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I think I just became a QBist ?

  1. Nov 24, 2013 #1


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    I think I just became a "QBist" ?

    Just finished a first reading of this paper:

    C.A.Fuchs, N.D.Mermin, R.Schack,
    "An Introduction to QBism with an Application to the Locality of Quantum Mechanics",

    Interesting that it has ideas that remind me of Rovelli's Relational QM and Relational EPR (which I find appealing), though Rovelli is not cited in the FMS paper.

    I like it because (imho) it cuts through a lot of the widespread BS that wafts around QM. :wink:

    (I mention the FMS paper here in BSTM, rather than the quantum forum, since it's a bit off the mainstream.)
  2. jcsd
  3. Nov 24, 2013 #2
    I don't have the patience to work out how "QBism" is different from the Copenhagen interpretation (in its original, epistemic form, which says that observables are what's real and that wavefunctions etc are just calculating devices), but if it helps QM make sense to you as a theory making probabilistic connections between states of the world that by classical standards are incompletely specified (e.g. because the definiteness of complementary observables is constrained by the uncertainty principle) - then good for you.

    Just don't kid yourself that such epistemic, instrumental, operational... interpretations of QM, make sense as a final statement about the nature of reality. You have to beware of this because authors of epistemic interpretations of QM, right back to Bohr himself, are always inventing convoluted rationalizations as to why certain questions don't need to be answered, why it makes sense to say that reality is objectively indefinite, and so on.

    Eventually, physics will have to return to objectivity to progress. Purely instrumental theories are necessarily "incomplete" (unfinished; not the full story about reality). The status of QM makes a lot more sense once you accept that it is incomplete as a theory of reality. Then you can accept it for what it is - something that works, but not an ultimate truth
  4. Nov 24, 2013 #3
    For balance, Chris Fields offer a critique of the QBism model presented by Fuchs based on the "measured" quantum reality of the wavefunction for the agent (the observer):


    At the end of the paper is this comment by Fields about QBism:

    "QBism provides no physical distinction between observers and the systems they observe, treating all quantum systems as autonomous agents that respond to observations by updating beliefs and employ quantum mechanics as a “users’ manual” to guide behavior. However, it treats observation itself as a physical process in which an “observer” acts on a “system” with a POVM and the system” selects a POVM component as the “observer’s experience” in return. This requirement renders the assumption that systems be well-defined - i.e. have constant d-impossible to implement operationally. It similarly forces the consistent QBist to regard the environment as an effectively omniscient observer, threatening the fundamental assumption of subjective probabilities and forcing the conclusion that QBist observers cannot segment their environments into objectively separate systems."
  5. Nov 24, 2013 #4
    maybe that 'aroma' wafting around stems from old intuitions which turned out to be predictive, extremely useful, but nobody still really knows why they work.....but suspect the theory is likely incomplete.....but the old gaffers were rather insightful!!

    With QM, the Schrödinger equation describes the continuous time evolution of a system's wave function and is deterministic. However, the relationship between that wave function and the observable properties of the system appear to be non-deterministic….and fundamentally inconsistent with many of our 'classical' prejudices....

    from Roger Penrose [the mathematical physicist] celebrating Stephen Hawking’s 60th birthday in 1993 at Cambridge England.....

    If we were not so involved, it would make rather funny reading.
  6. Nov 24, 2013 #5


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    Does Bayes rule really imply Bayesian updating?
  7. Nov 24, 2013 #6


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    We had an earlier thread here in BTSM about the Smerlak-Rovelli paper that you clearly are familiar with, that deals with non-locality in what i think is essentially the same way!
    Relational EPR
    Matteo Smerlak, Carlo Rovelli
    (Submitted on 10 Apr 2006)
    We study the EPR-type correlations from the perspective of the relational interpretation of quantum mechanics. We argue that these correlations do not entail any form of 'non-locality', when viewed in the context of this interpretation. The abandonment of strict Einstein realism implied by the relational stance permits to reconcile quantum mechanics, completeness, (operationally defined) separability, and locality.
    10 pages, published in Foundations of Physics 37:427-445,2007

    Thanks for pointing us to the Fuchs-Mermin-Schack! It's very well written, could be the clearest exposition of this idea so far, plus the catchy new name.

    The BTSM thread was
    Last edited: Nov 24, 2013
  8. Nov 24, 2013 #7


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    I see that DrDu has resurrected an old thread in the QM forum that was about the "Quantum Bayesian Interpretation of QM": https://www.physicsforums.com/showthread.php?t=692569

    I looked at the FS paper which started that thread (note: no Mermin). I quickly became bored with the philosophical style that DrDu refers to as speculations about what Feynman meant. I, too, have no time for that sort of thing.

    Probably, if I had put more time into the FS paper, and that old thread, I wouldn't have bothered to read the new FMS paper. But I like the FMS paper, from which I infer that N.D.Mermin must have had quite a large input into it.

    Anyway, maybe I shouldn't call myself a "QBist" after all -- since the original FS paper makes me feel like I'm suffocating. Similarly for Fields' critique.

    Er, it's not clear whether you actually read the FMS paper.(?) Most of your post #2 seems to be constructing a straw man to represent me. (I don't respond to straw men.)

    Yes, precisely. -- I had a feeling you'd recognize that. :wink:

    It's certainly clear -- I was able to read it easily in one sitting without getting bored, which is quite unusual (for me) with "interpretative" papers.

    But when looked again at the older FS paper on QBism (i.e., in a nonrelativistic context), I got depressed again. That name clearly has a sour taste for some.

    The value (imho) lies in the synergy that emerges when working with those ideas in the context of special relativity.
  9. Nov 24, 2013 #8


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    A quick look at Fuchs and Schack's "Quantum-Bayesian Coherence: The No-Nonsense Version" http://arxiv.org/abs/1301.3274 seems to suggest that Bayesian updating cannot be derived from Bayes rule, as they state it as an assumption.
    "Assumption 2: Principle of Reciprocity: Posteriors from Maximal Ignorance Are Priors." (Eq 122)
    "Resumption 1: Principle of Reciprocity: Posteriors from Maximal Ignorance Are Priors." (Eq 130)

    This is why I think collapse (or Bayesian updating) must be postulated, or additional axioms introduced if it is not postulated.

    I do like the idea of wave function collapse as Bayesian updating. It's intuitive (I've heard it informally many times before hearing of the QBist programme). Also, the Bohmian interpretation always seemed to me like Bayesian updating. So I googled, and found Howard Wiseman's "Grounding Bohmian Mechanics in Weak Values and Bayesianism" http://arxiv.org/abs/0706.2522 , which might be interesting to compare and contrast. Wiseman's remarks "As soon as an innocent observer were to open her eyes she would collapse her state of belief about x from Pprior(x; t) to a much sharper P(x; t), by observing the location of objects (from the pointer on a meter to the stars in the sky) relative to her. Note that this "collapse" is completely classical: it is just Bayesian updating of her beliefs about the positions of macroscopic objects."
    Last edited: Nov 24, 2013
  10. Nov 24, 2013 #9
    I can hardly bring myself to read it, it is so obnoxiously vacuous. It is effectively solipsistic. All the mysteries of quantum mechanics are to be resolved by focusing on the successive experiences of a single agent, and by refusing to think about any reality beyond that.

    The further I look into this paper, the worse it looks. For example, in the title they promise a demonstration that quantum mechanics is local. How does it work? Well, their method, as I mentioned, is to focus on the experience of a single agent. Nonlocality involves mysterious connections between two spatially separate locations; but a single agent is only ever in one place at a time, therefore nonlocality cannot arise - apparently other places just don't exist!

    Or look at the end of page 6 and beginning of page 7. EPR's perplexity arose because they insisted on thinking that physical theories make statements about physical reality. If only they had understood instead that physical theories are about "firmly held beliefs", then we could have avoided all that unpleasant agitation about the implications of quantum mechanics for the nature of physical reality. Under the QBist interpretation, QM is simply not about physical reality, therefore it has no such implications! Problems solved!
    What I wrote is a protest against a certain type of quantum sophistry and a warning against falling for it, intended for anyone reading a thread like this.

    On a second viewing, my judgement of this paper is even harsher. I take back what I said about how it might have some positive value.

    If people want QM to make sense, all they have to say is that it gives you the probability for going from one physical situation to another, but that it doesn't tell you what happens in between. So it's incomplete. It's not the final theory of physics. That's all that has to be said.

    (I put that in bold so that people who are looking for a quantum philosophy, a way to understand what QM means, have something to work with. Those sentences in bold - that is the attitude towards QM that I recommend.)

    But too many people want to turn QM into a philosophical idol, a new kind of science, in which its incompleteness is mysteriously a virtue. For example, in this paper, the authors want to turn QM into a sort of solipsistic anti-theory, in which the answer to various questions is just: un-ask the question, we should only care about the experience of a single observer, nothing else matters.

    I call this approach an anti-theory because a theory ventures to make statements about reality, and their whole approach is to eschew such statements on principle. They misunderstand the significance of the fact that everything comes to us through personal experience.

    Unless one intends to be a solipsist, with no explanation at all of the regularities in your experience, then there is more to reality than just your private sensations. And traditionally, a physical theory consists of some hypotheses, right or wrong, about what's going on in that greater reality.

    It's one of the distinctive oddities of quantum mechanics that (at least, according to Copenhagen) it does not present a hypothesis about what takes place between observable X taking the value x, and observable Y later taking the value y, it just presents a calculational procedure for obtaining the conditional probability Pr(Y=y|X=x).

    According to QBism - if these authors have portrayed it correctly - then QM is most truly itself when the observables are private sensations (qualia) of some individual, and when every other part of reality is regarded with Copenhagenist disinterest. That is the formula they propose to use, in order to resolve all the conundra of quantum mechanics.

    Technically one might object to this on the grounds that we just don't use QM in that way. In actual applications of QM, the observables are things like "the spin of the particle" or "the location of the particle"... not "my experience of the apparatus, after I cycled to the lab, unlocked the door, and sat down to check the readouts".

    And I've already stated the philosophical objection - unless you're a solipsist, there is more to reality than just your private stream of experience, and an interpretation of QM needs to say something about the theory's implications for the world outside your skull. Unless you're a solipsist, exclusively focusing on private experience is just a way to evade questions, not a way to answer them.

    strangerep, sorry if you experience my scorn for this outlook as a personal attack. You don't actually express your own ideas much, so I don't know your opinions. But the philosophy of this paper, when scrutinized in detail, is absurd and needs to be exposed as such. It is also pernicious to the discovery of truth, if people read a paper like this and come away thinking that various unanswered questions have been satisfactorily answered in it. That's why I say it has negative value, because it produces an illusion of conceptual progress.

    P.S. As atyy has just posted, it absolutely makes sense to think of the use of QM as part of a process of Bayesian updating. My point is just that this perspective doesn't deal with an ontological problem like the nature of quantum nonlocality, and yet the headline claim of this "QBist" paper is precisely that this problem has been dissolved.
    Last edited: Nov 24, 2013
  11. Nov 25, 2013 #10


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    How about being solipsist WITH an explanation of the regularities of one's experiences?
  12. Nov 25, 2013 #11


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    Are you sure? Hawking was not 60 in 1993!
  13. Nov 25, 2013 #12


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    I don't think so. Seems rather to parallel "Relational EPR" (Smer. Rov. 2006) which addresses the issue and is careful to explain that the view of reality presented is not solipsist. I think according to Aristotle reality consists of what we all see and can agree on.

    So the longstanding Mediterranean idea is that reality is determined by a community of observers who communicate among themselves.

    All Rel-EPR adds to that traditional picture, as I see it, is take note of the fact that each of the observers is subject to quantum mechanics (is a "quantum system") and their intercommunication is limited by the usual 1905 speed conventions.

    IOW there is something REAL out there, and don't let it throw you into a tizzy if it is a complexly entangled superposition. Your perception/intervention extracts classical pictures and facts. I accept you, as a fellow observer, are real. You are a quantum system too. I don't know what you just experienced because you haven't told me---the news hasn't reached me yet. But I acknowledge realty is real and making an impression on you, affecting your quantum state. Each of us "classicalizes" the reality we interact with.
  14. Nov 25, 2013 #13


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    Matt Leifer has a classification of realist and non-realist interpretations in http://mattleifer.info/2011/11/20/can-the-quantum-state-be-interpreted-statistically/

    "1. Wavefunctions are epistemic and there is some underlying ontic state. Quantum mechanics is the statistical theory of these ontic states in analogy with Liouville mechanics.
    2. Wavefunctions are epistemic, but there is no deeper underlying reality.
    3. Wavefunctions are ontic (there may also be additional ontic degrees of freedom, which is an important distinction but not relevant to the present discussion)"

    "Options 1 and 3 share a conviction of scientific realism, which is the idea that there must be some description of what is going on in reality that is independent of our knowledge of it. Option 2 is broadly anti-realist, although there can be some subtleties here[2]."

    His footnote [2] about whether the subtleties of option 2 being "anti-realist" is

    "The subtlety is basically a person called Chris Fuchs. He is clearly in the option 2 camp, but claims to be a scientific realist. Whether he is successful at maintaining realism is a matter of debate."

    Bolding is mine, not Matt Leifer's.

    Leifer has additional comments on the QBist proposal which seem very congruent with mitchell porter's:

    "I would classify the Copenhagen interpretation, as represented by Niels Bohr[3], under option 2. One of his famous quotes is:

    There is no quantum world. There is only an abstract physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature…[4]

    and “what we can say” certainly seems to imply that we are talking about our knowledge of reality rather than reality itself. Various contemporary neo-Copenhagen approaches also fall under this option, e.g. the Quantum Bayesianism of Carlton Caves, Chris Fuchs and Ruediger Schack; Anton Zeilinger’s idea that quantum physics is only about information; and the view presently advocated by the philosopher Jeff Bub. These views are safe from refutation by the PBR theorem, although one may debate whether they are desirable on other grounds, e.g. the accusation of instrumentalism."
    Last edited: Nov 25, 2013
  15. Nov 25, 2013 #14


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    AFAiCS it is in no way "incomplete". What do you want to be "in between", a classical trajectory?

    What happens in between observations/interactions is that the quantum system continues to evolve.
  16. Nov 25, 2013 #15


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    Many years ago, I had plenty of opinions, and expressed them readily. Most of those opinions turned out to be wrong. I now regard opinions and beliefs as being of little worth.

    So... my stance now could be summed up as: "evidence-based, plus Occam's razor", and "no, I don't want what they're smoking".

    Evidence that the Earth is not flat is absurd according to a flat-Earth believer.

    A suggestion that there is not one single over-arching "reality", but a multitude which nevertheless have interactions and hence partial correlations, thereby synthesizing an impression of a single reality, is probably absurd and pernicious to anyone who adheres to the loaded meaning of the word "reality" as "all that is".

    I prefer to keep an open mind and follow the evidence.

    (And now I wait for this thread to be terminated for being too philosophical...)
  17. Nov 26, 2013 #16


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    Today I followed the audio and slides of Tim Koslowski's ILQGS talk. The audio is faint part of the time (not speaking close enough to the microphone,…or a bad connection.) "What can we learn from Shape Dynamics?"
    He seems to be saying no single over-arching spacetime.

    This also seems to be the message from some work by Laurent Freidel and others called "relative locality".

    In neither case do they bother to deny a single overarching reality. It is just that the mathematical representation, the model, allows the possibility that there is no one single spacetime---geometric discrepancies are somehow allowed.

    According to Koslowski the conventional spacetime of General Relativity is recovered somehow, but not so precisely or completely as to prevent a test. I'll get the links to the slides PDF and to the audio, in case anyone is interested.

    An early "relative locality" paper: http://arxiv.org/abs/1101.0931
    Last edited: Nov 26, 2013
  18. Nov 26, 2013 #17


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    Ah, the elusive ultimate truth, is it even possible to attain, who knows.
  19. Nov 27, 2013 #18
    The relational interpretation, as I understand it, is that states of quantum objects are supposed to be relative to observers, in a way somehow analogous to the relativity of motion. But consider Schrodinger's cat. Either the cat is alive, the cat is dead, or there's more than one cat. It makes no sense to say that there is only one cat, and it's alive relative to those who see it as alive, and it's dead relative to those who see it as dead.

    As for reality, it just is what it is, regardless of whether we know the facts or accept them or guess them rightly or wrongly. Observers may share evidence or coordinate their guessing, but how can that have anything to do with e.g. whether physics is local? The Copenhagenist attempt to rationalize an epistemic, instrumental theory as the final word in physics ... perhaps I should call it the "old Copenhagen interpretation", since so many people now think Copenhagen means objective wavefunction collapse ... the old Copenhagen interpretation left a legacy of confused thinking which lives on in many newer interpretations.
    That sounds like "consciousness collapses the wavefunction", except that everyone is doing it in this version, not just the ghost of von Neumann.
    How does this work? I'll use Penrose's paraphrase of quantum mechanics according to von Neumann. There is a unitary evolution U between observations, and a quantum jump R (R for reduction, as in state vector reduction) at the moment of observation. I assume that the relational interpretation is supposed to avoid the need for R, because the states of quantum objects are relativized to observers somehow? So the only process is unitary U, but when we look at objects, it's as if R has occurred? Sorry, I don't see how this makes sense. Again I refer to the cat. Are you really going to say that its quantum state is |dead>+|alive>, but that it is dead "relative to" a person who sees it as dead, etc? What would that even mean?
    So what are these multiple realities like? What sort of things are they? How would you describe one of these partial realities to me? I can't entertain a hypothesis if I don't know what it is.
  20. Nov 27, 2013 #19


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    Hi Mitchell, I didn't mean that "consciousness" was involved. What I said about interacting quantum subsystems (like "you") was meant more generally:

    I acknowledge reality is real and making an impression on you, affecting your quantum state. Each of us "classicalizes" the reality we interact with.

    You don't have to be alive or conscious for your environs to make an impression on you. You could e.g. just be the electrode in a photocell.

    I'm trying to articulate the viewpoint I get from "Relational EPR". Maybe I don't understand the Smerlak Rovelli paper correctly. If someone else can interpret it better that would be most welcome!

    ==quote Mitchell==
    How does this work?

    You are asking about the MATHEMATICAL REPRESENTATION of reality without continuous trajectories. I'm not an expert and I don't follow QM foundations research at all closely, but my sense is that this is work in progress. One way it might "work", I suspect, is this:

    You represent reality by a C* algebra A and TIME as a one-parameter subgroup αt on A.
    A priori there is no Hilbert space, there is just the algebra of observables. A STATE is a positive functional ρ(a) defined on the algebra. The state can be used to construct the flow αt.

    So the flow of time depends on the state ρ. I'm simply speculating about how it might work out. There are continuous trajectories at the level of observables, in this picture, and there are transition probabilities I suppose, but in this picture there are no continuous trajectory *outcomes*. One cannot say what happened IN BETWEEN along the way :smile:

    Maybe you or Strangerep or someone else can improve on this. Strangerep mentioned the Smerlak Rovelli paper "Relational EPR" which gives what I think is a fairly clear well-thought-out description of how reality could be. When you ask "how it works" I think you are asking about the possible ways to make a mathematical implementation. Clearly it does not have to "work" in an ENGLISH COMMON-LANGUAGE description and according to metaphors from everyday life. Math as an invented language for describing how reality works is evolving/adapting is more suitable and we just have to wait and see where it goes. I've suggested one possible direction.
  21. Nov 27, 2013 #20


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    I don't know if RQM works or solves any problems, but Smerlak and Rovelli http://arxiv.org/abs/quant-ph/0604064 do have interesting interpretational comments, and claim to disavow solipsism. "It is far from the spirit of RQM to assume that each observer has a “solipsistic” picture of reality, disconnected from the picture of all the other observers. In fact, the very reason we can do science is because of the consistency we find in nature: if I see an elephant and I ask you what you see, I expect you to tell me that you too see an elephant. If not, something is wrong. .......... So, what happens if A and B compare notes? Have they seen the same elephant? .......... It is clear that everybody sees the same elephant. More precisely: everybody hears everybody else stating that they see the same elephant they see. This, after all, is a sound definition of objectivity."

    This actually reminds me a bit of Zurek's http://arxiv.org/abs/0707.2832 "It is therefore not clear whether one is forced to attribute "reality" to all of the branches of the universal state vector. .......... It can acquire objective existence only by "advertising itself" in the environment. This is obviously impossible for universal state vector - the Universe has no environment. Objective existence can be acquired (via quantum Darwinism) only by a relatively small fraction of all degrees of freedom within the quantum Universe ...."

    Zurek seems to be working within some sort of relative state interpretation, so in his tentative view the wave function of the universe is the deep quantum reality. While the classical reality is only available for some states and observers. At least that's what he seems to be aiming for. In http://arxiv.org/abs/1205.3197 , he, Jess Riedel and Zwolak write "Whether an essentially unique quasi-classical realm [41, 42] can be identified from such principles is a deep, open question [43, 44] about the quantum-classical transition."

    What is certainly not clear to me is whether RQM is really against what Smerlak and Rovelli call "Einstein's realism" that "there exists a physical reality independent of substantiation and perception.". Smerlak and Rovelli do write "RQM departs from such strict realism.". It is quite unclear to me why their view is incompatible with strict realism, except trivially. If by perception, they mean any interaction (they say "An atom interacting with another atom can be considered an observer") - then yes, things that don't interact at all can be considered not real. But even a strict realist would agree that it's not meaningful to talk about invisible fairies in the garden. So perhaps an error they make is that they take a realist view, but do not call it so.

    Also, I don't know if Rovelli really favours any one interpretation of QM. In his 2008 review http://relativity.livingreviews.org/Articles/lrr-2008-5/fulltext.html [Broken] of LQG he writes "Loop quantum gravity is a standard quantum (field) theory. Pick your favorite interpretation of quantum mechanics, and use it for interpreting the quantum aspects of the theory. I will refer to two such interpretations below. When discussing the quantization of area and volume, I will use the relation between eigenvalues and outcomes of measurements performed with classical physical apparatuses; when discussing evolution, I will refer to the histories interpretation. The peculiar way of describing time evolution in a general relativistic theory may require some appropriate variants of standard interpretations, such as Hartle’s generalized quantum mechanics [140], or a suitable generalization of canonical quantum theory [261, 243, 245, 242]."
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