QBism and Relational Quantum Mechanics compared (2108.13977v1)

Abstract: The subjective Bayesian interpretation of quantum mechanics (QBism) and Rovelli's relational interpretation of quantum mechanics (RQM) are both notable for embracing the radical idea that measurement outcomes correspond to events whose occurrence (or not) is relative to an observer. Here we provide a detailed study of their similarities and especially their differences.

• The paper offers a detailed comparative analysis of QBism and relational quantum mechanics by clarifying their differing treatments of quantum measurement and reality.
• It distinguishes between the subjective Bayesian view in QBism and the objective relational perspective in interpreting quantum states.
• The study highlights how each framework influences the understanding of shared reality and the normative role of quantum mechanics in guiding interactions.

Comparative Analysis of QBism and Relational Quantum Mechanics

The paper "QBism and Relational Quantum Mechanics compared" by Jacques Pienaar presents a detailed comparative analysis of two interpretations of quantum mechanics: the subjective Bayesian interpretation known as Quantum Bayesianism (QBism) and Carlo Rovelli's Relational Quantum Mechanics (RQM). Both interpretations embrace the principle of relative facts; however, they diverge significantly in their ontological commitments and philosophical implications. This essay attempts to provide an expert overview of the paper's contents, exploring the subtle nuances and distinctions between QBism and RQM.

Core Principles and Divergent Interpretations

Pienaar's analysis is structured around four core principles shared by what are termed "Copenhagenish" interpretations of quantum mechanics. These principles include the uniqueness of measurement outcomes, the epistemic character of the quantum state, the universality of quantum mechanics, and its completeness without hidden variables. The comparison elucidates how QBism and RQM, though aligned on these principles, diverge sharply in their philosophical underpinnings.

1. Uniqueness of Measurement Outcomes: While both QBism and RQM subscribe to the uniqueness of outcomes, their interpretation differs. In RQM, outcomes are derived from interaction events, emphasizing an impersonal, naturalistic viewpoint wherein interaction between systems constitutes an observer-system relationship. QBism, on the other hand, interprets measurement outcomes as subjective experiences of an agent, thereby attributing a personalized aspect to quantum phenomena.
2. Epistemic Nature of Quantum States: A crucial distinction arises in how quantum states are treated. In RQM, the quantum state represents information about one system relative to another, defined through an objective correlation of variables. QBism takes a subjective Bayesian stance, considering quantum states as expressions of an agent's personal beliefs or experiences, with no objective existence outside of those experiences.
3. Universality: For RQM, the universality of quantum mechanics is reflected in its capacity to describe the relationships between all parts of the universe but not the universe as a whole. QBism views quantum theory as a universally applicable decision theory—agents can apply it to any part of reality, including themselves, though not universally to the entire cosmos.
4. Completeness Without Hidden Variables: Both interpretations deny the existence of hidden variables but differ in their stance on uncertainty. RQM embraces objective uncertainty as a feature of relational interactions, while QBism interprets it as a facet of the agents' probabilistic beliefs about their experiences.

Implications for Quantum Laws and Shared Reality

The paper further examines how QBism and RQM regard the rules of quantum mechanics. QBism posits these as normative rules guiding rational action, representing decision-theoretic tools rather than laws of nature. RQM subscribes to an information-theoretic framework, treating quantum theory as an account of natural laws dictating information exchange between systems, independent of human agency.

Additionally, the paper addresses the philosophical issue of shared reality in the quantum field. QBism allows for subjective disagreement between agents on measurement outcomes due to its emphasis on personal experience and communication. Conversely, RQM posits that agreement is mandated through physical interaction, as communication is seen as a quantum mechanical process establishing consistent outcomes across observers.

Conclusion and Future Directions

The paper concludes by underscoring the irreconcilable ontological differences between QBism and RQM, despite their shared Copenhagenish roots. Each interpretation’s unique treatment of probability, measurement, and reality points to a divergent metaphysical understanding of the quantum world. Future research could aim to embody these differences within an ontological modeling framework that accounts for varied Copenhagenish perspectives, potentially offering a richer understanding of quantum theory's interpretative landscape.