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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."
- Schlosshauer (https://arxiv.org/pdf/1405.2390.pdf)
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: http://arxiv.org/pdf/1311.5253v1.pdf . 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).
I'm not sure how to interpret your question.
If you're asking:
"What is the case against the MWI interpretation of quantum theory?"
then I would probably cite difficulties in explaining why our branch's history appears to be Born-rule typical as a major argument.
If instead you're asking:
"What is the case for a non-ontological interpretation of the wavefunction?"
then the best I can do is attempt to summarise the arguments put forth in the above papers.
I'd like to try and flesh out the difference between your personal interpretation and (for example) QBism. In your nLab article you describe an objective Bayesian is someone who "who naturally thinks of Bayesian probabilities as reflecting knowledge rather than belief, betting commitments, etc". This suggests that it has to be knowledge about something; about some objective ontological process I assume. Is this ontological process still somehow "quantum" in nature? Is it perhaps a hidden variable of some kind? You didn't reply to my previous question about the PBR theorem, which seems relevant since it places strong restrictions on hidden-variable-type psi-epistemic interpretations of quantum theory. I'd be very interested in hearing a response to that if you have the time.
I don't know whether I'd consider myself a "fan" of any particular interpretation, but I think Quantum Bayesianism ranks highly in terms of the insight it sheds into the nature of quantum theory. I'd be interested in discussing or reading about the Bayesian interpretation in more detail, as I haven't had too much exposure besides Fuchs et al's papers and a couple of conference talks. For example, what is your take on the recent PBR theorem concerning the ontology of the quantum state, and would this depend on whether your Bayesianism is objective or subjective?
Do you have any resources you'd particularly recommend?
QBism is less an egocentric model of the universe, more an egocentric interpretation of quantum theory. It doesn't say that we cannot have an ontological model of the universe; it says that quantum theory ain't it.
However, I appreciate that this probably won't help with your lack of interest. Best of luck with everything.
That experiment sounds very problematic to me. He says "After you measure the electron’s spin about the x-axis, have someone fully reverse the physical evolution.... Such reversal would be applied to everything: the electron, the equipment, and anything else that’s part of the experiment.".
There is no explanation of the mechanics of how he thinks such a time-reversal could be implemented. We simply don't have the fine control over the quantum state of the entire measurement apparatus. In fact, the very assumption that quantum theory is even the true/applicable state of affairs at this macro scale is the kind of thing that many Copenhagenists dispute.
Conversely, if it were possible to have such a fine control over the entire system including the very equipment used to perform the measurement, well then, you might as well simply make a quantum measurement of the larger quantum system which includes that apparatus! There would be different outcomes depending on whether collapse has or has not yet occured.
It seems like whether or not this experiment even makes sense relies somewhat on whether MWI is true. Ultimately I think the very description of this experiment makes hidden assumptions, which beg the same question it is trying to answer.
You may be interested in (if you haven't already encountered) the "QBist" interpretation espoused by Fuchs, Mermin, Schack and others. Here are links to some appropriate papers by Fuchs, who in my opinion expresses the position most eloquently and efficiently:
http://arxiv.org/abs/1003.5209
http://arxiv.org/abs/1311.5253
http://arxiv.org/abs/quant-ph/0205039
I personally see QBism as quite a natural extension of classical Bayesianism to quantum mechanics, and I am surprised that it is not discussed at all in this community. Given the interest that Less Wrong members have in quantum theory and its foundations, I can only surmise that this niche is due to some kind of idolization of Eliezer and his views. I am somewhat placated by your inclusion of Kent's paper in your list of coherent anti-MWI arguments, although I would love to see more of the genuine academic debate surrounding the interpretation and foundations of quantum theory faithfully reflected in this forum.
This comment is old, but I think it indicates a misunderstanding about quantum theory and the MWI so I deemed it worth replying to. I believe the confusion lies in what "World" means, and to whom. In my opinion Everrett's original "Relative-State Formalism" is a much better descriptor of the interpretation, but no matter.
The distinct worlds which are present after a quantum-conditional operation are only distinct worlds according to the perspective of an observer who has engaged in the superposition. To an external observer, the system is still in a single state, albeit a state which is a superposition of "classical" states. For example, consider Schrodinger's cat. What MWI suggests is that quantum superposition extends even to the macroscopic level of an entire cat. However, the evil scientist standing outside the box considers the cat to be in state (Dead + Alive) / sqrt(2), which is a single pure state of the Cat System. Now consider the wavefunction of the universe, which I suppose must exist if we take MWI to its logical end. The universe has many subsystems, each of which may be in superpositions of states according to external observers. But no matter how subsystems might divide into superpositions of states, the overall state of the universe is a single pure state.
In sum: for the universe to "keep track of worlds" requires no more work than for there to exist a wavefunction which describes to state of the universe.
As I understand, it's less of a problem for a hardline Copenhagen interpretation because no definite ontological status is assigned to the wavefunction, or indeed the collapse of the wavefunction. CI can roughly be paraphrased as
"Consider this set of rules for predicting experimental outcomes. Look how well it works! Of course, we're not asserting anything about actual reality here".
One of those rules is the Born rule. Another is the fact that physical transformations correspond to unitary maps on the Hilbert space. All of them are postulated, and their correctness is a matter of experimental falsification/verification.
Conversely, MWI assigns definite reality to the wavefunction, but denies that collapse is a real process, and does not postulate any rules about predictions of experimental outcomes. Instead, the claim that a process of measurement inevitably results in a single result being recorded - with probability given by the square amplitude of the wavefunction - must be derived from the pre-existing structure of the theory (possibly with some reasonable assumptions about gambling commitments).
A conceivable alternative to MWI might have the Born rule as an additional postulate, supported only by experiment rather than following from the structure of the theory. I feel that this would be much less appealing to many of its advocates.
I'd like to know what you're implying with this post, but I'm unable to make a confident guess. Are you claiming that this WP quotation has something to do with many worlds?
I wonder if you would apply the same criticism to so-called "derivations" of quantum theory from information theoretic principles, specifically those which work within the environment of general probabilistic theories. For example:
http://arxiv.org/abs/1011.6451 ; http://arxiv.org/abs/1004.1483 ; http://arxiv.org/abs/quantph/0101012
The above links, despite having perhaps overly strong titles, are fairly clear about what assumptions are made, and what is derived. These assumptions are more than simply uncertainty and robust reproducibility: e.g. one assumption that is made by all the above links is that any two pure states are linked by a reversible transformation (in the first link, a slightly modified version of this is assumed). Of course, "pure state" and "reversible transformation" are well-defined concepts within the general probabilistic framework which generalize the meaning of the terms in quantum theory.
Since this research is closely related to my PhD, I feel compelled to give an answer your questions about uncertainty relations and complex numbers in this context. General probabilistic theories provide an abstracted formalism for discussing experiments in terms of measurement choices and outcomes. Essentially any physical theory that predicts probabilities for experimental outcomes (a "prediction calculus" if you like) occupies a place within that formalism, including the complex Hilbert space paradigm of quantum theory. The idea is to whittle down, by means of minimal reasonabe assumptions, the full class of general probabilistic theories until one ends up with the theory that corresponds to quantum theory. What you then have is a prediction calculus equivalent to that of complex Hilbert space quantum theory. In short, complex numbers aren't directly derived from the assumptions; rather, they can be seen simply as part of a less intuitive representation of the same prediction calculus. Uncertainty relations can of course be deduced from the general probabilistic theory if desired, but since they are not part of the actual postulates of quantum theory, there hasn't been much point in doing so. It bears mentioning that this "whittling down process" has so far been achieved only for finite-dimensional quantum theory, as far as I'm aware, although there is work being done on the infinite-dimensional case.
I am reminded of a series of documents uploaded to the arxiv earlier this year, each one reporting the results of a survey taken at a distinct conference, and supposedly revealing a "snapshot" of the participants' atitudes towards foundational issues (such as interpretations). Although the first document seems to be making some fairly strong claims about academic consensus, the following two are a little more conservative. The final one says something very similar to the original post here; their results suggest that,
'there exist, within the broad field of "quantum foundations", sub-communities with quite different views, and that (relatedly) there is probably even significantly more controversy about several fundamental issues than the already-significant amount revealed in the earlier poll.'
http://arxiv.org/abs/1301.1069