Guest post on Bohmian Mechanics, by Reinhard F. Werner

I went to a conference at the end of April in Bielefeld, entitled “Quantum Mechanics without observers III”, which was the meeting of a European network devoted to the Foundations of Quantum Mechanics. The network has a high percentage of members of the Neo-Bohmian school, so I was a bit of an outside observer, coming mainly to see what that school had been up to in the last couple of years. As the odd operational quantum mechanics guy at a convention of hard-core realists I was therefore part of a small minority. The following impressions were written up mainly as a feedback for the participants of the workshop, presenting an outside angle. I feel encouraged to send this out by a remark of Jürg Fröhlich at the workshop that we should not, out of misplaced politeness, refrain from criticising each other’s scientific positions. I am grateful to Tobias Osborne for posting this text on his blog. This will make it easier for me to collect the comments.

On one level the workshop felt to me like a fundamentalist congregation. To someone not sharing the belief in that particular brand of realism it was especially striking how this belief was enforced time and again by the usual forms of discourse in a one-faith community. Several speakers enjoyed drawing laughter by exposing supposedly absurd quotes from famous physicists, and the hagiography of John Bell kept an amazingly large number of speakers busy. Of course, this saint was claimed whole for the Bohmian camp, and any subtlety left in his writings thoroughly flattened. We heard a sermon on the question whether upon splitting a box with a quantum particle the particle is truly in one or the other box, and a chairman who actually asked the experimentalist speakers to declare their faith in this matter.

(I should add here that not all the Bohmian talks were bad. Stefan Teufel did a good job at presenting an argument. That’s how a workshop can become fruitful.)

So what is the Bohmian belief? I am one of those who see in “local realism” a conjunction of two concepts: locality and realism. Bell’s argument shows that this conjunction is not in agreement with the observed facts. The separation between the concepts is not difficult, something that I expect students to understand. Quantum mechanics as I understand it takes the local option, in the sense of not containing spooky signals. Of course, if you insist on a classical “realist” description they are all over the place. It is clear that if you are altogether unwilling to even debate realism (or “classicality”) you can soak your language in it to such a degree that it would seem like an undeniable demand of basic logic. But that is just sloppy thinking, which is not improved by any degree of shouting or religious devotion. “Realism” has a double meaning in this context. On one hand, it is a basic principle of science, the demand to check any claims against reality, to go for empirical content rather than storytelling. On the other hand, it stands for a particular way of constructing a theory, namely assuming that every individual system has an in principle complete description in terms of its properties (“classicality”). The irony of quantum mechanics is that it brings these two into conflict. Those insisting on the second kind of realism, like the Bohmian school, thereby lose sight of the first: Bohmian trajectories have no connection to empirical fact, and even the Bohmian theory itself claims no connection. So they are just a piece of fantasy. You may call the trajectories the reality givers (I even heard “realizors”) of the theory, and base an “ontology” on them. But they are still but a figment of your imagination.

It goes with this status that there is no way to answer questions about the possible structure of this reality, and to make basic theoretical choices except by appeal to lack of imagination. Why take Nelson’s diffusion constant equal to zero and not one (which gives the pleasing balance between forward and backward derivatives), or maybe 7? Why take wave functions as the description of single systems rather than density operators? I could give some arguments for that. You can drive Bohmian trajectories with density operators just as well, and they would tend to be less singular. Why go for position as the only “real” feature of particles, and not include other variables like spin and momentum, or maybe fewer, like one mystery Reality Bit, which nature chooses at random. All this is possible, and equally irrelevant. If you tell me you don’t believe in such arbitrary constructions, I can only say, “Fine, but then you should perhaps go one step further and scrap Bohmian positions and wave functions tagged on individual particles along with the rest.”

Let me try to explain it with a quote from Feynman (Messenger Lecture 1964, a few sentences after, and in elaboration of, his often quoted “I think I can safely say that nobody understands quantum mechanics”),

If you will simply admit that maybe [Nature] does behave like this, you will find her a delightful, entrancing thing. Do not keep saying to yourself, if you can possibly avoid it, `But how can it be like that?’ because you will get ‘down the drain’, into a blind alley from which nobody has escaped. Nobody knows how it can be like that.

(So my paraphrase of what he really says is “Nobody understands quantum mechanics IN CLASSICAL TERMS”.)

I read this as suggesting that one should suspend judgement on things like naive realism. Try to come to grips with what you can see in a lab. Try to build your concepts around that. Then it may be that realism at the quantum level is found to contribute to the enterprise, but on the other hand it might not. I think his warning can be exemplified well by the Bohmian community, going down that very drain 60 years ago, and indeed never coming back, except to declare that they saw Jesus at the bottom of it. To me physics is just too interesting to get seriously caught up in this. If you have heard Harald Weinfurter’s talk, ask yourself what the insistence that “spin isn’t real” could have added to the very competent explanations he gave. In order to make any analysis of a quantum optics experiment you do talk about spins and internal states of atoms and photons and polarization; positions are in no way privileged parts of the explanation. You will not be naive realist about any of these concepts. Certainly, no working physicist I know would think of measurement as uncovering predetermined values (a battle that Bell apparently still found necessary to fight). On the other hand, it seems to me that on philosophical grounds Bohmians (and the philosophers present at the workshop) would deny rationality of this discourse since it gives a shit about ontology. So what do you make of this? Is it really worth saving Physical Reality at the expense of real physics?

Let me give another example, from a conversation with Roderich Tumulka, who was quite patient with me. (The conversations on the side of the workshop were certainly more illuminating for me than most of the talks). I tried to describe in that conversation what kind of theorem I would call a solution of the measurement problem. Effective information loss is clearly part of it, as is the need to show that certain properties of macroscopic systems are stable against the way we interact with them. To me the question whether “the moon is there when nobody looks” is related to its quantum description only in the way that you have to PROVE IT from a better under understanding of macroscopic quantum systems. The trivial solution “Have no fear, its Bohmian particles are going to be somewhere” does not even begin to tackle this problem. Coming back to my description of the measurement problem, any Bohmian will understand that Roderich was probably not interested. To him my description was that of a lot of hard mathematical work, which in the end would fail to solve the MP, because there would still be a superposition; “effective collapse is not collapse”. At this point Nicolas Gisin dropped by and enlightened me: what I was after here (and to me would be all you could hope to rationally argue for) was merely “fapp” collapse. This sums it up nicely. To me the “fapp fixed outcomes” problem is a target on which even partial progress is highly welcome. It would require an increase of our understanding of complex systems and an improvement of our mathematical technique. Assuming that to be solved, there would be virtually nothing left of the measurement problem, except maybe a two line historical comment in a paper. The Bohmian perspective seems to be the opposite. You don’t care about the hard problem, but only about that last, utterly trivial bit.

The proposed Bohmian “solution of the measurement problem” is that the pointer is really somewhere, because the nuclei it is composed of are “really” somewhere, assembled in a pointerlike shape. The same observation is central to the claim of empirical equivalence between Bohmian Mechanics and Quantum Mechanics. At the end of the day everything is supposed to be recorded in a pointer position or ink on paper (There is something cozily old-fashioned about the insistence on position here. Magnetization of tapes, or storage on a USB stick or the colour of pixels on a screen are apparently unsuitable for macroscopic records.) This will be the same in quantum mechanics, and since these theories supposedly make the same predictions about positions, the two are “empirically equivalent”. Note how this argument grants that quantum mechanics had no measurement problem in the first place, since it apparently takes it as unproblematic that there will be agreement. The empirical content of Bohmian Mechanics entirely rests on this bridge. Again, it is left entirely to the quantum physicists to work out how stable pointer positions come about. Bohmian Mechanics will then extend a blessing of Reality. That’s all it does.

There was a time when the claim of empirical equivalence was made in a stronger form, namely that the two theories agree about the positions of quantum particles. This was part of the initial appeal of the theory: In spite of Heisenberg’s criticism of the notion of trajectories, here they are! Great! And if you look at positions at any one time, even the probabilities will come out right! Not any more, though, I am told. Position at the quantum level now shares the fate of spin: it is not real, but has to be indirectly inferred from an experiment (guaranteed as just mentioned to agree with quantum mechanics). Indeed, the agreement is shaky as can be. I produced a little example the other day (arXiv:0912.3740) showing that two-time correlations, which make sense in both theories, come out differently. So Alice and Bob must be forced to do their measurements at the same time, or agreement is lost. That was, of course, known to many Bohmians, although not often clearly stated. The answers I received about this told me to do it right and include the measurement devices, ultimately reducing everything to macroscopic pointers. That would bring back the empirical equivalence. Fair enough, but I am afraid this opens up a gap. If there is no direct connection between observable and Bohmian positions at the microscopic level, how am I justified to assume it at the macroscopic level? Should we invoke prestabilized harmony? Is this not rather like the measurement problem itself?

So far I have talked about Bohmian theory of measurement. Similar things happen on the preparation side. So what is the meaning of the wave function? To me this is almost the same as the question: What kind of justification can I reasonably give for choosing one wave function rather than another? In a controlled lab experiment the wave function (more likely the density operator) is an attribute of the preparation. I may have a theory about the preparation device, from which to justify an expression or maybe at least an ansatz. This could be tested by subsequent statistical measurements. Even outside the lab, under sufficiently clearly defined circumstances, there may be a justifiable ansatz. There would also be a dependence on those circumstances and how they are specified. The test for any ansatz would likewise be statistical experiments. So a measurement on the light of stars of a certain class, or on the microwave background may be a perfectly acceptable preparation procedure. Only tagging the wave function as some attribute on single particles is known to be a daft strategy, because it gets you into trouble if there is any entanglement. In any case, here is the absolutely most boring thing you can say about wave functions (It is anyhow false when applied to subsystems, i.e., anything below the universe level): “Every system really and truly has one, even if you can never find out which”.

Two lines I heard many times now are “Bohmian Mechanics is simple and beautiful, because it just needs 2 equations” and “Bohmian Mechanics explains Quantum Mechanics”. Now for the first, I could offer a further simplification: drop the Q-dot equation. The real reason the theory is simple is because you are very modest in your goals. If you don’t want to go into the details of the physics, it is easy to stay simple. If you just want Physical Reality restored to satisfy your philosophical needs, (“some Q is real, but spare me the details”) you can even drop the first equation, and leave it all to God: He knows what is real, and you can sleep reassured. Now that would be a really simple theory achieving as much in the field of Reality search as Bohmian mechanics. For the second claim, I see a pattern here: “To explain QM, invent P, to get ‘BM=QM and P’. From this you explain QM by forgetting P”. I am not impressed.

So what is the disagreement in the end? Should it be locality or realism? Should it be quantum mechanics in minimal statistical interpretation, with an operational stance, or Bohmian Mechanics, or maybe something else? I guess Bohmians and I agree that the choice is not between equally viable positions. We only disagree about which one it is. To me one is a sound basis for doing physics, including theoretical and mathematical physics with a foundational interest, and the other has turned out to be fairly sterile. In 60 years the number of interesting new physical or even mathematical problems from the Bohmian and Neo-Bohmian community has been rather modest. The workshop certainly didn’t convince me otherwise, although the hope was what made me come. Bohmian Mechanics feels to me like a theologian explaining the origin of the universe. He could say: “With all your physics, which anyhow does not cover the singularity, you cannot explain Why it happens, but theology can”. I can see that many people would go for that sleeping pill. But it is a really lousy contribution to cosmology nonetheless.

One last thing: I am always ready to play also with whacky ideas, like Bohmian trajectories. To me the trajectories used to be the most interesting part of the theory, even though the Bohmian community rarely seemed to bother to find out anything about them. What kind of physics would Bohm’s Demon see, by which I mean that hypothetical entity with direct access to the Reality of Bohmian trajectories, but to nothing else? So on Thursday evening I made a bet with Nicolas Gisin on a purely mathematical statement concerning Bohmian trajectories in the presence of detectors. He was on the anti-Bohm side, so I chose pro-Bohm, partly influenced by a heuristic argument Roderich Tumulka gave earlier that day. The stakes are a good bottle of wine, and I hereby put a second bottle as a prize for the person (most likely a Bohmian) who comes up with a pertinent theorem. I describe that in a separate post.

67 Responses to Guest post on Bohmian Mechanics, by Reinhard F. Werner

  1. Matt Leifer says:

    I am no Bohmian, but I have a fair bit more sympathy with them than you seem to, so I thought I would raise a few points in their defence.

    1. Bohmian trajectories can be measured, and have recently been measured, in experiments involving weak measurements. Now, I am no fan of the overinterpretation of weak values, but it seems to me that this provides at least some reason to take the trajectories seriously. After all, I can build a kind of detector that will display their values just as I can build a detector that displays the values of the more usual observables. Of course, this does not provide a reason to privellige position, since, fo example, I could play the same trick with momentum and find the “trajectories” that result from swapping the role of x and p in the Bohmian equations. Nevertheless, I think the dismissal of Bohmian mechanics as empirically irrelevant is a little harsh in light of this.

    2. Bohmian mechanics provides a counterexample to a lot of nonsense that people spout about quantum theory, e.g. in the double slit experiment people often say that it must be the case that the particle somehow “goes through both slits”. Even if we don’t take Bohmian mechanics seriously as a theory of the world, I still think it is a good intuition pump. It makes us think more carefully about the possible ways the world could be and still be compatible with the predictions of quantum theory. For example, thinking about the nonlocality of Bohmian mechanics was a key motivation for Bell in coming up with his eponymous theorem.

    3. If the goal of quantum foundations is just to get to the point where we are happy with what we can say about quantum theory and then leave it at that, then pretty much no interpretation of quantum theory is ever going to have much use. However, I see the goal as getting us to the point where we can understand where quantum theory might break down and what might replace it when it does. Different interpretations suggest radically different approaches to this. For example, operationalists might look to generalized probabilistic theories whereas Bohmians might look to nonequilibrium Bohmian mechanics. These make different predictions. In particular GPTs retain no-signalling but nonequilibrium BM predicts signalling. I think it is worth keeping all these alternatives around because ultimately one of them may prove to be correct if we ever enounter an experiment that cannot be explained by standard quantum theory.

    4. The constant hagiography of Bell amongst the Bohmians is a bit annoying. One sometimes gets the impression that nobody else has ever written anything sensible about quantum theory and that “Speakable and Unspeakable” is some sort of holy text. Nevertheless, Bell’s writings are a lot more sensible than those of his contemporaries and I am not sure if it is worse than the constant references to Bohr, Heisenberg, et. al. that we hear from the more mainstream community. I mean, how many papers have you read that claim “this is what Bohr really meant to say”. Personally, I would prefer it if we just got down to the business of trying to understand quantum theory as best we can from a modern point of view, but if you must beatify someone then there are worse choices than Bell.

    5. Regarding the reification of position, people do work on Bohm-like theories with other ontologies and take them seriously, particularly in the relativistic case. There are theories with field ontologies and theories with discrete ontologies in which spin is taken seriously. Admittedly, the choice of observable is still fairly arbitrary, but it is fair to say that, even amongst Bohmians, the usual position based model is regarded as more of a toy-theory that has to be replaced by something more sophisticated to account for quantum field theory.

    6. It is a shame that the Bohmian community is a bit insular and often adopts a kind of aggressive attitude towards everyone else. I have some sympathy though because many of the older advocates have been battling for years to even get people to admit that Bohmian mechanics is not ruled out by Bell’s theorem. If you have been ridiculed by the mainstream physics community for so many years then that is bound to cause some amount of insularity and aggression. However, given that quantum foundations is currently on the rise, I think it is time to get over this. I find it is less prevalent in the younger members of the community when you get them alone and I hope we can build on that.

    7. It is a fairly standard mantra that Bell’s theorem is based on the conjunction of realism and locality and so one can choose to reject one of them whilst keeping the other. As you say, Bohmians opt to throw out locality. As for the other position, i.e. locality without realism, I have a lot of trouble understanding what it is even supposed to mean. By locality, one could of course just mean that quantum theory obeys no-signalling or that observables defined on spacelike separated systems commute (which modulo infinite dimensional issues is just a less operational way of saying the same thing). However, this principle was never under question. Everyone, including Bohmians, agrees that the no-signalling principle holds, so if this were our definition of locality then Bohmians could claim to be local realists. Therefore, it seems to me that this is not an appropriate notion of locality to say we are upholding as a response to Bell’s theorem specifically. To reiterate, it is A notion of locality, but not one that is relevant to Bell’s theorem in any way. In fact, it seems to me that locality, in any sense that is even tangentially related to Bell’s theorem, requires realism for its very definition. You need to be able to say that there to be some things that objectively exist in the world in order to say whether changing them at one location affects them at some other. Hence, in my view, it is more accurate to say that holders of operational positions are rejecting both realism AND locality (in any sense that is relevant to Bell’s theorem).

    The only way I can see to possibly make sense of locality without realism as a response to Bell’s theorem is if we view quantum theory as a kind of generalized probability theory and then show that the quantum correlations follow from a kind of generalized version of Reichenbach’s principle of common cause. One could then make a generalization of Bell’s local causality condition out of this and claim that it holds for quantum theory. Now, one can do such a thing, but it is irrelevant to Bell’s theorem unless one can give a realist reading to the generalized probabilities that occur in that theory. I quite like this direction myself, but the interpretation of generalized probability is in a miserable state compared to the interpretation of classical probability so, at the moment, I would say that it is just a formal mathematical generalization.

    • Hi Matt,
      sympathy with (some) Bohmians is one thing. I am with you on that. My sympathy with Bohmian Mechanics, as a way of doing science is rather limited, however, as you can see. Concerning the points you raised:

      1. No, Bohmian trajectories have not been measured. What has been (weakly) measured is the probability current, from which you can calculate the Bohmian velocity. From that you can solve an ODE to get the trajectories printed in the paper. The trajectories are all in the post-processing, and no evidence whatsoever comes from these experiments to suggest that anything is following these paths. Nor has this been claimed.

      2. Indeed it was necessary to point out at some point (maybe in the 50s) that you can build hidden variable theories. Actually, one should point out that it is so easy that it is hard to do it in an interesting way. Of course, one should also point out the price that comes with such theories. BM as an intuition pump does not really talk to my intuition.

      3. I agree that anything goes in the creative process. However, you have to make personal choices. My intuition here would be that inventing a bunch of trajectories is crazy but way too boring to be crazy enough. Actually, I don’t expect much from generalized probabilistic theories either, even though I find that program much more interesting.

      4. I totally agree, only I would deny the “if” in your last sentence: there is no need to beatify anybody.

      5. Insisting on ontology may turn out to be as useful as insisting on mechanical models for the ether. But who knows.

      6. I just come from a week full of sect-like behaviour, which I guess was the punishment I deserve for being open enough to go there in the first place. More openness sure sounds like a good idea.

      7. This we would have to discuss in detail. Maybe we are talking about different theorems by Bell? In each of the seven or so proofs I can think of for what I would call Bell’s theorem, locality (in the sense of no-signalling or variants thereof) is a clearly visible assumption. So I don’t see how that fails to be relevant. Of course, the framework I would work in is probabilistic theories. I don’t see that “generalized probability” is some kind of new stuff here. Your favourite interpretation of classical probability works just fine.

      • Matt Leifer says:

        Regarding 7, maybe you are thinking of the decomposition of Bell’s locality condition into parameter independence and outcome independence? I agree that parameter independence looks superficially like a no-signalling condition, but it is a no-signalling condition at the ontological level rather than at the operational level. It says that, even if you knew the full state of reality, knowledge of Alice’s measurement setting would not affect the probabilities you would assign to Bob’s outcomes. This principle is not required for the operational no-signalling to hold, as the dependence may disappear upon averaging over the ontic states, as it does in Bohmian mechanics for example.

        If you are an operationalist then you cannot even formulate a principle that plays the same role as parameter independence because it requires the language of states of reality. Operational no-signalling is not a substitute for this as nobody thinks it is violated.

        I would also argue that adopting a Copenhagenish principle like Asher Peres’ “unperformed measurements have no results” does not help restore locality here, since this just means that there is only one possible ontic state and Bell locality would then imply that the measurements should be uncorrelated, i.e. we are essentially just back in the scenario of the original EPR argument.

        Regarding the interpretation of generalized probabilities, we are probably arguing about semantics. I agree that we can use interpretations that bear a strong family resemblance to the classical ones, but they must require at least a small modification because the standard accounts derive the Kolmogorov axioms from the a more basic interpretive idea so something has to give. For example, in subjective Bayesianism the Dutch Book argument is used to derive Kolmogorov (modulo issues about countable additivity). Of course, it is quite easy to identify what part of the derivation has to be given up. It is the idea that any system of bets can be decided simultaneously. Instead, we have to introduce the idea that deciding one bet may preclude the possibility of deciding another one and this loosens the constraints used in the derivation. Technically, you will get a state on a semiclassical test space where each test represents one of the mutually exclusive systems of bets that can be decided simultaneously. Such a state is just a conditional probability distribution. Now, you may say that this is the same interpretation as classical subjective Bayesianism but I would argue that it is different because we have modified it. Since many subjective Bayesians seem to think that their account has normative force, i.e. you must use classical probability on pain of irrationality, I think that it is appropriate to call this a different interpretation of probability.

    • Hi Matt,

      It is not correct to say that “I could play the same trick with momentum and find the “trajectories” that result from swapping the role of x and p in the Bohmian equations.” As I pointed out in my 2007 paper (and mentioned at the workshop), the weak value expression for trajectories is only consistent with QM probabilities if the Hamiltonian is at most quadratic in the variable conjugate to the variable you are considering. That is the case if you consider x (as H is at most quadratic in p, for all fundamental Hamiltonians we consider), but not for p (as H is not at most quadratic in x, e.g. a 1/|x_1 – x_2| potential).

      I see this as an advantage in the weak-valued veclocities as motivating BM, as it picks out x as the unique preferred variable.


  2. Tim Maudlin says:

    So here’s a clear indication that someone does not have anything clear and defensible to say: they resort to cheap rhetoric. Try to rewrite this without the completely empty and obnoxious references to “faith” and and “sects” and “whacky” and so on and see if anything coherent remains. Nothing does, except a confusion about the principles Bell used to derive his theorem. There is no supposition of “realism” in any sense in this theorem. If you think otherwise, point it out: it is, after all, a piece of mathematics.

    The remarks about the measurement problem also show a complete lack of understanding. Does there need to be “prestabilized harmony” between the positions of particles and the observable behavior of laboratory equipment as suggested above? Good lord: the laboratory equipment is supposed to be *made of* particles, so its behavior follows inexorable from the particle trajectories. If a pointer is made of particles and the theory implies that the particles all move to the right, then the pointer moves to the right without further ado, and this is observable behavior. This was said over and over at the conference. And this denseness from some someone who touts his “openness”. Is there any reader here who cannot follow this simple point. Why then cannot Werner?

    • Matt Pusey says:

      I think Reinhard made a valid point about the status of the (or perhaps a?) measurement problem in Bohemian mechanics, so let me try and put it a little more concretely.

      Bohmian mechanics indeed gives us a story why, immediately after a measurement interaction, a pointer either points left or right with the correct probabilities. However, this story is precious little comfort if the atoms of the pointer immediately start flying around all over the place. We actually need something much stronger: the pointer goes left of right, and then stays there for a long time, even if 5000 people, excited by the news that measurements have outcomes, come and look at the position of the pointer, some of whom photograph it or sneeze on it, etc etc. I don’t think Bohmian mechanics gets you this longer story without adding some extra arguments about the dynamics of the pointer’s wavefunction. These extra arguments will almost certainly involve something like “decoherence”. So I think it is completely fair to say that “Bohmian mechanics solves the measurement problem if and only if ordinary quantum mechanics FAPP-solves the measurement problem”.

      P.S.: It may well be the case that all Bohmians agree with the last statement of the previous paragraph. Although I didn’t hear any of them explicitly say it at the conference.

      • Travis Norsen says:

        Yes, all Bohmians agree that “something like ‘decoherence'” is involved and important. Putting this the way you did in your last sentence (before the PS), though, grants far too much to the non-Bohmian side of the argument. There is a very important sense of (or part of) “the measurement problem” which is solved *not at all* by what you there call a theory that “FAPP-solves the measurement problem”. The situation is simply this. We all see and know and agree that pointers not only point, but continue to point even when observers observe them or sneeze on them. Bohmian mechanics is, in fact, able to account for this, fully and precisely, using nothing but the fundamental (“microscopic”) dynamical postulates of the theory. Ordinary QM on the other hand simply cannot account for this fact — except by bringing in extra, inconsistent assumptions (about “measurement”, etc.) which are not formulated precisely (but are instead relegated to the loose surrounding talk) and whose status (therefore) always remains obscure.

        That, at least, is why Bohmians prefer Bohm’s theory. It’s not that it “really” solves (with some unspecifiably vague sense of “really”) a problem that OQM already solved for all practical purposes. It’s rather that Bohmian mechanics actually does something (and something rather important — account for the observed facts!) that OQM fails to do.

      • Tim Maudlin says:


        Why exactly do you think that the physics of a solid, like a pointer, cannot be handled by quantum theory? It is the interatomic Hamiltonian that keeps the pointer particles from flying apart. The entanglement of the wavefunction of these particles means there should be almost no amplitude for any state in which the particles are in distant regions from one another. Given this, the Bohmian particles will also stay together. Decoherence may play a role in explaining the suppression of terms in which the particles are far apart, but decoherence cannot solve the measurement problem (which is to have a unique outcome, corresponding to what is seen in the lab, represented in the physical description).

        We know what the linearity of the fundamental dynamical equation for the wavefunction implies if one considers the wave function of the whole experimental situation: a superposition of different macroscopic outcomes. That, together with the claim that the wavefunction is complete, produces the measurement problem. The Bohmian solution is simple: the wavefunction is not complete, and indeed, it is the wavefunction that is not at all directly observable. Rather, if the equipment is made of particles (whose *dynamics* depends on the wavefunction), then it is these particle positions, at macroscale, that are observable: this gives the empirical content of the theory in a principled way.

        Decoherence is a well-enough defined behavior of wave functions, and it happens according to a theory if the dynamical equations postulated by the theory imply it. No problem there. Decoherence does play a role, in the context of Bohmian mechanics (as one would expect) of explaining why it is difficult experimentally to produce certain interference effects even though, according to the theory, the wavefunction never collapses. As for the pointer not disintegrating, I would say that it is the interaction Hamiltonian between atoms that accounts for that. I don’t see any reason to think decoherence is very important, but even if it is: so what? Since (again) decoherence alone does not solve the measurement problem, it rather misses the point to make this remark.

      • Matt Pusey says:

        Tim: I don’t think that. Of course states with a particular well-defined pointer position will stay in that category under the pointer’s Hamiltonian.

        But the Hamiltonian acts on the wavefunction, not the Bohmian position. Even in Bohmian mechanics, the overall wavefunction actually involves a superposition over different pointer positions. We therefore need a mechanism to ensure that the particles in the pointer nevertheless behave, to an excellent approximation, as though they are guided by a wavefunction corresponding to a well-defined pointer position. I maintain that this mechanism will at some point make use of the same claims about ordinary QM that many would consider central to a QM-only FAPP-solution to the measurement problem. (Although I’m no longer sure whether or not decoherence would be a good name for the relevant such claims in this case.)

        Travis: I wasn’t trying to make any claim about the relative importance, conceptual soundness, or interestingness of the statements on either side of my iff. All I would say in that regard is that if you agree with my iff and you believe in Bohmian mechanics because you hope it solves the measurement problem, then I think consistency requires you to be interested in the thing on the right of the iff, i.e. the status of “something like ‘decoherence’” in ordinary QM.

        Finally I should say that I would view the truth of my iff as a *good thing* for Bohmian mechanics. My personal prejudice is that the correct solution to the measurement problem is probably going to make essential use of “something like ‘decoherence’”. Otherwise the presence of such helpful-looking effects precisely where they are needed would just be pure coincidence.

      • Tim Maudlin says:

        Matt (below):
        Right, the Hamiltonian acts on the wavefunction and the wavefunction guides the particles. But if you concede that “immediately after a measurement interaction, a pointer points either left or right”, then you already see that the theory has the pointer acting like a solid and not (e.g.) a gas, which is what your question seems aimed at. If the pointer particles were part of a gas, then sneezing would make an awful mess of things, and the wavefunction would still decohere! So decoherence per se does not explain the gas-vs.-solid issue, and we know what does. Decoherence only explains the difficulty of experimentally demonstrating interference effects. Schrödinger wasn’t worried about that at all: he was worried about getting the cat to end up either alive or dead. Of course, a many-worldser will deny that the cat does (objectively) end up just one or the other, and so has a very different take on things. But I assume no one here is arguing for many worlds. That is a different discussion.

      • Matt Pusey says:

        Tim, I concede that immediately after the measurement, the Bohmian positions have the pointer pointing either left or right. I do not concede that the wavefunction does.

      • Tim Maudlin says:


        Can you explain your issue more clearly? The very same sort considerations that get all the particles in the pointer to move together in the first place will keep them moving together. The idea that they could “start flying all over the place” suggests not a worry about decoherence but some other sort of worry, that I can’t follow at all. The thing that decoheres is wave function that (roughly) is a superposition of a piece with all the particles on the right and a piece with all the particles on the left, and no piece with some on the right and some on the left (“flying apart”). So (as I said) there is just zero amplitude to fly apart, so the Bohmian particles won’t. What’s the problem?

      • Travis Norsen says:

        Tim: I took Matt to be worrying about the possibility that the “empty” branches of the wf might again (later) overlap the branch with the actual config point in it. Then complicated interference effects might indeed lead to wild behavior for the particles. You might especially worry about this if you are thinking of the pointer as having a continuous set of possible final positions (so that, so to speak, the adjacent packets are “right next to each other”) but it is in principle an issue even with the binary (left/right) outcomes. Anyway, it is, I think, indeed because of decoherence that one knows one shouldn’t need to worry about this.

        But again (and Matt, this is in answer to your remark that you weren’t making any comment about the “relative importance, conceptual soundness, or interestingness” of the things on either side of your iff) the important point here is that decoherence in the context of OQM simply does not constitute a solution of the measurement problem. Bohmian mechanics (which includes Sch’s eq for the wf and hence includes decoherence) does solve it. What I was objecting to was the terminology of calling “a FAPP-solution” something that is, in fact, not a solution. That is misleading insofar as it implies that, whatever Bohmian mechanics adds, it is somehow imPractical or otherwise pointless or extraneous or metaphysical. The truth is that what it adds to OQM’s non-solution is: a solution.

      • Tim Maudlin says:


        Sorry, I’m not following you either! Even if one were able to keep the whole pointer-plus-particle system isolated from the rest of the environment, and focus things back together to get interference effects (one would have to not observe the outcome, or else the focussing involves interacting with one’s own brain in an invasive way), by what mechanism would the pointer particles start to fly apart? As I said, the interaction Hamiltonian among them prevents that. *The pointer as a whole* might behave unexpectedly (like a single Bohmian particle moving in a 2-slit experiment), but by what mechanism could the particles come apart from one another?

      • Travis Norsen says:

        Tim, my intention was primarily to bridge what seemed like a communication gap between you and Matt, not to say I disagreed with what you wrote before. But now that I’m stuck in the middle, I’ll say this about the actual issue: I think it depends on how you model the pointer. For example, in the simplest possible model, where the pointer is say just a single (heavy) free particle, which interacted with “the system” for some finite period of time, then post-interaction one has (say) two well-separated packets with the actual pointer position in one of them. But those packets will spread (slowly, but inevitably) and (if this was really a closed system) they will eventually overlap, and then (for example) an actual pointer position that would have stayed still (had that other distant packet not spread over to it) will instead suddenly veer off in a new direction. Now you want to say that it’s not just one particle, but a bunch of particles, which are effectively glued together. But then at very least there is some center-of-mass degree of freedom that will behave basically as I described before for the single particle. Whether these sorts of effects could in principle not only make the pointer as a whole veer off in some unexpected direction “for no apparent reason”, but also make the individual particles composing the pointer separate/explode/whatever, isn’t obvious to me. It seems like it would depend on details — like how tightly bound the particles are to each other, etc.

        But it seems to me the crucial point here is that it would be silly to get lost in a discussion of what, exactly, the conditions for that kind of weird behavior are, whether they’d be realized for a realistic (closed-system) pointer, etc. — because we already know *from decoherence* that even if those conditions were realized/realizable for a closed-system pointer, they will never occur in practice because, as you point out, there is light reflecting off the pointer that then interacts with the environment including our eyes, etc…

      • Travis Norsen says:

        Tim, more thinking out loud… Consider a Hydrogen atom in the ground state, and suppose it’s really an isolated system so we have some 6-D (two-particle) configuration space. Suppose it’s at rest, so the wave function has support in some little region in that 6-D space, and the actual bohmian configuration point is just some particular point within that region. Now if that’s really what \psi is, everything will just live happily ever after. But one could certainly have a different initial condition on \psi, which is exactly the same except that there’s a “rogue wave” off in some distant corner of configuration space which is, tragically, headed right for the region where the bohmian config point is. Now certainly it’s possible that, when the rogue wave gets there, the proton and electron will behave in surprising and unexpected ways — including the possibility that they will permanently dissociate (i.e., the atom will get ionized).

        That’s the kind of thing I was thinking might also be possible for the glued-together particles of your pointer. Now maybe there’s some reason why the type of “rogue wave” needed to “ionize” the atom/pointer/whatever can’t arise, or can’t arise from the kinds of interactions we’re imagining may have happened in the recent past, or whatever. If you have some such reason in mind, I hope you’ll explain it to me. But, in principle, this kind of thing is possible.

        But, in principle, we know from decoherence that we don’t need to worry about it.

      • Tim Maudlin says:


        Here’s a simple proof. There actually are interference experiments done with, e.g., buckyballs, where the wavefunction *does not* decohere (that’s the point) and the buckyballs don’t explode or come apart or anything. They just give positions that show interference effects.

      • Travis Norsen says:

        Yes, OK, good. So maybe the kind of case I (and Matt?) was worrying about isn’t possible. I’d like to understand more deeply why it’s impossible, if it is, rather than just have an example where it doesn’t happen — but the example is indeed relevant and helpful. In any case, there is at least the question of why pointers (despite staying rigidly glued together) don’t wiggle and jiggle erratically, due to these sorts of interference effects. I take it that is at least within the category of things Matt was worrying about. And I take it we all agree that it’s because of decoherence that (even) this won’t happen, according to bohmian mechanics. In other words, Matt was right that decoherence plays an important role in the bohmian understanding of the world — but wrong to (perhaps inadvertantly) suggest that decoherence alone could solve (or could mostly solve or could FAPP-solve) the measurement problem.

      • Matt Pusey says:

        Thanks for figuring what I was trying to say, or at least what I should have been trying to say. The buckyball example is useful, although its worth pausing to note that the stability of buckyballs will again follow from calculations within OQM.

        I’m not sure talking about “the measurement problem” is getting us very far because we probably have different ideas of what that means. Let’s consider the problem of why we see largely classical world we do. There are a whole bunch of questions you can ask within OQM about that: in what sense, and to what degree, do macroscopic systems immersed in an environment look like they’re following classical equations of motion? What are the practical measurements one can do on macroscopic systems? Are such practical measurements approximately non-disturbing and do they approximately commute with one another? Answering enough questions like this should build up a picture of the FAPP emergence of classical behaviour.

        Let me anticipate two objections to this picture:
        1) The link between the quantum descriptions of FAPP-classical systems and our actual observations comes only through the unprofessionally vague measurement postulate. This is true, although the idea of some of the questions above is it probably won’t turn out to matter very much exactly how you apply the measurement postulate, so its vagueness won’t cause any practical difficulties. One could even attempt to attack the vagueness with something like the following. Define a Practical Purposes parameter ε, which might be defined as something like the maximum error in your calculated probabilities, or 1/(current cost in USD for an adversary to create unexpected results). The idea being that small ε means you’re safe FAPP. Then for a given physical setup and purported use of the measurement postulate, it should be possible to bound ε by comparing to the results from a more conservative use of the postulate. I would expect a fairly sharp decrease in ε at the point where most physicists would expect to be able to use the measurement postulate. Jürg Fröhlich’s ideas about fundamental information loss raise the interesting possibility that you might even be able to get ε=0 in some situations.
        2) Even if you can get OQM to predict that everybody *sees* tables and chairs in the right places, doesn’t mean there actually *are* tables and chairs in the right places. Whether you find this distinction meaningful clearly depends on your philosophical positions.

        Now what does Bohmian mechanics bring to the table? It’s predictively equivalent to OQM, so if the FAPP-questions in my second paragraph have the wrong answer in OQM, they have the same wrong answer in Bohmian mechanics. For example, if OQM turns out to predict that tables will be seen to spontaneously explode every third Wednesday, then Bohmian mechanics will predict that tables spontaneously explode every third Wednesday. (Notice that it does allow us to remove the words “seen to”!) But if all the FAPP-questions turn out “right”, it may provide a solution to the two objections above. In that case, where has all the “workmanlike physics” (by which I mean identification of relevant degrees of freedom, understanding of their interaction, selection of approximations, handle-turning on calculations, etc) in explaining the emergence of classicality been done? Within OQM.

        Again, I’m not trying to advocate a position on the relative importance of “workmanlike physics” compared to more conceptual stuff. Personally, I think it’s only possible to be sure with the benefit of hindsight. Someone might have dismissed Einstein’s SR paper by saying that all the “workmanlike physics” had already been done by Lorentz, but the later appearance of GR would have proven that dismissal to be spectacularly stupid.

      • Tim Maudlin says:

        Hi Matt,
        In one metric we are at least within epsilon of agreement. After all, the whole point that Bell was making when he introduced “FAPP” was to insist that OQM is just fine FAPP: just fine for making actual predictions (using a collapse postulate tied to measurement) etc. And all of the spectacular predictions of OQM have been derived this way. The main questions are 1) can you do better than FAPP (and the vagueness in the physical theory that comes with it) and 2) is the desire to do better motivated by “physical concerns” or “philosophical concerns”.

        Some things that some of the early proponents of the theory (such as Bohr) said implied that one could not do better in principle: the observer or the concept of measurement had to be part of the axioms of the theory, for example. That’s why this conference, and the earlier ones, were called “Quantum Theory Without Observers”: the idea is to consider ways of getting observation and measurement out of the foundations of the theory. And we know by example that this is, pace Bohr, possible. We also know by example that some of the “proofs” that dissuaded people from trying to do this (e.g. von Neumann’s “proof”) did not prove anything like what they claimed. So the question is not can the observer and measurement be removed from the foundations of the theory, and FAPP be replaced by something sharp and clear and, in Bell’s words, not “unprofessionally vague”: we know it can be done in several quite distinct ways.

        So what about the motivations? Here there are many things to say, but let’s stick with just a few. First, it is historically true that most great physicists were concerned not with just a good predictive apparatus FAPP but with a precise physical theory. I would take the simple desire for an accurate and precise physical theory, rather than just a recipe for making predictions that is good FAPP, as itself someone physics should aim for. But let’s not be so defensive. Let’s ask whether anything good for “regular physicists” comes from these sorts of questions.

        Well, interest in understanding the EPR argument and whether locality could be ruled out empirically led Bell to his theorem, which in turn led to renewed interest in entanglement as a real physical feature of the universe, which led to quantum computation and quantum information theory. Not bad. Of course, Schrödinger had already realized that entanglement (not interference effects such as the two-slit experiment shows) was the fundamental thing to be found in quantum theory and not in classical theory. But note that Schrödinger, like Einstein, was a vociferous critic of OQM, and even to this day the point he was making with the cat example can be misunderstood. (In particular it is not about lack of interference and so is not solved by decoherence: it is about the theory being able to represent, in the physics, a unique outcome at the end…something denied by Many Worlds). Schrödinger’s critique of OQM, like Einstein’s (it was, of course, the EPR paper that inspired the cat paper) was brushed aside by the mainstream for many years. I myself think this shows that foundational issue are central to physics on any way of conceiving it.

  3. Travis Norsen says:

    Dear Prof. Werner, Thanks for sharing your perspective on the conference. I was also there. I guess you would classify me as a Bohmian. And I was probably the biggest contributor to what you described as “the hagiography of John Bell”. That’s by way of full disclosure of my perspective.

    I’d like to share some thoughts on your thoughts.

    To begin with, in a certain sense, I can appreciate that the workshop felt to you like a “fundamentalist congregation”. But this is only in the sense that I could also understand, for example, how a literal religious fundamentalist would feel that all the speakers at an evolutionary biology conference were rather dogmatic Darwinists. That is, shorn of the question-begging and biased rhetoric, all you are actually reporting is that you and the Bohmians have radically different ideological perspectives that make you and them (us?) assess various things in very different ways. Which side is the stale, unthinking, ignorant, dogmatic, unscientific one, is something that could only be decided by arguing the actual substantive merits of the various issues — not by the kind of inflammatory rhetoric that you use in your post.

    With no pretensions of completeness (a complete post-mortem on your post would require at least a whole book) let me then just note a couple of the substantive points of actual disagreement.

    First, Bell’s theorem. It is absolutely clear from your response (in particular point 7.) to Matt Leifer’s post that you don’t understand Bell’s theorem. “Signal locality” (or “local commutativity”) is simply not an assumption of Bell’s theorem (either/any of them) and nobody who had actually read Bell’s papers (in several of which he goes to great lengths specifically to *distinguish* “signal locality” from the locality assumption that is actually used in the theorem) could possibly harbor this misconception. Nor is “realism” (in anything but the most basic sense, denial of which would render “locality” — in any sense — completely meaningless, as Matt L already pointed out) an assumption of Bell’s theorem. I am not going to argue for these claims (about what is and isn’t assumed in the theorem) here; people can and should read Bell’s papers and decide for themselves. My point here is just that what you seem to regard as some kind of bizarre anti-scientific Bible-thumping behavior on the part of the Bohmians, is (or at least is seen from the other side as) being in fact an attempt to re-educate people who frankly abuse their prominence by perpetuating decades-old misconceptions about what Bell did and didn’t do.

    Second, and relatedly, you complained in particular about the talk (you chose to call it instead a “sermon”) of Jean Bricmont in which he tried to explain the EPR argument in its simplest version, what I dubbed in a paper “Einstein’s Boxes”. But again here your comments (in particular, your statement that a “chairman … asked the experimentalist speakers to declare their faith” in the matter of whether “a quantum particle … is truly in one or the other box”) show that you simply missed the point. Nobody thinks that this thought experiment (or any other) proves that the particle is “truly in one or the other box”. The claim is instead a disjunction, with which I think it would be rather difficult to disagree: either (prior to observation/measurement) the particle is “truly in one or the other box”, or not. Do you disagree with *that*? Assuming not, then the EPR conclusion follows immediately and trivially: if so, then QM is incomplete; whereas, if not, then the measurement intervention (say, on one of the boxes) causes the particle to materialize (or perhaps de-materialize) in (or from?) the other (distant!) box. That is, *either* the orthodox quantum description (of the state of affairs prior to any observation) is incomplete — *OR* the measurement intervention nonlocally influences distant physical goings-on. (Note that “nonlocally” is here being used in the Einsteinian sense of “spooky action at a distance”, *not* superluminal signalling.) That is what Bricmont was attempting to explain. And from where I sat, what appeared to you as inappropriately sermon-like behavior was in fact merely exasperation that several prominent and outspoken members of the audience could fail to grasp such a simple argument, especially after it had been laid out with such painstaking (indeed, even excruciating, plodding) care.

    Now a minor point, but one that warrants comment. Your claim to have shown that the pilot-wave theory makes the wrong predictions (for experiments involving measurements at distinct times) is ridiculous. When I heard you raise this in discussion on the first day (this is before I had any clue what the name of the guy raising this objection was) I immediately concluded that you were some crackpot who snuck in off the street. (Now, happily, I know that isn’t the case.) To give an analogy of the type you evidently appreciate, this is like the hypothetical religious fundamentalist at the evolution conference raising his hand and announcing that Darwinism is refuted because “according to Darwin all the monkeys should have turned into humans by now”. In short, it belies a fundamental failure to understand the theory in question. Darwinism, in fact, predicts nothing of the sort. Just like Bohmian mechanics, in fact, does not make the predictions you say it makes. Evidently you are aware of this fact, but attempt to spin things by suggesting that its not making these predictions is somehow an ad hoc defensive maneuver, characteristic of an empty dying research program, or some such. But that is simply absurd. I will leave it to you and other readers to play out the details.

    Let me end with a point where, actually, I think, the “operationalists” and the “Bohmians” agree: we are both “realists” about directly-perceivable, macroscopic things such as the disposition of laboratory measuring equipment. Where we differ, apparently, is in wanting to have a fundamental/microscopic theory (or at least a few respectable candidates for that status) which actually accounts for these uncontroversial macroscopic facts in a rigorous way (i.e., without a bunch of handwaving, “loose talk”, vague philosophizing, etc.). All of your criticisms of Bohm’s theory (that the trajectories are pointless, that the theory arbitrarily picks out some particular “observable” to bless with a realistic status, blah blah blah) are ultimately based simply on your failure to understand this basic motivation. Or maybe you understand it but just don’t share it. But then it would be much more useful, and much more honest, for you to level your criticisms at this goal — that is, explain in detail why it is unscientific/irrational/whatever to want a theory that is (to use some of Saint Bell’s inspired terminology) clean and professional.

    • Travis Norsen says:

      Oops, I see that while I was writing this long-winded response, Tim Maudlin managed to convey pretty much the same thing(s) but more tersely. So, yeah, what Tim said.

      • Tim Maudlin says:

        Obviously, I think that (barring Many Worlds, which needs a separate discussion) the phenomena require us to accept some real non-locality in the world, and so demand, in that sense, a new kind of physics. But the phenomena cannot require that physical theories have a new obligation to go beyond getting macroscopic behavior right to be taken seriously, without a fine analysis of the human nervous system. For we have the counterexamples to this being *required* at hand: Bohm, GRW, etc. So if *this* sort of novelty is somehow at least suggested by the phenomena, we should see why. Bell showed how non-locality can’t be avoided. Is there any similar argument that this sort of analysis of the nervous system is indicated?

    • Ok, I accept your point about the creationist at the biology workshop. If a group looks like a sect and quacks like a sect, it may still be the observer that is deluded. Let me just say that the religious analogies came to my mind because I found the arguments about as convincing as religious fundamentalist’s. Tim Maudlin, in a separate hate mail, insisted on confirming that impression by calling my text “the empty sneering of someone who does not follow clear arguments” and myself “unable to follow the simplest points due to your own blind adherence to an unprincipled and unclear dogma.” But let’s just leave that level and get to the arguments.

      You obviously had some problem in understanding why I found the two-time correlations interesting and why I think a Bohmian might want to think about it. I know I am risking here to revert to being the “crackpot who snuck in off the street”. Well, it is all about the question how seriously you should take Bohmian trajectories, and their relation to empirical facts. This is one issue where I find the Bohmian treatises I have read mostly fuzzy. Let me describe some options, ranging from taking them very seriously to not at all.

      (1) Bohmian trajectories are the Real Thing. Reality is properly perceived only by Bohm’s demon, the entity with direct access to them, although he cannot see wave functions or spin or anything else. Real physics is the the theory of what the demon sees, and this is what we should explore. Interesting questions here are: would the demon ever come up with wave functions? or spinor valued ones? Empirical content is not needed for this theory. What human observers think they see is their business and may be related to the positions of some electrons in their brains but otherwise uninteresting.

      (2) Same as (1) but the demon also sees the wave functions. Still at this level: a theory without observers. Theatre without an audience.

      (3) Positions are claimed to be the same quantities that quantum mechanics talks about. That is: Although measurement of things like spin is a tricky business and can be analyzed only by a complete theory of the measuring apparatus, position is different in that the quantum observable “position” is somehow synchronized with the Bohmian position. Personally, I do not think that position measurements are at all simple. Nevertheless, the claims of empirical equivalence between BM and QM have often been based on this agreement. In contrast to the previous levels, BM is now claimed to have some empirical content, supposedly in agreement with what is predicted by QM for the “observable” position. To be sure, BM also makes statements beyond that, particularly about trajectory properties referring to multiple times. But these are just for Bohm’s demon, i.e., not accessible to “observers”. I guess this more or less the line you take in your recent arXiv posts.

      (4) Position measurements are measurements. That is, we can only analyze what would be observed by using a complete quantum theory of the proposed measuring device. This will do dadadadada and decohere, but in the end give birth to a pointer (a little ornamented brass thing that you can sneeze on without destroying it). Forget about the positions of the quantum objects, but the pointers are Real, oh so Real. It is their position which is in agreement with QM. To put in the words of Tim’s hate mail
      “As was said over and over (although you apparently are unable to pay attention), in this theory
      laboratory equipment is made of particles and its observable behavior is obviously determined, in
      this theory, by where the particles go. This is exactly how one extracts the empirical content from
      the theory. This was explained over and over, and you are evidently unable to follow…”

      I would particularly like to draw your attention to Tim’s use of “obviously”, which I translate as “utter absence of any argument”. A possible argument would show that a human being’s or a camera’s interaction with macroscopic objects like pointers (“observation”) is such that this language can be justified. But, as I tried to explain in my post, this firstly does not involve the Bohmian particle positions of the pointer, and secondly would need some honest theoretical physics work, and not just some philosopher’s fart.

      The puzzling thing for me is how Bohmians like to oscillate between options (3) and (4). It is (3) for most of the advertising, and I heard some proposals at the workshop for looking at “first arrival events” which would indicate a naive type-(3) view. I don’t think this view is at all given up by the Bohmian community. My example of two-time correlations was targeted at (3) and only at (3). I think it shows that (3) has to be given up, and indeed that is the response I got, from Shelley Himself and others. So again, and very slowly so that even Tim can understand: If we give up (3), what the fuck is the justification for (4)?

      Now how is signalling related to Bell’s Theorem? There are many ways to sort this out. My favourite way of stating the conclusion is that you cannot have all three of the following
      (E) a correlation experiment violating the CHSH inequality
      (C) “Classicality”, or if you like “Realism” (probably you don’t like this substitution)
      (L) “Locality” again in a variety of possible explications.

      There are indeed many ways of making each of these points precise, leading to many “Bell’s Theorems”. So to me this is a genre, not a single statement. What John Stewart Bell thought is an interesting historical question (interesting fact: he personally talked Ed Nelson out of “Stochastic Mechanics” for reasons which all apply to BM), but for the moment I don’t care about this at all. Let me give you one piece of this genre:

      Assume the general CHSH setup: a source, two distant labs to which some things are distributed, and a choice of two settings of devices producing two outcomes on every shot. Assume (E) in the sense that from a set of four correlation experiments you find a combined CHSH correlation expression >2. Assume (C) in the (counterfactual) sense that Bob, in his lab, has found a joint measurement of the two devices of the basic setup. This possibility would be automatic in any classical theory, which is why I call it an assumption of type (C). The condition for any double-outcome measurement being called a “joint measurement of the given ones” is that the marginals come out right in any of the combinations allowed in this setup. (A very reduced, partial version of Fine’s “joint distributions exist”). Conclusion: Alice can signal to Bob on an admittedly noisy channel: send:=choose measurement 1 or 2. Receive:= compare outputs on joint measurement device, and guess “Alice sent 1” if “outputs equal” and “2” otherwise. This signalling is an instance of non(L), and the overall conclusion is (E)AND(C)==>non(L), as claimed.

      Another version: Henry Stapp claiming that (C) can be eliminated (so “quantum mechanics itself is non-local, not just its classical hidden variable extensions”). What he does: Phrase outcomes as events or “logical statements” in a predicate calculus, and assign probabilities to any conjunctions. To me that is just (C), loud and clear. Shitty paper, that.

      Another version: (L)=local commutativity in a quantum field theory. (E) obviously possible. Conclusion: (C) dead.
      (No real surprise here: for CHSH violation on vacuum you need non-jointly-measurable observables. See my papers in the early 80s. I had a long chat with John Bell about this 1988 in Dublin. He wanted to hear all of it, but gave up when he realized it was to demanding for general public talks.)

      And so on.

      • Travis Norsen says:


        You said you “found the arguments about as convincing as religious fundamentalist’s.” It seems, though, that you’re not responding to the actual arguments, but to the exasperation/frustration/hatemail that follows from your not grasping the arguments when they are presented. But let’s not argue about that. I gave the argument in my long comment above: in the “Einstein’s boxes” scenario it is clear that *either* the particle has a definition position already prior to observation (in which case there exist “hidden variables”) *or* there is a nonlocal influence in which observation of one box affects the physical contents of the other box. (See the post above for a fuller presentation of the argument, but basically it is just this trivial disjunction along with an obvious implication of each disjunct.) So, do you agree with the conclusion, that we have to choose between incompleteness (or ordinary QM) or nonlocality? If not, where is the error in the reasoning?

        On the two-time correlations thing, you said I apparently missed your point, but it seems after reading your elaboration that what I wrote the first time is exactly right. Your way of describing these different ways one might think about bohmian trajectories, your (1)-(4), is so warped by orthodox quantum philosophy that I have trouble following it. But it seems to me you are making an entirely trivial point: the (bohmian) trajectory of a particle can be influenced by its interacting with something (which is perhaps a “measuring apparatus”). You seem to think that this is somehow absurd on its face, such that bohmians ought to be somehow embarrassed by the need to “retreat” to (something like??) your (4). But (if I understand correctly) your (4) is simply the obviously true assertion that interactions between “quantum systems” and “measuring devices” *are interactions* and we should see what the theory in question says about what happens as a result of them. And as I gather you concede, Bohmian mechanics makes the correct predictions here. It’s just that, for some reason I cannot begin to understand, you think Bohmians should be embarrassed about this — should be embarrassed by the fact that their theory (a) is able to make such predictions without a bunch of BS philosophical handwaving, and (b) makes empirically correct predictions. I am truly at a loss trying to understand which part of this you think anybody should be embarrassed about. You ask “what the f*ck is the justification for (4)?” The justification is that if you want to know what a theory says will happen in some situation, you should use the theory to tell you what will happen (rather than making up some silly straw-man caricature of the theory, saying that that’s the *real* theory, showing that the caricature theory makes the wrong predictions, and then acting as if it’s your opponents who should be embarrassed).

        I don’t really want to get into a long discussion about Bell’s theorem, and anyway don’t really understand what your point was in that part of your reply. I guess you just wanted it to be known that you have read, and published, papers about Bell’s theorem? That was never in question. But you are on record as harboring at least the following two major misconceptions: (i) “no signalling” is a *premise* of Bell’s theorem, and (ii) “classicality” (or some undefined sort of “realism”) is a *premise* of Bell’s theorem. If you want to know what I think about these issues you can read my papers. Or Bell’s. Really though the point is just that you shouldn’t be surprised and offended that people (who appreciate Bell and his widely misunderstood theorem) try to educate you and others about it. You should be grateful.

      • Tim Maudlin says:

        OK Reinhard, so slowly that you can understand:
        No physical theory in the history of physics has given, or been expected to give, a physical analysis of how people can reliably see the macroscopic positions of large solid objects. It has always been sufficient, in the entire history of physics, to have a theory that accurately predicts the macroscopic positions of large solid objects. The evidence for Newtonian gravitation was the positions of the planets and the behavior of projectiles, not any analysis of the human optical system or a camera. If anyone doubts that humans can get reliable information about the positions of large macroscopic objects (like pointers, which you seem to hate, or cats, which might be alive or dead, or the location of ink on paper at macroscopic scale) then the entire logical structure of physics collapses and you will be nothing but a skeptic. If you disagree, give me a single counterexample from the entire history of physics of a physical theory that, in order to be taken seriously, had to give a complete physical analysis of how the human visual system gives reliable information about the macroscopic environment. Bohr certainly took this for granted, which was one thing everyone agreed with. Also his insistence that the disposition of physical laboratory equipment has to be taken into account when making predictions about what the outcome of an experiment is. Your rhetoric is self-defeating: first claiming that it is odd to have to take account of the laboratory equipment, which is an obvious thing to have to do, then complaining that the account does not go further into the human eye, which no other physical theory has been demanded to do.

        So: can Bohmian mechanics make, in an entirely principled way, predictions about the macroscopic positions of large solid objects? Yes: the large objects are made of small objects and the dynamics of the small objects is determined completely by the equations of the theory. The macroscopic positions of things follow obviously (and by “obviously” and mean obviously by simple coarse-graining of the microscopic description, the only thing anyone could possibly do in this case, which has nothing to do with any philosophical position and was done through the history of physics) from the microscopic position.

        So: the theory can also predict and explain correlations among macroscopic positions via correlations between their microscopic parts. And there is no reason in the world to think, and every reason to disbelieve, that an analysis of the physical interaction between the human visual system and the macroscopic positions of things (given the sorts of wave functions one would expect on the theory) would not yield reliable correlations between the positions of particles in the brain and the macroscopic disposition of particles in the environment. There is not a shred of evidence that Bohmian mechanics would have any more trouble giving such an account that any other physical theory in the history of physics. If you disagree, give an actual example. And if this does not explain how people can reliably come to know the outcomes of experiment, by getting the states of their brains correlated to the state of the environment, what in world would?

        By the way, as an operationalist, I thought you would appreciate an operational test for when someone has nothing cogent to say: just go erase the empty rhetoric and see what’s left. One might add the childish use of profanity and vulgarity. It’s a good operational test.

      • Matt Pusey says:

        Reinhard: As I said at the conference, I think the position is something like
        (3.5): the outcome of a position measurement is just the Bohmian position, but if you need to know the (possibly non-local) effect of the position measurement on the future evolution of the Universe, including the Bohmian positions of other particles, then you need to model the ornamented brass thing.

        Travis: Without wanting to set off another infinite loop, I’m afraid I don’t agree with you Einstein box disjunction. According to a studiously operational reading of QM, the story is as follows. Before the boxes are opened, the particle does not have a definite position (since this is not an operational notion). If we open box A and find the particle, a new fact is (perfectly locally) created: the particle has been found in box A. At this stage we can also predict that if somebody opens box B, they will not find the particle. But we can’t say the particle is not in box B, because that’s not an operational statement. Similarly if we do not find the particle in box A: there is now a fact that the particle has been found in box A, but there are still no facts about box B since nothing operational has been done to it.

        Tim: I’m not sure looking at previous physical theories is necessarily fair when none of them managed the trick of predicting violations of Bell inequalities without allowing signalling. I.e., it is at least logically possible that we’ve bumped into something about the world that requires us to adjust what we expect from current and future physical theories.

      • Tim Maudlin says:

        Matt (sorry to report, I hit the wrong button above)

        Obviously, I think that (barring Many Worlds, which needs a separate discussion) the phenomena require us to accept some real non-locality in the world, and so demand, in that sense, a new kind of physics. But the phenomena cannot require that physical theories have a new obligation to go beyond getting macroscopic behavior right to be taken seriously, without a fine analysis of the human nervous system. For we have the counterexamples to this being *required* at hand: Bohm, GRW, etc. So if *this* sort of novelty is somehow at least suggested by the phenomena, we should see why. Bell showed how non-locality can’t be avoided. Is there any similar argument that this sort of analysis of the nervous system is indicated?

      • Matt Pusey says:

        Sure, we have counterexamples to being able to this sort of distasteful analysis being *required*. But those counterexamples have problems of their own, the *requirement* of which is in turn ruled out by the counterexample of operational QM. So the possibility remains that the best understanding of nature might turn out to require such analysis.

        In my previous comment, the revision I had in mind was merely giving up our expectation that physical theories don’t require such analysis, which is weaker than expecting that they do require it.

      • Matt Pusey says:

        Oh dear. Scratch “to being able” from the first line above. I also forgot to respond to another of your questions: no, I don’t have a rigorous argument.

      • Travis Norsen says:

        Hi Matt. Yes, let’s avoid infinite loops. So, briefly. I think there’s an ambiguity in your sentence: “Before the boxes are opened, the particle does not have a definite position (since this is not an operational notion).” Saying “the particle was not there, before we looked” is just as operationally-meaningless as saying “the particle was already there, before we looked”. So I think what you really must have meant was not “before the boxes are opened, the particle does not have a definite position” but rather “I refuse on philosophical principle to discuss the question of what may or may not have been going on in the box prior to my observation of its contents.”

        What I’m getting at is that from a normal, common-sense, philosophically-realist (and I mean here: as opposed to the notion of “realism” that means something like hidden variables or determinism which people often think is an explicit assumption in Bell’s theorem) point of view, you don’t at all avoid the disjunction by saying “before the boxes are opened, the particle does not have a definite position”. That’s just one of the disjuncts! Namely, if it doesn’t have a definition position, then its position was somehow vague, it was somehow smeared out between the two locations, or … some such thing that means there is no fact about whether it is fully in the one box, ready to be observed there if/when it is opened, or not. But after (say) the distant box is examined, there *is* now uncontroversially a fact about that same thing. (Which implies that something over there changed as a result of something I did here.) So if you are coming at the issue in this philosophically-realist way, you don’t defeat/avoid the disjunction this way at all.

        To defeat/avoid it, you need to embrace a much more radical philosophical position, and embrace it consistently. I am happy to concede that, if you do this, you will be immune to the EPR argument. Of course, I think this anti-realist philosophy is stupid. Like Bell said about solipsism (which it amounts to, ultimately), it can’t be refuted, but if you take it seriously (and are consistent) you cannot take anything else seriously. But there’s no point arguing about that. Incidentally, I think I learned in one of the discussion periods at the conference that Tim and I disagree about this. I think I understood him to say that no “realism” assumption of any kind is needed to run this EPR/Bell argument. I agree in so far as “realism” means “deterministic hidden variables” or whatever it is people take themselves to mean when they speak of “local realism”. But I do think one has to allow for the meaningfulness of talking about (and making theories that say things about) unobserved/able things, or one can’t even get to the ground floor of the argument. So… if that was your point, Matt, I guess we agree.

      • Tim Maudlin says:

        I’m on board with everything but “distasteful”. I didn’t say it was distasteful, just that no physical theory to date has been demanded to provide it. And maybe a bit more. There is a real epistemological problem here: if you *sincerely doubt* that the deliverences of your senses about the macroscopic dispositions of things are reliable, then you will just have no way to claim you have any good empirical evidence for your theory, since evidence is always reported in these terms. So the most one can reasonably want is a sort of ex-post-facto consistency check: assume your senses are largely reliable, produce a physical theory on the basis of taking them to be, then show that such a theory can, in principle account for the reliability in physical terms. If it can’t you’re in trouble. No reason here to think, e.g, Bohmian mechanics can’t. But nothing distasteful here.

      • Travis Norsen says:

        Matt: “But those counterexamples have problems of their own, the *requirement* of which is in turn ruled out by the counterexample of operational QM.” Did you mean by this that “operational QM” is a counter-example to the claim that only nonlocal theories can explain the data (in Bell-type setups)? And in particular do you mean to suggest that “operational QM” is a local theory? And if so, does “local” in that sentence mean “locally causal” (i.e., “local” in the sense involved in the original claim: “Bell proved that local theories cannot account for the data”)? Or did you switch the meaning on us?

      • Matt Pusey says:

        Travis: “before the boxes are opened, the particle does not have a definite position”. That’s just one of the disjuncts!”

        No it isn’t. The disjunction was (a) there is a definite position beforehand, or (b) action at a distance. I’m claiming that strictly operational QM rejects both (a) and (b). I furthermore claim it does this whilst being 100% realist about things that are actually done and seen.

        This balancing act hinges on the (admittedly somewhat radical) idea that being able to predict something is not sufficient for their being a fact about that thing. Notice that EPR explicitly assume that it is sufficient. Chris Fuchs has a colourful way of looking at it: he considers it to come down to the difference between “probability 1” and “true”.

        I don’t dispute the meaningfulness of making physical theories wherein predictability does turn out to be sufficient for reality (classical mechanics is such a theory). But operational QM isn’t such a theory. So if that was your point, I guess we agree.

      • Matt Pusey says:

        Travis: In answer to your barrage of questions: No, no and no. I just said that operational QM is a counter-example the unspecified problems of the alternative theories being required.

        Full disclosure: As it happens, I _do_ currently think that operational QM is a counter-example to the claim that only theories with action-at-a-distance can explain the data. Therefore I clearly believe that “operational QM” is a theory without action-at-a-distance. Therefore by “action-at-a-distance” I clearly mean more than mere “failure to be locally causal”, since I vaguely remember some mention of a theorem that shows the latter is required.

      • Tim Maudlin says:

        As long as you acknowledge that your theory is not locally causal (or Bell local, for a better word), then there isn’t any issue. Of course, I find the “mere” in ‘mere “failure to be locally causal”‘ an odd qualifier: in the sense at issue for Bell, you acknowledge the theory is non-local. But be clear this is not just saying that the theory is not deterministic: non-deterministic theories can be Bell local.

        I rather doubt that any coherent theory can be made if the only things the theory treats as real are things that are actually seen. Most of the universe isn’t. As for “actually done”, if “doing” means action by an agent, then also most of the things in the universe are not things that are done. So it’s good to be realist about things that are seen and done, but any reasonable theory better be realist about quite a lot more.

      • Travis Norsen says:

        Matt: “No it isn’t. The disjunction was (a) there is a definite position beforehand, or (b) action at a distance.” No, you misunderstood. See my original comment here (May 14 … 2:28) where the disjunction is first put as: either the particle is definitely in one place or the other (prior to any observation) or not. What you said is (FAPP) the latter of these disjuncts. Then there is some further elaboration, showing that these two options map onto “incompleteness” and “non-locality” respectively. The point is that *if* the particle didn’t have a definite position prior to any observation — but (uncontroversially, right?) has one after the (perhaps distant) observation — then that implies spooky nonlocal action at a distance. At least, it implies it in any remotely-philosophically-realist type theoretical account, in which “not having a definite position prior to observation” means something definite, something physical, like perhaps that there’s a kind of fuzzcloud thing, half of which is in each box.

        Note that no appeal is being made to the infamous EPR “criterion of reality”. This is a total red herring, unfortunately introduced by Podolsky (who, as I assume you know, wrote the EPR paper without showing it to Einstein before it got published). The criterion also plays no role in Einstein’s other, later (and actually-Einstein-authored) discussions. So the real question is not whether OQM is a theory “wherein predictability does turn out to be sufficient for reality”. Who cares about that. The question is only whether it says anything about what’s going on physically at all (or is instead just a recipe for predicting future sensations or whatever). To the extent that it doesn’t, I am happy to concede that it thereby “eludes” the EPR dilemma — simply by refusing to engage in the discussion. But refusing to engage in a discussion is hardly the same as showing that there is a third possibility that was somehow neglected. Or to put it in Reinhard’s colorful terms, trying to refute EPR this way is nothing but a philosopher’s fart.

        As to my “barrage of questions”, thanks for answering. I’m honestly not sure why I suspected that the (still unspecified) problem (with dBB, etc.) that you had in mind was nonlocality. Maybe it would help if you just said what problem you did have in mind.

      • Matt Pusey says:

        Tim: I acknowledge the theory is not locally causal. Although I use it occasionally, I think “non-local” is bad shorthand: not everybody agrees what it means, and it to many it sounds much stronger than lack of local causality actually is.

        Replace “coherent” with “fundamental” and “reasonable” with “reasonable fundamental” in your second paragraph, and I agree. Nobody thinks that the theory of evolution is “incoherent” or “unreasonable” because it only applies to imperfectly self-replicating entities competing for resources.

        Travis: In that case your disjunction is a tautology, and my disagreement is with the second part of your “further elaboration”, specifically the supposedly uncontroversial part! In the story I told above, the box only definitely contains / doesn’t contain a particle after a local measurement of the box.

        Re EPR: Is this more hagiography? If Podolsky noticed a subtlety that Einstein didn’t, then so much the better for Podolsky, and so much the worse for Einstein.

        I’m open to the possibility that we will eventually find firm evidence of superluminal causation, in which case all this talk of predictability versus factuality will turn out to have been a philosopher’s fart. But I wouldn’t bet much money on it.

        I don’t have the energy to open a load more cans of worms by going through my problems with dBB. Ask me over a beer sometime. But your suspicion is not unreasonable: some of them relate to the over-enthusiastic way in which it fails to be locally causal.

      • Travis Norsen says:

        Matt: OK, we can just leave it there. But I have to say something about “hagiography”. It’s baffling that you’d even suggest this. Here’s the big picture. Both Einstein and Bell had profound foundational insights that were ignored, misunderstood, and distorted by almost everybody for several generations (including the present one). What apparently appears to you (and Reinhard and probably many others) as “hagiography” of Einstein and/or Bell is in fact simply an attempt to get the history right and set the record straight. I don’t expect you to just agree with that characterization. I just want to point out that it’s the same situation again as with Reinhard’s original post: we should debate the actual issues on which there is disagreement instead of slinging loaded rhetoric. In particular, if you want to actually know whether what I was doing before was some kind of irrational and arbitrary beatification of Saint Einstein — or instead simply correcting a long-standing historical misconception — you should go do a little historical research and find out whether what I claimed is factually/historically correct or not. (You might start by reading Arthur Fine and Don Howard. Reading what Einstein actually wrote, about the EPR argument, in say the Schillp volume, would also be useful.)

        I look forward to hearing more about your reservations about dBB over beers sometime.

  4. Dustin Lazarovici says:

    I hope you don’t mind if I blend into the discussion for a few short comments.

    1) I think that, stripped of all polemics, the point of Prof. Werner is this:
    What we are used to calling a “position measurement” in quantum mechanics is not always a measurement of the position of the Bohmian particles, or at least correlated with that “Bohmian position” in the way that one would naively expect. (This is correct.) So how can we claim that an observation of the pointer position is really an observation of a pointer constituted of Bohmian particles following a Bohmian trajectory?

    Altough the concern might seem weird to us “non-operationlists”, I can understand where it is comming from if one is used to the notion that “physicists measure operators”.

    I hope that Tim’s answer has cleared up the issue. The short answer is simply that this is really not a problem if you take the theory seriously and analyze what it is telling us about the distribution of stuff in physical space and about physical situations that many physicists would call a position measurement. But indeed, from the Bohmian perspective, a so-called “measurement of the position operator” is just an experiment whose outcome-statistics are described by a “position operator” and not necessarily a measurement suitable to provide information about the Bohmian trajectory. The empirical content of the theory is still provided by the fact that it describes the distribution of matter in space, constituting the quantum system, the experimental set-up and the devices that we use to read out the “result” of a measurement. And the virtue of the theory is still that it tells us that every measurment HAS a definite outcome and that it predicts the correct statistics and that it gives a precise and complete picture of what is going on in nature in that respective scenario.

    2) As much as I enjoy the exchange of Tim, Travis and Matt, I’m not sure that it is really helpful to claryfing the issues that started this conversations, since Prof. Werner’s misunderstandings of Bell’s theorem are clearly much more basic.

    3) I may have skipped over a few paragraphes, but has anyone given a clear account of what he means by “operational QM”, yet?

    I think that if the term refers to a certain ontological commitment, admitting only things or events that have been measured or observed, Tim and David have very conclusively explained why this doesn’t help to avoid the consequences of Bell’s theorem.

    I learned in my philosophy of science class that “operationalism” is a general stance on science, saying that the aim of science is not truth (or “approximate truth”) but just to be useful in making empirical predictions, advancing technology, elevating our standard of living, and so on… Operationalism in this sense also doesn’t help to avoid the consequences of Bell’s theorem, of course, but would simply deny the authority of scientific argument to tell us anything about what nature really is like.

    In the end, we may just have to accept that we are not all in the same business of understaing nature.



    • Tim Maudlin says:

      Hi Dustin,

      I agree with everything you said, but one bit might be misleading. It is true that one can describe experimental situations which one might *naively* take to be “position measurements” that do not, in fact, reveal the pre-existent position of the Bohmian particle. This is shown by physical analysis of the situation. But 1) in the case of large solid objects, e.g., tables and chairs and cats and pointers, to my knowledge this is just never the case (that has to do with decoherence…maybe that sheds some light) and 2) even in the case of single particles, it is usually not the case (e.g. a particle hitting a fluorescent screen). So in general, although not always, what seems to be a position measurement really is, i.e. reveals the pre-existent position (according to the theory).

      Note that this is not a comment about position operators, but about which experiments constitute real position measurements, i.e., physical ways of correlating the actual position of one thing with the actual position of another so the one is reflected by the other in the intuitive way.

    • Tim Maudlin says:

      One more point. Whatever “operationalism” as an *ontological* doctrine is supposed to be, it seems to be committed *only* to the “reality” of a very small collection of items…seemingly only laboratory operations. Whatever one thinks of this intrinsically as physics, it is clear that this sort of minimal ontology cannot possibly help *restore* locality in Bell’s sense. It is a simple logical consequence of his definition that if a theory is local (in his sense) and can predict the outcomes of experiments, then any theory with a strictly larger ontology will also be local (with respect to those experiments), but not conversely. Indeed, this was also the point of EPR: quantum theory, as it stood, was manifestly non-local, clearly had “spooky action-at-a-distance”, in Einstein’s sense. But in the EPR experiment, locality can be restored by *adding* to the ontology. That is why the EPR conclusion is that the quantum-mechanical description is not complete. One could not possibly restore the sort of locality Einstein wanted by *subtracting* from the theory! (And obviously, the sort of locality Einstein wanted had nothing at all to do with the absence of superluminal signaling. He knew perfectly well the EPR phenomena did not allow for that.)

  5. Dear discussion partners,

    Before I start repeating myself too much I will make this my last post. It was an interesting experience. The net outcome was perhaps only that I presented myself to the Bohmian community as a new target of ridicule or maybe didactic efforts. I obviously totally failed to make anyone in the Bohmian camp think a new thought, and I have to admit the language of my first post was not the best to achieve that. In the other direction, I did learn a few things about how the Bohmian view works. I also learned that in discussions like this it is good to have Matt Pusey around (thanks!).

    Just some short comments on things that came up.

    Tim got all worked up by my mentioning of “looking” at a record, visual cortex and so on. Of course, I agree there is no problem with any of this, firstly in the practice of science and secondly once you get a good understanding of macroscopic quantum objects. That will indeed provide the assurance that it does not matter which form the macroscopic records of measurement results take. Only: positions play no privileged role here. Spin (i.e., magnetization of a hard disc) works just fine. A Bohmian account has nothing to add to all this apart from what I called a Blessing of Reality, without which I will be just as happy.

    Boxes: My best theory of the particles , (“operational”) quantum mechanics, is fairly silent about this. It will tell us that in the particular situation described, my chance of finding the particle does not change with time and that due to conservation of particle number predictions about the remote friend’s box are ok. Since you are only discussing a single observable (in box or not in box) no harm is done by intermediate checks. If you wish, you can render this in plain English by saying that the particle is one of the boxes, and it is all exactly as for ping pong balls. But this is only because the situation was purposefully chosen so idiotically simple. Let us expand it a bit. The boxes become arms of an interferometer, and the incoming particles are entangled with a similar arrangement far away, so that there is a CHSH violation between the “welcher Weg” and the interference modes of operation of the interferometers. Then your insistence on the particles being in one arm or the other can lead you to make manifestly false predictions. I say “can lead ” and not “must lead”, because there is a Bohmian way out in which it is claimed that this presence of particles in the interferometer arms depends on how we close the interferometers, including the remote one. I don’t think the precise dependence has ever been spelled out (but then I don’t keep a full scan on the Bohmian production). However, rather resorting to a detailed analysis you can always invoke the observation that with a full description of the measuring devices you know you should get the quantum predictions. In this situation I choose to go for the option not involving spooky action at a distance. Operational quantum theory does not contain that in any way. It is always the hidden variable or other naive realist extensions that force this on you. In my experience you can live well without these appendices and still do good physics.

    Bell’s Theorem: Some contributors (especially Travis) have come to the conclusion that I haven’t understood anything about Bell’s inequalities. That is just ridiculous but it is quite possible that there are some language gaps here. After all, the conclusion you take home from such a result for guiding your further research may influence the way you present it in the first place. For me that pragmatic conclusion (NOT the conclusion of the theorem) is to drop “classicality” and opt for “locality”. I will not attempt to summarize the somehow opposite conclusion you take away. But as for the mathematical core of Bell’s Theorem we would certainly be able to reach agreement, even if that may require replacing all plain English terms by numbered assumptions. The version I mentioned was (sorry for repeating myself): Assume that (C) Bob has a joint measurement device replacing the two devices used in a CHSH test, and (L) this does not allow him to receive messages from Alice. Then the conclusion is non(E), in the form of the CHSH inequality. Since there is not a shred of evidence for (C) and likewise no evidence for non(L), the pragmatic decision I mentioned is to go for (L) and drop (C). I can only speculate what the Bohmian choice would be here, probably dropping both this form of (C), since the devices are actually in each other’s way, and also (L), because the local positions are supposed to feel the remote setting.

    So here is my attempt at a summary, with an attempt to avoid inflammatory language.
    (1) Bohmian mechanics adds nothing to the empirical content of quantum mechanics. This is not the aim and so it is unfair to expect anything in this direction. (2) The connection to empirical facts is always by describing the whole macroscopic setup in Bohmian terms. The results are then fixed in the positions of many particles. The connection between their Bohmian positions and laboratory-scale description of pointers is so obvious that one must not ask questions about this. (3) The actual physics of measuring devices is complicated and so is better left to the people in the physics department. For that practical purpose it is about the least helpful proposal one can make to analyse the measuring device in microscopic terms. Using this as the only theory of measurement (as seems to be the rule in Bohmian discussions of measurement) can only mean that Bohmians are utterly uninterested in such things. See (1): this is not what the theory is after in the first place.
    (4) Whether on the microscopic level Bohmian positions have any correlation with any empirical facts is an ill-posed question. Bohmian trajectories of an unobserved system have no resemblance to those one gets when the measuring devices are included. Here it is the theory “Bohmian Mechanics” which explains what can be measured: Bohmian positions cannot.
    (5)This is no trouble at all for the Bohmians, see (1). Trying to learn something about the trajectories of unobserved systems is a pointless enterprise.
    Trying to find out anything about the trajectories in a measuring device is much harder and even more useless. Wigner’s friend might be watching and there will be uncontrolled outside influences and all that. But we should take at face value that, whatever the outcome of such a computation, the fapp guys probably got it right and so the Bohmian trajectories just must be where we see the pointers when we visit the lab. Proof by inspection is entirely sufficient for that (and no, don’t ask about “inspection” or sneezing on the pointer here).
    (6) I hope that was a reasonable summary of what Bohmians are NOT after. I find it harder to describe what they are after. It is somehow related to Reality (capitalized if I may) and what there Really is. In spite of all that reliance on the work done in the physics department, there is presumably something that is lacking in everything mere physicists do. I suppose they are blinded by “unprincipled dogma” and anyway they do not know what they are talking “about”. I honestly don’t see anything lacking. I have done a fair bit of physics (by the way, mostly taking reality at the macroscopic level for granted) but I never found preconceptions about reality at the quantum level helpful. The reason might be that to me physics is about an *empirically guided* understanding of the world. If that is not what you are after we can leave it at that.

    • Travis Norsen says:

      I completely agree with Dustin’s nice comments, but wanted to add one note about the “boxes” argument. Reinhard, it seems that you have reverted here in your last post to your earlier misunderstanding — that somehow the boxes argument is supposed to convince you that the particle really has a definite position prior to measurement. That is simply not right. That is not what is being argued for. The only rational explanation I can come up with for why you’d continue to be confused about this is that the boxes argument is getting confused with the onslaught of “bohmian propaganda” in which the idea of particles having definite positions (even when not observed) is rather prevalent. So let’s change the example, back to the standard Bohm (1951) version of EPR involving a pair of spin-1/2 particles. This will be nice because I know you know that we Bohmians do not believe that spin is Real, i.e., we do not believe that spin 1/2 particles have definite pre-existing pre-measurement values for spin components. (See the pedagogical paper, “The pilot wave perspective on spin”, that I put on arxiv last week if there is any confusion about this, but I’m pretty sure everybody appreciates this.)

      So, run the same argument with spin. Two separated particles are in a state of total spin 0, alice and bob randomly choose some axes along which to measure the spins of their respective particles. When they both happen to choose (say) the z-axis, they always get opposite outcomes. Consider one of those cases. Neither Alice nor Bob has made their measurements yet. Suppose Alice makes her measurement first. After she makes it, there’s now a fact of the matter about how Bob’s measurement will come out. Now the question at issue is: did Bob’s particle already possess this definite value (somehow encoded in its structure or whatever, waiting to be revealed via interaction with Bob’s device) prior to Alice’s measurement? It’s yes or no. It either did or it didn’t. *That* is the conclusion. It’s a *dilemma*. The conclusion is not “Yes, it did!” The conclusion is rather only the (tautological, as Matt pointed out) statement that it either did or it didn’t. Good? Now, if it did, that’s a kind of “hidden variable”, which OQM knows nothing about. On the other hand, if it didn’t, then it evidently *acquired* this definite value as a result of Alice’s measurement (because we already agreed above it definitely *does* have it *after* her measurement). So the tautological dilemma translates directly into the dilemma: either OQM is incomplete (i.e., in particular, there exist deterministic hidden variables for these spin component measurements) or there is nonlocal action at a distance.

      Maybe you’ll be able to actually grasp the argument in that form since you know that I and other Bohmians *do not believe in deterministic hidden variables for spin components*?

      Finally, why does this matter? Because your wrong understanding of Bell’s theorem is based on failing to grasp this very argument. You continue to talk as if the upshot of Bell’s theorem is that we have to choose (at least) one of (what you only quite vaguely describe as) “classicality” and “locality” to reject. You have said that you choose to reject “classicality” and retain “locality”. But the “boxes” argument, or equivalently the EPRB argument I’ve recapped above, shows that in fact this is not a viable option at all. Because what you actually mean by “classicality”, when we hold hands and look at the mathematical derivations (of the inequality) that you undoubtedly have in mind, is nothing but the idea of deterministic hidden variables. But then it is immediately obvious that you cannot “save locality” by abandoning these hidden variables: having the hidden variables was the *only way* to account for the perfect correlations (when Alice and Bob both measure along z) without nonlocality! So you simply cannot account for all the QM predictions in a local way without these hidden variables. (That’s what the EPR type arguments show.) Nor can you account for all the QM predictions in a local way *with* these hidden variables! (That’s what Bell showed!) So as it turns out the hidden variables — your “classicality” — is a completely and total red herring. You can only account for the QM predictions with a *nonlocal* theory. That’s what Bell proved. And all of our threories (I mean Bohmian mechanics, OQM, etc… leaving aside MWI which is also nonlocal but in a distractingly different way) exemplify this. You only convince yourself otherwise by equivocating — by claiming your theory is local (but with a new and different meaning, a meaning for which Bohmian mechanics, too, is “local”).

      But now you no longer have any excuse for making these mistakes.

  6. Dustin Lazarovici says:

    Dear Prof. Werner,

    I’m sure someone more competent and more accomplished than me will answer to your last post, but I hope you allow me to give you my perspective as a student of Detlef’s and thus as part of the “next generation” of Bohmians.

    I really think that most Bohmians cherish any intelligent challenge of their favorite version of quantum mechanics. Unfortunately, most people’s dismissal of BM is based on ignorance and prejudice and dogma, rather than rational argument. Also in your contributions, I found it quite difficult to disentangle the two, although I think that you did raise some legitimate points that I am going to think about and I am thus grateful for the time you’ve taken to debate with us.

    It is certainly correct that BM may have to tell some story about if and how certain kinds of measurements or observations correlate with the position of Bohmian particles and that this story will have a lot of overlap with the one that non-Bohmians tell about decoherence and macroscopic localisation and so on. It is also correct that the Bohmian description of a measurement device will be extremely complicated, but then again, the description of a macroscopic object in terms of a microscopic theory will always be extremely complicated and require a good deal of approximation and idealization and FAPP-concepts. It is not true, though, that Bohmians don’t care about these things. Quite the opposite: the point is that BM allows in principle for a complete description in which the the quantum system and the measurement device and the laboratory equipment follow the same set of rules, whereas standard quantum mechanics doesn’t, at least not without running into the measurement problem.

    All in all, it seems to me that the center of the controversy is indeed Bell’s theorem and the problem is that, with all due respect, your understanding of Bell is purely and simply wrong. This is NOT a matter of language, or interpretation, or what you want to take away from the theorem, but simply a matter of following a subtle, yet precise and conclusive logical argument. And the fact that you are wrong about that argument can be seen, for instance, from the fact that your condition (C) doesn’t appear anywhere in Bell’s analysis and that your version of the “locality condition” (L) is not violated by Bohmian mechanics, although this theory does clearly and obviously an manifestly violate Bell’s notion of locality.

    I am not in the position to give you advice on literature, but if you feel motivated to revisit the issue, let me mention Tim’s book (quantum non-locality and relativity) or Travis’ papers (e.g. Bell Locality and the Nonlocal Character of Nature) or Bell’s own expositions (e.g. Bertlmann’s socks or la nouvelle cuisine) as great sources for that.

    If I may contribute one last thought: Most Bohmians I know are really not dogmatic about particles or trajectories, or determinism, or anything like that. The only thing that we are somewhat dogmatic about is ontology. We have a certain understanding of what a fundamental theory of nature should be like, which is that it must make an ontological commitment and tell us what there is in nature and how it behaves and allow us to derive correct empirical predictions from that. And we find that a formulation of QM in this vain gives us a much better physical understanding and avoids the usual problems and paradoxes.
    I get it that many physicsts may disagree with that, but there is something tragic and distrubing and debilitating about the way in which most people have falsely come to believe that the quantum phenomena force us to give up the idea that physics can develop a clear and complete and precise picture of nature. And to me there is also something suspicious about the way in which you seem to find that ideal not just superfluous or overly ambitious, but “silly” or even offensive.
    Again, if it all comes down to Bell, that is, if you believe that Bell has proven that quantum physics presents us with a choice: either give up the idea of a clear ontology and a complete and coherent physical description OR accept spooky action-at-distance, you are wrong about that – my respect for your accomplishments as a mathematical physicsts notwithstanding.


    Dustin Lazarovici

    • Dustin Lazarovici says:

      Just to make the last point a bit more precise:
      Neither is “classicality” or “realism” a clearly identifiable assumption that one could give up in order to save locality, nor is non-locality quite as “spooky” as you make it out to be, as it does not imply the possibility of faster-than-light signaling.

    • some random commenter says:


      I might not be qualified to take part in this discussion, but I’ll try anyway.

      Could it be, that the disagreement on Bell’s theorem simply stems from a different terminology?
      It seems to me, that your notion of locality (you are talking about Bell locality, right?) is different from Prof. Werner’s.
      He is dividing the condition of Bell locality into two separate conditions one named “locality” or (L) and another named “classicality” or (C).
      Whether one would want to do that, or how those conditions are exactly defined might be another question, but I doubt Prof. Werner would disagree with the statement, that a violation of Bell inequalities shows, that the theory can’t fulfill the condition of Bell locality.

      • Travis Norsen says:

        Interesting hypothesis. But I would bet Prof. Werner isn’t familiar with Bell’s careful formulation of local causality (i.e., “Bell locality”). It’s certainly not clear how a decomposition of this into (things that could reasonably be named) “classicality” and “locality” would go. Did you have something specific in mind here?

        As to your last statement, Prof. Werner made it abundantly clear that he thinks one doesn’t need to reject locality in response to the Bell experiments; one can reject instead “classicality”. It’s true that he never said this explicitly in terms of “Bell locality”. But it’s been made crystal clear by several posters here that that is the only sense of locality that is implicated here. So I think a more natural reading of the evidence would be that Werner *does* “disagree with the statement that a violation of Bell inequalities shows that the theory can’t fulfill the condition of Bell locality”.

  7. Travis Norsen wrote a recent review stating that Bell’s work shows a breakdown of local causality. To me this means qm is incomplete. Tim Maudlin wrote a book called the Non-Local Universe in which he tries to rationalize some of the absurd properties of non-locality (first chapter: superluminal; robust (independent of distance between EPR pairs), and discriminating (only acts between previously interacting particles).

    These points of view are miles apart. I side with Norsen because non-locality makes no sense and must be eventually be removed from physics as a bad idea gone viral.

    • Tim Maudlin says:

      Actually, Travis and I completely agree 100%: Bell’s theorem proves that there is non-locality. Quantum mechanics may well be incomplete, in the sense that there is more to physical reality than the quantum state, but that recognition does not in any way suggest, or provide a way to claim, that there is no non-locality. If you think what I write and what Travis writes are “miles apart” then you have failed to understand one or both of us. Go back and read again, and try to find where you think we disagree.

      • Ok, glad the two of you agree. I could follow few of the arguments after chapter 1 of your book. Do you believe you have answered “quantum weirdness”? If so, a nutshell rationale would be welcomed.

      • Tim Maudlin says:

        The various chapters of the book discuss different possible senses of “something going faster than light”, and of these senses, in which sense violations of Bell’s inequality require something to go “faster than light”. What we know is that there must be some non-locality, in a well-defined sense that Bell gives. You might look a the Scholarpedia article on Bell by Travis and Shelly Goldstein and others for a careful discussion. In general, this non-locality is implemented in quantum theory via the quantum state, which is itself not a local object. You could reject this picture (and Travis does have worries about the quantum state), but if you replace the quantum state with something else, that something else must also display some sort of non-locality in order to predict violations of Bell’s inequality.

  8. Just a comment concerning the article arXiv:0912.3740, by Kiukas & Werner.

    In Bohmian Mechanics, if the particles are entangled, then Alice’s measurement of the position of her particle yields the Bohmian position of her particle. Alice’s measurement changes the Bohmian trajectory of Bob’s particle. Now, when Bob measures the position of his particle, this measurement yields the Bohmian position of his particle. Of course, computing the Bohmian position of Bob’s particle ignoring Alice’s measurement does not yield the right prediction of Bob’s measurement. This would be like computing the trajectory of a golf ball flying around, ignoring the fact that it banged into a tree in mid flight, and then blaiming the theory that it didn’t make the correct prediction about where the golf ball is going to be found. (In this analogy, the “golf ball” is Bob’s particle and “banging into a tree” corresponds to Alice’s measurement of the position of her particle.)

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