Nonlocality: correlation vs causation

[Ken G]

But here's the $64,000 question: what determines which branch the two observers, A and B, are in?

 

[Demystifier]

But here's the $64,000 question: what determines which branch the two observers, A and B, are in?

That's easy: the initial particle positions.
Can I get my $64,000 now?

 

[bohm2]

You have understood 2. incorrectly. In particular, there is no problem of empty branches in Bohmian mechanics. What some view as a problem in Bohmian mechanics is nonlocality, which a solipsistic approach removes...Is it clearer now?

Thanks. This issue of "empty waves" is still confusing me because even some pro-Bohmian physicists (see quote above from Daniel J. Bedingham) have raised some issues on this topic. Even in Bohm's book that I read, he didn't give a very good explanation. The just of it, is the following criticism:

An obvious strategy for defeating the above argument in the Bohmian case is to claim that wavefunction-stuff is just not the kind of stuff from which objects like cats could be made, even in principle. One might even claim that the wavefunction is not any kind of “stuff” at all, but is merely a mathematical device for calculating the motions of the Bohmian particles. If either of these claims could be substantiated, then one would have a principled reason to deny that empty branches could contain cats, either dead or alive, or any other measurement outcomes for that matter.

Against this strategy, however, Deutsch writes of the empty branches (or “unoccupied grooves”) that “it is no good saying that they are merely a theoretical construct and do not exist physically, for they continually jostle both each other and the ‘occupied’ groove, affecting its trajectory” . Since empty branches interact with each other and with the occupied branch, and empty branches are nothing but aspects of the wavefunction, the wavefunction must be real a physical entity and not just a mathematical construct.

Empty Waves in Bohmian Quantum Mechanics
http://philsci-archive.pitt.edu/2899/

Valentini discussing Bohmian "empty waves" responds:

Furthermore, in realistic models of the classical limit, one does not obtain localised pieces of an ontological pilot wave following alternative macroscopic trajectories: from a de Broglie-Bohm viewpoint, alternative trajectories are merely mathematical and not ontological.

De Broglie-Bohm Pilot-Wave Theory: Many Worlds in Denial?
http://www.tcm.phy.cam.ac.uk/~mdt26/local_papers/valentini_2008_denial.pdf

But I thought the wave function even in Valentini's de-Broglian interpretation was not just mathematical but represents a new type of non-local "causal" agent as suggested in this video by him?

Valentini video- from Perimeter Institute The nature of the wave function in deBroglie’s pilot wave theory
http://streamer.perimeterinstitute.ca/Flash/3f521d41-f0a9-4e47-a8c7-e1fd3a4c63c8/viewer.html

 

[bohm2]

You have understood 2. incorrectly. In particular, there is no problem of empty branches in Bohmian mechanics.

Here's Bohm's explanation of the empty branches (it begins on p. 335):

While it is clear that what is automatically called the measurement process has thus been given an overall causal and objective description, one may nevertheless ask what is the meaning of all the “empty” wave packets (i.e., those not containing particles). Those still satisfy Schrödinger’s equation, but are nevertheless permanently inactive, in the sense that they never manifest themselves in the movements of the particles at all. Such packets seem to be floating, almost like wraiths in a strange multi-dimensional world. One can see, for example, in the “many worlds” interpretation of Everett, the problem is dealt with in a certain way, as each packet would correspond to a different universe, with its different measuring instrument (along with its different human observer as well). What do all these “empty” packets signify in the causal interpretation?...

As long as the measuring apparatus interacts reversibly with the classical system, channels that are inactive with regard to the particles are still potentially active. But as soon as the irreversible interaction with the registration device takes place, the channels not containing particles are permanently inactive. In the usual language, we would say that the information has been “lost”, but as with the diffusion of ink particles, it has merely ceased to be capable of acting in the manifest domain...

Before the decision is made, each of these possibilities constitutes a kind of information. This may be displayed virtually in imagination as the sort of activities that would follow if we decided on one of these possibilities. Immediately after we make such a decision, there is still the possibility of altering it. However, as we engage in more and more activities that are consequent on this decision, we will find it harder and harder to change it. For we are increasingly caught up in its irreversible consequences and sooner or later we would have to say that the decision can no longer be altered. Until that moment, the information in the other possibilities was still potentially active, but from that point on such information is permanently inactive. The analogy to the quantum situation is clear for the information in the unoccupied wave packet becomes more and more inactive as more and more irreversible processes are set in train by the channel that is actually active.

An ontological basis for the quantum theory
http://www.tcm.phy.cam.ac.uk/~mdt26/local_papers/bohm_hiley_kaloyerou_1986.pdf

I still don't understand what Bohm means here?

And here is Brown's criticism of Valentini's position:

According to Valentini, the unoccupied component is merely “simulating” the approximately classical motion of the atom. Valentini further claims in section 6 that the treatment of the analogous, and more pressing, case of a superposition of non-overlapping packets representing distinct macroscopic arrangements is conceptually just the same. But in both cases, this notion of simulation is hard to reconcile with the plausible claim in that, even in pilot-wave theory taken on its own terms, the intrinsic properties of quantum systems such as mass (both inertial and gravitational), charge and magnetic moment pertain to (at least) the pilot-wave. If in the second case the macroscopic systems involve contain human observers, and the superposition is defined relative to the appropriate decoherence basis, it is hard to see why phenomenologically the unoccupied component does not have the same status as it does in the Everett picture.

Comment on Valentini, “De Broglie-Bohm Pilot-Wave Theory: Many Worlds in Denial?”
http://www.tcm.phy.cam.ac.uk/~mdt26/local_papers/brown_on_valentini.pdf

 

[Ken G]

That's easy: the initial particle positions.
Can I get my $64,000 now?

No, because an initial particle position should not determine something about the observer, it's backward. That's the "empty branch" problem in spades.

 

[Demystifier]

because an initial particle position should not determine something about the observer,

I don't understand why do you say this. In the solipsistic HV interpretation the observer IS a bunch of particles, so particle positions do determine the observer.

 

[Ken G]

I don't understand why do you say this. In the solipsistic HV interpretation the observer IS a bunch of particles, so particle positions do determine the observer.

But that is not a closed stance, because you have the particle positions determining the observer, yet it is quite demonstrably true in physics that the observer determines the particle positions. Your tiger is chasing its own tail. Whether or not that is a serious objection depends critically on what goals you have for the picture you are espousing.

This is how I look at it: the sole reason for invoking classical trajectories in an interpretation of any theory of dynamics is that we understand them well. This is a valid reason for Bohmian interpretations-- the goal of an interpretation is to bring a physics theory into the realm of what we can understand. So we have a valid question-- can we find a way to think about the theory of relativistically consistent quantum mechanics using a picture of classical trajectories underpinning our understanding? That is a valid goal to establish, and that is what you have attempted to do, possible successfully-- I have not delved deeply enough nor are enough of an expert to say one way or the other. I certainly see no obvious flaw in the endeavor. My main point is that we must distinguish three very different things:

1) physics theories: these make predictions by borrowing from some formal mathematical structure, but take no stance on the meaning or lessons of the predictions, only their practical usefulness and demonstrable success.

2) interpretations of physics theories: these assist in the application of the theory, and can give us a sense of meaning behind the theory, but are not unique and are chosen based on pedagogical and philosophical considerations, not testable predictions.

3) claims on reality: it is natural for any physicist who adopts some form of realism to want to take successful theories and use them to make claims on how reality works. This can inform our search for new theories also, and generally borrows from particular interpretations of existing theories. But these are not the same as interpretations of theories, because they go beyond the theory and make assertions about reality, possibly in the form of lessons we have learned about reality (not about our theories). Some reject this step altogether, but most physicists wish to take their understanding of the theories and graduate them to an understanding of some aspect of reality.

Thus, I'm saying that anyone who would endeavor to explore the terrain of the requirements for relativistic QM and how it can be made consistent with classical trajectory concepts needs to decide which of those three areas they are shooting for, or what kind of combination of all three. It's important to distinguish claims made in the various areas, because they are judged by different criteria. I'm not completely clear the claims you are making about the solipsistic HV interpretation, as to which of those areas are being invoked, but I do think that certain criticisms of it might fall victim to misjudging the appropriate sector, thereby applying inappropriate criteria. Clarifying the sectors thus clarifies the appropriate criteria, and can effectively deflect much of the criticism.

 

[Demystifier]

yet it is quite demonstrably true in physics that the observer determines the particle positions.

It is true in experimental physics but not necessarily in theoretical physics, depending on how exactly one interprets the role of observer in theoretical physics. In particular, if one assumes that the known theoretical laws of physics (either deterministic or probabilistic) describe also the observer, then it is not true that observer determines the particle positions. Instead, particle positions are determined by the laws of physics and the corresponding initial conditions.

 

[Ken G]

It is true in experimental physics but not necessarily in theoretical physics, depending on how exactly one interprets the role of observer in theoretical physics.

Yes, I suppose I am exposing myself as an empiricist. But all the same, it is clear that what we choose to regard as "laws" we do so by virtue of agreement with experiment, not logical necessity (the ancient Greeks were a little unclear on that, but Galileo straightened them out). Thus, it is inescapable that our ability to interact with nature, and perceive and interpret that interaction, must enter into anything we can call physics.

What's more, In particular, if one assumes that the known theoretical laws of physics (either deterministic or probabilistic) describe also the observer, then it is not true that observer determines the particle positions. Instead, particle positions are determined by the laws of physics and the corresponding initial conditions.

But that is the logical morass I was referring to. Of course you may assume whatever you like, but physics is not about making assumptions, it is about verifying them. How are you going to verify that the particle positions are determined by "the laws of physics" (whatever that is) without first using an observer to establish that? And how is that observer going to establish that the particle positions, thus established by the observer, are what determines the observer? If we stick to what we can demonstrate, we cannot hold that particle positions determine observers.

 

[bohm2]

If we stick to what we can demonstrate, we cannot hold that particle positions determine observers.

But it seems reasonable to assume that the observers are made of that same kind of stuff as other stuff. Which is the reason some have difficulty with the orthodox view. As Maudlin writes:

The reason that this problem does not come up in practice is because the ‘standard’ interpretation is a legacy of the Copenhagen view, and the Copenhagen view does not postulate wavefunction monism. Copenhagenism insisted on the necessity of having a classical description somewhere, the description of the ‘measurement situation’: the infamous Copenhagen ‘cut’ was exactly between a quantum realm and a classical realm. And the classical description would, of course, be in terms of local beables, so there is no problem applying a spacetime transformation to it. Within this sort of a dualistic picture the problem of spacetime transformations of the wavefunction can be approached. The problem, of course, is that this sort of dualistic ontology is impossible to take seriously: no one ever thought that there were really two different sorts of physical systems, the classical and the quantum, that somehow interact. If that were the view, then the ‘cut’ would be a matter of physical fact: somewhere the classical and quantum bits of ontology would actually meet. Furthermore, it is evident that the ‘classical objects’, measuring apparatus and so on, are composed out of ‘quantum stuff’ (electrons, protons, and so on), so this cannot really be a dualistic ontology. In the confused morass of Copenhagenism, the observation that the ‘cut’ could, For All Practical Purposes, be moved about at will within a large range was taken to show that the cut itself corresponded not to a physical fact but to a convention, or something like that. But if the theory can be formulated without a cut at all, let it be so formulated. Having removed the cut and put everything in the quantum ontology, one would evidently remove all the local beables, and all the problems we have been discussing would return.

 

[Ken G]

But Bohr already found the refutation of that position when he said "there is no quantum world." The "cut" is more a Heisenberg creation, I believe-- Bohr never thought there was anything like we imagine on the other side of the cut. So Bohr would not hold that we are "composed of" quantum stuff, instead he would probably have said that whatever we are actually made of, we have no choice but to study it by conceptualizing it with the elements of the theory of quantum mechanics, and no choice but to test the success of that conceptualization but interacting with it via classical instruments. I don't think anything in that stance can be refuted, it is all simply true. The philosophical stance appears when one goes beyond what cannot be refuted, and says, "but it has to be more than that, it has to actually be 'quantum stuff' or quantum mechanics couldn't work so well." To that argument, I simply ask "how do you know?" After all, Newtonian gravity sure seemed like what gravity had to "actually be" to many who studied it throughout the eighteenth and nineteenth centuries, so weren't they asking "but if gravity wasn't really a force created by masses, then how could Newton's theory work so well?" In my opinion, physics really doesn't do ontology-- it just creates what it needs to imagine is ontology, and it suffices for awhile.

 

[Len M]

But that is the logical morass I was referring to. Of course you may assume whatever you like, but physics is not about making assumptions, it is about verifying them. How are you going to verify that the particle positions are determined by "the laws of physics" (whatever that is) without first using an observer to establish that? And how is that observer going to establish that the particle positions, thus established by the observer, are what determines the observer? If we stick to what we can demonstrate, we cannot hold that particle positions determine observers.

And isn’t this the crux – a crux that is more often and not ignored or brushed under the carpet by so many. The people doing the ignoring are often highly intelligent and knowledgeable about physics and on reading these accounts from the perspective of a non physicist, the confidence of the author just oozes out. Yet important questions (for me at any rate) about what actually the physics refers to in an ontological sense take on an almost default position by the author (i.e. the physics refers to nature as it exists without our involvement). To argue against that stance is taken by many (not all) as an argument against the physics and if you can’t take a proper part in that discussion because of the very technical nature of it, then objections of a philosophical nature are treated with some contempt.

I have noticed on a few threads now that extremely knowledgeable physicists just do not take into consideration the philosophical perspective that surrounds their very high order of physics. That perhaps is of no issue within a close minded group of similar people, but when that very high order of physics smothers any questions about the relationship of that physics to nature as it exists outside of the means in which the physics is practiced, then the losers are not the physicists but the rest of us reading these public forums (as well as “popular” accounts written by physicists) who are recipients of the physics that implies a “default” position (nature outside of our involvement is described by our physics). Some (including myself on a personal level) attempt to delve a little into the philosophy within physics, but it’s difficult. So I value the physicists who are prepared to do this and I value the extent that Ken G explores these issues on this forum.

 

[Ken G]

To argue against that stance is taken by many (not all) as an argument against the physics and if you can’t take a proper part in that discussion because of the very technical nature of it, then objections of a philosophical nature are treated with some contempt.

That's true even if you can take part in the technical discussion! The problem is, any "technical" discussion already starts with certain implicit assumptions about what physics should be regarded as trying to do, so if you deign to look under the hood of those assumptions, the technical discussion is not yet even relevant. But those with technical mastery hate to take such a "backward step" to look at their own implicit assumptions, so end up "proving" things without recognizing that their proofs are unjustified by different philosophical priorities. It's a big problem that creates a lot of misunderstanding and disagreements even among experts who are technically proficient but who are unwilling or unable to think philosophically. As one example, I offer the issue of interpretation of QM, where we find people who must be considered experts in quantum mechanics like Penrose saying that Deutsch is "not serious" about QM, while Deutsch says Penrose is "doing aesthetics not physics." Is one of them right, and the other a fool? I doubt it, I suggest they simply translate their own philosophical priorities into different conclusions.

 

[Demystifier]

Penrose saying that Deutsch is "not serious" about QM

Why is Penrose saying that Deutsch is "not serious" about QM?

 

[Ken G]

Penrose doesn't think any set of postulates that could create a theory like we should be trying to create for QM could entail a structure in which our perceptions of wavefunction collapse could stem from unitary evolution, and indeed this can be proven under certain assumptions (which is what I mean by the philosophical priorities that I assume Deutsch would reject). Penrose believes that "collapse" is a real dynamical effect, and as such must emerge dynamically from whatever postulates are used for QM. The direction he looks for this to be included in the postulates is in quantum gravity, because he thinks that wavefunction collapse is a kind of true dynamical instability that appears on scales above the Planck mass, owing to a kind of dynamical collapse of spacetime itself. In that way, he thinks QM needs to be made more consistent with dynamical theories of spacetime like GR, rather than the other way around.

As near as I can tell (best would be to have him comment on this, but it seems pretty clear this is what he is saying), he views as "not serious" anyone who is not looking for a dynamical explanation of wavefunction collapse (though I believe he does not require that people be looking at quantum gravity to be considered serious about QM, but he thinks serious efforts must point toward postulates in which everything that we perceive follows directly from the theory without any ad hoc steps). Of course this means he regards as non-serious any work from the perspective of CI (where the collapse is itself a postulate) and MWI (where collapse never happens in reality, but is perceived as a kind of error of interpretation by the perceiver), and maybe even Bohmian mechanics (where there is no collapse, the system always starts out collapsed but we don't know it). I'm not sure about Bohmian mechanics, because perhaps Penrose's picture can be made consistent with that, but there would seem to be some disagreement about whether or not the collapse has already occurred for quantum systems that have never interacted with anything on the scale of a Planck mass.

 

[Demystifier]

That's a very strange view of what it means to be not serious about QM.

By the way, Penrose is not fully satisfied with Bohmian mechanics, in my opinion for wrong reasons. Anyway, this is what he says about Bohmian mechanics:
"My difficulty is that there is no parameter defining which systems are, in an
appropriate sense, ‘big’, so that they accord with a more classical ‘particle-like’
or ‘configuration-like’ pictures, and which systems are ‘small’, so that
the ‘wavefunction-like’ behaviour becomes important"
[quote from "The Road to Reality", Sec. 29.9]

 

[Ken G]

That sounds like a general complaint that Penrose has about QM writ large, not just Bohmian approaches, and indeed the way he seems to criticize lots of QM interpretations is not that they are wrong, but rather that they "don't solve the problem." That appears to be why Penrose targets the Planck mass as an appropriate place for the "cut" (the other Planck scales are mind-bogglingly small, but the Planck mass is curiously large, only about the mass of a dust particle). Note also that Penrose doesn't think it is "serious" to leave the cut as a postulate, he wants a theory that describes the Born rule in a completely dynamical way, like how classical mechanics describes the cut between individual particle trajectories and fluid models. I applaud Penrose that he is willing to attempt that daunting task, though I agree that characterizing essentially everything else as "non-serious" is an unfortunate choice of words! I think he is trying to separate what he views as his own more mathematically rigorous mission from other kinds of inquiry, without being too insulting, but most people would take "non serious" as an insult.