An argument against Bohmian mechanics?

[Demystifier]

No rabbit in sight

Except that the appearance of a rabbit is what the whole trick is about.

 

[vanhees71]

Where is a trick in phenomena (no matter whether quantum or not)? It's what we observe nature does, and there's nothing else we can do. What do you want when you asking for knowledge how Nature does it? You'll never get an answer from the natural sciences, and philosophy leaves you most probably unsatisfied since it's not explaining anything either.

 

[Demystifier]

Where is a trick in phenomena (no matter whether quantum or not)? It's what we observe nature does, and there's nothing else we can do. What do you want when you asking for knowledge how Nature does it? You'll never get an answer from the natural sciences, and philosophy leaves you most probably unsatisfied since it's not explaining anything either.

So how would you qualify my attempted explanation of rabbit-from-the-hat in the entry 5)? Is it philosophy? Don't you do something similar when you see a performance by a magician?

 

[A. Neumaier]

without mathematics, just by visualizing localized wave packets and particle trajectories within them.

How do you do that without mathematics?

 

[rubi]

Which does not mean that one does not need to criticize the specific gaps in order to criticize the final conclusion of the argument.

Arnold already criticised some points in his post #16 and you didn't respond to it.

5) Try to devise a rational mechanism which could explain it.

That's what physicists try to do, but I don't think that anything about Bohmian mechanics can be considered rational. It's so irrational that most physicists even consider something as stupid as "shut up and calculate" to be more rational than BM. Arnold's list in post #84 can be continued almost indefinitely. In additional to what is absurd about BM, I also consider it highly irrational to expect naive 17th century ideas about physics to be the final word.

 

[Demystifier]

How do you do that without mathematics?

I meant without equations. I can visualize a wave packet and a trajectory without having an equation in my mind.

 

[Demystifier]

That's what physicists try to do, but I don't think that anything about Bohmian mechanics can be considered rational. It's so irrational that most physicists even consider something as stupid as "shut up and calculate" to be more rational than BM.

What is the most rational interpretation of QM in your opinion?

 

[Spinnor]

Does your attempted explanation involve some hidden variables, like those in 5) above?

I can not, right?

 

[Spinnor]

Does the rabbit have a definite position before it is pulled out of the hat ? Or is that interpretation dependent ?

That is classical physics, so it must.

 

[Demystifier]

I can not, right?

Why not? I think you can.

 

[vanhees71]

So how would you qualify my attempted explanation of rabbit-from-the-hat in the entry 5)? Is it philosophy? Don't you do something similar when you see a performance by a magician?

I'd say that's the most rational "explanation" of the phenomenon, and I'd consider it the most convincing one too. I, however, don't see in which this sense is an analogue to Bohmian mechanics. In your rabbit example you can check your assumptions and figure it out (provided the magician allows you to investigate his setup), while Bohmian mechanics claims unobservable trajectories and doesn't offer anything more than standard QT. I don't see, why I should evaluate the trajectories, if I can't check the result against experiment (except for the fun in the sense of a mathematical puzzle I solve for my pleasure).

 

[Demystifier]

I'd say that's the most rational "explanation" of the phenomenon, and I'd consider it the most convincing one too. I, however, don't see in which this sense is an analogue to Bohmian mechanics. In your rabbit example you can check your assumptions and figure it out (provided the magician allows you to investigate his setup), while Bohmian mechanics claims unobservable trajectories and doesn't offer anything more than standard QT. I don't see, why I should evaluate the trajectories, if I can't check the result against experiment (except for the fun in the sense of a mathematical puzzle I solve for my pleasure).

You have a point, but what if, for some reason, magician never allows you to investigate his setup? Would that change anything?

 

[vanhees71]

Well, then natural science must capitulate, i.e., its methods cannot be applied to the magician's trick, because he doesn't allow to apply them. Nature seems not to be that malicious since obviously she allows us to observe her.

 

[Spinnor]

Why not? I think you can.

I though Bell's theorem ruled out Hidden Variables?

 

[Demystifier]

I though Bell's theorem ruled out Hidden Variables?

No. It ruled out local hidden variables. Non-local hidden variables, such as those in Bohmian interpretation, are not ruled out.

 

[Demystifier]

Nature seems not to be that malicious since obviously she allows us to observe her.

Nature does not allow us to observe her hidden variables, which is precisely why they are called hidden.
More seriously, it's not that Bohmian hidden variables are hidden in an absolute sense. As I explained in
https://arxiv.org/abs/1309.0400 (Sec. 7.1)
the way they are hidden is analogous to the way how astrophysical dark matter is hidden from us.

 

[Demystifier]

Well, then natural science must capitulate, i.e., its methods cannot be applied to the magician's trick, because he doesn't allow to apply them.

But even then, you would use pure thinking to try to figure out how the magician does it, wouldn't you?

 

[vanhees71]

Sure, but it will stay speculative, if you cannot verify your thinking by observations. The same holds for Bohm's trajectories. They are ficticious, because they cannot be observed. I still don't understand what merit Bohm's additions may have for the understanding of QT.

 

[atyy]

Sure, but it will stay speculative, if you cannot verify your thinking by observations. The same holds for Bohm's trajectories. They are ficticious, because they cannot be observed. I still don't understand what merit Bohm's additions may have for the understanding of QT.

The aim of Bohmian mechanics is to remove the Heisenberg cut, which is needed in the orthodox interpretation eg. used by Landau and Lifshitz and Weinberg in their QM textbooks.

 

[Demystifier]

Sure, but it will stay speculative, if you cannot verify your thinking by observations. The same holds for Bohm's trajectories. They are ficticious, because they cannot be observed. I still don't understand what merit Bohm's additions may have for the understanding of QT.

Well, you confirmed that you would use pure thinking to try to figure out how the magician does it, even though it would stay speculative. What's the merit of that? Whatever the merit of speculation about magician tricks might be (and there must be some, because you confirmed you would do it), the merit of the Bohmian interpretation is completely analogous.

So if you want me to tell you what's the merit of Bohmian interpretation, first you must tell me what's the merit of speculations about magician tricks.

Or alternatively, if you insist that merit must be practical, then see the link in post #87.

 

[vanhees71]

The aim of Bohmian mechanics is to remove the Heisenberg cut, which is needed in the orthodox interpretation eg. used by Landau and Lifshitz and Weinberg in their QM textbooks.

How can Bohmian mechanics remove the Heisenberg cut (whatever you mean by that; I've neither found in LL nor Weinbergs QM textbook(s)) if it doesn't provide anything different from standard QM than the introduction of unobservable new elements?

 

[vanhees71]

Well, you confirmed that you would use pure thinking to try to figure out how the magician does it, even though it would stay speculative. What's the merit of that? Whatever the merit of speculation about magician tricks might be (and there must be some, because you confirmed you would do it), the merit of the Bohmian interpretation is completely analogous.

So if you want me to tell you what's the merit of Bohmian interpretation, first you must tell me what's the merit of speculations about magician tricks.

Or alternatively, if you insist that merit must be practical, then see the link in post #87.

I don't know, what's the merit of understanding a magicians trick. Perhaps, it's not even useful to find if you like to be rather entertained by the show? My point is that there's no additiona merit in Bohmian interpretation compared to the physical core of QT. It's maybe nice for some people who like to stick to classical pictures like trajectories of particles although they are contradicting observations and are thus in Bohmian mechanics not to be taken as observable. But that's rather introducing a "magician's trick" rather then undertanding what's behind it. For me the problem is, why I should think about QM as a "magician's trick" rather than just the so far best description of nature we have today and then add a "magician's trick" to figure out "what's behind it" (QM). In this sense it's somehow self-contradictory in its aims.

 

[atyy]

How can Bohmian mechanics remove the Heisenberg cut (whatever you mean by that; I've neither found in LL nor Weinbergs QM textbook(s)) if it doesn't provide anything different from standard QM than the introduction of unobservable new elements?

The Heisenberg cut is mentioned in LL as the classical measurement apparatus, and in section 3.7 of Weinberg's QM as "The discussion of probabilities in Section 3.1 was based on what is called the Copenhagen interpretation of quantum mechanics, formulated under the leadership of Niels Bohr. According to Bohr, “The essentially new feature of the analysis of quantum phenomena is ... the introduction of a fundamental distinction between the measuring apparatus and the objects under investigation."

Bohmian Mechanics removes the Copenhagen cut by introducing hidden variables, so there is no fundamental distinction between the measuring apparatus and the quantum system - if we make a big system consisting of the measuring apparatus (M) and the quantum system (S), we don't need another measuring apparatus to measure the combination of M and S in order to obtain predictions. In contrast, in Copenhagen, if we make a combined quantum system consisting of M and S, then we need yet another measuring apparatus to measure the combined M and S in order to obtain predictions.

 

[Demystifier]

I don't know, what's the merit of understanding a magicians trick. Perhaps, it's not even useful to find if you like to be rather entertained by the show?

Then why do you try to understand it? Seriously, if you really want to understand why some people care about Bohmian mechanics (which doesn't mean that you should care too), then you need to do some self-psychoanalysis and answer the question why do you care about trying to understand how the magician trick works. Only when you answer that question about yourself you will have chance to put yourself into the mind of a Bohmian.

 

[Demystifier]

How can Bohmian mechanics remove the Heisenberg cut (whatever you mean by that; I've neither found in LL nor Weinbergs QM textbook(s)) if it doesn't provide anything different from standard QM than the introduction of unobservable new elements?

How can you hope to understand how Bohmian mechanics removes the Heisenberg cut if you don't even know what Heisenberg cut is?

 

[vanhees71]

I know what the Heisenberg cut is, but I've no clue, where Weinberg and Landau-Lifshitz mentioned it. If you mean the classical behavior of macroscopic systems, including measurement devices, of course for me there is no such cut. Classical behavior of macroscopic systems is for me an emergent phenomenon due to the very coarse-grained observations I do on macroscopic objects. It's all "averaging" over the microscopic details, and thus engineers and experimental physicists can use the classical description to develop measurements devices, particle accelerators, rockets to fly to the moon, and so on.

 

[martinbn]

The question for everybody: What do you do when you see a magician trick?

I would try to figure it out. If I cannot, may be some else can. If no one can, then it should be called and viewed as an open problem. And there is nothing wrong with having open problems that stay unresolved for a long time. Look at Fermat's last theorem.

What you call a rational explanation in 5) to me seems to be just a guess. May be a good one, but still just a guess. If that's the case it should be considered a conjecture, not a solution to the problem.

 

[stevendaryl]

I know what the Heisenberg cut is, but I've no clue, where Weinberg and Landau-Lifshitz mentioned it. If you mean the classical behavior of macroscopic systems, including measurement devices, of course for me there is no such cut. Classical behavior of macroscopic systems is for me an emergent phenomenon due to the very coarse-grained observations I do on macroscopic objects. It's all "averaging" over the microscopic details, and thus engineers and experimental physicists can use the classical description to develop measurements devices, particle accelerators, rockets to fly to the moon, and so on.

Some of the arguments about interpretations of quantum mechanics are just about philosophical preferences, but I think some of them are genuine technical disputes, which presumably have technical answers, and are not just a matter of opinion. It's hard to tease apart the nuggets that are objective, though. But in this particular case, I think there is a technical question that you are assuming one answer to, and that others are assuming a different answer, and that is: to what extent does the classical world emerge from the quantum world by (mere) coarse-graining?

I use the word "mere" to mean "without adding additional assumptions about wave function collapse". I do not believe that the world that we see, in which macroscopic things have approximately definite positions and momenta at all times, follows by coarse-graining the microscopic description. The microscopic/macroscopic cut makes a bigger difference than that: on the microscopic side, we have deterministic evolution of quantum amplitudes, and on the macroscopic side, we have nondeterministic evolution according to the probabilities given by the Born rule. It seems to me not simply a matter of interpretation or metaphysics, but a technical question that should have a technical answer: Is the macroscopic side derivable from the microscopic side? (Many-Worlds seems like an attempt to do that) Or does the macroscopic side require additional assumptions (something akin to a collapse hypothesis, or something akin to the Bohmian assumption that particles have definition positions at all times)?

 

[RockyMarciano]

No. It ruled out local hidden variables. Non-local hidden variables, such as those in Bohmian interpretation, are not ruled out.

I wonder why all this interpretational debates seem to ignore the fact that we have relativistic QFT, and that it has been checked to make accurate predicitions to many decimal places. So any interpretation one may want to give to quantum theory must agree with the postulates that go into renormalized interacting 4-dimensional QFT, one of them is locality, so any interpretation that uses non-local hidden variables is not a plausible interpretation in the context of assuming the success of QFT as our most accurate quantum theory.
So the simplest argument against Bohmian interpretation(but not only to the Bohmian interpretation) besides the obvious of introducing ontologic elements that are not observable (commented by vanhees and others in previous posts) is just broadening the scope of the discussion to already checked improvements over NRQM such as QFT.

And please, do not introduce the fallacy that rejecting the specific "interpretational solution" of Bohmian mechanics as simply wrong due to the above arguments is equivalent to not recognizing that quantum theory as of now might not be the last word. It is just discarding that specific approach to alternatives.

 

[PeterDonis]

one of them is locality, so any interpretation that uses non-local hidden variables is not a plausible interpretation

This is not correct. The QFT version of "locality" is that operators at spacelike separated events must commute--in other words, the results of measurements at spacelike separated events cannot depend on the order in which the measurements are made. That is compatible with non-local hidden variables.

 

[RockyMarciano]

This is not correct. The QFT version of "locality" is that operators at spacelike separated events must commute--in other words, the results of measurements at spacelike separated events cannot depend on the order in which the measurements are made. That is compatible with non-local hidden variables.

Well, if you decide that non-local is compatible with local there seems to be not much more room for discussion. I would say that if non-local hidden variables are made compatible with relativistic locality they are actually local hidden variables.
Maybe you should define what you mean by local and nonlocal before claiming incorrectness. The locality I'm referring is no FTL signal transmission allowed, and the non-locality I refer to is FTL signal transmission allowed. Now the only way the nonlocality in Bohmian interpretation could be different that my definition would be if it was not a realistic/deterministic interpretation, which it is claimed that it is by its proponents...

 

[stevendaryl]

I wonder why all this interpretational debates seem to ignore the fact that we have relativistic QFT, and that it has been checked to make accurate predicitions to many decimal places.

Well, as I understand it, the locality problems with quantum field theory are not really any different than for quantum mechanics. Roughly speaking, we have two parts to quantum mechanics:

1. Calculate amplitudes for various outcomes using Schrodinger's equation.
2. Use the amplitudes to compute probabilities for measurement outcomes.

QFT modifies #1, but it doesn't change #2, which is where the interpretation difficulties arise.

 

[RockyMarciano]

Well, as I understand it, the locality problems with quantum field theory are not really any different than for quantum mechanics. Roughly speaking, we have two parts to quantum mechanics:

1. Calculate amplitudes for various outcomes using Schrodinger's equation.
2. Use the amplitudes to compute probabilities for measurement outcomes.

QFT modifies #1, but it doesn't change #2, which is where the interpretation difficulties arise.

But it adds something to the locality problem discernment, according to Bell's theorem you still have the option when disposing of local realism to choose between discarding realism(classical determinism) or discarding locality, and so the Bohmian interpretation would choose to discard locality and keep classical determinism. But the QFT that produces high precision predictions is a local theory and leaves no choice but giving up classical determinism.

 

[RockyMarciano]

In other words in the obligation(per Bell's theorem) to discard local hidden variables, one could try to split a local part and a hidden variables part, the former referring to local causality and the latter to classical determinism(aka realism), so one could choose to renounce to causality and keep realism or viceversa keep causality and get rid of realism but couldn't keep both. Now QFT clearly keeps causality and by having spacelike separated operators commute it clearly renounces to classical determinism with predetermined spacelike values for events(since they commute they can't be ordered and there cannot be predetermination),

Given this I think in the light of QFT certain interpretations like BI, TI or MWI and others simply don't hold water, they are in clear conflict with what we know about QFT.

 

[atyy]

I know what the Heisenberg cut is, but I've no clue, where Weinberg and Landau-Lifshitz mentioned it. If you mean the classical behavior of macroscopic systems, including measurement devices, of course for me there is no such cut. Classical behavior of macroscopic systems is for me an emergent phenomenon due to the very coarse-grained observations I do on macroscopic objects. It's all "averaging" over the microscopic details, and thus engineers and experimental physicists can use the classical description to develop measurements devices, particle accelerators, rockets to fly to the moon, and so on.

Yes, of course if you believe that, then Bohmian Mechanics, Consistent Histories, Many Worlds and other approaches that attempt to solve the measurement problem are pointless. However, the standard view (LL, Dirac, Weinberg) is that you are wrong (ie. it is not a matter of taste, you are simply wrong - your averaging does not work). LL mentions the Heisenberg cut in his QM text (p3 of the 1977 English translation, 3 ed, Pergamon), Weinberg mentions the Heisenberg cut in section 3.7 of his 2013 QM text.