# Why is the pilot-wave theory controversial ? Is it?

1. Jul 6, 2012

### nonequilibrium

Why is the pilot-wave theory "controversial"? Is it?

The wikipedia page on the pilot-wave theory (aka the de Broglie-Bohm interpretation of QM), being http://en.wikipedia.org/wiki/Pilot_wave , says things like
According to whom is this not a valid challenge? Of course, that there are people who do not accept it as their interpretaion, I'm fully aware of, but are there any respectable physicists who actually deny it being a valid challenge? That implies the idea of it not being consistent and/or not reproducing regular QM predictions (which is simply not true)...

EDIT (mostly for mods): see my 2nd post here for a justification for posting this in the science forum (as opposed to the philosophy forum)

Last edited: Jul 6, 2012
2. Jul 6, 2012

### Maui

Re: Why is the pilot-wave theory "controversial"? Is it?

Anything unobservable is controversial, even unscientific. As an extention, all hidden variable theories are controversial.

3. Jul 6, 2012

### nonequilibrium

Re: Why is the pilot-wave theory "controversial"? Is it?

Hm, I definitely don't want to make this a thread about philosophy of science, but I'll respond in the way that I think stays on-topic. I hope to stay on the "science" track in my answer.

The view you express seems very naive to me. The best reply I can think of is Einstein saying "it's the theory which determines what is measurable". This is a subtle quote, but what lies at its core is the realisation that what is "observable" or not is actually a very subtle matter. Two points:

* In one sense according to your logic any interpretation is controversial since they all differ on matters not directly "observable" (that's what makes them interpretations), and if you truly stand by that, then I think we're talking past each other (i.e. then it's becoming a semantics issue);

* In another sense (and more importantly!) every observation is embedded in a theory; in other words, you cannot meaningfully talk about the result of a measurement without associating to it an underlying theory, and in this process even elements which are naively regarded as "unobservable" are crucial. This is what the Einstein quote is talking about and I think it's most clearest with an example: the Copenhangen interpretation says "one cannot tell which slit the particle went through" (without adding extra measurement devices beyond the slits) whereas, rather surprisingly, in the pilot-wave theory you can always say which slit the particle went through (specifically: if you measure it at the lower side of the slit, it went through the bottom slit, and vice versa). This example indicates that the whole notion of "observation" is far more subtle (i.e. theory-dependent) than a naive interpretation might suggest.

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DISCLAIMER

Note in all this that my intention is not convincing anybody of the de Broglie-Bohm interpretation as such (although, of course, I would welcome that) --otherwise this should go into the philosophy of science forum--, I just want to debate about its validity as an interpretation among the other interpretations (which seems to be debated by the wikipedia article).

Specifically, I don't want to talk about arguments concerning the metaphysical niceness of the pilot-wave theory or whatever, I want to talk about facts (as I believe this post is trying to do) such as issues of "observability" or "consistency" etc. That's why I believe it belongs in the science forum.

4. Jul 6, 2012

### Maui

Re: Why is the pilot-wave theory "controversial"? Is it?

Well, yes... the relationship between qm and the outside world is rather controversial, so the interpretations are not to blame. However, some interpretations are more scientific than others - e.g. the standard minimalist interpretation is closer to the "shut up and calculate" approach without positing unobservable influences to please someone's pre-conceived notions of reality.

No. The point of a theory is to make predictions, what underllies them is currently too big of a question. The rest is philosophy.

5. Jul 6, 2012

### nonequilibrium

Re: Why is the pilot-wave theory "controversial"? Is it?

You can't just say "No" when I've given an explicit example. At least address the example. Or did you not read the whole part of my second point?

EDIT: and by the way your first paragraph indicates you deem pretty much all interpretations (except what you regard as the minimalist interpretation) controversial. In that case you're talking about preferring one interpretation over another, that's not what I'm talking about (that would be a philosophical discussion, I'm having a scientific discussion). Then again, your objection about observability is on-topic, and regarding that I stand by what I said in this post (i.e. before the "EDIT").

Last edited: Jul 6, 2012
6. Jul 6, 2012

### Maui

Re: Why is the pilot-wave theory "controversial"? Is it?

It's hard for me to see your point, given the commonly accepted ambiguity of the term 'measurement'. What does this prove or imply?

7. Jul 6, 2012

### nonequilibrium

Re: Why is the pilot-wave theory "controversial"? Is it?

That the interpretation has an effect on what you observe.

For example, according to the pilot-wave theory, the two-slit experiment (the standard set-up) can be seen as a measurement of "through which slit did the particle pass". This is not the case according to the orthodox interpretation, where the measurement of the position of the particle on the screen does not tell you which slit it went through.

My intention is to back up my claim that your view is too naive: you say the Bohmian particles are not observable, indicating they have no influence on what you measure. The above convinces me, and hopefully you too, that it does. Hence the argument in your first post is not valid.

8. Jul 6, 2012

### Maui

Re: Why is the pilot-wave theory "controversial"? Is it?

No, you are misunderstaning. The unobservable part(and controversial) is the implicate order that Bohm talked about(the hidden variable). See here

Anything that's not observable cannot be regarded as scientific, hence it's a philosophical position.

9. Jul 6, 2012

### nonequilibrium

Re: Why is the pilot-wave theory "controversial"? Is it?

You seem to be mixing things. The words "the implicate order" are indeed words used by Bohm but are totally not necessary for the pilot-wave theory; they're just extra philosophical baggage which do not translate into any math (and let's subsequently ignore those words since they're not relevant in a physics forum). The "hidden variables" however refer to the postulated point particles, which are essential to the pilot-wave theory. As is often noted (e.g. by J.S. Bell), "hidden variable" is a misnomer, since according to the pilot-wave theory they're actually the measurable thing, and more concretely my aforementioned example shows the so-called "hidden variable" does have an influence on observation.

Anyway, to keep it more precise, generally under "the de Broglie-Bohm interpretation of QM" or "the pilot-wave theory" one understands the two axioms that associated to any (one-particle) system there is
A) a wavefunction $\psi(\mathbf x,t) = R e^{iS}$ (using complex notation), governed by the Schrödinger equation;
B) a point particle with position $\mathbf X(t)$ and with law of motion $m \mathbf{ \dot X} (t) = \nabla S(\mathbf x,t) |_{\mathbf x = \mathbf X(t)}$

(sometimes Born's rule, i.e. that the modulus squared gives the probability of finding a particle, is listed too, but most pilot-wave theorists agree that this is actually a theorem derivable from the previous two axioms)

I understood your first post as saying that the entities listed in axiom B are "not observable and hence have no influence on matters of observation" and my subsequent posts were an attempt to convince you otherwise. I hope there's no mix-up.

Last edited: Jul 6, 2012
10. Jul 6, 2012

Re: Why is the pilot-wave theory "controversial"? Is it?

I think you're treading on thin ice here. As Mr.Vodka said, quantities are only observable within a theory. To measure distance with a metre stick you need certain assumptions about space (e.g. that a metre here is the same as a metre somewhere else).

11. Jul 7, 2012

### Demystifier

Re: Why is the pilot-wave theory "controversial"? Is it?

Any interpretation of QM is controversial, simply because we don't have a proof that it is THE right interpretation.

12. Jul 7, 2012

### nonequilibrium

Re: Why is the pilot-wave theory "controversial"? Is it?

Demystifier, I don't think you've read it carefully. It says that even the fact that it's a valid challenge as an interpretation is controversial. This has nothing to do with accepting it as a correct interpretation or not, but more fundamentally with its validity as an interpretation. The former is a more philosophical matter, the latter a more scientific matter.

13. Jul 7, 2012

### bohm2

Re: Why is the pilot-wave theory "controversial"? Is it?

I thought it was because non-locality is more explicit in pilot-wave theories (moreso than the more popular orthodox/epistemic view) and since it is directly in conflict with relativity then it's more controversial? Personally, I don't see any problem with non-locality or whatever stuff a future physics may need to postulate in order to explain phenomena.

14. Jul 7, 2012

### nonequilibrium

Re: Why is the pilot-wave theory "controversial"? Is it?

Ah yes that makes more sense.

EDIT: on the other hand, since QM is a non-relativistic theory, why should anyone use relativistic arguments in discussions about its interpretation?

15. Jul 7, 2012

### Maui

Re: Why is the pilot-wave theory "controversial"? Is it?

How does the math matter in ANY interpretation, instead to match predictions with experiment?? Remember that's an interpretation we are talking about.

Then we are indead talking of completely different things. I was pointing out that the question of whether a random outcome is predetermined by a nonlocal theory is philosophical, and it can be potentially intractable. I have no idea what you mean by hidden variables being observable/measureable.

No, that's not what i meant to say. Particles are of course observable and have observable influence on measurement results. The way i understand the BI(and i believe it's not controversial at all) is that the behaviour of particles is fundamentally deterministic(as opposed to in qm) and the determinism stems from hidden variables(unobservable hidden reality - hence my comment of the implicate deterministic order that manifests as random). The motivation for this weird interpretation was to show that it is in principle possible, not that it's correct or sound or reasonable. It's not otherwise possible to directly reconcile realism with probabilities(later attempts like MWI seem to do better than BI but it also has its share of issues).

Last edited: Jul 7, 2012
16. Jul 7, 2012

### Staff: Mentor

Re: Why is the pilot-wave theory "controversial"? Is it?

Exactly. All interpretations of QM suck - but in their own special way - you simply pick the one that to you sucks the least. To me the pilot wave theory sucks at a number of levels - an inherently unobservable pilot wave, the reintroduction of a preferred frame, and the difficulty extending it to QFT. But having discussed it extensively in the past the reasons it sucks for me are precisely the reason others like it - to each his/her own.

Thanks
Bill

Last edited: Jul 7, 2012
17. Jul 7, 2012

### Staff: Mentor

Re: Why is the pilot-wave theory "controversial"? Is it?

There is no conflict between realism and probabilities. A world out there external and independent of us can be probabilistic. If you flip a coin and cover it with your hand you know it is heads or tails - just not which it is. The issue with realism and QM is the superposition principle where a particle for example can literally be in two positions at once which is the antithesis of any sane view of reality - and even logic. It's not like flipping a coin - it's not in one position or the other but you don't know which - it literally is in both positions. However now we understand QM better, especially decoherence, how the world of everday experience with its usual rules of logic and probability emerge is not as big an issue and I believe with further research will eventually be totally resolved - it almost is now - evidently anyway - I am delving into the full detail of decoherence right now to discover what the unresolved issues are - as far as I can tell its simply a few technical issues of how it works in all situations - the current models are not as general as they should/could be.

What you may be referring to is what is called naive realism where the world is both non contextual and value definite - that has been dealt a harsh blow - but not reality per se:
http://en.wikipedia.org/wiki/Naïve_realism

Thanks
Bill

Last edited: Jul 7, 2012
18. Jul 7, 2012

### nonequilibrium

Re: Why is the pilot-wave theory "controversial"? Is it?

It's getting kind of boring having to repeat the same thing over and over, but: you're missing the point, indicating you haven't read properly what you're replying to. The "controversial" comment is concerning the validity of it being an interpretation. Please try to respect the distinction between this and actually preferring a specific interpretation. It's the same distinction between "this sentence is nice to read" and "this sentence is not a correct sentence" (e.g. due to grammatical issues).

And as for your other comment: isn't it rather inconsistent to label the pilot-wave unobservable since the pilot-wave is actually the well-known $\psi$? I might agree with the statement that the psi function is not directly observable, however this would actually be an argument pro the pilot-wave theory, since it's the only interpretation that does not just consist of a psi function. Perhaps you meant the postulated point particles are unobservable, as opposed to the pilot-wave. In that case I redirect you to my 2nd post in this thread where I try to argue that it does have observable consequences, in a sense.

19. Jul 7, 2012

### Staff: Mentor

Re: Why is the pilot-wave theory "controversial"? Is it?

I thought my reply was perfectly clear and directly related to the quote I replied to - its a valid interpretartion - but like all interpretations sucks in some way. The controversy is the same controversy any interpretation has - how to decide via experiment which is correct. Until you can do that its simply which appeals to your aesthetics of science better. Personally I think the pilot wave interpretation is a load of rubbish - its like a re-introduction of an aether and a step backwards - but opinions are like bums - everyone has one - it doesn't make it correct.

According to the pilot wave theory it really exists out there - and guides a real particle with a real and actual position - but so far no one has ever figured out how to directly observe it - that is not generally considered a good scientific ontology. It's like the aether of LET compared to SR. Both theories are valid but most people reject LET because of the inherently unobservable aether. A few still cling to it because they like physical causes for things like length shortening rather than it simply being the result of geometry like it is in SR. In the ensemble interpretation it simply is a device for calculating probabilities. Other interpretations have a different view as well - but the most common view is its simply a calculational device not having any direct existence eg Copenhagen and Consistent Histories view it that way.

Thanks
Bill

Last edited: Jul 7, 2012
20. Jul 7, 2012

### nonequilibrium

Re: Why is the pilot-wave theory "controversial"? Is it?

Alright, then it was just off-topic.

I see what you mean now. I think that's a good argument (although it's still off-topic, since it then becomes about preferring a certain interpretation, and that brings in the danger of getting this thread moved over to the philosophy forum). However, viewing the pilot-wave as physical is not a necessary part of the pilot-wave theory. Actually, I think it's more logical to say it's not physical. One argument is that it is not acted back upon (no action-reaction principle) by the point particle; this is not a very strong argument, but it's an intuitive one. In this sense the role of the pilot-wave is like that of the Hamiltonian function in classical mechanics: it determines the evolution without it being influenced by that evolution. A stronger argument is that the pilot-wave is a function on $\mathbb R^{3N}$ (for N spin zero point particles in 3D). In the general N particle case this cannot be interpreted as a function on space, unlike for example the electric field, which can indeed be interpreted as physical. (This makes the pilot-wave look even more like the Hamiltonian in classical mechanics.)