arXiv:1705.03483

Notwithstanding Bohr, the Reasons for QBism

Christopher A. Fuchs

Published May 9, 2017• 73 citations

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."

DOI: 10.48550/arxiv.1705.03483

Related papers

QBism, Polishing Some Points

Christopher A. Fuchs, Blake C. Stacey

QBism pursues the real by first eliminating the elements of quantum theory too fragile to be ontologies on their own. Thereafter, it seeks an "ontological lesson" from whatever remains. Here, we explore this program by highlighting three tenets of QBism. First, the Born Rule is a normative statement. It is about the decision-making behavior any individual agent should strive for, not a descriptive "law of nature." Second, 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, which implies a more radical kind of indeterminism in quantum theory than other interpretations understand. Third, quantum measurement outcomes just are personal experiences for the agent gambling upon them. Thus all quantum measurement outcomes are local in the sense of the agent enacting them. Through these tenets, we explain four points better than previously: 1) how QBism contrasts with Bohr's concern over unambiguous language, 2) how QBism contrasts with the Everett interpretation, 3) how QBism understands the meaning of Bell inequality violations, and 4) how QBism responds to Wigner's "suspended animation" argument. Finally, we consider the ontological lesson of the tenets and ask what it might mean for the next one hundred years of quantum theory and humankind more generally.

quant-pharXiv:2512.14122Dec 17, 20250 citations

QBism, Where Next?

Christopher A. Fuchs

This chapter expresses what a breath of fresh air it has been since a few phenomenological philosophers have started to engage with QBism. In service of the newfound discussion, the aim of this exposition is to lay out the structure of QBism as clearly as possible for that audience. In the process, we arrive at eight tenets for QBism: (1) A quantum state is an agent's personal judgment. (2) A quantum measurement is an agent's action upon its external world. (3) Quantum measurement outcomes are personal to the agent performing the action. (4) The quantum formalism is normative rather than descriptive. (5) Unitary evolution too expresses an agent's degrees of belief. (6) Even probability-one assignments are judgments without ontic content. (7) Subjective certainty about what an outcome will be does not negate that unperformed measurements have no outcomes. And, (8) quantum theory is a single-user theory for each of us. We then analyze the Wigner's friend thought experiment in light of the eight tenets and indicate that a still more extended analysis is potentially QBism's surest path to uncovering an ontology to go with quantum theory's normative structure. We conclude with a small discussion of how the philosophy of Maurice Merleau-Ponty may be relevant to this quest.

arXiv:2303.01446Routledge eBooksNov 20, 202339 citations

Is QBism a Possible Solution to the Conceptual Problems of Quantum Mechanics?

Hervé Zwirn C

QBism is a recently developed version of Quantum Bayesianism. QBists think that the primitive concept of experience is the central subject of science. QBism refuses the idea that the quantum state of a system is an objective description of this system. It is a tool for assigning a subjective probability to the agent's future experience. So quantum mechanics does not directly say something about the "external world". A measurement (in the usual sense) is just a special case of what QBism calls experience and this is any action done by any agent on her external world. The result of the measurement is the experience that the agent gets from her action on her personal world. For Qbists, this way of seeing the measurement helps rejecting any non-local influences. We will examine if these claims are robust and will show that a certain number of issues needs to be clarified in order to get a coherent picture. Depending on the way these necessary clarifications are made, the global image given by Qbism can vary from pure instrumentalism to interpretations making the observer play a radical role.

arXiv:1912.11636Dec 25, 20192 citations

“B” IS FOR BOHR

Ulrich Mohrhoff

It is suggested that the "B" in QBism rightfully stands for Bohr. The paper begins by explaining why Bohr seems obscure to most physicists. Having identified the contextuality of physical quantities as Bohr's essential contribution to Kant's theory of science, it outlines the latter, its proper contextuality (human experience), and its decontextualization. In order to preserve the decontextualization achieved by Kant's theory, Bohr seized on quantum phenomena as the principal referents of atomic physics, all the while keeping the universal context of human experience at the center of his philosophy. QBism, through its emphasis on the individual experiencing subject, brings home the intersubjective constitution of objectivity more forcefully than Bohr ever did. If measurements are irreversible and outcomes definite, it is because the experiences of each subject are irreversible and definite. Bohr's insights, on the other hand, are exceedingly useful in clarifying the QBist position, attenuating its excesses, and enhancing its internal consistency.

arXiv:1905.07118WORLD SCIENTIFIC eBooksOct 1, 20230 citations

QBism on Locality and Nonlocality

John B. DeBrota, Christopher A. Fuchs, Rüdiger Schack

Recently Pienaar (2026), building on work of Cavalcanti (2021), has shown that QBism cannot always assume distinct observers' quantum-measurement outcomes---say, of Wigner and his friend---are embedded in a single spacetime. This follows from QBism's rejection of the `Absoluteness of Observed Events' assumption in the Bong et al. no-go theorem. Thus, QBism has no choice but to treat the notion of spacetime every bit as personalistic as it treats quantum states and quantum measurement outcomes. In a way, this is not a surprise to QBists, as they have taken the notion of `personalist spacetimes' to be the ansatz most compatible with their other views since at least 2009. But it does enjoin us to finally make crystal clear the sense in which QBism is a purely local interpretation of quantum mechanics despite this new theorem and despite quantum theory's age-old violation of Bell's inequalities. With the extra clarity we also hope to poise QBism for a distinctly new way to approach issues at the interface of quantum theory and gravity.

quant-pharXiv:2608.05368Aug 7, 20260 citations

"B" is for Bohr

Ulrich Mohrhoff

It is suggested that the B in QBism rightfully stands for Bohr. The paper begins by explaining why Bohr seems obscure to most physicists. Having identified the contextuality of physical quantities as Bohr's essential contribution to Kant's theory of science, it outlines the latter, its proper contextuality (human experience), and its decontextualization. In order to preserve the decontextualization achieved by Kant's theory, Bohr seized on quantum phenomena as the principal referents of atomic physics, all the while keeping the universal context of human experience at the center of his philosophy. QBism, through its emphasis on the individual experiencing subject, brings home the intersubjective constitution of objectivity more forcefully than Bohr ever did. If measurements are irreversible and outcomes definite, it is because the experiences of each subject are irreversible and definite. Bohr's insights, on the other hand, are exceedingly useful in clarifying the QBist position, attenuating its excesses, and enhancing its internal consistency.

May 17, 20190 citations