Exploring Bohmian Mechanics: Questions for Non-Physicists

[nomadreid]

One of the favourite themes for threads which create long discussions here is on Bohmian mechanics. Unfortunately, most of them operated under the assumption that the reader is familiar with the intricacies of this ... interpretation? theory? I do not wish to rehash all that has been written on this theme, but rather, as someone who is neither a physicist nor well familiar with Bohmian mechanics beyond the superficial text descriptions without going through all the maths in Wikipedia https://en.wikipedia.org/wiki/De_Broglie–Bohm_theory or Stanford Encyclopedia https://plato.stanford.edu/entries/qm-bohm/ (this latter being obviously written from a partisan point of view), I would like to pose two relatively elementary questions to better understand the dispute.
First, I read that an objection to Bohmian mechanics is that it is non-local. Yet I thought that quantum mechanics, with its handling of entanglement, was necessarily non-local. So how is Bohm's theory different in this respect?
Secondly, I read in Wikipedia that there is a 2016 article https://en.wikipedia.org/wiki/De_Broglie–Bohm_theory#cite_note-67 which points out an experimental difference between a prediction made by mainstream QM and the Bohmian mechanics. (see attachment). If this is the case, could not one settle the dispute through straightforward experimental data?
(Yes, I know that there is a whole discussion https://www.physicsforums.com/threads/an-argument-against-bohmian-mechanics.898028/ about https://arxiv.org/abs/quant-ph/0001011, but that article predates the one I am asking about.)
From these questions you can see that I am not able to wade through all the massive amount of arguments presented on either side, but I would be grateful for any indication that would at least allow me to get my toes wet.

 

[DrChinese]

Here's a couple of very simple comments to start things off:

1. "objection to Bohmian mechanics is that it is non-local" - Yes, you could say QM itself is non-local because systems of entangled particles do not have a spatial limitations that you might expect. In other words, distance is not a factor as it would be in a local theory. But there is no specific element or mechanical model that drives this as there is in the Bohmian world (where particle position is king, and every particle instantaneously interacts with every other particle).

Keep in mind that there are ways to get the apparently "non-local" effects of QM without having instantaneous action at a distance. 2. There have been a number of experiments that rules out Bohmian type theories. However, none of them are of a type that Bohmians would agree are decisive. And generally, those experiments do not specifically attempt to address Bohmian Mechanics. There are some exceptions. Not sure if you have seen these 2. These types, along with the ones like you mentioned about trajectories, are often brought out.

https://arxiv.org/abs/1410.2014
Suarez (2014) A Michelson-Morley-type experiment is described, which exploits two-photon interference between entangled photons instead of classical light interference. In this experimental context, the negative result (no shift in the detection rates) rules out David Bohm's postulate of an infinite-speed time-ordered "quantum potential", and thereby upholds the timeless standard quantum collapse. ... Therefore Bohm’s “preferred frame” assumption can be considered falsified by experiment to the same extent as relativity is considered to be confirmed by it.

https://arxiv.org/abs/1002.1390
Gisin (2010) Local variables can't describe the quantum correlations observed in tests of Bell inequalities. Likewise, we show that nonlocal variables can't describe quantum correlations in a relativistic time-order invariant way.

At this point, there is not much on either side to convince those of opposite view.

 

[nomadreid]

Thank you very much, DrChinese. I have downloaded the articles you mentioned and will be looking at them with interest. As far as I understand, the proponents of Bohmian mechanics would say that the results which are refuted by experiment are not actually implied by Bohmian mechanics. (Or, to put it another way, they say that Bohmian mechanics is an interpretation, whereas the others try to show that it is an independent theory.) This state of affairs is strange to someone like me who is used to working in pure mathematics, where, for any dispute as to whether a theory has a certain consequence can be (attempted to be) resolved by going back to the axioms and rules of inference; the stereotype one has of physics is that the experiments are the final arbiter, but it appears that physics contains more ambiguities than a non-physicist expects. I will be reading these papers to see if I can understand where this ambiguity lies.

As far as the difference between non-local and action at a distance, I tend to think of the former in the case of entanglement as arising because the two particles are really a single entity and hence not independent, and the latter as being a relation between two independent particles, in the same way that causation is. I am not sure this is correct, though.If it is, my vague impression is that QM offers the former but not the latter, whereas BM offers also the latter. Would this be way off the mark?

 

[Demystifier]

First, I read that an objection to Bohmian mechanics is that it is non-local. Yet I thought that quantum mechanics, with its handling of entanglement, was necessarily non-local. So how is Bohm's theory different in this respect?

Standard QM has nonlocal correlations. Bohm's theory has nonlocal causation. Some people think that nonlocal correlation is much weaker form of nonlocality than nonlocal causation. I disagree, I think that's essentially the same:
https://www.physicsforums.com/threads/nonlocality-correlation-vs-causation.506860/

article https://en.wikipedia.org/wiki/De_Broglie–Bohm_theory#cite_note-67 which points out an experimental difference between a prediction made by mainstream QM and the Bohmian mechanics. (see attachment).

I have already explained why this and many similar arguments are wrong, and I don't like to repeat myself. Let me just say that all such arguments don't take into account the dynamics of quantum measurements.

 

[nomadreid]

Thanks, Demystifier. Yes, I have seen your posts in past discussions of Bohmian mechanics, but my limited understanding of the background brought me to post this request for more basic input. Your article on correlation versus causation is interesting, something I must mull over. I have come across your refutation of several other articles, such as the 2010 one, but I have not yet come across your refutation of the 2016 paper which I mentioned (and I don't recall seeing the refutation of the 2014 paper, which doesn't mean I didn't see it.] But you indicated that there are no new arguments in papers indicating measurable test to distinguish QM from BM (that is, that they still don't take into consideration the dynamics of QM). However, if there are any new arguments in the later papers, and you have the links to their refutations handy,I would highly appreciate getting those links. If not, do you consider that there could be any such measurement test that took the dynamics of QM into account? If not, then BM would be one more interpretation along with Copenhagen, many-worlds, etc., right?
[Ah, in my last post I put "see attachment", and forgot to remove it.]
PS I also downloaded your xxx.lanl.gov/abs/1112.2034, and will be looking at that as well.

 

[Demystifier]

If not, then BM would be one more interpretation along with Copenhagen, many-worlds, etc., right?

Right.

 

[Demystifier]

PS I also downloaded your xxx.lanl.gov/abs/1112.2034, and will be looking at that as well.

That's good, from this paper one can understand what is the real reason that Bohmian mechanics produces the same measurable predictions as standard QM.