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Sunday, October 30, 2022

What was the recent Nobel Prize in Physics really about?(Guest Post)

 David Marcus was a Math major a year ahead of me at SUNY Stony brook (he graduated in 1979,

I graduated in 1980). He then got a PhD from MIT in Math, and is a reader of this blog.  Recently he emailed me that he thinks the current Nobel Prize Winners in Physics do not understand their own work. Is it true? Let's find out!

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(Guest blog from David Marcus)

2022 Nobel Prize in Physics Awarded for Experiments that Demonstrate Nonlocality

The 2022 Nobel Prize in Physics was recently awarded to experimenters who demonstrated that the world is nonlocal. The curious thing is that neither the writers of the Nobel Prize press release nor the recipients seem to understand that this is what they demonstrated.


For example, the press release (see here) says: "John Clauser developed John Bell's ideas, leading to a practical experiment. When he took the measurements, they supported quantum mechanics by clearly violating a Bell inequality. This means that quantum mechanics cannot be replaced by a theory that uses hidden variables." That is not what the experiments mean, and the statement is false.

The word "locality" means that doing something here cannot instantly change something other there.

The experimental setup is the following: You prepare two particles, A and B, and send them in opposite directions so that they are far apart. You and your colleague do experiments on each particle at the same time. If you and your colleague perform the same experiment, then, from your experiment on A, you can predict with certainty the result of your colleague's experiment on B (and vice versa).

In a paper in 1935, Einstein, Podolsky, and Rosen pointed out that, assuming locality, the experimental results at A and B must be determined by the source that prepared the particles. They didn't actually say, "assuming locality", but they implicitly assumed it. (If you disagree with them, please offer an alternative.)

In 1964, John Bell published his paper. In it, he considered three of the experiments that could be done on the particles A and B. Assuming the results are determined by the source (which follows from Einstein, Podolsky, and Rosen's argument), he derived an inequality on the correlations between the results of the three experiments on the two particles. The math is simple; for details, see here.

The Nobel Prize winners did experiments, and their results violated Bell's inequality (or similar inequalities). Hence, the world is nonlocal.

The simplest theory that agrees with experiment is Bohmian Mechanics. This is a deterministic theory of particles whose motion is governed by a wave (the wave function being the solution of the Schrödinger equation). Of course, Bohmian Mechanics is nonlocal, as is the world.

62 comments:

  1. One false statement from one winner does not equate to the title of this article.

    ReplyDelete
  2. "...the world is nonlocal. The curious thing is that neither the writers of the Nobel Prize press release nor the recipients seem to understand that this is what they demonstrated."

    I smell an axe being ground. Everyone who has read much of any of the popular lit on QM understands that it's the nonlocality that's the problem. Maybe we understand it incorrectly and/or incompletely, but we get it that what Bob does affects what Alice sees, and hidden variables can't explain it.

    So the idea that the Nobel committee and the recipients don't understand it is problematic.

    ReplyDelete
    Replies
    1. > hidden variables can't explain it.

      The standard meaning of "hidden variables" is anything in addition to the wave function or quantum state. With this meaning, hidden variables can explain it. The "hidden variables" are the positions of the particles.

      Delete
    2. For some strange reason, many physicists try to avoid saying "QM is nonlocal". See for example
      https://www.youtube.com/watch?v=gNAw-xXCcM8 (Murray Gell-Mann - The experimental confirmation of quantum mechanics (165/200)):
      "John Bell, ... introduced words that are sort of prejudicial like 'non-local', ..., what they mean is that a classical interpretation of what's happening would have to be either non-local or involved negative probabilities or both. That's not the same as saying that it actually is non-local, ... it’s a matter of giving a dog a bad name and hanging him, as far as I can tell."

      Even Bell himself was careful not to claim that "QM is provably nonlocal":
      https://youtu.be/TUyp2uC6VJc?t=1616 (Adam Becker - " 'Not Merely False, but Foolish': The History of Bell’s Two Theorems"):
      "I don't know any conception of locality which works with quantum mechanics. So I think we're stuck with nonlocality." (Bell, 1990.)

      It can be really surprising when you find-out after a complicated dance around the word "nonlocal" that some physicists actually understands the situation in the same way that you do, but somehow prefers to use "less concrete" words for it:
      https://www.physicsforums.com/threads/nature-physics-on-quantum-foundations.1045477/post-6812667

      Delete
    3. Gell Man and many others were quite confused ab Bell; but Bell himself was explicit ab nonlocality - even in what you quote. It is discussed in my book making sense of QM

      Delete
    4. I highly recommend the books "Making Sense of Quantum Mechanics" and "Quantum Sense and Nonsense", both by Jean Bricmont.

      Delete
    5. Doesn't "So I think we're stuck with nonlocality" clearly say that Quantum Mechanics is nonlocal? If you read Bell's writings, he couldn't be clearer about this.

      The derivation of Bell's Inequality from locality is not difficult to follow. The experiments of the Nobel Prize winners have confirmed that Bell's Inequality is violated. Hence we are stuck with nonlocality.

      Delete
    6. In John Bell's paper "Bertlmann's socks and the nature of reality", Bell gives the example of Dr. Bertlmann who likes to wear two socks of different colors. Seeing one sock, we know that the second sock is a different color. This example illustrates the reasoning of Einstein, Podolsky, and Rosen. That is, the ability to predict something (the outcome at B, the color of the other sock) implies (assuming locality) that the something must be determined by the source (the preparation of the particles, Dr. Bertlmann putting on his socks).

      After reading Bell's paper, I suggest that you find a copy of the book "The Quark and the Jaguar" by Murray Gell-Mann. On page 173, regarding Dr. Bertlmann, it says, "... Yet no signal is propagated from one foot to the other. Likewise no signal passes from one photon to the other in the experiment that confirms quantum mechanics. No action at a distance takes place. (The label 'nonlocal' applied by some physicists to quantum-mechanical phenomena like the EPRB effect is thus an abuse of language. What they mean is that if _interpreted classically in terms of hidden variables_, the result would indicate nonlocality, but of course such a classical interpretation is wrong.)"

      Delete
    7. David Markus wrote: "Hence we are stuck with nonlocality."

      OK, now you make me think I misunderstood you. This sounds like what I take the current state of things to be. (Sorry to have been grumpy. And thanks for the book recommendations.)

      Delete
    8. > David Markus wrote: "Hence we are stuck with nonlocality."

      But those are Bell's words, without a clear indication that David Markus agrees that Bell is right in being careful about not claiming too much.

      > OK, now you make me think I misunderstood you. This sounds like what I take the current state of things to be.

      I am not sure whether you really misunderstood him.

      David Markus said:
      > Doesn't "... stuck with nonlocality" clearly say that Quantum Mechanics is nonlocal? If you read Bell's writings, he couldn't be clearer about this.

      This sounds to me like accusing Bell of being overly cautious in the way he choses his words.

      My own point was less that "QM is nonlocal" would be true, but that "nonlocal randomness" is "unproblematic", and therefore all this fighting over the word "nonlocal" is unnecessary.

      gentzen said:
      > But in the simplest form of the notion, one could imagine it indeed as if the random outcomes only got determined at the moment where they got recorded. The notion is unproblematic even in this "simple but unrealistic" form.

      vanhees71 said:
      > That's my point! It's "unproblematic", but particularly this is what's denied by all those who think that there's a "measurement problem", because it's very hard to accept that ...

      Delete
    9. > without a clear indication that David Marcus agrees that Bell is right in
      > being careful about not claiming too much.

      Bell said that violation of a Bell inequality implies that the world is nonlocal. Bell was correct.

      The "measurement problem" is that people say the cat is both alive and dead. Bohmian Mechanics solves this problem.

      Delete
    10. > Bell said that violation of a Bell inequality implies that the world is nonlocal. Bell was correct.

      So you definitively don't agree with Bell to be careful what you claim. I doubt that DJL shares your assessment.

      Bell knew that he could not proof that "QM is nonlocal" beyond doubt. Bell openly admitted that he didn't get Bohr's objection (ot EPRB), and he did anticipate the objection currently defended by people like Tim Palmer, Sabine Hossenfelder, or Gerard 't Hooft.

      Delete
    11. > So you definitively don't agree with Bell to be careful what you claim.

      Bell was perfectly clear that his inequality (and similar inequalities) follow from locality.

      In "Bertlmann's socks and the nature of reality" (Chapter 16 in the book "Speakable and Unspeakable in Quantum Mechanics", 2nd edition, page 143), Bell wrote, "It is important to note that to the limited degree to which _determinism_ plays a role in the EPR argument, it is not assumed but _inferred_. What is held sacred is the principle of 'local causality' - or 'no action at a distance'" ... "It is remarkably difficult to get this point across, that determinism is not a _presupposition_ of the analysis. There is a widespread and erroneous conviction that for Einstein^10 determinism was always _the_ sacred principle.The quotability of his famous 'God does not play dice' has not helped in this respect. Among those who had great difficulty in seeing Einstein's position was Born."

      Footnote 10 says, "And his followers. My own first paper on this subject (Physics 1, 195 (1965).) starts with a summary of the EPR argument _from locality to_ deterministic hidden variables. But the commentators have almost universally reported that it begins with deterministic hidden variables."

      On page 144, Bell writes, "These references to Born are not meant to diminish one of the towering figures of modern physics. They are meant to illustrate the difficulty of putting aside preconceptions and listening to what is actually being said. They are meant to encourage _you_, dear listener, to listen a little harder."

      In the essay "Speakable and Unspeakable in Quantum Mechanics" (Chapter 18 in the book "Speakable and Unspeakable in Quantum Mechanics", 2nd edition, page 171), Bell wrote, "[The de Broglie-Bohm] picture, and indeed, I think, any sharp formulation of quantum mechanics, has a very surprising feature: the consequences of events at one place propagate to other places faster than light. This happens in a way that we cannot use for signalling. Nevertheless it is a gross violation of relativistic causality. Moreover the specific quantum phenomena that require such superluminal explanation have been largely realized in the laboratory...especially by Aspect, Dalibard, and Roger, in Paris in 1982 (Phys. Rev. Lett. 49, 1804 (1982))."

      So, Bell clearly stated that Quantum Mechanics is nonlocal. And, he was aware of the status of the experimental confirmations of nonlocality. (He also comments on the experiments in some of the other essays in the book.)

      Delete
    12. > Bell openly admitted that he didn't get Bohr's objection (to EPRB)

      You are referring to what Bell wrote in Appendix 1, "The position of Bohr", in "Bertlmann's socks and the nature of reality": "While imagining that I understand the position of Einstein, as regards the EPR correlations, I have very little understanding of the position of his principal opponent, Bohr." Bell goes on to quote Bohr and try to make sense of what Bohr wrote. Unfortunately, what Bohr wrote makes no sense, so that is why Bell could not understand it.

      For more on Bohr's writings, see "The Sokal Hoax: At Whom Are We Laughing?" by Mara Beller, Physics Today, September 1998, https://bohmian-mechanics.net/sokalhoax.html .

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    13. > Sabine Hossenfelder, or Gerard 't Hooft.

      Sabine Hossenfelder and Gerard 't Hooft defend what is called "superdeterminism": It is a local hidden variables theory, if you wish, but the values of the hidden variables depend on which experiments will be done on the particles in the future. So, particles don't have to have fixed values of hidden variables for all possible directions of spin measurements but only for the direction which will be actually used in the experiment.

      But what if we switch the direction in which the spin will be measured while the particles are in flight (which is what Aspect does)? Ah, they will say, that's impossible because of a gigantic conspiracy going back to the initial conditions of the universe that implies that those particles will have values of their hidden variables adjusted to the switched direction.

      They disguise this by saying that they are just not assuming "statistical independence". Sabine Hossenfelder wrote that "statistical independence" is "an obscure assumption" (http://backreaction.blogspot.com/2021/12/does-superdeterminism-save-quantum.html). Let's see how obscure it is:

      If you test a drug, you assume that the people to whom you give the active substance and those to whom you give the placebo are independent. But, what if those to whom you give the placebo would be sick anyway and those to whom you give the active substance would do well anyway? So, you can deny the efficacy of any test by making such assumptions.

      Or take thousands of students and ask each a question taken at random from a set of questions. Suppose they all answer correctly. You might think that they know the answers to all questions, right? But maybe not. Maybe it just so happens that each student knows the answer to just one question, the one that they are asked.

      Delete
    14. The question is less "what is true", but whether DHL would really agree to all your "statements of definite facts". At least to me, your behavior "still" feels like axe grinding:

      DHL said:
      > I smell an axe being ground.
      > ...
      > So the idea that the Nobel committee and the recipients don't understand it is problematic.

      > OK, you know more about physics than the bloke who wrote the intro to Bell's collected papers. Great. I still smell an axe grinding.

      David Marcus said:
      > Sabine Hossenfelder and Gerard 't Hooft defend what is called "superdeterminism": It is a local hidden variables theory, if you wish, but ...

      Frankly, I don't appreciate that you excluded Tim Palmer from your answer. Excluding it from the quote could be excused by the "arbitrary" line break, but excluding him from the answer inicates that this was a conscious decision. I guess the reason to exclude him was that he is "less (in)famous" than the other two.

      In order to argue that Bell knew objections, I was forced to quote the objections known to him, independent of whether I am fond to discuss them or not. So even if Asher Peres' "Unperformed experiments have no results" would be an objection (not sure, didn't think about it in detail), I could only quote it (to support my claim that Bell knew objections), if I would find a place where Bell is discussing (or at least mentioning) that objection.

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    15. > In order to argue that Bell knew objections

      I'm sorry, but I'm not sure what your point is. Are we discussing what Bell knew or how the world works? Wikipedia says that Gerard 't Hooft discussed superdeterminism with Bell. That's fine, but how is it relevant?

      I wasn't familiar with Tim Palmer. I see now that he has an article with Sabine Hossenfelder promoting superdeterminism:

      https://www.frontiersin.org/articles/10.3389/fphy.2020.00139/full

      So, my comments about superdeterminism apply to Tim Palmer too.

      Superdeterminism attempts to save locality by allowing conspiracies. Besides the fact that the proponents of superdeterminism only have toy models, the cure is worse than the disease.

      For more on this, see the discussion of the "no conspiracy" condition in

      http://www.scholarpedia.org/article/Bell%27s_theorem#The_CHSH.E2.80.93Bell_inequality:_Bell.27s_theorem_without_perfect_correlations

      Here is Bell on this topic ("Bertlmann's socks and the nature of reality", Chapter 16 in the book "Speakable and Unspeakable in Quantum Mechanics", 2nd edition, page 154):

      "But this way of arranging quantum mechanical correlations would be even more mind boggling than one in which causal chains go faster than light. Apparently separate parts of the world would be deeply and conspiratorially entangled, and our apparent free will would be entangled with them."

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    16. > > In order to argue that Bell knew objections
      >
      > I'm sorry, but I'm not sure what your point is. Are we discussing what Bell knew or how the world works?

      Why should we discuss "how the world works"? Your post made statements like "...the world is nonlocal. The curious thing is that neither the writers of the Nobel Prize press release nor the recipients seem to understand that this is what they demonstrated." Then I contributed the observation "For some strange reason, many physicists try to avoid saying 'QM is nonlocal'." and especially the quote:
      "I don't know any conception of locality which works with quantum mechanics. So I think we're stuck with nonlocality." (Bell, 1990.)

      I interpret this quote in the way that Bell was not convinced to have proven "the world is nonlocal" beyond reasonable doubt.

      I warned DJL that you probably interpret this quote as if Bell would agree with you. And when you "doubled down" on your opinion (that of course Bell agrees with you), I tried to give examples of objections "known to"/discussed by Bell.

      I did read the book "Speakable and Unspeakable in Quantum Mechanics" a long time ago, it was a joy to read, but I no longer remember most details. I now watched
      https://www.youtube.com/watch?v=TvDCMS9bk74 (Prof. Harvey Brown: The evolution of Bell's thinking about the Bell theorem)
      in the hope to learn "concrete" reasons why Bell did not want to claim that "the world is nonlocal" has been proven beyond doubt.

      That certainly was interesting, and showed that Bell was still fighting with that stuff. Turns out "nonlocal randomness" was problematic for him after all. Either you had anthropocentric notions like no-signaling, or a prefered reference frame like in Bohmian mechanics, or ... worse (like MWI, super..., ...)

      So maybe there is a reason after all why physicists avoid the word "nonlocal" to the point where it get comical.


      Regarding "axe grinding": My own axe to grind would be that Nobel prices should be awarded before the recipients are retired or have died (like Bell in this case). I now read in Jean Brickmont's book "Making Sense of Quantum Mechanics" that David Bohm "was elected Fellow of the Royal Society only in 1990, at the age of 72, and died two years later". I immediately wondered whether he was elected before or after Bell's death. Bell died 1 October 1990... Anyway, a wise decision by the Royal Society.

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    17. > Why should we discuss "how the world works"?

      Because by now there have been enough experiments that confirm that the inequalities are violated. And because the Nobel Prize was given to three of these experimenters.

      > "I don't know any conception of locality which works with quantum
      > mechanics. So I think we're stuck with nonlocality." (Bell, 1990.)
      > I interpret this quote in the way that Bell was not convinced to have
      > proven "the world is nonlocal" beyond reasonable doubt.

      There are two separate issues: what the theories say and what the experiments have confirmed.

      > I did read the book "Speakable and Unspeakable in Quantum Mechanics" a
      > long time ago, it was a joy to read, but I no longer remember most
      > details.

      If you reread Bell's book, you will see that he was clear that quantum mechanics, the theory, is nonlocal. Perhaps he was too polite for your taste in the way that he phrased this.

      > I now watched https://www.youtube.com/watch?v=TvDCMS9bk74 (Prof. Harvey
      > Brown: The evolution of Bell's thinking about the Bell theorem)

      In the video, Harvey Brown says that the assumption for the inequality is "local hidden variable" or "deterministic hidden variable". Bell could have been talking about Harvey Brown in footnote 10 of "Bertlmann's socks and the nature of reality": "... My own first paper on this subject (Physics 1, 195 (1965).) starts with a summary of the EPR argument _from locality to_ deterministic hidden variables. But the commentators have almost universally reported that it begins with deterministic hidden variables."

      Harvey Brown also says (at the very end) that Many Worlds is a quantum mechanics theory that is local. I don't understand his explanation of why he thinks this, but it doesn't really matter since Many Worlds has so many defects as a physical theory.

      I recommend reading Bell's book rather than watching this video. You should also read Jean Bricmont's books.

      Harvey Brown also discusses reconciling Lorentz invariance with nonlocality. On this topic, see "Can Bohmian mechanics be made relativistic?", D. Dürr, S. Goldstein, T. Norsen, W. Struyve, and N. Zanghì, Proceedings of the Royal Society A 470, doi:10.1098/rspa.2013.0699(2013), http://arxiv.org/pdf/1307.1714

      Delete
    18. > In the video, Harvey Brown says that the assumption for the inequality is "local hidden variable" or "deterministic hidden variable". Bell could have been talking about Harvey Brown in footnote 10 ...

      Well, in the paper linked below the video (https://arxiv.org/abs/1501.03521), at the bottom of page 4, Harvey Brown said:
      "Bell started his 1964 paper by recalling that the 1935 Einstein-Podolsky-Rosen (EPR) argument demonstrates, using the strict, or perfect correlations involved in entangled systems, that if action-at-a-distance is to be avoided, the standard quantum mechanical state could not be a complete description of the systems in question. In particular Bell seems to have accepted that the perfect anticorrelations for parallel spin components in the singlet state of spin-1/2 systems imply that a deterministic underpinning (‘causality’) is necessary if locality in this sense is to prevail, and that this is the reason why a local deterministic hidden variable theory is the focus of his 1964 paper."

      For me, this proves beyond reasonable doubt that you are accusing Harvey Brown of commiting a mistake that he did not commit!

      > I recommend reading Bell's book rather than watching this video.

      Well, I already watched that video, so it is simply impossible for me to follow your recommendation. I have no intention of rereading the entire book anyway (which would take me much longer than the 30 min it took me to watch that video at 2x speed). But I will reread "Bertlmann's socks and the nature of reality". However, we both expect that this will probably not change my interpretation of Bell's words:

      > If you reread Bell's book, you will see that he was clear that quantum mechanics, the theory, is nonlocal. Perhaps he was too polite for your taste in the way that he phrased this.

      If Bell really wrote words that he didn't mean, then he would indeed be "too polite". My interpretation is that Bell is polite enough to write exactly what he means, in a language that is easy to understand.

      > You should also read Jean Bricmont's books.

      So far, the parts of his books that I read left a very positive impression on me. However, I didn't read what he says about the de Broglie-Bohm theory yet. I will try to postpone this as long as possible. No need to spoil that positive impression too early.

      > > Why should we discuss "how the world works"?
      >
      > Because by now there have been enough experiments that confirm that the inequalities are violated. ...

      I meant, I don't get why the two of us should discuss it in this comment thread. Of course, it could be fun to discuss such topics, in "some appropriate setting". For me personally for example, PhysicsForums provides such an "appropriate setting".

      Delete
    19. > So even if Asher Peres' "Unperformed experiments have no results" would
      > be an objection (not sure, didn't think about it in detail),

      See "Quantum information and general relativity" by Asher Peres, https://arxiv.org/abs/quant-ph/0405127 . Asher Peres is mixing up what each experimenter knows at different times with what the world is doing.

      For example, Peres writes, "When Alice measures her spin, the information she gets is localized at her position, and will remain so until she decides to broadcast it. Absolutely _nothing_ happens at Bob’s location."

      But, Alice can predict the result of Bob's experiment. Either the results of the experiments were determined at the source or something did happen at Bob's location.

      Delete
    20. > Well, in the paper linked below the video
      > (https://arxiv.org/abs/1501.03521), at the bottom of page 4, Harvey Brown
      > said:

      Footnote 8 is a good example of the tortured logic in this paper: "See Maudlin (2010) for making the case that Bell viewed determinism as a strict consequence of the EPR argument based on locality (no action-at-a-distance). Certainly after 1964, Bell was explicit: ‘It is important to note that to the limited degree to which _determinism_ played a role in the EPR argument, it is not assumed but _inferred_.’ Bell (1981, p.143), original emphasis. However, disagreement has arisen in the literature as to the precise logic of Bell’s 1964 theorem, with particular reference to the role of determinism. Some commentators see determinism as one of the assumptions of the theorem (view 1), while others see it as a consequence of the assumptions, which include the existence of perfect (anti)correlations. Clearly, if the former position is correct, then the empirical violation of the Bell inequality implies _either_ indeterminism or nonlocality (in the 1964 sense of the term). A recent careful textual analysis of Bell’s writings in the context of this debate is due to Wiseman (2014), who provides grounds for thinking that in later life Bell’s own reading of his 1964 logic—in line with view 2—is questionable. Note that nothing in our paper hangs on this debate."

      So, Harvey Brown knows that Maudlin already pointed out that he's wrong, but he's sticking to his guns.

      I suggest everyone just read Bell's book rather than trusting Harvey Brown to read it for you. I've read Bell's book, and I couldn't recognize what Bell wrote in the Harvey Brown video. And the comment about Many Worlds at the end of the video capped it off.

      Delete
    21. I just noticed that Section 9 of Harvey Brown's paper (https://arxiv.org/abs/1501.03521) explains why he thinks Many Worlds is local. Brown notes that Maudlin explained why he is wrong (Tim Maudlin. 2010. "What Bell Proved: A reply to Blaylock". American Journal of Physics, 78(1), 121-125.) But, Brown still thinks that he is right.

      It appears that Brown is denying that Alice and Bob really did experiments and got results when they were far apart (page 22):

      "Given the initial entangled singlet state, we can make formal statements about what the probability distribution over joint measurement outcomes for spacelike separated measurements is. In general these joint-probabilities will be non-trivial and in fact Bell-inequality violating. But physically, there is nothing for such joint probabilities to be joint probabilities of until one reaches the overlap region of the future light-cones of the measurement events, since it is only in this overlap region that measurement outcomes for the individual measurements on either side can become definite with respect to one another, in general. Before then, the joint probabilities are only formal statements, regarding what one would expect to see, were one to compare the results of measurements on the two sides."

      Delete
    22. > Footnote 8 is a good example of the tortured logic in this paper: "See Maudlin (2010) for making the case that Bell viewed determinism as a strict consequence of the EPR argument based on locality (no action-at-a-distance).

      > So, Harvey Brown knows that Maudlin already pointed out that he's wrong, but he's sticking to his guns.

      > I suggest everyone just read Bell's book rather than trusting Harvey Brown to read it for you. I've read Bell's book, and I couldn't recognize what Bell wrote in the Harvey Brown video.

      I have now started "to look into" Bell's again. I still don't see the huge contradictions with the video that you notice, but maybe this is not important, see next point.

      > And the comment about Many Worlds at the end of the video capped it off.

      I initially brushed off those remarks about Many Worlds as irrelevant. That was a mistake, they should have triggered me to check whether Harvey R Brown is a well known MWI proponent, and if yes which "school". "Faculty of Philosophy and Wolfson College, Oxford" indicates that he indeed might be a well known MWI proponent of "Simon Saunders' decoherence/emergence" school. Indeed, he even cowrote a paper with David Wallace. And he seems to have a track record of not getting even Tim Maudlin's clearest points (like "Note that Brown and Wallace see fit to avail themselves of the terminology ‘configuration space’ while simultaneously denying the very existence of the particles that might be in any configuration and, presumably, the low-dimensional space that the multiplicity of particles might commonly inhabit.").

      Therefore, I should have interpreted Brown's reference to Tim Maudlin as a way for Brown to express his disagreement. Instead I interpreted it as an acknowledgment of Maudlin's point. Sorry. So I was definitively wrong when I wrote:

      > For me, this proves beyond reasonable doubt that you are accusing Harvey Brown of commiting a mistake that he did not commit!

      I hope you understand that I don't want to publicly embarrass myself even more. So I will stop this "public discussion" here, especially since also don't want to be forced to publicly make statemtents about people like Harvey Brown, even more since I don't really know them. We can continue such discussion in a private setting, if you like.

      Delete
    23. > I have now started "to look into" Bell's again. I still don't see the
      > huge contradictions with the video that you notice

      Brown says that Bell's hypothesis for the inequality in Bell's 1964 paper is local/deterministic hidden variables. This is wrong. Bell is perfectly clear that the only hypothesis is locality. From then on, the logic in the video is all confused. If you want to discuss the implication of the inequality being violated, you need to get the hypothesis correct.

      I summarized Bell's logic in my original blog post.

      Delete
  3. OK, you know more about physics than the bloke who wrote the intro to Bell's collected papers. Great. I still smell an axe grinding.

    ReplyDelete
    Replies
    1. Bell's book, "Speakable and Unspeakable in Quantum Mechanics", is must reading. Bell is perfectly clear that the only hypothesis needed for his inequality is locality. The second edition of the book has an introduction by Alain Aspect. It is not clear what Aspect thinks. I won't quote anything from the introduction because I don't want to take it out of context. But, it appears that Aspect is saying that the required hypothesis is local hidden variables. The date on the introduction is 2003. John Bell died in 1990.

      Delete
  4. the winners may not say things as false as the committee; they may say that there are no local HV, which is true but understates the significance of their result, as DM shows

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  5. I've never found these experimental results to be very interesting, as they completely align with predicted correlations. The interpretation of the results, however... that's the juiciest stuff in the world. I still favor Einstein's view that Bob's measurement does not impact Alice's measurement. The Elitzur–Vaidman bomb-tester thought experiment, to me, is more intriguing, stretching beyond our intuition of what locality does or does not imply. I would love to know what Bohr and Einstein would have written about these two experiments.

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    Replies
    1. If Bob's measurement does not affect Alice's measurement, how do you explain that they always get opposite results if they do the same experiment?

      Delete
    2. Um, different anonymous here...

      Getting opposite results isn't the problem. After all that is what you would expect classically. It is required by the need to conserve spin. The problem is with how often they get the same results when they do slightly different experiments.

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    3. > Getting opposite results isn't the problem. After all that is what you
      > would expect classically.

      I'm sorry, but I don't know what you mean. Are you saying that the values are determined when the particles are prepared by the source?

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    4. >I'm sorry, but I don't know what you mean. Are you saying >that the values are determined when the particles are >prepared by the source?

      No, that is what is happening classically. If I have a left shoe and a right shoe and I send one off without looking at it I can know which shoe it was by looking at the one I have. The shoes are (anti)correlated. this was set when the state was created.

      Einstein used this to argue that the state of particle pairs must be set at creation with hidden variables. This was the EPR paradox.

      Bell proved that this could not work by considering what happens when you do an off axis measurement of the particle/cat/shoe. This off axis measurement has no meaning classically.

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    5. So, Bob's measurement does affect Alice's measurement.

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    6. Depends on what you mean by "affect". There is no causal connection and you cannot steer the distant particle into a particular state so there is no way to send information faster than light. There is a correlation that offends our classical instincts. You want to call that nonlocal? Go ahead but that's just linguistics. Causality remains local.

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    7. If Bob does his experiment, then this determines Alice's result. So, Bob's experiment affects Alice's experiment. And, the effect happens far away in less time than it would take light to travel. It is true that we can't use this to send a message. But, it is still a nonlocal causal effect. "Causality" is not the same thing as messaging. If you change the meaning of the words "causal" and "nonlocal", then it will be hard to communicate.

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    8. In physics messaging pretty much is causality. If I send you a message telling you to do something then I have a causal effect on you. If I can send a message back in time then we have cause before effect. All it would take to stop the Kennedy assassination is an FTL message. That is causality.

      Pilot waves travel FTL meaning they can send information back in time. The only way to protect causality is to make them impossible to observe even in principle. But if you make the mechanism invisible there is no way to distinguish it from other models like The transactional interpretation or even invisible blue faeries. You can make up anything you want.

      If Bob does his experiment it need have no effect on Alice until information about Alice's results arrive. Then it is a local thing.

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    9. If the world is nonlocal, then different observers will disagree which of two events is the cause and which is the effect. This is certainly odd. But, all versions of Quantum Mechanics predict this nonlocality. And, the experiments of the Nobel Prize winners confirm it.

      > Pilot waves travel FTL

      If by "Pilot waves" you mean the solutions to the Schrödinger equation, then yes, the wave is a function of the instantaneous position of all the particles. But, the Schrödinger equation is in all theories of quantum mechanics. So, this is not a feature only of Bohmian Mechanics.

      Bohmian Mechanics only has a little nonlocality in it. It has just enough to agree with experiments.

      > If Bob does his experiment it need have no effect on Alice until
      > information about Alice's results arrive. Then it is a local thing.

      You appear to be saying the following: The Nobel Prize winners are just imagining that their experiments violated Bell's inequality. The records of their experimental results changed when they brought the records from A and B together to one place.

      Perhaps the world is this malicious, but there is no reason to suppose that it is.

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    10. As Tim Maudlin has observed, the Big Bang and earthquakes have effects but cannot be used to send messages. To send messages, you need to control the source, and this cannot be done in Quantum Mechanics.

      Quantum teleportation is based on the fact that one does send information.

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  6. Wow - Bill and Lance, did you really invite someone onto your blog to "explain" this result, and along the way baldly assert that pilot-wave theory is the one true QM interpretation? ("The simplest theory that agrees with experiment is Bohmian Mechanics").

    Many Everettians would disagree with both this choice of interpretation and the assertion that it's the simplest theory that agrees with experiment.

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    1. You can say why some Everttians disagree, or why you disagree, to further the conversation.

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    2. Bohmian Mechanics is simple: There are particles, and the particles move ("mechanics"). There are two equations: The Schrödinger equation and an equation that gives the velocities of the particles. That is the whole theory. In particular, there are no measurement postulates, no collapse rules, no observers, no separation of the world into quantum and classical, and no cats that are both alive and dead.

      The Everett Many-Worlds theory is not adequate: It does not give the correct probabilities for results of experiments (i.e., the Born rule). It does not have a clear ontology, i.e., where do the tables, chairs, and cats come from? A wave function defined on a high-dimensional space is not a cat.

      The multiplication of "worlds" in the Everett Many-Worlds theory is weird. Bohmian Mechanics is not weird: "Matter moves" (to quote Detlef Dürr). You might think nonlocality is weird, but the world is nonlocal, so that is not a defect of the theory. We've had nonlocal theories before, e.g., Newtonian gravity.

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    3. "Bohmian Mechanics is simple: There are particles, and the particles move ("mechanics"). There are two equations: The Schrödinger equation and an equation that gives the velocities of the particles."

      The Many-Worlds interpretation is simple, and has one equation (Schrodinger's).

      "[Everett Many-Worlds] does not give the correct probabilities for results of experiments"

      Yes it does - the major interpretations all agree on the experimental results. This is why experiments haven't resolved the the disagreements between adherents of different interpretations.

      "The multiplication of 'worlds' in the Everett Many-Worlds is weird."

      There is no QM interpretation that does not contain weirdness (to our macroscopic intuitions). Multiplication of worlds is weird; non-locality is weird; non-determinism is weird; wave-function collapse is weird. Pick your poison of weirdness, and pick at least one.

      Many-worlds implies a very large multiverse, but it's the multiverse that's large, not the theory (which is just the Schrodinger equation), so Occam's Razor doesn't apply. It is similar to how a simple theory of cosmology leads us to believe in a spatial universe that's much larger than the observable universe. A smaller universe would require a more complicated theory.

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    4. > the major interpretations all agree on the experimental results.

      As I wrote, the Everett Many-Worlds theory does not give the Born rule for the probabilities of experimental results.

      > There is no QM interpretation that does not contain weirdness (to our
      > macroscopic intuitions). Multiplication of worlds is weird; non-locality
      > is weird; non-determinism is weird; wave-function collapse is weird. Pick
      > your poison of weirdness, and pick at least one.

      Some things are imposed on a theory by experiment. Nonlocality has been confirmed by experiment. Bohmian Mechanics has none of the other things that you listed.

      Where are the tables and chairs in the Everett Many-Worlds theory?

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    5. > The Many-Worlds interpretation is simple, and has one equation
      > (Schrodinger's).

      In addition to the Schrödinger equation, you need to provide rules for when the "worlds" split into more "worlds", and you need to explain where the tables and chairs come from. Things sometimes seem simple if you skip stuff.

      Many Worlds says that when there is a macroscopic superposition in the wave function, the superposed terms split into different "worlds". Many Worlds gets the probability of a result by considering what faction of the "worlds" have that result and appealing to the large of large numbers to argue that we are in one of those "worlds".

      If an experiment has two possible outcomes (say), then this procedure gives that each outcome will have a probability of 1/2. But the Born rule says that the probability of each result in an experiment is proportional to the square of the amplitude of the wave function, and these probabilities can be different from 1/2. For example, we can do an experiment where one result has a probability of 1/4 and the other 3/4.

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    6. Many Worlds and the Born Rule:
      https://arxiv.org/abs/1504.05259

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    7. "Where are the tables and chairs in Everett Many-Worlds theory?"

      I really am not sure what you mean by this, as tables and chairs are human labels applied to very macroscopic phenomena, and as such will not have complete consensus on the labels, and will also be nebulous at the borders (is a chair that's missing one of its legs still a chair? etc).

      Where are the the tables and chairs in the Standard Model?

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    8. > Many Worlds and the Born Rule: https://arxiv.org/abs/1504.05259

      This article is about decision theory, not physics. The Born rule tells us the probability of getting a particular experimental result. This article does not give a way of deriving the Born rule probabilities from the Many Worlds theory.

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    9. > Where are the the tables and chairs in the Standard Model?

      In Bohmian Mechanics (and Newtonian Mechanics), there are particles (electrons, protons, neutrons, atoms, molecules, etc.). The tables and chairs are made of these particles. Bohmian Mechanics includes the Schrödinger equation. The Schrödinger equation is a function of time and 3N dimensions. In Bohmian Mechanics, N is the number of particles. The masses of the particles {m_j} appear as coefficients in the equation.

      In Many Worlds, there is a wave function that is a solution of the Schrödinger equation. There are no particles. There is only the wave function. Since there are no particles, it is not clear what N is supposed to be, but presumably it is a large number. Similarly, since there are no particles, it is not clear what the {m_j} are supposed to be. At various times (not clearly specified), the wave function splits so that there is more than one wave function (more than one set of 3N dimensions).

      But, tables and chairs are made up of matter in 3-dimensional space. A function on 3N space (for a large N) is not a table or a chair. So, where are the tables and chairs in the Many Worlds theory?

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  7. I think most physicists say that QM is not nonlocal because there is no causal link from one particle to the other. All you have is a correlation that is impossible from a classical point of view. But quantum mechanics isn't classical. Get over it. It also allows things like alive/dead cats so spooky correlations shouldn't be that hard to swallow.

    The problem with Bohmian Mechanics is that it adds nothing to QM. You have all the framework of QM plus an extra appendage to carry your philosophical baggage for you. Ugly and pointless and without any way to test. It does not even do a good job of carrying philosophical baggage because nonlocality introduces things like backwards causality that are at least as hard to take as spooky correlations.

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    1. Doing the experiment at A, does cause the same experiment at B to have the opposite result, and vice versa.

      I'm not sure what a "classical point of view" is. The correlation is impossible assuming locality. That is how we know that the world is nonlocal.

      Schrödinger added his cat to the experiment because he thought it would be obvious that a cat can't be both alive and dead. He would be turning over in his grave if he knew that people think it makes sense for his cat to be both alive and dead.

      Bohmian Mechanics adds quite a lot to textbook Quantum Mechanics: It provides a clear ontology (particles move). It explains how the double slit experiment works (a particle guided by a wave). It also removes a lot. It removes the measurement postulates. It removes observers from the theory. It removes cats that are both alive and dead.

      Of course, Bohmian Mechanics does not remove nonlocality because the world is nonlocal. If it removed nonlocality, then it would disagree with the world.

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    2. First lets get some terminology.

      By classical I mean viewed from a Newtonian perspective. The world isn't classical so your classical instincts can steer you wrong. You need to be careful.

      Before Einstein locality was not a necessary concept. The whole universe could be considered local since there was no known limit to the speed information could be transmitted and no known reason to think there was such a limit. Locality becomes important with the limit on the speed of light and speed of causality.

      Schrödinger very well knew that people were thinking it makes sense for his cat to be both alive and dead. They told him. This isn't some new thing.

      Bohmian Mechanics does add a clear ontology to QM. But it is an empty ontology. Superluminal invisible blue fairies would also add a clear ontology. Both must be superluminal to violate causality and both must be invisible because if you could see them you could send messages faster than light and backwards in time. Once you have something that can travel in time and is invisible you can explain absolutely anything. And thus it explains absolutely nothing.

      Decoherence removes the observer and measurement problem.

      The world is not nonlocal in the sense that no information can travel faster than light. You can postulate invisible blue fairies that travel faster than light and tell particles how to move. But then to protect causality you have to make them undetectable even in principle. They are only there to satisfy your classical instincts.

      I also reject many-worlds for much the same reason. Parallel worlds are just another version of blue fairies. You can postulate alternate universes but unless you can tell me how to get there I have no reason to accept them as ontological things. I could as well believe in blue fairies.

      Also there is a way to think about this that avoids nonlocality entirely. Say I have a quantum pair of shoes. I send one to you that is in a superposition of left/right. I keep the other shoe that is in an anticorrelated state of left/right. Now you may think that if you look at your shoe you collapse the wave but from my perspective that isn't what happens. From my perspective both you and the shoe are in a state of superposition. You could send a letter to me telling me which shoe you have but that letter would also be in a state of superposition. The wave collapse only happens when I open the letter. At that point the letter, me and my shoe must correlate but that's ok since we interacted locally.


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    3. That is not what the word "ontology" means. The ontology is the stuff in three-dimensional space, e.g., chairs, tables, cats. In Bohmian Mechanics, this stuff is particles (e.g., electrons), and the chairs, tables, and cats are made up of these particles.

      You seem to think "ontology" is the way that the particles affect each other. That is not what "ontology" means. (The particles affect each other via Schrödinger's wave equation.)

      I don't understand what your shoe example has to do with Bell's inequality. Are you simply denying that it is possible to have things happen at different places far apart?

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    4. Ontology is the branch of metaphysics dealing with the nature of being. If I believe in ghosts I think they are ontological things. If I believe in blue faeries then they are real things. If either mediate interactions between particles then they furnish a clear ontology for those interactions. I can also be a mathematical Platonist in that I believe math exist out there in some sense. That would make it an ontological thing. And the wave function, I can be epistemic and believe it is just a representation on knowledge or I can be ontological and believe that it is an actual thing in itself. I tend to the epistemic but with some quantum weirdness added.

      In Bohmian Mechanics Pilot waves are ontological things that furnish a clear ontology. I really don't understand your objection here.

      The shoe example simply shows that the wave function of the distant shoe does not have to collapse when you observe the local shoe. It only collapses when information about it arrives locally. Then it is constrained to collapse in a way consistent with local information. You need to look into decoherence.

      As a practical matter you cannot put a cat in a superposition by placing it in a box. It isn't isolated enough and there is no practical way to make a box that could isolate it. But if the cat is a light year away then it is effectively isolated from you and so can be in superposition. It cannot collapse until information about it arrives locally. Along with decoherence you need to look into quantum computing.

      But don't take any of this too literally. Wave collapse itself is just a verbal short cut. Reality is just a ray in Hilbert space.

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    5. When discussing physical theories, the ontology is what the theory says the chairs are made out of. In Bohmian Mechanics, the chairs are made out of particles.

      The wave function is a function on a high-dimensional space. It is not a function on three space. So, it is not the stuff that chairs are made out of.

      Bohmian Mechanics gives formulas for how these particles in three space move in three space.

      There is no mention of "collapse" in the derivation of Bell's inequality. Bell's inequality follows from locality. The Nobel Prize winners did experiments that violate Bell's inequality. Therefore the world is nonlocal.

      If you do a different experiment, e.g., moving your shoes here and there, then it may be possible to give a local explanation for your experiment. This just means that you haven't done the correct experiment to demonstrate nonlocality.

      > Reality is just a ray in Hilbert space.

      So, matter is not made out of atoms, electrons, etc.? We'd better recall all the physics textbooks.

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    6. Specifying the ontology is a prerequisite for discussing locality. This is because locality is about what happens to the stuff in three space. If your theory does not have anything in three space, then it is neither local nor nonlocal. But then you need to explain where the tables and chairs come from.

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  8. I keep reading `Bohmian Mechanics' as `Bohemian Mechanics' and then think of the song `Bohemian Gravity'
    https://www.youtube.com/watch?v=2rjbtsX7twc

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  9. It is also called the "de Broglie-Bohm theory" for Louis de Broglie and David Bohm.

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  10. I find these results definitely uninteresting. They are difficult experiments, but they do not increase our understanding of nature, unlike other Nobel winning experiments such as the fractional quantum Hall effect, neutrino mass, parity violation, etc... As a computer scientists, you should definitely understand why this is uninteresting: (1) first year undergraduate quantum mechanics already predicts the results of these experiments (2) if the experiments gave a result different from predicted, then quantum mechanics is wrong (3) however, many other predictions of quantum mechanics were already verified to ridiculously high accuracy, most notably things like the electron magnetic moment (4) so, if the experiments give a result different from predicted, then nature is described by some completely crazy theory which fits both (2) and (3). So, (5) if there is some local hidden variable theory, this might imply the existence of a model which is both efficiently classically simulable and which agrees with orthodox-QM in the range of experiments that we actually care about, such as chemistry, solid state, and high energy physics. So, why has no one found such a model and the corresponding algorithm?

    Being excited about the experiments feels like being excited each morning that your "prediction that the sun would rise" was verified. Of course, physics must be tested by experiment, but that doesn't mean you need to test every single thing all the time. The theory has already passed much more stringent and quantitative tests.

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  11. > if there is some local hidden variable theory

    The experiments rule out much more than "local hidden variable" theories. They rule out all local theories.

    The best understanding we have of nature is Bohmian Mechanics. It agrees with experiment and let's us draw pictures of what the particles are doing. The following article reports experimental evidence that photons follow Bohmian trajectories:

    Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer
    Sacha Kocsis et al.
    Science 332, 1170 (2011);
    DOI: 10.1126/science.1202218

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    1. "> if there is some local hidden variable theory

      The experiments rule out much more than "local hidden variable" theories. They rule out all local theories."

      If this is meant as a rebuttal of Unknown 11:33, it seems a non-sequitor. Remove the phrase "hidden variable" from that comment and everything goes through unchanged. I welcome someone writing down a local theory which correctly calculates both g-2 for the electron and the binding energy of molecules. If they do, then turn it into an algorithm.

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    2. Each person can decide what they find interesting. But, experimental confirmation that the world is nonlocal does increase our understanding of nature.

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