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The QBist stance is that we "know" very little about the underlying reality. One of the only things that Chris Fuchs is willing to accept as an objective property of a quantum system is its Hilbert space dimension.

I doubt it's sensible to talk about an interpretation of MWI. MWI says that the wavefunction is a real physical object and wavefunction splitting is something that's genuinely physically occurring. QBism denies that the wavefunction is a real physical object.


The QBist aim is not to provide an ontological description of the universe. Rather, it is to persuade you that whatever such a description is, quantum theory ain't it.

"The professed goal is to strip away all those elements of quantum theory that can be interpreted in subjective, agent-dependent terms. The hope is that whatever remains will hint at something essential and objective about nature."

I've read the quantum theoretic parts of the sequences: Eliezer doesn't really make a case for why Born probabilities arise. Indeed this is one of the major open problems with the MWI.


Depends what you mean by "about". The (strong) Qbist perspective is that probabilities, including those derived from quantum theory, represent an agents beliefs concerning his future interactions with the world. If you're looking for what these probabilities tell us about the underlying "reality" then that's an open question, which Fuchs et al are still exploring.


A Qbist would say they represent the map. The complex vector formalism of quantum theory is simply a convenient/elegant manual for predicting the outcomes of one's future interactions with nature. It may be able to tell us something about the territory, but is not the territory itself.


I'd be grateful if someone could give/suggest a reason for the downvotes.


Whilst the BBC may lean towards the left and be a powerful organisation, I doubt that the effect it had was anywhere near as great as the combined forces of the Daily Mail, The Sun, the Daily Telegraph, and The Times. These newspapers, which account for a huge percentage of the UK's circulation, conducted what I can justifiably describe as a co-ordinated campaign of vitriolic rhetoric against the Labour party and Ed Miliband's character. It's telling that 2 of the 4 are owned by Rupert Murdoch, who has a lot to gain from a continued Conservative government.

There is simply no way the BBC would be able to pull the same kind of trick, as their reputation rests on them at least appearing politically neutral. It can't have been that hard to come up with some derisive slogan regarding the welfare cuts Cameron is inevitably going to try and make, but the country (both left and right) would be up in arms if it was seen to be the opinion of the BBC. The only left-leaning newspaper with a comparable circulation is the Daily Mirror, which ran the headline "Keep Cameron Out". I haven't seen this article myself, but I doubt it has the same attacks on character as the right-wing papers.


A belated thank you for your replies. I feel like I'm starting to get the hang of what it means to take seriously the idea that probabilities are epistemological. It's difficult, moving between papers espousing differing interpretations, because their very language tends to presuppose some ontological commitment or other.


As far as I can tell, all he does in his experiment is label one of a pair of electrons as the "Observer" and exclaim that Many-World has been proven because this "Observer" electron enters into a superposition with the other electron. The problem is that literally every other interpretation of quantum theory would make the same predictions for this experiment, however you label the electrons.


To clarify: do you believe that there is something ontological in the system which is assigning probabilities of measurement outcomes in some way, when you make a measurement of the obervable O?


I agree with you up until your last paragraph: the strength of Fuchs' papers are not in their direct criticism of Everettian interpretations (Kent's papers are a lot better at that).

For your last paragraph, I think Fuchs would take umbrage at the idea that you are necessarily "modeling the thing" when you assign a quantum state to a given system. I don't think he believes that systems have a "true ontic state" of which quantum states are representative. Rather, the quantum state is merely a representation of an agent's beliefs about the future outcomes of their interventions/measurements into the universe. Nevertheless, Fuchs claims to be a scientific realist.

I'm deliberately using the word "think" a lot here because I'm not confident of relaying Fuchs' views faithfully (this isn't directly my area of research). I haven't adopted a QBist interpretation (or any other), but from what I've read I feel it's worth serious discussion.

You also mentioned theorems constraining ontology. You may be interested in Fuchs' take on Bell's Theorem: . I have been to a talk where he explains why the PBR theorem doesn't impact his interpretation, although the details evade me (and I can't find anything written about it by him online).

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