak to Bohr's interpretation of quantum mechanics. It is something quite distinct. Along the way, we lay out three tenets of QBism in some detail: 1) The Born Rule---the foundation of what quantum theory means for QBism---is a normative statement. It is about the decision-making behavior any individual agent should strive for; it is not a descriptive "law of nature" in the usual sense. 2) All probabilities, including all quantum probabilities, are so subjective they never tell nature what to do. This includes probability-1 assignments. Quantum states thus have no "ontic hold" on the world. 3) Quantum measurement outcomes just are personal experiences for the agent gambling upon them. Particularly, quantum measurement outcomes are not, to paraphrase Bohr, instances of "irreversible amplification in devices whose design is communicable in common language suitably refined by the terminology of classical physics." Finally, an explicit comparison is given between QBism and Bohr with regard to three subjects: a) The issue of the "detached observer" as it arose in a debate between Pauli and Bohr, b) Bohr's reply to Einstein, Podolsky, and Rosen, and c) Bohr's mature notion of "quantum phenomena." At the end, we discuss how Bohr's notion of phenomena may have something to offer the philosophy of William James: A physics from which to further develop his vision of the world---call it an ontology if you will---in which "new being comes in local spots and patches."

Motivation & Objective

• Clarify how QBism differs from Bohr and the Copenhagen interpretation.
• Articulate three core tenets of QBism and their philosophical implications.
• Explain how Bayesian probability and quantum information theory shape QBism.
• Trace the historical and conceptual development from Bohr, Wheeler, and Pauli to QBism.
• Discuss how Bohr’s notion of phenomena can be reconciled with or reinterpreted by QBism.

Proposed method

• Present three central tenets of QBism (Born rule as normative, subjectivity of all probabilities, personal experience of measurement outcomes).
• Describe how QBism uses Bayesianism and quantum information theory to reformulate quantum states and measurements.
• Argue that quantum states represent personal beliefs, not ontic properties of systems.
• Compare QBism’s stance to Bohr on detached observers, EPR, and phenomena.
• Discuss historical influences (Bohr, Wheeler, Pauli) and contemporary pragmatist philosophy (William James) that shaped QBism.

Experimental results

Research questions

• RQ1 How does QBism conceptually distinguish probability, measurement, and state from traditional interpretations?
• RQ2 In what ways does QBism revise Bohr’s notions of observation and phenomena?
• RQ3 How does QBism address EPR-like correlations and the notion of an observer in quantum mechanics?

Key findings

• QBism treats the Born rule as a normative guideline for an agent’s decision making rather than a law of nature.
• All quantum probabilities, including probability-1 assignments, are subjective in QBism, so quantum states lack ontic hold on reality.
• Quantum measurement outcomes are personal experiences shared between the agent and the external system, not irreversible amplifications described in classical terms.
• QBism integrates Bayesian personalism with quantum information to separate subjective and objective aspects of the theory, guiding ongoing foundational development.
• The work positions QBism as distinct from Bohr while preserving useful aspects of Bohr’s ideas, and links its outlook to pragmatist philosophy and Wheeler’s observer-participancy view.