Debating Quantum Reality — My Exchange With Dr Christopher Fuchs, Founder of QBism
Recently, I was surprised and privileged to receive a mail from none other than Dr. Christopher Fuchs, a leading physicist and one of the founding members of QBism. This was regarding a recent blog that I wrote about the “Quantum Mechanics and The Crisis of Reality and Meaning”, where I spoke about how quantum mechanics transcends the simplistic notions of waves or particles and instead forces us to grapple with profound philosophical and scientific questions about reality, existence, and measurement. In the blog I spoke about how QM reveals the limitations of classical thought and compels us to confront whether the universe is inherently deterministic, probabilistic, or fundamentally unknowable.
First, Dr. Fuchs is a pioneering thinker in the foundations of quantum mechanics, whose work has significantly shaped how we view quantum theory through a Bayesian lens. He has contributed extensively to the conceptual development of quantum information theory and has been instrumental in reinterpreting the quantum formalism to center on subjective probabilities. As a Fellow of the American Physical Society and a recipient of the International Quantum Communication Award, Dr. Fuchs’s contributions have not only advanced theoretical physics but have also deepened the philosophical dialogue around the nature of reality.
QBism (Quantum Bayesianism) was primarily developed by Christopher Fuchs, along with Carlton M. Caves and Rüdiger Schack. While the term “QBism” emerged later to distinguish it from earlier Bayesian interpretations of quantum mechanics, Dr. Fuchs is widely regarded as the leading figure and primary advocate for QBism.
In my blog, I had written the following about QBism “Quantum Bayesianism (QBism) exemplifies this, interpreting ψ as a tool for updating subjective probabilities based on measurement outcomes. In QBism, probabilities are Bayesian constructs, reflecting an agent’s beliefs rather than objective randomness. This approach reframes quantum mechanics as a theory of knowledge, not existence, but at the cost of sidestepping what quantum mechanics might reveal about reality itself.”
Dr Fuchs did not seem to appreciate this framing of mine. His email was both a defense of QBism’s philosophy and a challenge to how I framed my critique. What follows is an exchange that dives into the heart of quantum mechanics’ most profound questions. Does quantum mechanics describe reality as it is, or does it reflect the limits of our knowledge? Below, I share his original email, my response, and my reflections on how these conversations shape our understanding of science. I will write a follow up blog on QBism and my thoughts on it.
Dr. Fuchs Mail to Me:
Dear Mr. Preetham,
I enjoyed reading your piece in Medium on QM and the Crisis of Reality. But about QBism, you wrote:
“This approach reframes quantum mechanics as a theory of knowledge, not existence, but at the cost of sidestepping what quantum mechanics might reveal about reality itself.”
If so, then why would I write things like this over and over and over,
“QBism comes from humble beginnings. It was not born fully formed as Botticelli’s Venus was, nor as the Bohmian and Everettian interpretations of quantum mechanics purport to be. Most strictly, QBism has always been a research program. Its long goal — to say something deep about the character of reality — was always at the top of the mind, but after 96 years of the quantum debate, a slow and careful methodology seemed called for. Less cheap, guesswork ontology, more surgical dissection of the theory and an honest reckoning with what its structure has been trying to tell us all along. QBism’s tack was to ask over and over, what is it about the world that makes us well-advised to use the calculus of quantum mechanics for structuring our probabilities? Our Bayesian probabilities. Said this way, it became clear that the pertinent way to move forward was to get the “epistemics” of the theory right before anything else: Getting reality right would follow for those who had patience enough to pass the marshmallow test.” [From https://arxiv.org/pdf/2303.01446]
I guess I just wish science writers would take the time to read actual QBist writings rather than rely on the misrepresentations of still other science writers. What else could be the source of the continuing mischaracterizations?
With best wishes,
Chris Fuchs
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Christopher A. Fuchs
Professor of Physics
University of Massachusetts Boston
100 Morrissey Boulevard
Boston, MA 02125, USA
My Response and Clarification:
Dear Dr. Fuchs,
Thank you for your email and the detailed paper, which offers an insightful exposition of QBism’s epistemic foundations and its trajectory toward uncovering an ontology grounded in normative structures. I value the opportunity to engage with your perspective and refine the articulation of my critique.
- Epistemic Prioritization and Its Implications
Your work provides a compelling argument for prioritizing epistemic clarity, as seen in the treatment of the Born rule as a normative consistency condition (Section 2.5). The interpretation of ψ as an agent’s catalog of expectations rather than a representation of reality contrasts sharply with interpretations like Bohmian mechanics or GRW, which embed ψ into an ontological framework. I acknowledge that QBism’s stepwise epistemic methodology avoids speculative ontology and aims to construct a solid foundation. However, this approach delays direct engagement with the immediate ontological implications of quantum mechanics that other interpretations address more explicitly. My critique that QBism prioritizes epistemics over ontological questions stems from this comparative perspective.
While QBism carefully deconstructs classical assumptions about measurement and objectivity, its deliberate methodological restraint leaves questions about reality’s nature for later exploration. By rejecting ψ as an ontological entity, QBism advances a pragmatic theory of knowledge. Yet, as you acknowledge in Section 3, the project remains in search of an ontology to complement its epistemic scaffolding. This creates a tension. How does QBism reconcile its epistemic focus with the broader philosophical challenges that quantum mechanics raises?
2. Comparisons With Realist Interpretations
A comparative analysis highlights the distinct strengths and trade-offs of QBism. Bohmian mechanics restores classical causality by introducing hidden variables, albeit at the cost of nonlocality. GRW incorporates stochastic collapses into ψ, offering a resolution to the measurement problem but leaving open questions about the physical origins of collapse. Many-Worlds resolves collapse altogether through deterministic evolution but demands a multiverse of branching realities. Each of these approaches makes ontological claims that, while speculative, engage directly with the metaphysical implications of quantum mechanics.
QBism’s unique stance avoids these speculative pitfalls by treating ψ as subjective, personal, and normative. However, the agent-specific perspective raises its own challenges. The subjective nature of quantum states and measurement outcomes (Sections 2.2 and 2.4) challenges classical notions of objectivity but leaves open questions about intersubjective consistency and the emergence of classicality. These unresolved issues are crucial for situating QBism within a broader understanding of quantum mechanics.
3. The Role of Normative Structure
Your formalization of the Born rule through reference apparatuses and SIC POVMs (Section 2.5.1) provides a mathematically elegant framework for connecting probabilities across different measurement contexts. This innovation highlights QBism’s strength as a normative theory. However, the epistemic nature of these structures contrasts sharply with realist interpretations that situate ψ within a physical framework. For example, Bohmian mechanics and GRW use ψ to bridge the quantum-classical divide, while QBism interprets it purely as an agent’s tool. I wonder if this divergence reinforces my critique. QBism’s focus on epistemics defers engagement with the ontological questions that quantum mechanics inherently raises.
Your argument that QBism aims to reverse-engineer ontology from epistemic constraints is valid (i am fully aligned here) , but it also postpones addressing some of the most pressing philosophical questions in the field. By comparison, interpretations like Bohmian mechanics or MWI make bold ontological claims, enabling a more immediate exploration of quantum reality’s structure. While QBism’s restraint is methodologically sound, it creates a gap in addressing reality’s fundamental nature.
4. Clarifying My Position
Upon reflection, I recognize that my phrasing could more accurately convey my perspective. QBism does not “sidestep” reality in a dismissive sense but instead prioritizes an epistemic reconstruction of quantum mechanics with the aim of addressing ontological questions at a later stage. However, in the broader interpretative landscape, this approach contrasts with frameworks that address ontological questions head-on, even at the cost of speculative assumptions. My critique remains that this epistemic-first methodology, while rigorous, leaves unresolved the deeper ontological implications that quantum mechanics demands we confront.
Dr. Fuchs, I respect the intellectual depth and rigor of QBism as a framework and its potential for reshaping how we understand quantum mechanics. However, I also stand by the necessity of situating QBism within the broader interpretative context, where ontological questions are not merely deferred but actively explored alongside epistemic ones. I appreciate your thoughtful critique and would welcome further discussion on how QBism might bridge its epistemic scaffolding to the broader ontological challenges it seeks to address.
PS: I will be posting your initial email to me, which provides valuable clarity, along with my response in a subsequent article for my readers (This will show that your points stand). Additionally, I will publish a follow-up article after understanding your paper more slowly and thoroughly to clarify my points and refine my position.
Thank you for taking your time in educating me.
Best regards,
Freedom Preetham
I will post any response I receive in subsequent blogs, but this is the gist of the exchange so far.
