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

[audioloop]

Is realism compatible with true randomness?
http://lanl.arxiv.org/pdf/1012.2536.pdf

i concur.
so to have properties, you need objects, cos if you talk about outcomes you need them, there are no properties without objects.
and by the way probabilities are just epistemic.

 

[nonequilibrium]

Now that I look over that Einstein quote I'm not sure that Einstein's quote used by Fuchs to question non-locality is accurate. Consider again:

Is Einstein arguing that non-locality makes experimentation impossible or is it non-separability that is his concern? Not being able to individuate systems spatio-temporally as per Einstein's quote appears to have more to do with non-separability rather than non-locality, I think?

I appreciate the distinction you're trying to draw, but I'm having trouble sorting out what the essential difference would be between (non-)separability and (non-)locality. Let me give it a try: in the case of nonlocality (but separability), things can *be* separate entities, but not *act* like separate entities. But what about the old adagium: if it acts, tastes and sounds like X, it is X. Maybe it's more fertile to figure out what the key argument is Einstein is making, as opposed to what case/object/concept he is applying that argument to. Intuitively, I had understood it as the idea that if an experimentator builds a machine and sets up and experiment, he must be able to do so independently of the state the system is in. (And if that is indeed the core idea, then I suppose it's both applicable to non-separability and non-locality, whatever the difference may be). What do you think?

EDIT: and if that is the core issue, it would also be in confrontation with determinism (?), hence my comment.

 

[audioloop]

Now that I look over that Einstein quote I'm not sure that Einstein's quote used by Fuchs to question -locality is accurate. Consider again:

Is Einstein arguing that non-locality makes experimentation impossible or is it non-separability that is his concern? Not being able to individuate systems spatio-temporally as per Einstein's quote appears to have more to do with non-separability rather than non-locality, I think?

http://arxiv.org/pdf/quant-ph/0205039v1.pdf

"What relation is there between the “state” (“quantum state”) described by a function ψ and a real deterministic situation (that we call the “real state”)? Does the quantum state characterize completely (1) or only incompletely (2) a real state?
One cannot respond unambiguously to this question, because each measurement represents a real uncontrollable intervention in the system (Heisenberg). The real state is not therefore something that is immediately accessible to experience, and its appreciation always rests hypothetical. (Comparable to the notion of force in classical mechanics, if one doesn’t fix a priori the law of motion.) Therefore suppositions (1) and (2) are, in principle, both possible. A decision in favor of one of them can be taken only after an examination and confrontation of the admissibility of their consequences.
I reject (1) because it obliges us to admit that there is a rigid connection between parts of the system separated from each other in space in an arbitrary way (instantaneous action at a distance, which doesn’t diminish when the distance increases). Here is the demonstration: A system S₁₂, with a function ψ₁₂, which is known, is composed of two systems S₁, and S₂, which are very far from each other at the instant t. If one makes a “complete” measurement on S₁, which can be done in different ways (according to whether one measures, for example, the momenta or the coordinates), depending on the result of the measurement and the function ψ₁₂, one can determine by current quantum-theoretical methods, the function ψ₂ of the second system. This function can assume different forms, according to the procedure of measurement applied to S₁.
But this is in contradiction with (1) if one excludes action at a distance. Therefore the measurement on S₁ has no effect on the real state S₂, and therefore assuming (1) no effect on the quantum state of S₂ described by ψ_2. I am thus forced to pass to the supposition (2) according to which the real state of a system is only described incompletely by the function ψ₁₂.
If one considers the method of the present quantum theory as being in principle definitive, that amounts to renouncing a complete description of real states. One could justify this renunciation if one assumes that there is no law for real states—i.e., that their description would be useless. Otherwise said, that would mean: laws don’t apply to things, but only to what observation teaches us about them. (The laws that relate to the temporal succession of this partial knowledge are however entirely deterministic.)
Now, I can’t accept that. I think that the statistical character of the present theory is simply conditioned by the choice of an incomplete description."

 

[bohm2]

I (And if that is indeed the core idea, then I suppose it's both applicable to non-separability and non-locality, whatever the difference may be). What do you think?

I think you're right. It seems that both non-separability and non-locality may have bothered Einstein. I have come across stuff suggesting that Einstein's argument for the incompleteness of QM was based on both separability and locality principles (although in that quote, it's not very clear). The definitions I've come across seem to vary depending on author source. Here's one:

Separability Principle: Spatiotemporally separated systems possesses their own separate, individual real physical states, of such a kind that the composite state of a joint system is wholly determined by the separate states of the component systems.

Locality Principle: The real physical state of a system in one region of spacetime cannot be influenced by events in a region of spacetime separated from the first by a spacelike interval. (No action at a distance.)

I'm guessing non-locality would be far more controversial because QM is considered non-separable. I've seen papers that even sub-categorize different degrees of non-locality (weak versus strong). I have no idea what they mean. In one paper the author writes:

Second, concerning the metaphysical implications of quantum non-locality it has been argued that while parameter dependence2 requires a causal relation (action at-a-distance), outcome dependence is best understood as a non-causal connection (non-separability / holism). Since one cannot take refuge in outcome dependence any more: does that mean that we necessarily have to accept action at-a-distance? If yes, between which variables? Or can the idea of a nonseparability be made intelligible even for parameter dependent theories?

A stronger Bell argument for quantum non-locality
http://philsci-archive.pitt.edu/906...er_Bell_argument_for_quantum_non-locality.pdf

 

[Demystifier]

yet

i agree.

Elegance and Enigma: The Quantum Interviews.
Maximilian Schlosshauer.

Jefrey Bub We don’t really understand the notion of a quantum state, in
particular an entangled quantum state, and the peculiar role of measurement in taking
the description of events from the quantum level, where you have interference
and entanglement, to an effectively classical level where you don’t. In a 1935 article
responding to the EPR argument, Schrödinger characterized entanglement as “the
characteristic trait of quantum mechanics, the one that enforces its entire departure
from classical lines of thought.” I would say that understanding the nonlocality associated
with entangled quantum states, and understanding measurement, in a deep
sense, are still the most pressing problems in the foundations of quantum mechanics
today.

Sheldon Goldstein I think it would be better, however, to respond to the following question: what have been the most pressing problems in the foundations of quantum mechanics?
And to this I suppose the standard answer is the measurement problem, or, more or
less equivalently, Schrödinger’s cat paradox.
If one accepts, however, that the usual quantum-mechanical description of the
state of a quantum system is indeed the complete description of that system, it seems
hard to avoid the conclusion that quantum measurements typically fail to have results.

Daniel Greenberger For reasons I’ll explain in my answer to the Question
(see page 152), I don’t think the measurement problem will be solvable soon, or possibly
Ever. We will probably have to know more about nature for that.

Lucien Hardy the most well-known problem in quantum foundations is the
measurement problem—our basic conception of reality depends on how we resolve
this. the measurement problem is tremendously important.

Anthony Legget To my mind, within the boundaries of “foundations of
quantum mechanics” strictly defined, there is really only one overarching problem: is
quantum mechanics the whole truth about the physical world? that is, will the textbook
application of the formalism—including the use of the measurement axiom.

Tim Maudlin the most pressing problem today is the same as ever it was: to
clearly articulate the exact physical content of all proposed “interpretations” of the
quantum formalism. this is commonly called the measurement problem.

Lee Smolin the measurement problem—that is to say, the fact that there are
two evolution processes, and which one applies depends on whether a measurement
is being made. Related to this is the fact that quantum mechanics does not give us a
description of what happens in an individual experiment.

Antony Valentini the interpretation of quantum mechanics is a wide open
Question… ..It would also be good to see further experiments
searching for wave-function collapse…

David Wallace I think anyone’s answer to this is going to depend above all on
what they think of the quantum measurement problem. After all, the measurement
problem threatens to make quantum mechanics incoherent as a scientific theory—to
reduce it, at best, to a collection of algorithms to predict measurement results. So the
only reason anyone could have not to put the measurement problem right at the top
of the list would be if they think it’s solvable within ordinary quantum mechanics.
(Someone who thinks it’s solvable in some modifed version of quantum mechanics—
in a dynamical-collapse or hidden-variables theory, say—ought to think that
the most pressing problem is generalizing that modified version to account for all of
quantum phenomena, including the phenomena of relativistic feld theory.)

They all more or less agree what the main unsolved problem of QM is.
But note that those respectable physicists are not chosen randomly. They all have something in common - they are all doing research in quantum foundations.

On the other hand, physicists doing research in quantum applications (rather than foundations), which is actually what most quantum physicists do, typically do not see the measurement problem as a serious problem.

And that gives one of the most frequent answers to the "why BM is considered controversial" question. Most physicists do not see the use of BM, and for them it's often a sufficient reason to consider it "controversial". That's why it is important to stress the existence of the book
https://www.amazon.com/dp/9814316393/?tag=pfamazon01-20

 

[bhobba]

They all more or less agree what the main unsolved problem of QM is. But note that those respectable physicists are not chosen randomly. They all have something in common - they are all doing research in quantum foundations.

On the other hand, physicists doing research in quantum applications (rather than foundations), which is actually what most quantum physicists do, typically do not see the measurement problem as a serious problem.

And that gives one of the most frequent answers to the "why BM is considered controversial" question. Most physicists do not see the use of BM, and for them it's often a sufficient reason to consider it "controversial". That's why it is important to stress the existence of the book
https://www.amazon.com/dp/9814316393/?tag=pfamazon01-20

Abso-friggen-lutely.

I have often seen it mentioned BM has much more traction as the preferred or one of the preferred interpretations by philosophers and those working on foundations.

Thanks
Bill

 

[Demystifier]

Abso-friggen-lutely.

I have often seen it mentioned BM has much more traction as the preferred or one of the preferred interpretations by philosophers and those working on foundations.

But have you noticed that the book I mentioned shows that BM is actually USEFUL for practical (not merely philosophical or foundational) physical problems?

 

[bhobba]

But have you noticed that the book I mentioned shows that BM is actually USEFUL for practical (not merely philosophical or foundational) physical problems?

Well not having read it I don't have any first hand experience but I take your word for it. And yes it is interesting.

Thanks
Bill

 

[Demystifier]

For any further discussions on whether BM is controversial, I think it is important to distinguish four different types of controversy existing in the physics community:

1. Is BM self-consistent?

2. Is BM consistent with observations?

3. Is BM useful?

4. Is BM simple/beautiful/natural enough?

Practical physicists usually do not have complaints on 1, 2, or 4, but they often argue that BM is not useful. Since most physicists are practical, the controversy of type 3 can be considered the most prevalent. Yet, physicists who find BM unuseful are usually silent about that and simply ignore BM without spelling it out. As a consequence, the type 3 controversy often looks much less prevalent than it really is.

Ironically, despite of being most prevalent, type 3 controversy is the most certainly unjustified. Namely, the book I mentioned definitely demonstrates that, at least in some cases, BM is useful. It is certainly less useful than some more standard techniques of solving QM problems, but the controversy concerns the question is whether it is useful at all. And it definitely is.

Type 2 controversy seems most prevalent in public discussions. But whenever one finds argument that BM is not consistent with predictions of standard QM (and thus with observations), it always turns out that one does not understand the general proof that measurable predictions of BM always agree with those of standard QM. It is like searching for a perpetuum-mobile without understanding the general theorem of energy conservation.
Thus, type 2 controversy is unjustified as well.

Objections of the type 1 do not seem to exist in physics community. It seems that more or less all physicists agree that BM at least does not have internal inconsistencies.

What remains are type 4 controversies. While BM certainly has some advantages over other interpretations concerning their simplicity, beauty and naturalness, it also has some disadvantages of that type. What people disagree on is whether the advantages are stronger than the disadvantages. And frankly, there is no simple and objective way to answer who is right. Therefore, type 4 controversy is the only type of controversy which is really justified.

 

[audioloop]

For any further on whether BM is , I think it is important to distinguish four different types of controversy existing in the physics community:

1. Is BM self-consistent?

2. Is BM consistent with observations?

3. Is BM useful?

4. Is BM simple/beautiful/natural enough?

Practical physicists usually do not have complaints on 1, 2, or 4, but they often argue that BM is not useful. Since most physicists are practical, the controversy of type 3 can be considered the most prevalent. Yet, physicists who find BM unuseful are usually silent about that and simply ignore BM without spelling it out. As a consequence, the type 3 controversy often looks much less prevalent than it really is.

Ironically, despite of being most prevalent, type 3 controversy is the most certainly unjustified. Namely, I mentioned definitely demonstrates that, at least in some cases, BM is useful. It is certainly less useful than some more standard techniques of solving QM problems, but the controversy concerns the question is whether it is useful at all. And it definitely is.

Type 2 controversy seems most prevalent in public discussions. But whenever one finds argument that BM is not consistent with predictions of standard QM (and thus with observations), it always turns out that one does not understand the general proof that measurable predictions of BM always agree with those of standard QM. It is like searching for a perpetuum-mobile without understanding the general theorem of energy conservation.
Thus, type 2 controversy is unjustified as well.

Objections of the type 1 do not seem to exist in physics community. It seems that more or less all physicists agree that BM at least does not have internal inconsistencies.

What remains are type 4 controversies. While BM certainly has some advantages over other interpretations concerning their simplicity, beauty and naturalness, it also has some disadvantages of that type. What people disagree on is whether the advantages are stronger than the disadvantages. And frankly, there is no simple and objective way to answer who is right. Therefore, type 4 controversy is the only type of controversy which is really justified.

what about the Seevinck criterion and bohmian mechanics ?

 

[Demystifier]

what about the Seevinck criterion and bohmian mechanics ?

What is Seevinck criterion?

 

[bohm2]

Maybe someone can answer these questions since it kind relates to the question of controversy with at least some versions of the pilot-wave model, I think. And I'm a bit confused:

1. Are locality and separability logically independent of one another?
2. Which of the two would most find more controversial (non-locality or non-separability?)
3. Which of these 2 principles does entaglement violate?
4. Are all pilot-wave models both non-local and non-separable?

I'm guessing it's possible for a model to be non-local but separable (Newtonian mechanics, comes to mind). So what would be a non-separable but local model? Is entaglement an example?

 

[audioloop]

What is Seevinck criterion?

http://philsci-archive.pitt.edu/2191/1/holism_shpmp.

about holism in bohmian mechanics.

...is that some property of a whole would be holistic if, according to the theory in question, there is no way we can find out about it using only local means, i.e., by using only all possible non-holistic resources available to an agent. In this case, the parts would not allow for inferring the properties of the whole, not even via all possible subsystem property determinations that can be performed...

--
but it doesent seem, cos
http://arxiv.org/pdf/1207.2794.pdf
It is tempting to interpret the fact that for entangled
quantum systems the velocity for particle A depends on
the position of particle B as an immediate demonstration
of the non-locality of BM. However, this is in fact not conclusive.
BM is deterministic. This means that without
external intervention the positions of the particles at all
times are uniquely determined by their initial positions
plus the initial wave function
--

...Because of this failure of a one-to-one correspondence there are interpretations of quantum mechanics that postulate dierent connections between the state of the system and the dynamical properties it possesses. Whereas in classical physics this was taken to be unproblematic and natural, in quantum mechanics it turns out to be problematic and non-trivial. But a connection must be given in order to ask about any holism, since we have to be able to speak about possessed properties and thus an interpretation that gives us a property as signment rule is necessary...

...Another interesting theory that also uses a state space with a Cartesian product to combine state spaces of subsystems is Bohmian mechanics (see e.g.Durr, Goldstein, & Zangh (1996)). It is not a phase space but a conguration space. This theory has an ontology of particles with well dened positions on trajectories Here I discuss the interpretation where this theory is supplemented with a property assignment rule just as in classical physics (i.e., all
functions on the state space correspond to possible properties that can all be measured).

 

[Demystifier]

Audioloop, I still don't understand what exactly bothers you.

 

[Demystifier]

1. Are locality and separability logically independent of one another?

They are different, but not independent.

2. Which of the two would most find more controversial (non-locality or non-separability?)

Nonlocality is more controversial. I think nobody doubts that QM is non-separable.

3. Which of these 2 principles does entaglement violate?

All experts agree that entanglement violates separability, while they do not agree whether it violates locality.

4. Are all pilot-wave models both non-local and non-separable?

Yes.

So what would be a non-separable but local model?

E.g. http://xxx.lanl.gov/abs/1112.2034

 

[ZapperZ]

Please note an important part in the PF Rules:

References that appear only on http://www.arxiv.org/ (which is not peer-reviewed) are subject to review by the Mentors. We recognize that in some fields this is the accepted means of professional communication, but in other fields we prefer to wait until formal publication elsewhere.

Topics in QM are still heavily dependent on peer-reviewed publication. So please use only such sources and not unpublished ones.

Zz.

 

[Fredrik]

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?

It sounds like what you have in mind when you say "QM" is the quantum theory of a single spin-0 particle in Galilean spacetime. ("The Schrödinger equation and stuff"). But "QM" can also refer to the mathematical framework in which quantum theories are defined ("Hilbert spaces and stuff"), and it can certainly handle special relativistic theories.

Edit: I see that there are almost a hundred posts after the one I replied to, so someone has probably said this already.

 

[Demystifier]

Jklasers, what essay are you referring to?

 

[Cthugha]

That post by jklasers was a full quote from some of the first google hits on pilot wave theory. As the other posts from the same user looked similar, I suppose it is a spambot which was not allowed to post his links yet.

 

[bohm2]

What is Seevinck criterion?

I've read a few of his papers and I have trouble understanding his arguments. Seevinck appears to argue that Bohmian mechanics, at least, the DGZ version (where ψ is nomological) is not "holistic". But I don't believe Seevinck has looked closely at the different interpretations of pilot wave theories although he does acknowledge this possibility in a footnote. He writes:

Indeed, in Section 4 classical physics and Bohmian mechanics are proven not to be epistemologically holistic, whereas the orthodox interpretation of quantum mechanics is shown to be epistemologically holistic without making appeal to the feature of entanglement, a feature that was taken to be absolutely necessary in the supervenience approach for any holism to arise in the orthodox interpretation of quantum mechanics...It was shown that all theories on a state space using a Cartesian product to combine subsystem state spaces, such as classical physics and Bohmian mechanics, are not holistic in both the supervenience and epistemological approach. The reason for this is that the Boolean algebra structure of the global properties is determined by the Boolean algebra structures of the local ones.

Holism, physical theories and quantum mechanics
http://mpseevinck.ruhosting.nl/seevinck/gepubliceerde_versie_Holism.pdf

 

[Demystifier]

I don't see why it is even important whether it is holistic or not.

 

[bohm2]

I don't even understand the difference between nonseparability versus holism. I always assumed that the two meant the same thing. But it seems there are different types/degrees of non-separability/holism and different degrees of non-locality as suggested in the paper I posted above and also in the Stanford piece by Richard Healey:

Holism and Nonseparability in Physics
http://plato.stanford.edu/entries/physics-holism/

And where does non-local "directional" quantum "steering" fit into the picture? I'm guessing this is a very "weak" form of non-locality? So in terms of controversial from most to least:

strong non-locality>weak non-locality> steering>non-separability/holism

I still don't understand how some authors can argue that Bell's inequality excludes not just local but even weakly non-local theories while others argue that it only rules out separability/non-holism.

 

[audioloop]

I 't even understand the difference between nonseparability versus holism. I always assumed that the two meant the same thing. But it seems there are different types/degrees of non-separability/holism and different degrees of non-locality as suggested in the paper I posted above and also in the Stanford piece by Richard Healey:

Holism and Nonseparability in Physics
http://plato.stanford.edu/entries/physics-holism/

And where does non-local "directional" quantum "steering" into the picture? I'm guessing this is a very "weak" form of non-locality? So in terms of controversial from most to least:

strong non-locality>weak non-locality> steering>non-separability/holism

I still don't understand how some authors can argue that Bell's inequality excludes not just local but even weakly non-local theories while others argue that it only rules out separability/non-holism.

Holism is more than Non-Separability as Contextuality is more than Non-Locality
---
"a physical theory is holistic if and only if it is impossible in principle to infer the global properties,
as assigned in the theory, by local resources available to an agent, there is no way we can find out about it using only local means, i.e., by using only all possible non-holistic resources available to an agent. In this case, the parts would not allow for inferring the properties of the whole, not even via all possible subsystem property determinations that can be performed"
Seevinck. (Epistemological Holism, physical property holism).
unlike of Ontological Holism, Nonseparability.


how, when and what it can posit the cause of explanation (determination) of values
or are values just "parts" of a single fact or process (as matter of fact, not so composed) ?Hierarchical Status

Holism -> Non separability -> Contextuality -> Nonlocality

Syntactical Reality .

 

[Demystifier]

Sure, it's really a nice little idea. From the little I know of quantum gravity, it seems the interest originates from there, in an attempt to derive the time-dependent Schrödinger equation from a time-independent universal wavefunction, this by treating spacetime as a macroscopic quantity.

Let's keep it simple, keeping the idea clear: the set-up is a two-particle system, the first with coordinates q, the latter with coordinates Q. The "universal" wavefunction is the time-independent \Psi(Q,q) satisfying E \Psi = \hat H \Psi. We now suppose that the Q-particle is macroscopic, such that we know its (Bohmian) position Q(t) at all times. We now want to treat the subsystem q quantum-mechanically. To do this, it is logical to define the conditional wavefunction \psi(q,t) := \Psi(Q(t),q). Note that the conditional wavefunction is now time-dependent since we've evaluated the universal wavefunction in the Bohmian trajectory for the macroscopic particle. It's not hard to prove/see that this conditional wavefunction and the universal wavefunction predict the same physics for the small particle.

Now due to the postulates of pilot-wave theory we know \dot Q(t) in terms of \Psi. Consequently, using the chain rule, we can calculate i\partial_t \psi(q,t). One gets that in highest order of M, being the mass of the macroscopic particle Q, we get that i\partial_t \psi = \hat H' \psi where \hat H' denotes the appropriate Hamiltonian for the subsystem. The math is a bit cumbersome, however I worked it out in a bachelor (i.e. undergraduate) project I made; I will PM it to you.

Summarizing, in the case of a time-independent Schrödinger equation, we can derive the time-dependent Schrödinger equation for a subsystem in case the environment is macroscopic.

Another, in my view less compelling, approach is taken by Goldstein in e.g. http://arxiv.org/pdf/quant-ph/0308039v1.pdf (page 21). The above approach, the one I outlined, I haven't seen as such in print. I think perhaps Kittel talks about it in his quantum gravity book, but I'm really not sure, this is more of a guess. Anyway I don't claim priority on this one, the suggestion mainly came from my advisor for the project (Ward Struyve), and I don't know where he got his juice, although there is a link with Tejinder Pal Singh as I outline in my project. I'll send the PM in a moment. (Anyone else interested is free to PM me, of course.)

These beautiful ideas are now further further elaborated in
http://arxiv.org/abs/1209.5196