I noticed that Quantum Bayesianism (Qbism) seems to solve a number of issues in QM like non-locality, decoherence and the measurement problem. But I am not sure if physicists and philosophers would consider it a viable way to interpret QM.

SEP, Quantum-Bayesian and Pragmatist Views of Quantum Theory gives some arguments against Qbism (e.g. the fact that it leads to instrumentalism etc.), but I would like to understand some things. Could Qbism treatment of probability be considered a good way to understand probability in QM for physicists? Are all the issues of QM effectively solved by Qbism? Is Qbism treated as a major interpretation of QM since it solves a variety of issues? Or it is just perceived as another minor interpretation that physicists and philosophers would not opt for?

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    – J D
    Commented Mar 29, 2020 at 17:22

2 Answers 2


What the issues are, and what "effectively solving" them means is largely in the eye of the beholder, people decide among interpretations largely based on their personal core beliefs about realism, determinism, the role of science and the like. Quantum Bayesianism, with its mix of realism about physics with anti-realism about the structure of quantum theory, is mostly attractive to the philosophically sophisticated. In some ways Qbism overtook purely statistical interpretations that brought rather loose Copenhagen to its logical "shut up and calculate" conclusion (to put it crudely). But while it puts some meat on statistical bones, many questions remain as to what really stands behind the QM formalism much of which is instrumentalized by Qbism.

A good review of features and challenges is Quantum Bayesianism: A Study by Timpson, who argues that it fits well with the Nancy Cartwright's style of ontology, with entities having independent causal powers, whose actions are "average out" statistically to give rise to the observed physical laws ("dappled world" in Cartwright's metaphor). Alas, this is at odds with the dominant view of the fundamental laws of nature, considered strict, not statistical, and limits attractiveness of Qbism to mainstream physicists and philosophers. McArthur gives some strong criticisms of the "dappled world" picture from the mainstream positions in Contra Cartwright.

Schlosshauer, Kofler and Zeilinger conducted a poll of physicists, philosophers, and mathematicians, who attended a conference Quantum Physics and the Nature of Reality in 2011. The interpretational preferences are ranked on p.8:
Copenhagen 42%
Informational 24%
Everett 18%
Objective collapse 9%
Quantum Bayesianism 6%
Relational 6%
Bohmian mechanics 0%
Other 12%
No preference 12%
The poll is non-scientific and the sample size is small (33, 27 physicists), so this should be taken with caution. Qbism registers, but not too highly, and that it is behind objective collapse, which is a revision of quantum mechanics rather than its interpretation, underscores its marginal status. On the other hand, it does better than once popular Bohmian mechanics.

  • Thanks @Conifold just another question you told this “many questions remain as to what really stands behind the QM formalism“ so, what stands behind the QM formalism for Copenhageners? And another question what is the “dominant view of the fundamental laws of nature”?
    – PavlovOlga
    Commented Mar 30, 2020 at 11:18
  • @PavlovOlga I once asked the first question myself, What is the philosophy behind the Copenhagen interpretation of quantum mechanics? One secret of Copenhagen's endurance is strategic vagueness. The dominant view is that the universe as a whole is "driven" by strict exceptionless laws, there are no individual "drivers" with causal powers. That some laws are probabilistic only means that some events are uncaused, but there is no way to "influence" them one way or the other, laws are causally complete. McArthur discusses it in more detail.
    – Conifold
    Commented Mar 30, 2020 at 12:18
  • @conifold- What axiomatic foundations serve as a basis for these 'uncaused' laws? CMS
    – user37981
    Commented Mar 31, 2020 at 11:23
  • @CharlesMSaunders That's what the "theory of everything" is supposed to tell us one day, I guess.
    – Conifold
    Commented Mar 31, 2020 at 11:33

Could Qbism treatment of probability be considered a good way to understand probability in QM for physicists?

How QBism understands probability is exactly the same as how probability is understood in QM; in both cases it is epistemic. Moreover, QBism

maintains that rather than (either directly or indirectly) representing a physical system, a quantum state represents the epistemic state of the one who assigns it concerning that agent’s possible future experiences. It does this by specifying the agent’s coherent degree of belief (credence) in each of a variety of alternative experiences that may result from a specific act the agent may perform.

Thus they argue:

this makes the Born Rule of quantum theory not a law of nature but an empirically motivated norm of rationality a wise agent should follow in addition to those whose violation would render the agent’s degrees of belief incoherent


QBists deny that the Schrödinger equation is a dynamical law governing the evolution of an objective quantum state. For them it merely provides a diachronic constraint on an agent’s epistemic state.

This appears a wholly epistemic interpretation and this is why this view has been generally dismissed as 'instrumentalism, solipsism, anti-realism, mysticism, and even psychology', in a word, anti-realist; and this is why, Fuchs, one of the founders of this view suggested the term 'participatory reality'; though one has to ask the question, who participates in reality when there is no-one around doing the measuring (ie participating) - for example, say 4 billions years ago, when the earth had no-one on it?

Fuchs has also suggested in his essay, QBism and the Greeks that:

QBism's agent-centered worldview can be seen as a development of ideas expressed in Schrödinger's essay "Nature and the Greeks".

I don't see Schrodinger touch upon any such ideas in this essay; however, he does discuss mind in his discussion of Parmenides:

But, at least, it is said to deal with human beliefs (doxai); they are in the mind (noien), which is identified with existence (enai); have they then not a certain existence as phenomena of the mind? These are questions we cannot answer, contradictions we cannot remove. We must be content to remember that he who touches for the first time a deep, hidden truth that is contrary to universally accepted opinion usually overstates it in a way that is likely to involve him in logical contradictions.

and the atoms in this essay, but not the nature of matter qua matter; this he discusses in a different essay, Science and Humanism, where he states:

I am quite sure it is the same dog, the dog that I first saw more than fifty years ago on my father's desk. But why am I sure of it ? That is quite obvious. It is clearly the peculiar form or shape that raises the identity beyond doubt, not the material content. Had the material been melted and cast into the shape of a man, the identity would be much more difficult to establish. And what is more : even if the material identity were established beyond doubt, it would be of very restricted interest. I should probably not care very much about the identity or not of that mass of iron, and should declare that my souvenir had been destroyed.

I consider this a good analogy, and perhaps more than an analogy, for pointing out what the particles or atoms really are. For we can see in this example as in many others how in palpable bodies, composed of many atoms, individuality arises out of the structure of their composition, out of shape or form, or organization, as we might call it in other cases. The identity of the material, if there is any, plays a subordinate role ... when you come to the ultimate particles constituting matter, there seems to be no point in thinking of them again as consisting of some material. They are, as it were, pure shape, nothing but shape.

This is underlined by the title of this section: Form, not substance, is the fundamental concept.

In fact in later life:

Schrödinger’s own metaphysical outlook, as expressed in his last book, My View of the World (1961), closely paralleled the mysticism of the Vedanta.

Fuchs view is actually much closer to Wigners as expressed in his essay Remarks on the Mind-Body Question but differs in that there is no wave collapse, merely an epistemic 'updating of his/her beliefs in response to a new experience.'

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