Notwithstanding Bohr, the Reasons for QBism

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."

The Open Systems View

“…Heisenberg (1959,1971), Grete Hermann (2017), Wolfgang Pauli (1994), John von Neumann (1955Neumann ( [1932), Carl Friedrich von Weizsäcker (2006Weizsäcker ( [1985), and others that are together sometimes referred to as the Copenhagen group of interpretations, though they do not constitute a single unified view (see Howard, 2004). More recent offshoots of the orthodox branch include the objectivist neo-Copenhagen interpretations of, for instance, Brukner (2017); Bub (2016Bub ( , 2017; Bub & Pitowsky (2010); Demopoulos (2012Demopoulos ( , 2018; Janas et al (2022); Pitowsky (1989Pitowsky ( , 2006; 50 the subjectivist interpretation called QBism (Fuchs, Mermin, & Schack, 2014;Fuchs, 2017); Richard Healey's (2012; pragmatist interpretation; Carlo Rovelli's (1996; relational interpretation, and others. 51 Orthodox interpretations, which deny that we should think of a system as having an observer-independent state, will truly seem radical for someone who finds it hard to imagine describing reality in an observer-dependent way.…”

“…51 Given its radical subjectivity, it may seem strange to call QBism an orthodox interpretation, but this is entirely appropriate given our visualization of interpretations of quantum mechanics as a phylogenetic tree. QBism begins with the (objectivist) ideas of Bohr, Pauli, John Archibald Wheeler (1983), and others, and then diverges from them (Fuchs, 2017), especially as it regards the interpretation of probability (which for QBists is subjective). As far as we can tell, however, QBism and all of the other views we call orthodox share the very general features that we lay out in this section.…”

Unobservable entities in QBism and phenomenology

“…It is worth explaining in more detail what the QBists mean by this. As recalled in Fuchs (2017), the principle originates from a remark by Wolfgang Pauli in a letter to Niels Bohr in 1955, where Pauli suggests that ". .…”

“…the claim that only one's own mind is knowable through experience. However, in taking both the subjective and transcendent aspects of "experience" as equally fundamental, QBism contradicts the traditional view that ontology is something a priori objective, and consciousness is something reducible to non-conscious ontological elements (Fuchs, 2017;Pienaar, 2021). In order to successfully defend its unorthodox vision of "ontology" against more traditional forms of scientific realism, QBism would benefit from a metaphysical framework capable of accommodating the unique demands that it places upon "experience".…”

“… See Fuchs and Schack (2013, 2014);;Fuchs (2010Fuchs ( , 2016Fuchs ( , 2017; DeBrota and Stacey (2019);Fuchs and Stacey (2019).…”