# Pilot wave theory, fundamental forces

I read a short high-level article about the pilot wave interpretation of quantum mechanics and I have some questions.

Is there a good way to formulate that theory so that the only force on a particle is from the pilot wave (inertia, gravity, EM, ... move/effect the wave which in turn effects the particle)? Seems like people would have tried this, but I can't find anything when searching the web.

Also, the article claimed that pilot wave theory provides new, testable predictions. Where I can find more information about that?

Demystifier
Gold Member
Is there a good way to formulate that theory so that the only force on a particle is from the pilot wave (inertia, gravity, EM, ... move/effect the wave which in turn effects the particle)? Seems like people would have tried this, but I can't find anything when searching the web.
According to the pilot wave theory, the only force on a particle is from the pilot wave. It is quite obvious from most treatments of the theory, but I don't know any reference in which this point is particularly emphasized.

Also, the article claimed that pilot wave theory provides new, testable predictions. Where I can find more information about that?
It would help if you could specify which article are you talking about.

According to the pilot wave theory, the only force on a particle is from the pilot wave. It is quite obvious from most treatments of the theory, but I don't know any reference in which this point is particularly emphasized.
Hi Demystifier,

But surely when you analyze the force equations there is a $$-\nabla V$$ term as well as the quantum force term $$-\nabla Q$$ (where V and Q are the respectively the classical and quantum potentials). This implies that the particles attract/repel each other as well as being pushed around by the pilot-wave, no?

Zenith

Regarding question 1: As Zenith said, that article seemed to imply that gravity, ... act on the particle (mathematically through a potential V). Demystifier, do you know where I can look to find the formulations in which the only force on the particle is the pilot wave?

Regarding question 2: The article was by Mike Towler at Cambridge University, but I can't find the link now. However, I don't know if that's relevant -- the article just mentioned (in a bullet) that pilot wave theory provides new, testable predictions, but it did not say what they were. I would like a reference to find out what they are.

Thanks

Regarding question 2: The article was by Mike Towler at Cambridge University, but I can't find the link now.
I've referred to the article that I think you mean in recent threads. You can find it at :

http://www.tcm.phy.cam.ac.uk/~mdt26/PWT/towler_pilot_waves.pdf" [Broken]

He also has a full on-line graduate course in pilot-wave theory at:

http://www.tcm.phy.cam.ac.uk/~mdt26/pilot_waves.html" [Broken]

However, I don't know if that's relevant -- the article just mentioned (in a bullet) that pilot wave theory provides new, testable predictions, but it did not say what they were. I would like a reference to find out what they are.
If you look in the sidebar link "Further Reading" in Towler's course there are links to hundreds of relevant papers. As regards testable predictions, presumably he means Valentini's non-equilibrium stuff leading to observable consequences in the cosmic microwave background etc. (though there are some other more flaky ones such as detecting possible violations of Pauli's exclusion principle, and/or using "lasers" - mounted on the head of a shark? - to detect whether particles held in traps are absolutely at rest in violation of Heisenberg uncertainty principle).

Looking at Towler's list you might read Valentini's recent "Beyond the quantum" article in Physics World, or the following three articles:

Inflationary cosmology as a probe of primordial quantum mechanics A. Valentini (2008).
De Broglie-Bohm prediction of quantum violations for cosmological super-Hubble modes, A. Valentini (2008).
Astrophysical and cosmological tests of quantum theory, A. Valentini (2007).

For the laser stuff, see the book "Quantum Cauasality" by Rigg.

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Great, thanks for the info Zenith.

Also, if the above refs don't include a formulation in which the only force on a particle is the pilot wave, I would still like to know if anyone knows of a ref for that.

Demystifier
Gold Member
Hi Demystifier,

But surely when you analyze the force equations there is a $$-\nabla V$$ term as well as the quantum force term $$-\nabla Q$$ (where V and Q are the respectively the classical and quantum potentials). This implies that the particles attract/repel each other as well as being pushed around by the pilot-wave, no?
That is certainly true. However, when I think about the pilot wave theory, I like to think of the wave function, and not of the quantum potential, as the fundamental quantity. The wave function guides the particle and the wave function by itself does not distinguish between classical and quantum force. All "force" is described by the wave function. (See however my next post which clarifies it more carefully.)

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Demystifier
Gold Member
Demystifier, do you know where I can look to find the formulations in which the only force on the particle is the pilot wave?
ANY paper on pilot wave theory describes how the motion of the particle is described only by the pilot wave (and the initial position of the particle). However, it is actually incorrect to say that the pilot wave determines the force. Namely, by definition a force is a quantity that determines acceleration, while the pilot wave determines the velocity. The initial velocity is not arbitrary in pilot wave theory, which is why it is somewhat misleading to formulate pilot wave theory in terms of forces and quantum potentials. The quantum potential is useful only to demonstrate similarity between classical mechanics and pilot wave mechanics, but the quantum potential does not have a fundamental role in pilot wave theory.

http://xxx.lanl.gov/abs/0912.2666

The quantum potential is useful only to demonstrate similarity between classical mechanics and pilot wave mechanics, but the quantum potential does not have a fundamental role in pilot wave theory.
I see your point, but perhaps it's slightly misleading to present this as the settled view of the pilot-wave community. I know that the Goldstein group that you link to present it in this way, but many others (e.g. Peter Holland, Basil Hiley, and Peter Rigg, to name three authors of pilot-wave textbooks) argue quite vehemently the opposite position. This is particularly the case if one argues that the wave field is a repository of energy, along the lines I did in https://www.physicsforums.com/showthread.php?p=2369492#post2369492".

Holland and Hiley in particular have some serious-sounding arguments in their recent papers - which I could look up if I could be bothered - in which they claim to prove that the quantum potential is fundamental.

For the moment let's just say we don't know who's right - so I don't think it's true to say definitively, as you do, that the quantum potential does not have a fundamental role.

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Demystifier
Gold Member
Holland and Hiley in particular have some serious-sounding arguments in their recent papers - which I could look up if I could be bothered - in which they claim that the quantum potential is fundamental.
I would like to see these papers if you know the exact references.

Anyway, what is your opinion? What is more fundamental, wave function or quantum potential?

I would like to see these papers if you know the exact references.
Ditto.

Anyway, what is your opinion? What is more fundamental, wave function or quantum potential?
I'd like to chime in on this question.

As an historical aside, the inequivalence between the Schroedinger equation and the Madelung equations was actually discovered twice in different (but related) contexts; the first time was by Takehiko Takabayasi in 1952, who showed that Madelung's hydrodynamic equations are not equivalent to Schroedinger's equation without the (in his own words) "ad-hoc" BSW quantization constraint on the velocity potential S(x,t) in Madelung's equations. Takabayasi also tried to argue that Bohm's 1952 causal interpretation of QM, which made use of Madelung's equations, was also inequivalent to QM, but this turned out to be wrong as we now know. The second time was by Timothy Wallstrom in 1988, in the context of stochastic mechanical derivations of the Schroedinger equation. Wallstrom showed that even though stochastic mechanical theories such as Edward Nelson's can derive the Madelung equations (and, consequently, the quantum potential), they do not derive the Schroedinger dynamics for a single-valued wavefunction without also imposing the ad-hoc BSW constraint on the velocity potential S(x,t) in the stochastic mechanical equations of motion. You can read more about all this in Wallstrom's concise 1994 paper:

Inequivalence between the Schrödinger equation and the Madelung hydrodynamic equations
Phys. Rev. A 49, 1613–1617
http://pra.aps.org/abstract/PRA/v49/i3/p1613_1

In my opinion, if one could find a dynamical justification for the BSW quantization constraint from the dynamics of the particles in stochastic mechanical theories, then one could reasonably claim that the quantum potential is more fundamental than the wavefunction in the context of such theories. In fact, if stochastic mechanical theories could successfully derive the Schroedinger equation, then even the deterministic pilot-wave theories would be "coarse-grained" approximations to the stochastic mechanical theories, and it would only appear on the coarse-grained level that the dynamics of the pilot-wave (wavefunction) and particles are Aristotelian. Moreover, the wavefunction would have to then be interpreted as an epistemic mathematical construct, rather than an ontic field. The quantum potential, on the other hand, would still be interpreted as an ontic potential energy field. So the success or failure of stochastic mechanical derivations of the Schroedinger equation clearly has direct and significant implications for your (Demystifier's) question.

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zenith8 said:
Holland and Hiley in particular have some serious-sounding arguments in their recent papers - which I could look up if I could be bothered - in which they claim that the quantum potential is fundamental.
I would like to see these papers if you know the exact references.

Hi Demystifier,

Sorry for the slight delay. I was out of town for a few days and the thread slipped off the bottom of the page..

Just based on a quick search in "Further Reading" on http://www.tcm.phy.cam.ac.uk/~mdt26/pilot_waves.html" [Broken], the following papers indicate what I mean (links on the page):

Schroedinger revisited: an algebraic approach, M.R. Brown and B.J. Hiley (2004).
See p. 9, paragraph 4

From the Heisenberg picture to Bohm, B. Hiley (2002)
Section 3, p. 7 onwards

Hamiltonian theory of wave and particle in quantum mechanics I: Liouville's theorem and the interpretation of de Broglie-Bohm theory, P.R. Holland (2001).
Section 1.2, p.6 "The role of the quantum potential"

Plus see the book by Rigg in the Textbook section at the top.

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Hi Demystifier,

Sorry for the slight delay. I was out of town for a few days and the thread slipped off the bottom of the page..

Just based on a quick search in "Further Reading" on http://www.tcm.phy.cam.ac.uk/~mdt26/pilot_waves.html" [Broken], the following papers indicate what I mean (links on the page):

Schroedinger revisited: an algebraic approach, M.R. Brown and B.J. Hiley (2004).
See p. 9, paragraph 4

From the Heisenberg picture to Bohm, B. Hiley (2002)
Section 3, p. 7 onwards

Hamiltonian theory of wave and particle in quantum mechanics I: Liouville's theorem and the interpretation of de Broglie-Bohm theory, P.R. Holland (2001).
Section 1.2, p.6 "The role of the quantum potential"

Plus see the book by Rigg in the Textbook section at the top.
On my behalf, thanks for these refs, Zenith.

By the way, not to be pushy, but are either of you (Zenith and Demystifier) interested at all in discussing the question (about the fundamentality of the wavefunction vs quantum potential) that I suggested an answer to? I was really expecting that it would be discussed.

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Demystifier
Gold Member
Zenith, thanks for the references.

Maaneli, #11 was a great post. I mostly agree with it.

Zenith, thanks for the references.

Maaneli, #11 was a great post. I mostly agree with it.
Thanks. Out of curiosity, which parts do you disagree with?

Demystifier
Gold Member
Thanks. Out of curiosity, which parts do you disagree with?
Well, I think it is not completely clear whether the Walstrom argument is correct or not. See Sec. IV of
http://xxx.lanl.gov/abs/quant-ph/0609109

Well, I think it is not completely clear whether the Walstrom argument is correct or not. See Sec. IV of
http://xxx.lanl.gov/abs/quant-ph/0609109
Ah, I've studied that paper recently, and I found several problems with Smolin's arguments. First, his arguments are only applied to the artificial and trivial case of the Schroedinger equation on a circle, whereas the arguments of Takabayasi and Wallstrom apply to the Schroedinger equation in 2 dimensions or greater. And for even just 2 dimensions, Smolin's claim of a well defined mapping between solutions of the Nelson equations and solutions of the Schroedinger equation, is problematic. For example, Valentini and Bacciagaluppi have pointed out that for just one node in a 2 dimensional wavefunction, moving the line across which psi is discontinuous will in general produce a different wavefunction, so that the mapping between solutions of the Schroedinger and Nelson equations is not well defined; and for the case of more than one node, the mapping seems even more ill-defined.

Additionally, Valentini and Bacciagaluppi pointed out that even for the case of the circle, it is problematic to allow discontinuous wavefunctions to be physical wavefunctions, since, as is well known, discontinuous wavefunctions can have divergent values of observables such as the variance of the total energy, the mean kinetic energy, etc.. This is why physical wavefunctions are required to be continuous, or more precisely, that their first derivatives be square-integrable, so that the wavefunctions form a Sobolev space. Smolin does not address this point, aside from a brief comment on page 9 where he asserts that the expectation value of the Nelsonian energy is well defined. But even if so (and he doesn't explicitly show this for the general case), how is this to be reconciled with the fact that the standard definitions of operator expectation values (using the derived discontinuous wavefunctions) are divergent for the aforementioned observables? And if Smolin is going to use the Nelsonian definition of energy expectation values, instead of the standard quantum mechanical definitions, how can he claim that Nelson's theory derives standard quantum mechanics? Smolin does not address any of these inconsistencies.

Lastly, Wallstrom explicitly showed in his 1994 paper that if one allows S(x,t) to be arbitrarily multi-valued in Nelson's equations (so that the derived wavefunctions are arbitrarily multi-valued, as Smolin wants to allow), then this leads to non-quantized values of angular momentum for the case of a 2-dimensional central force problem. In other words, Nelson's stochastic mechanics would not be empirically equivalent to standard quantum mechanics, because it would predict non-quantum values of angular momentum for a well established quantum mechanical situation.

Demystifier
Gold Member
Thanks Maaneli. Are you talking about the book by Valentini and Bacciagaluppi, or about another reference I am not aware of?

Thanks Maaneli. Are you talking about the book by Valentini and Bacciagaluppi, or about another reference I am not aware of?
Not the book, private communications. But Valentini did tell me that he plans to publish these criticisms in his next book.

I see.

By the way, in August there will be a workshop on de Broglie-Bohm theory, for the case you are interested:
http://www.vallico.net/tti/tti.html

I know. I'm one of the invited speakers. See the list of invitees.

I know. I'm one of the invited speakers. See the list of invitees.
Not fair. No-one invited me. Even after I blew one of the organizers in the stationery cupboard after I saw him give a lecture.

And whenever Dr. Chinese tells people who to ask about pilot-wave theory, he always says, "search for posts by Demystifier". Don't know why I bother.

Feeling neglected. Sulk.

Even after I blew one of the organizers in the stationery cupboard after I saw him give a lecture.
:rofl: Are you serious by any chance???

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:rofl: Are you serious by any chance???
As far as I know, I'm the only female in the entire world interested in quantum foundations. There are at least fifty men and no women on the list of invitees at the de Broglie-Bohm conference. Obviously even my extreme measures didn't help..

As far as I know, I'm the only female in the entire world interested in quantum foundations. There are at least fifty men and no women on the list of invitees at the de Broglie-Bohm conference. Obviously even my extreme measures didn't help..
There are a few other women interested in quantum foundations. Off the top of my head, Vishniya Maudlin, Hilary Greaves, Doreen Fraser, Jenann Ismael, and Ruth Kastner. But none of them (with the exception of Vishniya) are especially interested in pilot-wave theory. And Vishniya is likely to come anyway with her husband, Tim Maudlin.