Howard Wiseman


    Does quantum theory apply to observers? A resurgence of interest in the long-standing Wigner’s friend paradox has shed new light on this fundamental question. Brukner introduced a scenario with two separated but entangled friends. Here, building on that work, we rigorously prove that if quantum evolution is…[Read more]

  • Well of course if you use different assumptions you can do the EPR argument very simply. The challenge is to read the EPR paper, and try to extract a rigorous argument from it, phrase by phrase, word by word, even. Or maybe I don’t understand what you mean by “primitive concepts”.

    Glad you agree with my own first paragraph. But I disagree with…[Read more]

  • Matt

    Sure, I totally agree that the DAG intuition is why Bell thought he had proven determinism from predictability. But I guess we both agree that it would be completely unreasonable to allow that as an implicit assumption in a theorem, since orthodox quantum theory violates it. As I’ve always said, I am interested in Bell’s *theorem* of 1964,…[Read more]

  • Dear Travis,

    Some brief (I hope!) replies.

    1. I agree. We can set out our arguments and other people will have to decide themselves.

    2. Yes I really believe that it is very far from clear that Bell is defining ‘locality’ by his [2]. But I don’t think there’s anything more either of us can say on this.

    3. I agree with you that…[Read more]

  • Hi Travis,

    I don’t have anything to add to my reply under my article about why I think Bell gives his own definition of `locality’, and uses the referencing of Einstein as an appeal to authority to justify the reasonableness of making an assumption like this.

    In my formal reply I will give the quotes and both our opinions, and readers can…[Read more]

  • Hi Matt,

    I agree with some your comments here. But:

    1. Your discussion about DAGs is not really relevant to the physics community in 1964. Pearle’s first book was only published in the 1980s. No-one in physics was thinking this way in the 1960s. And in any case, the DAG model of causality is one that orthodox quantum mechanics fails to obey. So…[Read more]

  • Sorry, another comment: Travis admits that Bell’s words in the EPR paragraph cannot be read as being a statement of local causality. That is why he says the paragraph “leaves something to be desired,” that it “disappoints” and is “problematic”. If it was all plausible that Bell’s words could be interpreted as being “local causality” then Trav…[Read more]

  • Hi all,
    Just a quick comment to say that, as per my original paper, and my reply paper, I do not at all agree that
    “… we can only speculate on how the 1964 Bell would have formally defined locality in a stochastic hidden variable theory. Since both parameter independence and local causality both reduce to exactly your equation (3) in the…[Read more]

  • Dear Travis,

    Thanks for the comment — it will be useful in preparing the revised version of my IJQF-paper, the reply to you. Here let me just say this:

    1. I will admit I slightly overstepped in my 2014 paper in saying that there is only one plausible reading of `locality’ in Bell’s 1964 paper”, namely Parameter Independence (PI). lndeed, I…[Read more]

  • Yes. That is my polemical reply to the titular question in Travis Norsen’s self-styled “polemical response to Howard Wiseman’s recent paper.” Less polemically, I am pleased to see that Norsen has made significant concessions on my position that Bell’s 1964 theorem is different from Bell’s 1976 theorem, and that the former does not include Bell…[Read more]

  • H. M. Wiseman

    Yes. That is my polemical reply to the titular question in Travis Norsen’s self-styled “polemical response to Howard Wiseman’s recent paper.” Less polemically, I am pleased to see that Norsen has made significant concessions on my position that Bell’s 1964 theorem is different from Bell’s 1976 theorem, and that the former does not include Bell’s one-paragraph heuristic presentation of the EPR argument. In his response, Norsen admits that “Bell’s recapitulation of the EPR argument in [the relevant] paragraph leaves something to be desired,” that it “disappoints” and is ”problematic”. Moreover, Norsen makes other statements that imply, on the face of it, that he should have no objections to the title of my recent paper (“The Two Bell’s Theorems of John Bell”). My principle aim in writing that paper was to try to bridge the gap between two interpretational camps, whom I call ‘operationalists’ and ‘realists’, by pointing out that they use the phrase “Bell’s theorem” to mean different things: his 1964 theorem (assuming locality and determinism) and his 1976 theorem (assuming local causality), respectively. Thus it is heartening that at least one person from one side has taken one step on my bridge. That said, there are several issues of interpretation where Norsen and I still disagree, and these I address after discussing the extent to which we now agree. The most significant points of disagreement are: the indefiniteness of the word ‘locality’ prior to 1964; and the assumptions Einstein made in the paper quoted by Bell in 1964 and their relation to Bell’s theorem. Wiseman – Reply

    Note: This paper has also been posted/discussed in John Bell Workshop 2014.

    • This paper has been sent out to peer review.

    • Dear Howard,

      Rather than write another detailed rebuttal to the many points you raise here, I would like to simply re-frame the big picture and encourage interested people to make up their own minds on the basis of what has already been written.

      I will cut right to the chase. You wrote, in your original paper, that “there is only one plausible reading of `locality’ in Bell’s 1964 paper”, namely Parameter Independence (PI). This, in my view, is simply preposterous. PI is a statement about probabilities, but nowhere in his 1964 paper — either in his English prose or in his equations — does Bell indicate that what he means by `locality’ has something to do with probabilities. There is simply no evidence for your interpretation. Furthermore, your interpretation flies in the face of the direct, conclusive evidence that does exist: that Bell references Einstein’s Autobiographical Notes, 3 times, by way of specifying what he means by `locality’.

      I think any honest reader of Bell’s 1964 paper would have to recognize that Bell uses `locality’ to mean what Einstein expresses in his Autobiographical Notes — the condition you call “no telepathy” and concede has “the same force as local causality”. And once that is established, everything else falls nicely into place: Bell’s equation (1) and the surrounding prose are describing a narrower implication of locality, an implication relevant to the sort of deterministic theory that Bell (following Einstein) regards as required to locally explain the usual perfect correlations and which therefore figures crucially in the novel 2nd part of Bell’s overall two-part-argument.

      The fundamental dispute (between your so-called operationalists and realists) is about whether Bell *assumed* determinism, as a premise, in 1964, or instead saw determinism as following from locality via something like the EPR argument. But this dispute is trivially resolved: Bell says explicitly, in his EPR-recapitulation paragraph, that predetermined outcomes *follow* from locality and perfect correlations. Of course there are further questions about that argument (Is it really valid? Does Bell capture it adequately in his 2-sentence recapitulation? How exactly is `locality’ formulated in this argument? Are there any other prerequisite assumptions beyond `locality’ and perfect correlations?) but there can be no question about whether Bell took Einstein to have previously made a valid argument along these lines.

      And so the operationalists in question were simply *wrong* to think that determinism was, for Bell, an independent assumption which could be rejected instead of locality. All of this was of course made much clearer in Bell’s subsequent papers. But the problem was never that Bell proved two different things in two different decades, or changed what he meant by `locality’ and then dishonestly attempted to cover his tracks, or failed at first to even indicate that his novel result was the second part of a two-part-argument (i.e., that his novel result was crucially based “On the Einstein-Podolsky-Rosen paradox”). The problem was simply that the operationalists in question apparently never bothered to read and understand what Bell actually did.

      (Note that I am not saying that, had these operationalists actually understood what Bell was doing in his 1964 paper, they would have agreed with it. If they had understood that Bell’s notion of `locality’ and the crucial first part of the two-part-argument were sourced back to Einstein’s Autobiographical Notes, and followed up with that reference, they probably would have disembarked from the train of thought at the first sentence of the relevant passage: “Physics is an attempt conceptually to grasp reality as it is thought independently of its being observed.” But then we would hear them expressing a philosophical disagreement, about the very meaningfulness of Einstein’s — and Bell’s — notion of `locality’, or about the appropriateness of Einstein’s and Bell’s visions of the purpose and aim of science… not claiming, as you claim they do, that Bell assumed determinism and/or meant, by `locality’, Parameter Independence or — even less plausibly — No Signalling.)


    • Dear Travis,

      Thanks for the comment — it will be useful in preparing the revised version of my IJQF-paper, the reply to you. Here let me just say this:

      1. I will admit I slightly overstepped in my 2014 paper in saying that there is only one plausible reading of `locality’ in Bell’s 1964 paper”, namely Parameter Independence (PI). lndeed, I contradict myself later (p. 18) of that 2014 paper (as I note in my IJQF submission here) by noting an alternative reading and saying “but to avoid further terminological complication I will not pursue that line of argument.” But the important thing to note is that NONE of the plausible readings of Bell’s words amount to local causality, or anything like that, which would be necessary for him to be able to derive predetermination from predictability. So Bell’s vagueness does not help your argument at all. The notion of PI is simply the broadest interpretation of his words.

      2. Yes, Bell references Einstein 1949 three times in sentences that relate to locality. But he never unambiguously says that he is using the quote to define locality. He never says, for example. “Now we make the locality hypothesis of Einstein [2].” or even “Now we make the hypothesis [2].” He does say “Now we make the hypothesis [2], and it seems one at least worth considering, that if the two measurements are made …” but here
      “[2]” is just an interruption, in the same way that “and it seems one at least worth considering” is an interruption. In this, and every other, instance, the reference “[2]” could be omitted from the sentence, and it would actually improve the grammatical and scientific clarity of the sentence. So the reference “[2]” really seems only a motivation or context for the definition Bell actually gives for locality. I’d agree he probably thought his definition was the same as that in [2], but this is just more evidence that he didn’t spend enough time thinking about the EPR paragraph, as it was just background to his actual theorem. In any case, there is nothing in Bell’s writing that would support *replacing* the definition Bell gives for locality with that in [2], as you would like to do.

      3. I’m rather surprised that you are still misquoting me, and misrepresenting Einstein, by claiming that the Einstein quote in question has “the same force as local causality”. I will not say any more about this, as this lapse of yours is discussed already in my submission here.

      4. Your own paper discusses how it is not true that “everything else falls nicely into place”. Bell’s EPR paragraph is quite flawed, as you admit. Again, I won’t say anything more.

      5. You say “there can be no question about whether Bell took Einstein to have previously made a valid argument along these lines.” I think it is clear, from his flawed presentation of it, that Bell in 1964 did not understand the EPR argument at a technical level. Therefore I am not so surprised at all the evidence suggesting that Bell did not consider this argument, on which he had such a shaky grip, to be part of his 1964 proof. That evidence, presented in my 2014 paper, is repeated in summary on page 3 of my submission. As noted there, you ignored much of it in your IJQF comment on my paper.

      6. The “further questions” you mention in relation to this argument are of course of vital interest to those who want to understand Bell’s theorem. Your seeming suggestion to ignore them and include the EPR argument as given by Bell as part of Bell’s 1964 theorem would convert it into Bell’s 1964 non-theorem. Personally I think that would be a far greater disservice to Bell than any criticism I have made of his rigour or memory.

      7. Your final paragraph suggests that are actually opposed to finding a form of Bell’s theorem that operationalists would agree with. For you, operationalism itself is wrong and thus there can only be one Bell’s theorem – the form that operationalists find unacceptable. Since my aim was to seek common ground between realists and operationalists, I doubt that anything fruitful will come form further discussion on this forum. But thanks again for the new insights into your point of view — they will help in the revision of my paper.


    • Howard, some further comments in response to your comments. (I follow your numbering scheme.)

      1. My view, as you must know by now, is that Bell’s *definition* of “locality” is provided by the passage he quotes from Einstein, and that some of his later words must be understood not as attempts to provide a general definition, but instead as characterizing the specific implication of locality (to deterministic theories) that is relevant to the second part of his two-part argument. It’s not that I think that “Bell’s vagueness” is helping my argument — I don’t actually think there is any vagueness. He cites the passage from Einstein by way of explaining what he means by “locality”. You and I will never convince each other; interested people will have to decide for themselves whether my reading is plausible.

      2. I find your comments here rather silly. Do you really think that your feelings about how “the grammatical and scientific clarity” could have been improved by omitting the references to Einstein, constitute a justification for simply ignoring those references? It seems quite clear to me that, in all three places where Bell cites that passage from Einstein, he is doing so in order to explain what he means by “locality”.

      3. I understand that you think it is misleading to suggest that (what you call) “no telepathy” *alone* “has the same force as local causality” since, you want to say, it is really only “no telepathy” *plus* “the assumption … that systems have real factual situations” which, together, “have the same force as local causality”. I agree that this other realism-type assumption is needed. But I think we disagree about the logical-hierarchical relation of the two assumptions. In my view (which is also Einstein’s view and Bell’s view) “the assumption that systems have real factual situations” is a pre-requisite for discussing locality. That is, one could not maintain a meaningful notion of locality (such as “no telepathy”) while rejecting this elementary sort of realism. Indeed, as I pointed out in my comment above, the relevant section of Einstein’s Autobiographical Notes begins with a statement about this sort of realism being an essential aspect of what “Physics is”. I thus find it quite misleading to list this as an assumption at the same level as locality, as if one could meaningfully choose to maintain *either* (by abandoning the other). You probably disagree with all this. I’m not trying to convince you to agree with me; I’m just explaining why, from my point of view, it is in fact quite accurate to identify Einstein’s formulation of locality (from the Autobiographical Notes) with what Bell would later call “local causality”. (Note by the way that an identical “realism” assumption underlies Bell’s “local causality”.)

      4. I wouldn’t (and didn’t) describe Bell’s EPR paragraph as “quite flawed”. For sure it could have been clearer, more precise, more formal, and longer. And, looking back, I wish it had been all of those things, so that (ongoing!) misunderstandings had simply been pre-empted. But there’s a big difference between writing something that is “quite flawed” and simply failing to adequately foresee and pre-empt misunderstandings.

      5. Your suggestion that “Bell in 1964 did not understand the EPR argument at a technical level” is based on your (I think, preposterous) interpretation of what Bell meant by “locality”. Of course we agree that Parameter Independence + Perfect Correlations do not imply deterministic hidden variables. And so we agree that *if* Bell took the EPR argument to be this, and thought it was a valid argument, he “did not understand the EPR argument at a technical level”. But of course I don’t agree that he did take the EPR argument to be this. Instead I think Bell understood the EPR argument to be the one given by Einstein — the one that Bell quotes from and cites 3 times — in which “locality” most certainly does not mean Parameter Independence.

      6. I wasn’t suggesting we *ignore* the “further questions” about the EPR part of the argument. They’re important questions. I just don’t think we should let a (subjective and highly anachronistic) obsession with “theoremhood status” blind us to parts of Bell’s paper which — whatever else we might think of them — were clearly important parts of what Bell was doing in 1964. The very phrase “Bell’s theorem” is a modern invention, and it is anachronistic to approach Bell’s 1964 paper as if Bell’s goal in the paper had been to “prove Bell’s theorem”. At the time, he wasn’t thinking in those terms at all. He was, rather, thinking that Einstein had already argued — convincingly to him — that deterministic hidden variables were required to explain certain QM-predicted correlations locally. He had also noticed and been impressed by the grossly nonlocal character of Bohm’s 1952 hidden variable theory, and wondered whether (as had been suggested earlier by Einstein et al.) it would be possible to construct an empirically viable, but *local*, hidden variable theory. After some failed attempts, he began to suspect that this might actually be impossible, and then managed to construct a proof that, indeed, it was impossible. The paper lightly sketches all of this background, and then presents the proof of the new result. To ignore or suppress this background, because it’s not presented in a rigorous way and hence not part of “Bell’s 1964 theorem” and hence irrelevant to the thing we are celebrating the 50th anniversary of, is just absurd. One shouldn’t let a contemporary name for an important historical discovery blind one to the actual content of that discovery.

      7. Is it really news that, for me, “operationalism itself is wrong”? Of course. Just as, for (you?) operationalists, “realism itself is wrong”. I wouldn’t exactly say I’m “opposed to finding a form of Bell’s theorem that operationalists would agree with” — I just think it’s hopeless since Bell’s theorem, as I understand it, centrally involves an inherently “realist” notion of locality (what you call “no telepathy” or “local causality”) which no operationalist could ever endorse as meaningful. Yes, this means that, in my opinion, your “aim” (“to seek common ground between realists and operationalists”) was misbegotten and doomed to failure from the beginning.


    • Dear Travis,

      Some brief (I hope!) replies.

      1. I agree. We can set out our arguments and other people will have to decide themselves.

      2. Yes I really believe that it is very far from clear that Bell is defining ‘locality’ by his [2]. But I don’t think there’s anything more either of us can say on this.

      3. I agree with you that ‘no-telepathy’ as Einstein phrased seems dangerously close to being meaningless without “independent real situations”. But then Einstein explicitly mentions “either … or … ” and “both alternatives”. So I’m not going to agree with you, but I can’t say exactly what Einstein had in mind.

      4. Not perfect = flawed.

      5. Yes, my suggestion that “Bell in 1964 did not understand the EPR argument at a technical level” is based on my “preposterous” interpretation of what Bell meant by “locality”. But not only on that. Also on the fact that Bell does not use any of the technical terms that EPR use. See p. 10 of my 2014 paper.

      6. This is actually a good point. Should we be celebrating “50 years of Bell’s 1964 paper”? It’s not too late, since it was actually published in 1965, which seemed to be common knowledge in the 1960s and 1970s. 🙂 I completely agree with what you say about Bell presenting the background. And it is very important for understanding the motivation for his work. It’s just that I would keep it as background, not move it into the foreground as part of his theorem/result/proof.

      7. No, this is not news to me, but this is not a private conversation so some of my comments may be for the benefit of any others (hello there!) who may be reading it. And no, I do not count myself an operationalist by any means. I am just trying to present things in a neutral way. However, this last point at least raises something new: Do you not have to admit that there *is* a theorem that uses parameter independence (an inherently operational notion of locality) plus predetermination to prove the same thing you can prove by whatever your favourite assumptions for Bell’s theorem are? So you are saying that theorem, despite its similarities, is not Bell’s theorem? Whose theorem is it? Perhaps CHSH’s theorem? (This is not an arbitrary suggestion — I’ll explain a little more in the longer version of my formal reply to you.) Would you be comfortable with that, as long as Bell’s name was not involved?

    • 3. Fair enough.

      6. Thanks. I suppose my feeling is that you are not managing to “keep it as background”, but are instead setting it completely aside as irrelevant. (Which makes sense, insofar as you define your mission as examining exclusively the thing you segregate off as “his theorem/result/proof”.) But then you are simply repeating and sanctioning precisely the misunderstanding that Bell later complained was “almost universal”. That is, as I see it, the problem all along has been that your “operationalists” failed to notice, or understand, or appreciate, the parts of Bell’s 1964 paper that are here being described as “background”. I am happy to agree that Bell shares in the blame for this, in the sense that he could have presented this “background” material more clearly, more explicitly, more consistently, more carefully, more formally, etc. But whatever. The real point is just that, to fix the problem, what’s needed is to clarify, to bring out more explicitly and consistently and carefully and formally, this background stuff — especially to the people who missed or misunderstood it originally. Telling the operationalists that, actually, they were right to miss it because it’s not really there — when it plainly is, even if in a disappointingly brief/informal/unclear/imprecise format — doesn’t seem right. Not, at any rate, if one’s goal is to spread a better understanding of Bell’s 1964 paper “AS IT IS WRITTEN”. =)

      7. First an aside: I dispute, actually, that PI is an “an inherently operational notion of locality”. But as to your main question, yes, sure, it’s possible to derive a “Bell type inequality” from lots of different sets of assumptions. And in general I would not think it appropriate to call all of them “Bell’s theorem” merely on the grounds that they arrive at some formula that is somehow obviously in the same category as Bell’s (original) inequality (or even identical to it). As to the specific example you mention, since I don’t think that Bell ever once constructed an argument whose premises were determinism and Parameter Independence, I would find it inappropriate to refer to that argument as “Bell’s theorem”. Off the cuff, it seems likely that Jarrett, or perhaps Shimony, first constructed such an argument, so probably (if you think the argument deserves a special name) it should get one of their names… or CHSH or whoever first actually ran it.

    • Report of Referee A

      This manuscript is in reply to Norsen’s submission “Are there really two different Bell’s theorems?”, itself a response to the (current) author’s “The two Bell’s theorems of John Bell”. Since Norsen’s submission confronts the authors position directly it seems only fair that he has an opportunity to reply. Furthermore the manuscript is engagingly written and makes several interesting points on related topics, for example I particularly enjoyed the concluding discussion of who’s position does more credit to Bell. Overall then I recommend that if Norsen’s submission is published, this reply is as well.

      I would ask the author to reconsider some aspects of the second page, which in places teeters dangerously on the line between merely paraphrasing Norsen (for rhetorical effect) and putting words into his mouth. Additional caution seems wise in light of the author’s complaint that Norsen “misrepresents” him.

      More specifically, since Norsen does not use the word “flawed” in his paper, it seems a little strong to say he has admitted Bell’s EPR paragraph is flawed. How about something like this: “Some of the flaws in Bell’s argument in that paragraph, from ‘locality’ to determinism, are now acknowledged by Norsen. He therefore does not object to somebody like me withholding the word ‘theorem’ from that part of Bell’s work.”? Ultimately the point is best made by letting the Norsen quotes on page 4 speak for themselves.

      Similarly, I don’t see what in Norsen’s paper licenses saying “In Norsen’s view [Bell’s 1981 commentary] is proof enough that this is a misunderstanding.” Norsen does dwell on Bell’s 1981 commentary, and calls it an “important piece of evidence”, so why not just say that?

      I would like to conclude with a remark on section 4. I have made it clear in my report on Norsen’s submission that I find some of the “Are there really two different Bell’s theorems?” quotations on pages 8 and
      9 of the submission to be potentially misleading and hence unacceptable for publication. Hopefully Norsen will rectify these issues, and the present manuscript can then be updated in light of this. I think readers will appreciate it if we are able to avoid a discussion on what Bell and Einstein said degenerating into a discussion on what Wiseman said about what Bell and Einstein said!

    • Author’s response:

      We thank the referee for the fair-minded and thoughtful report. We have made several changes in response to the report, as detailed below. We have also made numerous minor changes to reflect the change from I to we (as heralded in the previous version) and to improve the text in other ways. Finally, we have made two substantial additions belonging to none of the above categories: a long paragraph at the end of Sec. 2.1, and a short paragraph at the end of section 3. These strengthen our argument through further analysis of the text of Bell’s 1971 paper. We apologise to the referee for not having thought to include such arguments until now.

      For the convenience of the referee, all changes are in mauve, except for added quotes in the appropriate colour (which usually have some mauve text in the vicinity) and for trivial corrections, clarifications, and textual reorganisations. We will of course remove this mauve colouration upon acceptance.

      The referee says:
      [S]ince Norsen does not use the word “flawed” in his
      paper, it seems a little strong to say he has admitted Bell’s EPR
      paragraph is flawed. How about something like this: “Some of the flaws
      in Bell’s argument in that paragraph, from ‘locality’ to determinism,
      are now acknowledged by Norsen. He therefore does not object to
      somebody like me withholding the word ‘theorem’ from that part of
      Bell’s work.”? Ultimately the point is best made by letting the Norsen
      quotes on page 4 speak for themselves.

      We have followed the referee’s advice closely.

      Similarly, I don’t see what in Norsen’s paper licenses saying “In
      Norsen’s view [Bell’s 1981 commentary] is proof enough that this is a
      misunderstanding.” Norsen does dwell on Bell’s 1981 commentary, and
      calls it an “important piece of evidence”, so why not just say that?

      Again, we have followed the referee’s advice closely.

      I would like to conclude with a remark on section 4. I have made it
      clear in my report on Norsen’s submission that I find some of the “Are
      there really two different Bell’s theorems?” quotations on pages 8 and
      9 of the submission to be potentially misleading and hence
      unacceptable for publication. Hopefully Norsen will rectify these
      issues, and the present manuscript can then be updated in light of

      We are pleased that Norsen has removed one unreasonable claim (that I accused Einstein of having made a huge blunder), enabling us to remove one of the (originally) five points of Section 4. However, another unreasonable accusation he made has merely been reworded as a rhetorical question and a statement of suspicion. We felt this still required a response, so have rewritten, rather than removed, the first point of Section 4. In response to new statements and a new, lengthy, footnote by Norsen on the subject of what is ‘plain’ in Einstein’s 1949 work, we have added a sentence.

      I think readers will appreciate it if we are able to avoid a
      discussion on what Bell and Einstein said degenerating into a
      discussion on what Wiseman said about what Bell and Einstein said!
      We agree, but we have to comment on what Norsen said Wiseman said Einstein said when he (Norsen) misrepresents us. In particular we were disappointed that Norsen did not remove his gross misquotation (on p. 9) of a passage on p.11 of my 2014 paper. Until he does so, we will need to address that issue as we do, albeit briefly, in the Second point of section 4.

    • Report of Referee A:

      My overall impression of the manuscript I gave in my previous report still holds for this revised version. The authors have revised it carefully in response both to my comments and the changes Norsen has made to his paper. The new discussion of Bell’s 1971 paper is also a useful addition. Although my aspiration for the debate not to be about what Wiseman said in 2014 has not been fully realised, I think we are probably as close as is reasonably possible, and so I support publication.

  • Likewise (regarding posting early). Thanks for the stimulating piece Harvey and Chris – some interesting history and perspectives.

    I have a shorter question/comment than Travis: Why go so far as Everett’s theory as an example violating local causality (LC) but respecting locality/no-action-at-a-distance? Why not simply consider operational q…[Read more]

  • Michael J. W. Hall, Dirk-André Deckert, and Howard M. Wiseman,
    Quantum phenomena modelled by interactions between many classical worlds. Phys. Rev. X 4 041013 [17 pages] (2014).

    At the request of Dr Shan Gao, I’m posting here a short discussion of this new paper.

    This work is motivated by the quantum measurement problem, which suggests that orthodox quantum mechanics does not provide a realistic model of the universe. We postulate an ontology which is radically different from any other interpretation. In particular there is no wave-function, only a vast collection of essentially classical worlds, all real, which interact. Our world is but one of them. We call our theory an approach to, rather than an interpretation of, quantum mechanics, because it has the potential to lead to new predictions. It also has potential as a numerical tool.

    For commentaries, see:

    1. Bill Poirier, The Many Interacting Worlds Approach to Quantum Mechanics
    Phys. Rev. X 4 040002 (2014) [Commentary as part of Editorial]

    2. Alexandra Witze, A quantum world arising from many ordinary ones? Nature News (24 October, 2014)

    3. me, “When parallel worlds collide … quantum mechanics is born“, The Conversation (24 October, 2014)

    Our many interacting worlds (MIW) approach is most closely related to some earlier approaches using the hydrodynamic formulation of QM, which made a tentative connection between streamlines and worlds (see the commentary by Bill Poirier for references.) We postulate discrete interacting worlds, and are not tentative in our interpretation. The paragraphs below are based on those on page 2 of the published paper.

    At the current stage, the MIW approach is not yet well enough developed to be considered on equal grounds with other long-established realistic approaches to quantum mechanics such as the de~Broglie-Bohm (dBB) and many-worlds (MW) interpretations. We have outlined the theory only for scalar non-relativistic particles, and for fields. We have done explicit calculations reproducing quantum statistics only for a single particle. Nevertheless it is worth comparing its ontology with those of these better known approaches.

    In our MIW approach there is no wave-function, only a very large number (not a continuum!) of classical-like worlds with definite configurations that evolves deterministically. Probabilities arise only because observers are ignorant of which world they actually occupy, and so assign an equal weighting to all worlds compatible with the macroscopic state of affairs they perceive. In a typical quantum experiment, where the outcome is indeterminate in orthodox quantum mechanics, the final configurations of the worlds in the MIW approach can be grouped into different classes based on macroscopic properties corresponding to the different possible outcomes. The orthodox quantum probabilities will then be approximately proportional to the number of worlds in each class.

    In contrast, the dBB interpretation postulates a single classical-like world, deterministically guided by a physical universal wave function. This world — a single point of configuration space — does not exert any back reaction on the guiding wave, which has no source but which occupies the entire configuration space. This makes it challenging to give an ontology for the wave function in parts of configuration space so remote from the `real’ configuration that it will never affect its trajectory. Furthermore, this wave function also determines a probability density for the initial world configuration. From a Bayesian perspective on probability this dual role is not easy to reconcile.

    In the Everett or MW interpretation, the `worlds’ are orthogonal components of a universal wave function . The particular decomposition at any time, and the identity of worlds through time is argued to be defined (at least well-enough for practical purposes) by the quantum dynamics which generates essentially independent evolution of these quasiclassical worlds into the future (a phenomenon called effective decoherence). The inherent fuzziness of Everettian worlds is in contrast to the corresponding concepts in the MIW approach, of a well-defined group of deterministically-evolving configurations. In the MW interpretation it is meaningless to ask exactly how many worlds there are at a given time, or exactly when a branching event into subcomponents occurs, leading to criticisms that there is no precise ontology. Another difficult issue is that worlds are not equally `real’ in the MW interpretation, but are `weighted’ by the modulus squared of the corresponding superposition coefficients. As noted above, in the MIW approach all worlds are equally weighted, so that Laplace’s theory of probability is sufficient to account for our experience and expectations.


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