Understanding Bohmian Mechanics of Instrumentalists

[Lynch101]

TL;DR Summary

I'm hoping to gain a better understanding of the arguments for and against Demystifier's paper as well as the implications of certain, specific claims.

I recently read @Demystifier's paper entitled, Bohmian Mechanics for Instrumentalists and I found it quite interesting. There is a danger that I am guilty of a certain amount of confirmation bias, as I find that deterministic interpretations are more closely aligned to my own logical reasoning. In the interest of better understanding the paper itself, and opening that confirmation bias to challenge, I would love to get the thoughts and ideas of @Demystifier and others.

There are a number of ideas in the paper that I would ultimately like to discuss, including the point made about the creation and annihilation of particles, but I think the best place to start might be with what seems - based on my limited understanding - like a fairly critical point; namely, the idea that Bohmian Mechanics produces the same observational predictions as standard QM. It is also a statement I have heard previously in relation to deterministic interpretations of QM, that they do not give rise to the same predictions.

To make a measurable prediction, one must first specify how exactly the arrival time is measured [39], which requires a formulation of the problem in terms of a perceptible. When the problem is formulated in that way, BM makes the same measurable predictions as standard QM, despite the fact that there is no time operator in standard QM

@Demystifier, in your Insight article, How I Stopped Worrying and Learned to Love Orthodox Quantum Mechanics, you mention

the last specialized paper on Bohmian mechanics I have written, a referee found a deep conceptual error that I was not able to fix.

I understand that the paper you are referring to there is not the same paper as per the title of this thread. I'm wondering if this conceptual error applies equally to the 'Bohmian Mechanics for Instrumentalists' paper?

My reading of your insight article is that the conceptual error only applies to a latter attempt to simplify your original work, is that correct? If so, I take this to mean that, although not as 'elegant' as you would like it to be, BM can still be shown to make the same predictions as standard QM. Am I correct in that?

In the paper you mention

The general recipe for making such a false “measurable prediction” out of BM is to put too much emphasis on trajectories and ignore the perceptibles. A lot of wrong “disproofs of BM” of that kind are published in the literature

but from my understanding, your paper seeks to address these 'wrong disproofs' - is that correct? Are you aware of any challenges to your subsequent paper?

 

[Demystifier]

I'm wondering if this conceptual error applies equally to the 'Bohmian Mechanics for Instrumentalists' paper?

No it doesn't.

My reading of your insight article is that the conceptual error only applies to a latter attempt to simplify your original work, is that correct?

The error only applies to those older works where I attempted to make Bohmian mechanics fundamentally relativistic covariant. In my IBM (instrumental Bohmian mehanics) approach, I gave up making BM fundamentally relativistic covariant, so the error does not apply to IBM.

If so, I take this to mean that, although not as 'elegant' as you would like it to be, BM can still be shown to make the same predictions as standard QM. Am I correct in that?

That's correct.

In the paper you mention but from my understanding, your paper seeks to address these 'wrong disproofs' - is that correct?

I address them only by explaining why they are wrong.

Are you aware of any challenges to your subsequent paper?

What subsequent paper? In http://de.arxiv.org/abs/2003.14049 I propose how some sort of Bohmian trajectories could be measured, but this would not prove that Bohmian interpretation is right.

 

[Lynch101]

No it doesn't.

The error only applies to those older works where I attempted to make Bohmian mechanics fundamentally relativistic covariant. In my IBM (instrumental Bohmian mehanics) approach, I gave up making BM fundamentally relativistic covariant, so the error does not apply to IBM.

That's correct.

I address them only by explaining why they are wrong.

Thank you, that was my interpretation.

What subsequent paper? In http://de.arxiv.org/abs/2003.14049 I propose how some sort of Bohmian trajectories could be measured, but this would not prove that Bohmian interpretation is right.

Sorry, I meant if you were aware of any challenge of your IBM paper?

The chronology I had in mind was:
1) 'disproofs' of Bohmian mechanics
2) Your subsequent paper addressing the error in those disproofs
3) A challenge to your IBM paper?

 

[Demystifier]

I see. AFAIK there are no such challenges.

 

[Lynch101]

In your paper you mention:

For truly fundamental particles it makes sense to assume that they cannot be created and destroyed. On the other hand , it is known that relativistic QM and QFT naturally lead to particle creation and destruction (see e.g. [44,. Hence it seems reasonable to assume that the truly fundamental particles, if they exist, are described by non-relativistic QM

Again, this makes perfect sense to me and it seems to speak to the age-old philosophical question of why there is something instead of nothing, assuming that I am understanding it correctly.

The idea that a particle can be 'created' and 'destroyed' seems to me, to be antithetical to the very fundamental principle of determinism as expressed by Laplace:

We ought to regard the present state of the universe as the effect of its antecedent state and as the cause of the state that is to follow.

'Created' seems to imply that a particle is not the effect of its antecdent state, while a particle that is 'destroyed' wouldn't be "the cause of the state that is to follow". Am I correct in that characterisation of the notions of 'create' and 'destroy'?

Of course, this would be perfectly in-keeping with the notion of an indeterministic interpretation but it would seem to require an incredibly 'extraordinary' explanation. Again, relating it back to the age-old question, how can a particle be created from absolutely nothing? How can something come from absolutely nothing? How can a particle then simply cease to have any properties whatseover?

Those are the kind of questions I would have when I hear about particles being 'created' and 'destroyed'. Is that along the lines of what you mean by the statement, 'the truly fundamental particles, if they exist, are described by non-relativistic QM'?

Is there a principle in physics that matter/energy can neither be created or destroyed, or am I misinterpreting that from another aspect of physics?

 

[PeterDonis]

The idea that a particle can be 'created' and 'destroyed'

Is not what QFT actually says. What QFT actually says, at least in its standard interpretation is that "particle" is just a name for certain kinds of states of quantum fields, and processes that are sometimes described as "particles being created or destroyed" are just particular kinds of quantum field processes. The quantum fields themselves don't get created or destroyed, and there are no violations of conservation laws.

@Demystifier in his paper is adopting a different, nonstandard interpretation of QFT according to which (if I understand correctly) the quantum fields are a calculational device and the particles are the actual physical things. On this interpretation, yes, "particle creation and destruction" can be viewed as a problem. But this problem can be avoided by simply not adopting his interpretation.

Is there a principle in physics that matter/energy can neither be created or destroyed

Modern physics generally regards conservation laws as arising from symmetries, on the basis of a theorem, called Noether's Theorem, which connects the two. For example, energy conservation arises from time translation symmetry, momentum conservation arises from space translation symmetry, and angular momentum conservation arises from rotational symmetry. Models in QFT have these symmetries and the corresponding conservation laws.

 

[Lynch101]

Is not what QFT actually says. What QFT actually says, at least in its standard interpretation is that "particle" is just a name for certain kinds of states of quantum fields, and processes that are sometimes described as "particles being created or destroyed" are just particular kinds of quantum field processes. The quantum fields themselves don't get created or destroyed, and there are no violations of conservation laws.

Ah, OK. This sounds more like what I had heard before. I had heard of particles being created and destroyed but usually in conjunction with 'particles' being described as 'excitations' in the quantum field.

I'm probably wrong in thinking this, but this representation of QFT seems to sound somewhat deterministic, where 'particles' are the effect of the antecedent state of the quantum field. This is kind of what I imagine when I think of Bohmian Mechanics, that the pilot wave is just one feature of the quantum field and the 'particle' itself another feature. Somewhat like waves on the ocean, where the wave is a feature of the ocean and the foam is another feature, whose path is determined by the wave.

Probably a very basic and possibly misleading analogy?

@Demystifier in his paper is adopting a different, nonstandard interpretation of QFT according to which (if I understand correctly) the quantum fields are a calculational device and the particles are the actual physical things. On this interpretation, yes, "particle creation and destruction" can be viewed as a problem. But this problem can be avoided by simply not adopting his interpretation.

This then sounds like an instrumentalist interpretation.

Can QFT be thought of in those two ways?

Modern physics generally regards conservation laws as arising from symmetries, on the basis of a theorem, called Noether's Theorem, which connects the two. For example, energy conservation arises from time translation symmetry, momentum conservation arises from space translation symmetry, and angular momentum conservation arises from rotational symmetry. Models in QFT have these symmetries and the corresponding conservation laws.

Ah, thank you. This definitely puts more flesh on the bones of what I had heard previously.

 

[PeterDonis]

this representation of QFT seems to sound somewhat deterministic

Only if you don't consider what happens when measurements take place--for example, when a particle detector detects a particle. QFT works the same as ordinary QM in that respect: it only tells you the probabilities for detectors at various different spacetime events to detect a particle; it doesn't tell you with certainty which ones will or will not detect a particle. Similar remarks apply to other kinds of measurements in QFT that are not usefully described as "detecting particles".

Can QFT be thought of in those two ways?

Most QM interpretations can be applied to QFT the same way they are applied to ordinary QM.

The Bohmian interpretation is normally considered to have issues with relativity and hence with QFT, because nobody has figured out how to make a Lorentz invariant formulation of it. @Demystifier is taking the opposite approach to that issue: he is saying that QFT itself is not fundamental, and that the fundamental theory that underlies it (and to which Bohmian mechanics is an approximation) is actually not Lorentz invariant.

 

[Demystifier]

@Demystifier in his paper is adopting a different, nonstandard interpretation of QFT according to which (if I understand correctly) the quantum fields are a calculational device and the particles are the actual physical things. On this interpretation, yes, "particle creation and destruction" can be viewed as a problem. But this problem can be avoided by simply not adopting his interpretation.

You misunderstood one crucial detail. The fundamental particles are the actual things, but not particles like electrons, photons etc. The latter are interpreted as quasiparticles, analogous to phonons, so there is no problem of creation and destruction of them. The real things are some hypothetical fundamental particles described by non-relativistic QM, so they are not created and destroyed.

Or to put it more blatantly, in IBM (instrumental Bohmian mechanics) electrons, photons etc. do not have trajectories. Only fundamental particles have trajectories. But we still can't observe those fundamental particles (we would need a stronger particle collider for that), so in practice we don't need to worry about details of those fundamental particles and their Bohmian trajectories. In practice we can use the standard instrumental QM/QFT, while Bohmian mechanics can be used for a conceptual explanation of those instrumental rules.

 

[EPR]

You misunderstood one crucial detail. The fundamental particles are the actual things, but not particles like electrons, photons etc. The latter are interpreted as quasiparticles, analogous to phonons, so there is no problem of creation and destruction of them. The real things are some hypothetical fundamental particles described by non-relativistic QM, so they are not created and destroyed.

Or to put it more blatantly, in IBM (instrumental Bohmian mechanics) electrons, photons etc. do not have trajectories. Only fundamental particles have trajectories. But we still can't observe those fundamental particles (we would need a stronger particle collider for that), so in practice we don't need to worry about details of those fundamental particles and their Bohmian trajectories. In practice we can use the standard instrumental QM/QFT, while Bohmian mechanics can be used for a conceptual explanation of those instrumental rules.

You just invented a new physics of your own. How is this science? How are electrons quasi particles? How do you know this? What is this assertion based on?

 

[Demystifier]

You just invented a new physics of your own. How is this science? How are electrons quasi particles? How do you know this? What is this assertion based on?

It's a hypothesis. Hypothesis is legitimate in science.

 

[Lynch101]

Only if you don't consider what happens when measurements take place--for example, when a particle detector detects a particle. QFT works the same as ordinary QM in that respect: it only tells you the probabilities for detectors at various different spacetime events to detect a particle; it doesn't tell you with certainty which ones will or will not detect a particle. Similar remarks apply to other kinds of measurements in QFT that are not usefully described as "detecting particles".

Most QM interpretations can be applied to QFT the same way they are applied to ordinary QM.

This would seem to go back to a key issue of instrumentalist vs deterministic interpretations, where the probabilistic predictions in the deterministic interpretation are due to a lack of information, but the process itself is fundamentally deterministic. Where "particle" is just a name for a state of the quantum field, and the quantum fields themselves don't get created or destroyed, this would seem to necessitate that present states are the effect of antecedent states of the quantum field and therefore deterministic.

For the process to be truly probabilistic would require, as mentioned in the previous thread, that deterministic process to be interrupted. The literal creation and destruction of particles would offer such interruption and would seem to be what a truly probabilistic process would necessitate.

The Bohmian interpretation is normally considered to have issues with relativity and hence with QFT, because nobody has figured out how to make a Lorentz invariant formulation of it. @Demystifier is taking the opposite approach to that issue: he is saying that QFT itself is not fundamental, and that the fundamental theory that underlies it (and to which Bohmian mechanics is an approximation) is actually not Lorentz invariant.

This sounds like a completely valid approach, but I'm clearly not in a position to evaluate it. Are there any issues with this approach or are there any challenges to it, that you are aware of?

I have read various statements about the completeness of quantum mechanics and the impression I got is that it is generally accepted that standard QM is incomplete but, according to certain no-go theorems, no more complete theory is possible. Is that accurate, or have I misrepresented something there?

 

[Lynch101]

You misunderstood one crucial detail. The fundamental particles are the actual things, but not particles like electrons, photons etc. The latter are interpreted as quasiparticles, analogous to phonons, so there is no problem of creation and destruction of them. The real things are some hypothetical fundamental particles described by non-relativistic QM, so they are not created and destroyed.

Or to put it more blatantly, in IBM (instrumental Bohmian mechanics) electrons, photons etc. do not have trajectories. Only fundamental particles have trajectories. But we still can't observe those fundamental particles (we would need a stronger particle collider for that), so in practice we don't need to worry about details of those fundamental particles and their Bohmian trajectories. In practice we can use the standard instrumental QM/QFT, while Bohmian mechanics can be used for a conceptual explanation of those instrumental rules.

In IBM, would particles still be considered 'excitations' of the quantum field? Would there be a fundamental, Universal quantum field?

Could this be how to interpret the following statement in the paper?

What we propose here is that the Earth (and everything else) is made of ether

 

[PeterDonis]

The fundamental particles are the actual things, but not particles like electrons, photons etc. The latter are interpreted as quasiparticles, analogous to phonons, so there is no problem of creation and destruction of them. The real things are some hypothetical fundamental particles described by non-relativistic QM, so they are not created and destroyed.

Ah, ok.

 

[PeterDonis]

Where "particle" is just a name for a state of the quantum field, and the quantum fields themselves don't get created or destroyed, this would seem to necessitate that present states are the effect of antecedent states of the quantum field and therefore deterministic.

We've already been through this in another thread. Your "therefore" here is a non sequitur.

 

[PeterDonis]

Are there any issues with this approach or are there any challenges to it, that you are aware of?

The obvious potential issue with it is that it depends on Lorentz invariance not being fundamental, i.e., Lorentz invariance would have to be violated on some small enough scale. We have no evidence of that so far.

I'm not aware of any challenges in the sense of other papers making arguments against those made in @Demystifier's paper

 

[PeterDonis]

I have read various statements about the completeness of quantum mechanics and the impression I got is that it is generally accepted that standard QM is incomplete but, according to certain no-go theorems, no more complete theory is possible.

You'll need to be more specific and give some references here. I'm not aware of any no-go theorems that prove that no more complete theory than QM is possible.

 

[Lynch101]

We've already been through this in another thread. Your "therefore" here is a non sequitur.

I don't think we resolved that particular point, but maybe not best to get back into it here.

 

[PeterDonis]

I don't think we resolved that particular point

We didn't "resolve" it in the sense that we never reached agreement. But we certainly discussed it to the point where further discussion is not going to be productive.

 

[Lynch101]

The obvious potential issue with it is that it depends on Lorentz invariance not being fundamental, i.e., Lorentz invariance would have to be violated on some small enough scale. We have no evidence of that so far.

Would that be necessary for the position that QFT is not fundamental or more specifically to support the Bohmian picture?

 

[Lynch101]

You'll need to be more specific and give some references here. I'm not aware of any no-go theorems that prove that no more complete theory than QM is possible.

Again, this could be me conflating different information that I have heard. I'll have to go back over some articles/papers etc. that I have [attempted to] read.

 

[PeterDonis]

Would that be necessary for the position that QFT is not fundamental or more specifically to support the Bohmian picture?

For the Bohmian picture. Other viewpoints on QFT not being fundamental (such as various possible quantum gravity theories like string theory or loop quantum gravity) don't necessarily require Lorentz invariance to be violated at any scale.

 

[Demystifier]

For the Bohmian picture. Other viewpoints on QFT not being fundamental (such as various possible quantum gravity theories like string theory or loop quantum gravity) don't necessarily require Lorentz invariance to be violated at any scale.

Yes, but I would add that Refs. [48-50] are highly cited papers proposing Lorentz invariance violation for reasons that have nothing to do with Bohmian mechanics and quantum interpretations. My point is, there are several independent arguments for theoretical plausibility of Lorentz invariance violation at some scale.

 

[Demystifier]

In IBM, would particles still be considered 'excitations' of the quantum field? Would there be a fundamental, Universal quantum field?

No. More precisely, particles of the Standard Model (SM) are excitations of the SM fields, but, according to IBM, SM is not fundamental, fundamental particles are not excitations of a field and there is no fundamental universal quantum field.

 

[Lynch101]

No. More precisely, particles of the Standard Model (SM) are excitations of the SM fields, but, according to IBM, SM is not fundamental

Ah, I get that, thank you.

fundamental particles are not excitations of a field and there is no fundamental universal quantum field.

This is a very crude question and probably doesn't have a simple answer, but I will ask it anyway, in case it might point in some direction: what is the pilot wave "made of", or what "stuff" does the particle travel on? I was thinking in terms of a fundamental field of some sort but if not that, are there statements that can be made about it?Just on a previous point. I referenced this part of your paper:

For truly fundamental particles it makes sense to assume that they cannot be created and destroyed. On the other hand , it is known that relativistic QM and QFT naturally lead to particle creation and destruction (see e.g. [44,. Hence it seems reasonable to assume that the truly fundamental particles, if they exist, are described by non-relativistic QM.

Peter noted:

@Demystifier in his paper is adopting a different, nonstandard interpretation of QFT according to which (if I understand correctly) the quantum fields are a calculational device and the particles are the actual physical things. On this interpretation, yes, "particle creation and destruction" can be viewed as a problem. But this problem can be avoided by simply not adopting his interpretation.

Is there any particular reasoning for adopting this non-standard interpretation of QFT?

 

[Demystifier]

Is there any particular reasoning for adopting this non-standard interpretation of QFT?

Yes, there is quite a general point of view that all known QFT's are just effective theories emerging from a more fundamental theory. In particular, QFT's in condensed matter are known to emerge from QM of atoms.

 

[Demystifier]

what is the pilot wave "made of", or what "stuff" does the particle travel on?

Those are supposed to be fundamental, so the theory does not offer an answer to that question.

 

[kith]

This is a very crude question and probably doesn't have a simple answer, but I will ask it anyway, in case it might point in some direction: what is the pilot wave "made of", or what "stuff" does the particle travel on? I was thinking in terms of a fundamental field of some sort but if not that, are there statements that can be made about it?

The problem is more fundamental than this. It is difficult to separate the pilot wave from the particle in the first place, see this discussion. I don't think that conceptualizing it similar to a particle traveling on a water wave or a charged particle being influenced by an electromagnetic wave is valid.

 

[Lynch101]

Yes, there is quite a general point of view that all known QFT's are just effective theories emerging from a more fundamental theory. In particular, QFT's in condensed matter are known to emerge from QM of atoms.

Is that interpretation of QFT, with particles being created and destroyed, necessary to maintain a fundamentally indeterministic interpretation? To me the alternative would seem to be deterministic, where particles are just a different state of an underlying field, with particles "emerging" from and "returning" to that field.

 

[Demystifier]

Is that interpretation of QFT, with particles being created and destroyed, necessary to maintain a fundamentally indeterministic interpretation? To me the alternative would seem to be deterministic, where particles are just a different state of an underlying field, with particles "emerging" from and "returning" to that field.

Indeterminism is possible even without creation and destruction of particles. But I agree, if there is no fundamental creation and destruction, then determinism seems much more natural.

 

[Lynch101]

Indeterminism is possible even without creation and destruction of particles. But I agree, if there is no fundamental creation and destruction, then determinism seems much more natural.

That is the part that I have trouble reconciling. How is indeterminsim possible without fundamental creation and destruction of particles?

I can see how you would have an indeterministic theory, where indeterministic predictions are a result of a lack of information, however nature would still be fundamentally deterministic in this picture.

 

[Demystifier]

That is the part that I have trouble reconciling. How is indeterminsim possible without fundamental creation and destruction of particles?

I can see how you would have an indeterministic theory, where indeterministic predictions are a result of a lack of information, however nature would still be fundamentally deterministic in this picture.

I meant in theories/interpretations different from the Bohmian one. For instance, the Nelson interpretation works with particle trajectories similar to the Bohmian ones, except that the equation for the particle velocity has one additional stochastic term.

 

[Lynch101]

I meant in theories/interpretations different from the Bohmian one. For instance, the Nelson interpretation works with particle trajectories similar to the Bohmian ones, except that the equation for the particle velocity has one additional stochastic term.

Oh yes, I think you mentioned that. I must look for more on it. Would you have any suggestions for resources on it?

My thinking might be too far in the direction of philosophy because I'm thinking in terms of states being the result of antecedent states. Without the creation and destruction of particles I can't see how such a physcially deterministic chain be broken.

 

[Demystifier]

My thinking might be too far in the direction of philosophy because I'm thinking in terms of states being the result of antecedent states.

In stochastic processes this is not so. See e.g. https://en.wikipedia.org/wiki/Stochastic_process#Random_walk

 

[Lynch101]

In stochastic processes this is not so. See e.g. https://en.wikipedia.org/wiki/Stochastic_process#Random_walk

Thanks Demystifier.

Just looking more into this now. Is Brownian motion a representative example of a stochastic process, that would be analogous in this case?