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There's a nice rhetorical flourish to the idea that Bayesianism is a "more sophisticated interpretation of probabilities" (my emphasis). All I need to know now is whether sophistication makes something more or less likely to be Really True or, I suppose, inadequately to the sophistication of QBism, a Real Belief. Qualms about rhetoric aside, updating beliefs seems a sometimes worthwhile way to shut up and calculate, whether one commits to realism of any kind or not.

Question: is there any literature on how QBism applies to a QFT context?

Only nature could produce a reality so simple, a single cell amoeba could deal with it.

One real visible Universe must have only one reality. Simple natural reality has nothing to do with any abstract complex codswallop musings such as this utterly ridiculous article describes. No infinite visible part of surface could ever be in multiple invisible places at the same unnatural time. As I have thoughtfully pointed out in my brilliant essay, SCORE ONE FOR SIMPLICITY, the real Universe consists only of one unified visible infinite surface occurring in one infinite dimension, that am always illuminated by infinite non-surface light. Reality am not as complicated as theories of reality are.

Joe Fisher, Realist

I think that QBism is the most coherent view of quantum phenomena, and reality, so far. Certainly, QBism's view of quantum phenomena corresponds to our everyday perceptions (and the experimental results involving observers and quantum systems) that some parameters of our actions are free and that we can act and genuinely make a difference to the world. Opposed to QBism is the view that some parameters of our actions are random (e.g. as a result of our inhabiting a randomly selected multiverse outcome), or the view that all parameters of our actions are fully determined by the system of laws-of-nature, so that even though you might think that you are acting to make a difference to reality (e.g. because of climate change), all parameter numeric values now, and in the future, are already fully determined.

One thing that I'm not clear about is the issue of "locality". If 3D space is due to relationship, as opposed to 3D space being a fundamental primitive of the universe-system, then law-of-nature relationships are the precursor to 3D space. If law-of-nature relationships are the precursor to 3D space, then it is possible for at least some law-of-nature relationships to be independent of 3D space.

Bayesian mechanics like all statistics have much allure because all observations when it comes right down to it are a matter of statistics. The real issue is the roles of the classical noise of chaos versus quantum phase noise.

The author does mention a nice product state (Eq. 13 in the cited paper), which looks like biphoton gravity, but Bayes is not quite up to gravity yet.

Just like the allure of the symmetry of the Standard Model, Bayesian statistics may simply be a complexification of the underlying simplicity of aethertime.

Good luck!

Having now read the Fuchs article that I didn't at first notice the link to, arXiv:1003.5209v1 [quant-ph], I noted that "A knee-jerk reaction in many physicists upon hearing these things is to declare that dimension as a capacity collapses to a triviality as soon as it is spoken. "All real-world systems possess infinite-dimensional Hilbert spaces. And it doesn't take quantum field theory to be completely correct to make that true; a simple one-dimensional harmonic oscillator will do. It has an infinite-dimensional Hilbert space."" I don't have precisely that reaction, but quite close. What seems in doubt to me is whether there is such a thing as a "real-world system", presumably isolated, or, if there is, whether such a thing has a determinate dimensionality except as an approximation or FAPP.

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I'm seeing more and more buzz about QBism, but still no explanation of it that says anything to me. Ok, it's revolutionary, unorthodox, controversial. So are a lot of things. Per this article, the central insight is that "the outcome of a measurement can only be thought of as a new experience that the observer ought to take into account in any new beliefs." I guess I'd need a lot more than that, to even start getting interested.

So what is QBism?

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    Thanks Georgina, I'll read that article.

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    Thanks for that link, Georgina. I quote from that article, amongst several other quotable Fuchsisms, "As QBism understands a quantum measurement outcome, it's personal. No one else can see it." I see this quote as problematic as a description of experimental practice. In days of Bohr, indeed someone looked at a meter needle and no one else looked, but now it's almost universally a computer that looks and anyone can look at what the computer recorded.

    Still the best example (IMO, it's nontrivial, at the right level for QBism in that it's modeled by a low-dimensional Hilbert space, but starting to be a little old) is the collection of datasets generated by Gregor Weihs in the experiment described at length in his thesis and in PRL (for which see https://arxiv.org/abs/quant-ph/9810080). Anyone can look at a classical copy of the same data Gregor looked at (he's become unwilling to let just anyone analyze the data, but for a few years it was freely available), which for each run of the experiment consists of two lists of times at which Alice's and Bob's detectors, 400 meters from each other in the apparatus, switched from current off to current on (plus which detector current changed and the state of the polarizer at the time).

    If one feels that one has to think in terms of wave function collapses, Gregor didn't cause any collapses, perhaps we could say that he set up a computer to cause some collapses. We can, however, consider the circuits of Gregor's experiment in more detail, asking what the detector signals would have looked like at the picoAmp and picosecond scale, say (Terabytes of data per second), in which there would be identifiable changes of state --collapses, much less data to record-- but changes of the detector current are nonetheless rather closer to continuous --not so much collapses.

    There is a question how to manage the relationship between the experimental datasets and the expected values that are generated by Hilbert space models, where we might choose Bayesian methods as one particular approach, but there are many ways in statistics to estimate model parameters.

    Someone might feel like responding substantively here on FQXi? Is there other literature on QBism that discusses experiments in a more world-like-way than Chris Fuchs uses here?

    Having now read the comments on the Quanta article you linked to, Georgina, just one comment there alludes to the qualm I express above, "There is an objective reality; it's on a server somewhere. We each have our own copy to hack as we want. Why the quantum? Because pixels." At least, the first sentence, maybe the rest is too tongue in cheek, too little detail. One could say that an experiment only becomes part of science when it's published in a journal, ideally with supplementary datasets that allow the conclusions to be discussed in detail; then, it's on a server.

    I also looked at a paper linked to by Blake Stacey there, https://arxiv.org/abs/1301.3274. The idea that QBism wants to take the KFC approach, that QBism's B stands for whatever I or you or Chris Fuchs says it stands for, falls rather by the wayside in the focus on SICs, more than hinted at at the end of the Quanta article. QSICism, hinting at "thus" or "just as", has a problematic sense. It doesn't have the same ring, but the mathematics, in contrast to the metaphysics, is kinda-maybe QPEism, Quantum Parameter Estimation-ism, allowing that one might use a Bayesian approach, but also allowing other statistical methods, when constructing Hilbert space models for experimental descriptions, which can be said to determine a prior before the experiment is run, and the resulting datasets.

    The question of when the wave-function can be said to have collapsed is of interest to me. I have submitted a paper to viXra that posits that rather than an unmeasured electron having multiple spins represent-able by the wave-function, that it is encountering conditions within the apparatus that induce the detected behaviour. I have called that provocation rather than measurement. The outcome becomes inevitable at provocation but is not known about until the observer receives information, by which the result is known. Thus there is an un-named duration in which the wave-function of multiple states no longer applies prior to the observer knowing the result. However if the wave-function is considered to be only a state of knowledge and not the reality of the external system outside of the mind, then the wave-function can not be said to collapse until the state of observer knowledge is updated by the new information. If the state observed is considered to have been provoked, the wave-function was a representation of the many possibilities that might have been, the multiple (not yet existent) possibilities, eliminated by the provocation. The term wave function redundancy might be useful to denote the situation post provocation but prior to the observer update and final designation of collapse or no further applicability of the wave-function of multiple states to the observer observed system ensemble.

    Also interesting is whether probabilities have any claim to being a part of reality. They relate to possibilities and potential. I have put those two on the border of Object reality between what does and does not yet exist on my diagrams. Yet there are two kinds of possibilities or potential. 1. might be-s that do not yet exist ( as shown on my diagrams) and another class, 2. unknowns that do exist. eg. the unmeasured spin of a globe. In the case of electron spins I think it is not that they do all co-exist but that the possibility of getting a detection of any of them is not excluded by the existing Object reality, until provoked by interaction with the apparatus, into producing one 'spin" outcome. So the first kind of possibilities.

    'Exist' is the wrong word to use for spin of a globe, a kind of motion not a thing. I should have said- 'Unknowns that do exist or are happening'.

    Georgina, I doubt I can be very helpful, but I'll try... Am I right that your intention is to call the events that are recorded as part of an experiment "provocations" instead of "measurements"? It's not clear to me from your discussion how one differs from the other. In particular I can't see any instrumentalist difference, in that the same datasets will be recorded for later analysis, but I also can't see in your account what difference of metaphysics you intend?

    Reading my PoV into your account, measurement events are a consequence of including a macroscopic, meta-stable thermodynamic system as part of an experimental apparatus. An event happens when the electric resistance (or some other thermodynamic property) of this relatively large assemblage changes, and the amplified current is enough to be recorded, together with a time-stamp, in a computer memory somewhere. If the macroscopic, meta-stable thermodynamic system were not there (as a provocation, if you will, but I don't like this coining much), there would have been no event to record in the computer memory, and hence no datasets that we can use to verify that quantum theory correctly predicts events. One has to keep in mind, however, that not all the usefulness of quantum theory derives from records of discrete events.

    I think I read your second comment just to say that you're not sure whether probabilities are "real". FWIW, my take is that probability distributions are parts of mathematical models, which have a typically slightly equivocal, less than completely formal relationship to datasets containing raw experimental results (which are still mathematical objects, but with significantly less mathematical structure; they're typically just large but finite lists of numbers). The World --perhaps we might even call it The Real World-- is not to me obviously a Mathematical Object, though perhaps it is; however I've never been able to see a way to determine whether it is or is not.

    Whether this is helpful or not, Best Wishes.

    Joe, I am deleting your posts because you are blanket posting the same message over and over again. I find your behaviour inconsiderate and tedious. I have tried discussing your ideas with you and have read and commented on your essay. You have not been ignored or had your idea suppressed. Your essay has been accepted for the contest and is available for anyone who is interested.Its just that enough is enough.

    My feeling about Qbism is that it is in a long line of ideas about physics that leap off into philosophy. Quantum interpretations are all ways of trying to make metaphysical or existential categories fit with quantum mechanics. In general these fit into two sets ψ-ontology and ψ-epistemology. The big example of the first of these are the Everett-DeWitt Many Worlds Interpretation,. While less popular the Bohm interpretation is also in this set. The main example of the latter is the Copenhagen interpretation. Qubism is pretty clearly a case of ψ-epistemic interpretation. These all have some utility in working certain problems. I also think they are all ultimately flawed. The reason is that with quantum mechanics we can only talk about outcomes that are real valued as having ontology. Quantum mechanics is complex valued, which means there are formal aspects of QM that can't be expressed in ordinary language that fits within these categories completely.

    ψ-epsitemic interpretations have the problem that Heisenberg pointed out with Bohr's insistence on a quantum vs classical divide. Where does this exists? The Schrodinger cat and other arguments are meant to illustrate this difficulty. Much the same holds with Qubism, where the observer's perspective is considered central. However, who or what is an observer? I can consider myself to be an observer, but I am different with every breath I take, or with each meal or every time I use the bathroom. Where is the cut-off with the observer? How is the observer that distinguishable from the rest of the world? If I find cosmic ray tracks in multi-billion year old rock am I really measuring a wave function collapse? I could say in some way that I plus the rock over billions of years are a composite as an observer. However, does that really change what happened quantum mechanically with the reduction of states in the rock? At some point this starts to sound less like physics and more like philosophy.

    Quantum interpretations are starting to multiply like bunnies, and there seems to be no clear way to indicate which one is (ones are?) correct. The workshops and conferences on these things grow in size and number, but as I sense things they all seem to be like meetings of philosophers arguing the truth of some point based on clever use of language. Polls are sometimes taken at meetings or conferences of physicists on QM interpretations. There is always a split and this changes a bit with time, but no interpretation appears headed for becoming the clear winner. Some are very popular right now, such as MWI, and others are in the dumps such as Bohm's QM. Bohm's QM has problems but it is not as horrible as people think.

    There is in a sense one interpretation, which is the null-interpretation. QM can only interpret itself, not us. I think this is tied into the prospect that a quantum measurement as a quantum system plus quantum apparatus (apparatus ultimately made of quantum states etc) is a quantum information or quantum Turing case of Godel's theorem. QM lacks the power to predict a specific outcome, only probabilities for the outcome of some observable (Born rule etc), and there does not exist a quantum interpretation that will close off QM in a consistent manner with measurement or the existence of a classical (classical-like really) world. The so called quantum measurement problem will never be solved.

      Peter,

      Measurement is of an already existing or happening, property or behaviour. Provocation causes the behaviour or property of the thing to come in to being or happen.The difference is not in the results but in how we think about what is going on.

      Lawrence -never is very definite. Would you rather there was no discussion and no new thoughts on the matter? As there are already bunnies galore and no clear winner.

      Thanks, Georgina. Do you take "existence" to be a property that only happens after provocation? If an individual event is provoked, does the particle that is said to cause the event exist only at the moment of the event, not before or after? If so, is a particle a synonym for an event? Or how is a particle different from an event? Too many questions! Choose any or none.

      There's a slight feel of contextuality to your proposal, though enough differences are apparent to me that I won't ask you to delineate what differences you see.l

      I take existence to be only at one time, not spread over time. I am not using events as I think you intend the word to mean , i.e. as space-time co-ordinates. My proposal is that provocation induces the state measured, that state being a behaviour in response. the electron isn't carrying X,Y and Z spins but if its just a random electron, just moving. I imagine how it happens to be moving interacts with the fores encountered to give an outcome then referred to as a spin.

      It seems to me the wave function collapse is a switching of model used to represent reality. The interaction with the apparatus that makes the outcome inevitable is something happening in the foundational Object reality that is not observable so separate from the observer awareness pre- and post result ( and pre and post models of reality.) Just because the "redundancy" is taking place prior to information receipt by the observer doesn't mean it isn't happening. I'm not sure if that is at all similar to your line of thought re. the computer.