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European Journal for Philosophy of Science

QBism and the limits of scientific realism

David Glick

Published May 24, 2021• 46 citations
Publisher

Abstract

Abstract QBism is an agent-centered interpretation of quantum theory. It rejects the notion that quantum theory provides a God’s eye description of reality and claims instead that it imposes constraints on agents’ subjective degrees of belief. QBism’s emphasis on subjective belief has led critics to dismiss it as antirealism or instrumentalism, or even, idealism or solipsism. The aim of this paper is to consider the relation of QBism to scientific realism. I argue that while QBism is an unhappy fit with a standard way of thinking about scientific realism, an alternative conception I call “perspectival normative realism” may allow for a reconciliation.

DOI
10.1007/s13194-021-00366-5

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Abstract QBism is an agent-centered interpretation of quantum theory. It rejects the notion that quantum theory provides a God’s eye description of reality and claims instead that it imposes constraints on agents’ subjective degrees of belief. QBism’s emphasis on subjective belief has led critics to dismiss it as antirealism or instrumentalism, or even, idealism or solipsism. The aim of this paper is to consider the relation of QBism to scientific realism. I argue that while QBism is an unhappy fit with a standard way of thinking about scientific realism, an alternative conception I call “perspectival normative realism” may allow for a reconciliation.
This article explores the relationship between Kantian epistemology and QBism, a modern interpretation of quantum mechanics. QBism reconceives scientific objects not as independent entities but as phenomena synthesized through the observer's active role. It emphasizes the interdependence of knowledge and observation, rejecting the notion of a pre-existent, absolute reality. Much like Immanuel Kant's assertion that the thing-in-itself lies outside human cognition, QBism denies ontological status to the unobservable or unexperienced, grounding objectivity in intersubjectivity: the shared cognitive structures of human agents. Within this Critical framework, I address an objection raised by Carlo Rovelli, who critiques QBism for anchoring reality to the observer, claiming it risks falling into idealism by positioning the subject outside nature. Rovelli defends a relational view, situating the observer within the world as describable by the same physical laws that apply to all objects. In response, I argue that the QBist observer represents a universal human subject, not a single determinate agent and that intersubjective agreement ensures objectivity without collapsing into solipsism.
Abstract The theories of pre-quantum physics are standardly seen as representing physical systems and their properties. Quantum mechanics in its standard form is a more problematic case: here, interpretational problems have led to doubts about the tenability of realist views. Thus, QBists and Quantum Pragmatists maintain that quantum mechanics should not be thought of as representing physical systems, but rather as an agent-centered tool for updating beliefs about such systems. It is part and parcel of such views that different agents may have different beliefs and may assign different quantum states. What results is a collection of agent-centered perspectives rather than a unique representation of the physical world. In this paper we argue that the problems identified by QBism and Quantum Pragmatism do not necessitate abandoning the ideal of representing the physical world. We can avail ourselves of the same puzzle-solving strategies as employed by QBists and pragmatists by adopting a perspectival quantum realism. According to this perspectivalism (close to the relational interpretation of quantum mechanics) objects may possess different, but equally objective properties with respect to different physically defined perspectives. We discuss two options for such a perspectivalism, a local and a nonlocal one, and apply them to Wigner’s friend and EPR scenarios. Finally, we connect quantum perspectivalism to the recently proposed philosophical position of fragmentalism.
Abstract The approach to quantum mechanics that we present in this book aligns fairly well with the QBist position in quantum foundations, which takes quantum mechanics as describing the interaction of an agent with the world: A theory mainly offers a way for the agent to manage her expectations about the outcomes of her interactions, not a picture of what nature is really like. We introduce QBism briefly and situate the framework within a range of options for integrating agency and quantum mechanics. We agree with QBism on many fundamental issues, but we do not subscribe to QBism wholesale. We agree with QBism’s pragmatic outlook that takes scientific theories to be tools for agents to use, and with the QBist goal of developing a participatory realism. But we are critical of the particular idealisations that QBism in its present form subscribes to. Most importantly, we are critical of the structural choice not to resolve the agent nor the world in any detail. In our view, scientific agents learn by modelling. Models are tools that agents can use and develop in dealing with the world. Guided by concrete examples, we describe a scientific modelling framework that allows for loops of three different degrees of difficulty and complexity. These range from quantum state updating, which neatly fits the QBist approach of working within a fixed probability space, via a broader class of updates including so-called Hamiltonian learning, up to the most difficult cases in which true scientific creativity is at play. All three levels of difficulty need to be acknowledged for a full picture of real scientific agency. At the end of the chapter, we describe the project of modelling scientific agency itself, which leads to the topics of Chaps. 5 and 6 .
QBism is currently one of the most widely discussed 'subjective' interpretations of quantum mechanics. Its key move is to say that quantum probabilities are personalist Bayesian probabilities and that the quantum state represents subjective degrees of belief. Even probability-one predictions are considered subjective assignments expressing the agent's highest possible degree of certainty about what they will experience next. For most philosophers and physicists this means that QBism is simply too subjective. Even those who agree with QBism that the wave function should not be reified and that we should look for alternatives to standard Psi-ontic interpretations often argue that QBism must be abandoned because it detaches science from objectivity. The problem is that from the QBist perspective it is hard to see how objectivity could enter science. In this paper, I introduce and motivate an interpretation of quantum mechanics that takes QBism as a starting point, is consistent with all its virtues, but allows objectivity to enter from the get-go. This is the view that quantum probabilities should be understood as objective degrees of epistemic justification.
QBists insist that theirs is a realist position while denying that the quantum formalism represents a mind-independent world. The insistence is hard to place. The realism debate is throughout a dispute about represented content, so a theory that denies standing in the representation relation appears not to take up a position within that debate at all. This paper examines the most developed attempt to make good on the QBist claim--David Glick's perspectival normative realism--and argues that it fails instructively. Glick seeks the ground of the Born rule's objectivity among states of affairs and construes an agent's perspective as a location within an already-constituted world; finding no state of affairs fit to bind every agent at once, he must book the ground as brute. Phenomenology supplies the third thing his picture lacks. A horizon is not a position within the world but the condition under which a world is given. Read as an agent's weighting--her curvature--of such a horizon, the quantum state is neither a defective description nor a brute posit, and the Born rule articulates the invariant lawfulness by which any sense confers a curvature at all.
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.