# I A skeptic's view on Bohmian Mechanics

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1. Jan 11, 2017

### A. Neumaier

Reading this paper, I found their reference to a blog article by Reinhard Werner, one of the leaders in quantum information theory, on Bohmian mechanics, with sharp comments and questions such as the following:
At the end he poses http://www.itp.uni-hannover.de/~werner/Bohm.html [Broken], for which one can earn a bottle of good wine. I don't know whether @Demystifier likes wine, but perhaps he likes to decide the question with a mathematical proof.

And then follows on the blog a lively debate ....

Last edited by a moderator: May 8, 2017
2. Jan 12, 2017

### Demystifier

I don't like wine, but I think that the only hard part of the problem is to solve the Schrodinger equation for that case, which does not depend on whether you use or not use Bohmian mechanics. Once you have the solution of the Schrodinger equation (which may be a task for any quantum physicist), the questions specific to the Bohmian interpretation should be easy to answer.

Last edited by a moderator: May 8, 2017
3. Jan 12, 2017

### A. Neumaier

But any quantum physicist is unlikely to be interested in it, since it is a question about the behavior of Bohmian particles.

That you (and anyone else in the Bohmian camp) take so little interest in actually discussing and solving these kind of problems reinforces the observation by Werner that Bohmian mechanics did not generate interesting results from a technical point of view but understands itself just as a commentary on quantum mechanics, without which most physicists can happily live.

Last edited: Jan 12, 2017
4. Jan 12, 2017

### ShayanJ

This is confusing! Most physicists are already living happily with quantum mechanics, without MWI, or your thermal interpretation or decoherence or even without thinking what interpretation they're using or whether the interpretation they're using is consistent or not. So how is it that the same thing is interpreted as the shortcoming of BM?

P.S.
I don't like BM, but people are obviously being unfair to it!

5. Jan 12, 2017

### A. Neumaier

I agree. There is no need for an interpretation beyond what everyone happily lives with, unless one can answer questions that need a lot of math of the kind needed for solving the Bohmian Detector Problem. Someone happy to live without an answer to this within the Bohmian interpretation will also be happy without the Bohmian interpretation itself. I don't understand why you find this confusing!

Note that I spent myself a lot of time with the Bohmian interpretation before I rejected it as superficial, essentially for the reasons given by Werner. It didn't add any understanding but wasted a lot of my time.

My thermal interpretation is nothing new, it just gives explicit words for what people happy with QM are using anyway when interpreting their theories and results. Not a single additional feature is present. And it poses some highly nontrivial mathematical problems involving many-body problems related to measurement, whose solution I am working on. Having not solved them is one of the reasons I delay publication of a formal account of the interpretation.

Last edited: Jan 12, 2017
6. Jan 12, 2017

### Demystifier

As I stressed several times, there are some technical physical practical results of BM:
https://www.amazon.com/Applied-Bohmian-Mechanics-Nanoscale-Cosmology/dp/9814316393

Nevertheless, I agree that most of the work on BM (or any other interpretation beyond the minimal pragmatic one) is just a commentary on quantum mechanics, without which most physicists can happily live. But some people need more to reach happiness, and BM (just as all other interpretations beyond the minimal pragmatic one) is devised for them.

Last edited by a moderator: May 8, 2017
7. Jan 12, 2017

### Demystifier

Just because it is a question about BM doesn't make it relevant for BM.

8. Jan 12, 2017

### ShayanJ

I didn't say that. Interpretations are not there to make such people happy. They're there to make other people happy. I can't stop myself from comparing it to quantum gravity. Why do we feel the need to quantize gravity? Because general relativity predicts singularities and we're not happy with it? So just be happy with it, problem solved! Because black holes don't seem to conserve information and we're not happy with it? So just be happy with it, problem solved! Because gravity seems so much different from other interactions and we're not happy with it? So just be happy with it, problem solved! Is this the kind of physics you like?

9. Jan 12, 2017

### A. Neumaier

I know. But I said that. All researchers choose what they like to do research on and what they are prepared to believe on hearsay, or defer judgement. This is about happiness, not about physics.

Those problems will eventually be solved where enough people are unhappy with them that one of them finds the energy and creativity to actually do the work.

10. Jan 12, 2017

### Demystifier

There is no such interpretation with which everyone is happy.

11. Jan 12, 2017

### A. Neumaier

You seem not to be familiar with figures of speech.

12. Jan 12, 2017

### Demystifier

Maybe I could learn it by writing a paper entitled "Why everyone needs Bohmian mechanics".

13. Jan 12, 2017

### ShayanJ

Well, you said "I agree", so I thought that's what you got from what I said!
Anyway, that's exactly what I wanted to say. The physics community now is divided. The majority of physicists are happy with quantum mechanics and see no problem in it and just want to use it. There are some people who see some problems and want to solve them. These are not only Bohmians. Everyone who does some research on foundational issues is in this camp. So its not that there are problems that only Bohmians see, its just that Bohmians have their own way of looking at these problems. And I really don't see anything different here and that's what confusing me. Why everyone treats Bohmian mechanics so much different than other interpretations? Its not better than others but its not worse too!

14. Jan 12, 2017

### A. Neumaier

The minimal interpretation is the common intersection between all and hence a necessary part of every interpretation - even of the Bohmian, which reduces itself to it when measurement is taken into account. Therefore it deserves a special place. The Copenhagen interpretation is the oldest, and hence also has a special place. Among the other interpretations, the Bohmian gets special attention on this forum because one of the active members, demystifier, has a strong position about it and another one (myself) has a diametrically opposite point of view.

I don't like many worlds or consistent histories, but for lack of a highly interested opponent they don't get the share of critical remarks from me that they would deserve. I don't like Copenhagen either, but since it has in atyy a strong defender on this forum, I had enough motivation to be at times very critical of it.

15. Jan 12, 2017

### A. Neumaier

The problem is that there are very different ideas of what it means to have solved them. For me, there are unsolved problems in measurement theory but solving them means attacking difficult many-body problems., For others, the only unsolved problems can be handled in a paper of 10 pages or less, something done already by Bohm or Everett. In my opinion, they scratch the surface only, and leave the real work undone - as Werner also complained.

16. Jan 12, 2017

### ShayanJ

That's right. And I'm sure you, as an experience researcher, know better that me that its important than all these people try to solve the problems they see, the way they see fit. That's because basic research is like that, we don't know what we're doing! its not always clear what is a problem and what is not. And its not always clear what is a solution and what is not. So you try to solve the problems you see the way you see fit, the same with Demystifier.

17. Jan 12, 2017

### stevendaryl

Staff Emeritus
Werner wrote:

I'm quoting it because it is something that I had noticed about Bohmian mechanics. The rough argument that Bohmian mechanics is observationally equivalent to standard quantum mechanics involves just showing that Bohmian trajectories lead to the same probability current as standard quantum mechanics. But that's not the end of the story, because the quantum recipe goes beyond probability currents. There is also the update of the wave function: Von Neumann's collapse hypothesis. After you measure a composite system to have eigenvalue $\lambda$ of some operator $O$, then from that moment on, you use, not the original wave function, but the projection of the wave function onto the subspace of wave functions with eigenvalue $\lambda$. The measurement problem is the question of whether and how to understand this apparent collapse with the smooth evolution of the wave function. Decoherence and Many-Worlds and so forth are different ways to understand what's going on during measurement.

The benefit of Bohmian dynamics is supposed to be that there is no collapse, and the only updating is ordinary updating of a probability distribution to reflect new information, plus the change of the quantum potential due to changes in system setup. To show that Bohmian mechanics really agrees with observation, though, requires showing that the apparent collapse is explainable using pure quantum mechanics (via decoherence or whatever). I'm just repeating what Werner said at this point, but it does seem to me that a rigorous proof that Bohmian mechanics is consistent with our observations requires essentially solving the measurement problem for standard quantum mechanics first.

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18. Jan 12, 2017

### stevendaryl

Staff Emeritus
I appreciate this comment very much. In a certain sense, the disagreements among the various interpretations of quantum mechanics, and the feeling that there is no completely satisfying interpretation involve the question of whether and how quantum mechanics explains the behavior of measurement devices involving an astronomical number of particles. That's not likely to ever be done except in some approximation, and there is a danger that the nature of the approximation already assumes something of what is to be proved. (For example, if you treat measurement devices using classical mechanics.)

19. Jan 12, 2017

### Demystifier

It's illusory to search for a rigorous proof, simply because the measurement problem necessarily involves a very large number of degrees of freedom ($10^{23}$ or so). This is like searching for a rigorous derivation of classical statistical mechanics from classical mechanics, and it is known that a rigorous derivation of classical statistical mechanics does not yet exist.

20. Jan 12, 2017

### stevendaryl

Staff Emeritus
That's true. My point is this:

A lot of the appeal of Bohmian mechanics is that it doesn't have a measurement problem. It doesn't give a special role for measurement interactions. In contrast, the standard "recipe" for quantum mechanics does treat measurement as something different. By the standard recipe, I mean the practical rules of thumb for applying quantum mechanics, which are basically:
• Treat measuring devices classically.
• Treat microscopic systems quantum mechanically (that is, they evolve smoothly according to Schrodinger's equation) between observations.
• Apply the Born rule for measurement results.
• If there are multiple measurements performed on the same system (or on different components of a composite system), then use the "collapsed" wave function after a measurement.
The standard recipe is what empirical tests of quantum mechanics are really testing. The "measurement problem" to me is the problem of explaining why the standard recipe works without treating measurement and measurement devices as something special. (Or alternatively, spelling out why they are special.) Showing that Bohmian mechanics (or any other no-collapse interpretation of quantum mechanics) is empirically equivalent to the standard recipe requires solving the measurement problem.