I QFT made Bohmian mechanics a non-starter: missed opportunities?

  • #51
Fra said:
To give a mathematical theory of the "inside observer" and it's interactions, is of course an open issue, i have no answer.
Then you are verging on personal speculation, which is off limits here. You do give a reference, which is good, but please stay within the bounds of what is in the literature.
 
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  • #52
PeterDonis said:
Then you are verging on personal speculation, which is off limits here. You do give a reference, which is good, but please stay within the bounds of what is in the literature.
(I am aware of this by now, so even if I had some answers, I would not put it on here, at least not until it's published elsewhere.)

The role I rather take on here, is to try to broaden the perspectives on how we see and understand theories in general. Quite often, different stances or interpretations tend to fail to see the logic in the other perspective, so discussions rarely lead anywhere. Sometimes it stops by agreeing to disagree on a definition of a word. I have been fascinated by seeing similarities rather than differences. Different stances have different primary notions (or ontics that Demysitifer calls it), and with it comes also correspondingly different guiding principles or constraints. I am thinking that it may exist dualities here, so that the different approaches are simply different, but they aim to understnad the same world, so we might benefit from understanding the relations between perspectives.

Constraints vs Emergence is one such thing, which seems to have a flipside, which is finetuning vs evolution, which makes a difference to some when it comes to explanatory value. I think these questions appear in different disguises in all the different research programs.

In string theory we have the fine tuning among all the backgrounds that one arries at by a constraintn based approach. Does it have to be manually "tuned" or is there a better way?

In Bohmian mechanis, the primary picuture superficially violates the observer equivalence of SR, but perhaps it is instead to be understood not as a constraint, but as emergent, or are there constraints to arrive a similar situations as in string theory?

and the list of comparastions go on, but what is a common denominator here? can we associate elements of different apparoches to each other and gain insight? Probably each approach has it's advantages and disadvantages.

/Fredrik
 
  • #53
Demystifier said:
Right now I am working on an opportunity that was missed so far to better understand foundations of quantum statistical mechanics (mixed states, thermal mixed states, etc.) from a Bohmian perspective.
Finished: :smile:
https://arxiv.org/abs/2308.10500
 
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  • #54
vanhees71 said:
Otherwise I can invent any kind of phantasy stuff, I like for some personal reasons, and claim it's needed to understand "what's really happening", but which cannot be observed.
You don't need to claim that it's needed for everybody. You can claim that you understand it better when you imagine that this fantasy stuff is real. And maybe some (not all) other guys will also like your fantasy and conclude that they also better understand it when they imagine that the fantasy stuff is real. In this way, the fantasy may become a useful thinking tool, a mental trick, that transforms an abstract entity into something that some humans can grasp more intuitively. So who cares that this fantasy is invented, if the fantasy serves a practical purpose?

Mathematics is full of useful invented fantasies, like infinity, complex numbers, real numbers, perhaps even integers. Some mathematicians believe that these things exist on their own and that mathematicians discover them, rather than invent them. Other mathematicians disagree. But at the end of the day that's irrelevant, what matters is that these things are useful as thinking tools.

The same is the case with quantum interpretations. If, for instance, someone likes to think that paths in Feynman path integrals are "real", because such a way of thinking makes path integrals more intuitive to him, nobody forces you to think that way. At the end of the day, people with different styles of thinking need to agree on measurable predictions. But how they arrived to them, did they use a minimal style of thinking which you prefer, or a style dressed with additional fantasy stuff, who cares? Why it bothers you that some people like to think in a less minimal style, if they all eventually arrive to the same results?
 
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  • #55
And even those who say "shut up and calculate," aren't. When I'm asked "which quantum interpretation do you think is the right one," I say, "I think you have a mistaken idea about what an interpretation is for."
 
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  • #56
Demystifier said:
Mathematics is full of useful invented fantasies, like infinity, complex numbers, real numbers, perhaps even integers. Some mathematicians believe that these things exist on their own and that mathematicians discover them, rather than invent them. Other mathematicians disagree. But at the end of the day that's irrelevant, what matters is that these things are useful as thinking tools.
Are fantasies and fictions the same thing? Not sure whether infinities per se are a fantasy, or whether the fantasy are just the inexhaustibly many different types of infinity.

Let me link to my old comment and my old FOM post on Mathematical fictionalism vs. physical fictionalism:
Like many other mathematicians, I believe in a principle of 'conservation of difficulty'. This allows me to believe that mathematics stays useful, even if it would be fictional. I believe that often the main difficulties of a real world problem will still be present in a fictional mathematical model.
...
From my experience with physicists (...), their trust in 'conservation of difficulty' is often less pronounced. As a consequence, physical fictionalism has a hard time
 
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  • #57
Demystifier said:
In this way, the fantasy may become a useful thinking tool, a mental trick, that transforms an abstract entity into something that some humans can grasp more intuitively.
One of the most fruitful "fantasies" is of course the field concept, or originally Faradays "lines of force" that @vanhees71 still finds hard to grapple with.
 
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  • #58
I've no problem with the field concept but only with the depiction as field lines. For me they are no more than the lines pointing in direction of the field to be depicted. How dense the field lines are depends on my choice of how dense I plot them to get an overview about how the field is directed. It's no clear measure for the field strength so.
 
  • #59
I think the most useful element of a field line comes in the form of effective stresses. Electromagnetic field lines act as though they had a tension to them, producing a restoring force when they bend. This allows a nice picture for the electromagnetic oscillations of light, as well as the propagation of Alfven waves in magnetohydrodynamics. Imagining that the field is threaded with lines is thus actually pretty useful, because the mathematics of Maxwell can often appear quite similar to the mathematics of strings under tension. That is in a sense similar to the concept of a quantum mechanical interpretation, we are looking for a kind of conceptual analog where the mathematics can be framed similarly, so we can borrow from insights we have developed in other contexts.
 
  • #60
gentzen said:
Are fantasies and fictions the same thing?
Fiction, of course, in this context is a better word, but @vanhees71 used the word "fantasy" to emphasize the negative connotations associated with it, so I used the same word in a reply.
 
  • #61
vanhees71 said:
I've no problem with the field concept but only with the depiction as field lines. For me they are no more than the lines pointing in direction of the field to be depicted. How dense the field lines are depends on my choice of how dense I plot them to get an overview about how the field is directed. It's no clear measure for the field strength so.
I guess you are just not a visual type, for you visualizations of abstract ideas do more harm than good. If I'm right, you probably don't like all these pictures of spacetime in special and general relativity, because they are all misleading if taken too literally.
 
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  • #62
Minkowski diagrams and their relatives in GR are indeed more complicated than thought. Already for reading a Minkowski diagram you have to abandon your well-trained thinking in terms of the Euclidean plane and substitute it with the Minkowski pseudo-metric. E.g., you have to construct the right "tic marks" for the axes of inertial frames with hyperbolae. Sometimes such spacetime diagrams may give an intuitive picture about some things like the relativity of simultaneity etc. As you say, one should be very careful in taking them too literally.
 
  • #63
Demystifier said:
I guess you are just not a visual type, for you visualizations of abstract ideas do more harm than good. If I'm right, you probably don't like all these pictures of spacetime in special and general relativity, because they are all misleading if taken too literally.
That is a bad example. The spacetime diagrams are not fantasies or a thinking tool, they are an accurate discription of spacetime.
 
  • #64
martinbn said:
That is a bad example. The spacetime diagrams are not fantasies or a thinking tool, they are an accurate discription of spacetime.
I didn't say they are fantasies. I conjectured that @vanhees71 is not a visual type, and proposed a test of my conjecture. If my conjecture is true, he should dislike these diagrams even though they are not fantasies.
 
  • #65
Demystifier said:
I didn't say they are fantasies. I conjectured that @vanhees71 is not a visual type, and proposed a test of my conjecture. If my conjecture is true, he should dislike these diagrams even though they are not fantasies.
And how is this related to the thread?!
 
  • #66
martinbn said:
And how is this related to the thread?!
If he is not a visual type, while Bohmian interpretation is intuitive precisely because it's visual, it explains why he doesn't find the Bohmian interpretation intuitive.
 
  • #67
Demystifier said:
If he is not a visual type, while Bohmian interpretation is intuitive precisely because it's visual, it explains why he doesn't find the Bohmian interpretation intuitive.
No, it doesn't, that's why I said it was a bad example. I like space-time diagrams, and I find them intuitive and usefull. But I don't feel the same way about BM.
 
  • #68
martinbn said:
No, it doesn't, that's why I said it was a bad example. I like space-time diagrams, and I find them intuitive and usefull. But I don't feel the same way about BM.
I think you dislike BM for different reasons than him. All likers of BM are alike, each disliker of BM dislikes it in his own way. :oldbiggrin:
 
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  • #69
Spacetime diagrams, when properly read, provide a picture of observable things ("events"). Bohmian trajectories are illusions. They are not depicting anything observable and may lead to false intuitions.
 
  • #70
vanhees71 said:
Bohmian trajectories ... may lead to false intuitions.
Of course they can. But any intuitive idea may lead to false intuitions, if you don't understand it properly.
 
  • #71
Demystifier said:
I think you dislike BM for different reasons than him. All likers of BM are alike, each disliker of BM dislikes it in his own way. :oldbiggrin:
I agree, there are many reasons why BM is not good.
 
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  • #72
martinbn said:
I agree, there are many reasons why BM is not good.
What's your most important one?
 
  • #73
Demystifier said:
What's your most important one?
It fails in one of its main claims, to give a better explanation of observed phenemena than QM.
 
  • #74
vanhees71 said:
Spacetime diagrams, when properly read, provide a picture of observable things ("events"). Bohmian trajectories are illusions. They are not depicting anything observable and may lead to false intuitions.
How is your favored interpretation of QM not an "illusion", or depicting "observable" things?
 
  • #75
My favored interpretation is the minimal statistical interpretation, and it describes just the observable things and doesn't invent some unobservable, unnecessary elements just to solve some philosophical pseudo-problems like trajectories in BM.
 
  • #76
I think it should be pointed out that there are (at least) three different arenas in which one can discuss the value, or lack thereof, of interpretations of physical theories. These separate arenas present yet more ways for people discussing interpretations to talk past each other:
1) an interpretation as a guide to correctly applying the theory
2) an interpretation as a means of getting a subjective sense of "what is really happening"
3) an interpretation as a way to see potential incompletenesses that could point to the next theory

For example, if one is looking only at (1), and has no trouble applying the theory with only minimal interpretation, then they see no value in any but a minimal interpretation, obviously. But that does not address (2) or (3). Of course, such a person may have no need for (2), that might not be what proffers the fascination of physics to them. They might also be skeptical of (3), though Einstein himself provided several spectacular examples of making progress in science via this direction (as well as one equally famous failure to do so.) So arena (1) relates to issues like how visual vs. abstract is one's thinking, (2) relates to the subjective and philosophical reasons that various different people do science, and (3) relates to how closely one thinks science needs to be, or at least can be, connected to philosophy. So any discussion about which interpretation is "better" really must start with addressing these much deeper issues, or it will just be people talking past each other.
 
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  • #77
vanhees71 said:
My favored interpretation is the minimal statistical interpretation, and it describes just the observable things and doesn't invent some unobservable, unnecessary elements just to solve some philosophical pseudo-problems like trajectories in BM.
Can you give an example where you analyze a phenomenon only in terms of observable things that are not "illusory"?
 
  • #78
martinbn said:
It fails in one of its main claims, to give a better explanation of observed phenemena than QM.
"Better" is a subjective notion, and in this case totally vague because you don't present any explanation why do you think so. One could write exactly the same sentence for any other interpretation of QM, and nobody would have any clue why one thinks so. I'm sure you can do better than this.
 
  • #79
vanhees71 said:
philosophical pseudo-problems
Are you using "philosophical" and "pseudo" as synonyms? Or are there philosophical problems (in physics) which are not pseudo-problems?
 
  • #80
AndreasC said:
Can you give an example where you analyze a phenomenon only in terms of observable things that are not "illusory"?
I don't do this, but my experimental colleagues. They build the LHC and detectors, where they register particles using various types of detectors and store the result to analyze it later on the computer. These are the observations to be described by our theories. On the most fundamental level it's the Standard Model of elementary particles, which is despite being challenged for a long time with all these experiments, still found to be always valid. In my special field, relativistic heavy-ion collisions, the theoretical tools are derived from this fundamental level in various ways: equilibrium and non-equilibrium relativistic many-body theory (Kadanoff-Baym equations), quantum-transport theory and corresponding numerical simulations, relativsitic hydrodynamics derived from it and corresponding numerical simulations, lattice QCD at finite temperature and baryo-chemical potential... All these theoretical tools use the minimal interpretation of Q(F)T. There's no need for extra philosophical quibbles. The purely physical problem is hard enough to tackle without superfluous additions.
 
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  • #81
Demystifier said:
All likers of BM are alike, each disliker of BM dislikes it in his own way. :oldbiggrin:
martinbn said:
I agree, there are many reasons why BM is not good.
The correct conclusion should have been that dislikers of BM are unhappy. :oldbiggrin:

Which in fact is not far from truth. At this forum, dislikers of BM seem quite frustrated by the fact that some people feel happy by having BM as their way of thinking.

“If we cannot disprove Bohm, then we must agree to ignore him.”
- Oppenheimer
 
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  • #82
vanhees71 said:
I don't do this, but my experimental colleagues. They build the LHC and detectors, where they register particles using various types of detectors and store the result to analyze it later on the computer.
When they analyze it on computer, they use many unobservable ideas. Their analysis contains a lot of theoretical bias.
 
  • #83
martinbn said:
It fails in one of its main claims, to give a better explanation of observed phenemena than QM.
Better than what?
  • Better than Ballentine's statistical interpretation, which is also the interpretation favored by vanhees71? (The one which is most easily applied to repeatable experiments, but which can cause disagreement when it comes to applying it to non-repeatable scenarios, like to actual measurements of sufficiently complex samples, and especially to cosmology and the universe as a whole.)
  • Better than the "literal" variant of Copenhagen presented in some of Mermin's papers? (Papers like In praise of measurement (2006) or Copenhagen Computation: How I Learned to Stop Worrying and Love Bohr (2003) that advocate that a literal "discontinuous collapse" interpretation of measurement works perfectly fine in quantum computer science.)
  • Better than the "moderately" epistemic variant of Copenhagen that Rudolph Peierls explained in his 1991 paper In defense of "measurement"? Or better than the "radically" epistemic QBism?
  • Better than [... please insert some no-nonsense interpretation that you actually have in mind here...]?
 
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  • #84
vanhees71 said:
My favored interpretation is the minimal statistical interpretation, and it describes just the observable things and doesn't invent some unobservable, unnecessary elements just to solve some philosophical pseudo-problems like trajectories in BM.

The thing I don't understand is why are you wasting so much time actively participating in lengthy discussions about philosophical pseudo-problems. I would assume that you at least does not consider them as philosophical pseudo-problems.
 
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  • #85
weirdoguy said:
The thing I don't understand is why are you wasting so much time actively participating in lengthy discussions about philosophical pseudo-problems.
He would like to stop it, but he can't, it's stronger than his rational will, because he's a philosopher in his heart. :smile:
 
  • #86
gentzen said:
Better than what?
I think he meant better than shut up and calculate.
 
  • #87
Demystifier said:
I think he meant better than shut up and calculate.
Oh, good point, I didn't expect this. I guess I could even agree to the "calculate" part. I don't like the "shut up" part, but vanhees71 doesn't subscribe to that part anyway. Let him calculate and explain the interesting part of the physics, i.e. the non-pseudo stuff. As for martinbn, well, he can speak for himself.
 
  • #88
gentzen said:
I don't like the "shut up" part, but vanhees71 doesn't subscribe to that part anyway.
There are two kinds of shut up and calculate guys. Those who shut up and calculate, and those who tell others to shut up and calculate.
 
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  • #89
vanhees71 said:
I don't do this, but my experimental colleagues. They build the LHC and detectors, where they register particles using various types of detectors and store the result to analyze it later on the computer. These are the observations to be described by our theories. On the most fundamental level it's the Standard Model of elementary particles, which is despite being challenged for a long time with all these experiments, still found to be always valid. In my special field, relativistic heavy-ion collisions, the theoretical tools are derived from this fundamental level in various ways: equilibrium and non-equilibrium relativistic many-body theory (Kadanoff-Baym equations), quantum-transport theory and corresponding numerical simulations, relativsitic hydrodynamics derived from it and corresponding numerical simulations, lattice QCD at finite temperature and baryo-chemical potential... All these theoretical tools use the minimal interpretation of Q(F)T. There's no need for extra philosophical quibbles. The purely physical problem is hard enough to tackle without superfluous additions.
But I don't really see how any of that speaks to the power of one interpretation or the other. I don't really see what you mean by saying that these theoretical tools "use" one interpretation or the other. Supposing the interpretations are all observationally equivalent, then at that level you would be working with the same equations. Theoretically, you could have started from the Bohmian picture or whatever other interpretation you may think of, derived the same things, and then claim you don't need to tackle with the superfluous quibbles of the MS interpretation. At the end of the day, as you said, at the LHC they receive data that they analyze with computers. At that level, all of the entities in any interpretation, including your preferred one, are intermediate "fictions" to explain the results. They don't somehow make direct observations of particles etc, there is a huge theoretical structure at play to even produce these "observations", and that includes a ton of what you call "fictions". So I don't think saying a theory has an "unobservable" element is a strong enough argument on its own.

In general I think it's usually a little bit suspect when some idea is called "minimal", or "simplest possible" and thus supposedly better. It reminds me of how dubiously Occam's razor is always invoked.
 
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  • #90
Demystifier said:
There are two kinds of shut up and calculate guys. Those who shut up and calculate, and those who tell others to shut up and calculate.
But vanhees71 is neither of those "shut up" guys. Neither does he shut up himself, nor does he tell you or me to shut up. He openly admits that he doesn't like BM, but that is different from telling somebody to shut up.
 
  • #91
AndreasC said:
But I don't really see how any of that speaks to the power of one interpretation or the other. I don't really see what you mean by saying that these theoretical tools "use" one interpretation or the other. Supposing the interpretations are all observationally equivalent, then at that level you would be working with the same equations. Theoretically, you could have started from the Bohmian picture or whatever other interpretation you may think of, derived the same things, and then claim you don't need to tackle with the superfluous quibbles of the MS interpretation. At the end of the day, as you said, at the LHC they receive data that they analyze with computers. At that level, all of the entities in any interpretation, including your preferred one, are intermediate "fictions" to explain the results. They don't somehow make direct observations of particles etc, there is a huge theoretical structure at play to even produce these "observations", and that includes a ton of what you call "fictions". So I don't think saying a theory has an "unobservable" element is a strong enough argument on its own.

In general I think it's usually a little bit suspect when some idea is called "minimal", or "simplest possible" and thus supposedly better. It reminds me of how dubiously Occam's razor is always invoked.
Well, a big part of my choice of the minimal statistical interpretation, which is very similar to a flavor of Copenhagen, which neither assumes a collapse (which cannot occur for causality reasons and it's not needed at all to use the quantum formalism to compare what's predicted concerning physical observables and what's found when measured) nor a quantum-classical cut (for which there is not the slightest evidence; rather the classical behavior of macroscopic objects is well-understood as an effective description of coarse-grained macroscopically relevant observables), is Occam's razor.

In the case of BM first of all there's no need to calculate the Bohmian trajectories, because it's not needed to confront the theory with experiment. All our colleagues measure are cross sections and related quantities, and these are just about the statistical properties of the outcome of measurements on ensembles of equally prepared collision systems (at the LHC either 2 protons ->X or 2 nuclei -> X, where there are measurements where X is resolved in to identified particles).

Another case against BM is that there's no satisfactory Poincare covariant Bohmian interpretation of relativistic QFT, while standard QFT in the minimal interpretation is Poincare covariant.

The minimalism I like is, that as a natural scientist one should look at what's observed/observable and not at some philosophical fictions, which never can be objectively tested. The latter are simply not within the realm of the natural sciences, which may be unsatisfactory for some philosophically inclined, who think there's more to be known about Nature than what's objectively accessible to observations, but these are questions more in the direction of a personal world view or religion.
 
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  • #92
vanhees71 said:
Well, a big part of my choice of the minimal statistical interpretation, which is very similar to a flavor of Copenhagen, which neither assumes a collapse (which cannot occur for causality reasons and it's not needed at all to use the quantum formalism to compare what's predicted concerning physical observables and what's found when measured) nor a quantum-classical cut (for which there is not the slightest evidence; rather the classical behavior of macroscopic objects is well-understood as an effective description of coarse-grained macroscopically relevant observables), is Occam's razor.
Hmm, I kind of object to that, because I don't think Occam's razor makes any kind of rigorous sense almost every time it is applied, and even when it does, there is no guarantee it's not misleading.
vanhees71 said:
In the case of BM first of all there's no need to calculate the Bohmian trajectories, because it's not needed to confront the theory with experiment.
Ok but where's the problem? Don't calculate them if you don't want!

vanhees71 said:
Another case against BM is that there's no satisfactory Poincare covariant Bohmian interpretation of relativistic QFT, while standard QFT in the minimal interpretation is Poincare covariant.
Right, that is a completely different and more valid argument, although @Demystifier disagrees that it matters. Maybe they are right, I don't know, haven't read the papers.

vanhees71 said:
The minimalism I like is, that as a natural scientist one should look at what's observed/observable and not at some philosophical fictions, which never can be objectively tested.
The issue I have with that is that I don't see how you can coherently partake in some kind of "fiction designation" and "fiction counting" in these theories. Also it's kind of weird because while arguing in favor of a specific interpretation, it feels like you are arguing against the very concept of the argument. I mean, if all that matters is the calculations you make, how can you possibly argue in favor of one or the other interpretation? You can make the same calculations with any of them.
 
  • #93
gentzen said:
But vanhees71 is neither of those "shut up" guys. Neither does he shut up himself, nor does he tell you or me to shut up. He openly admits that he doesn't like BM, but that is different from telling somebody to shut up.
He is a polite guy, this is why he doesn't directly tell anybody to shut up. But it seems to me that this is what he means.
 
  • #94
AndreasC said:
although @Demystifier disagrees that it matters. Maybe they are right, I don't know, haven't read the papers.
The https://arxiv.org/abs/2205.05986 is really a light read, from a technical point of view. Just saying.
 
  • #95
vanhees71 said:
In the case of BM first of all there's no need to calculate the Bohmian trajectories, because it's not needed to confront the theory with experiment.
This by itself is not really an objection however, because there are lots of other examples in physics where we calculate things that we don't need to calculate because we don't measure them. A classic example is the "force of gravity", taught to essentially every single physics major in the world. Not only don't we measure that, our best theory of gravity says it is actually impossible to measure it, but this does not prevent every first year physics course from going on about it! The reason is clear, it is regarded as a nice fiction, which means the term "fiction" cannot be categorically perjorative. The true skeptic regards all of physics as similarly a "nice fiction," and expects every single current physics law to be regarded as incorrect at some point in the future.
vanhees71 said:
Another case against BM is that there's no satisfactory Poincare covariant Bohmian interpretation of relativistic QFT, while standard QFT in the minimal interpretation is Poincare covariant.
I would agree that we don't want our interpretations of some theory to impose limitations the theory itself does not impose. Still, even if it is true that BM cannot be made to interpret relativistic QM (a point that I believe @Demystifier has provided counterargument to, ironically in the spirit of saying that BM might not be able to match the "fictions" of relativity, like Poincare invariance, but can be made to match the observables), one could still correctly say that BM interprets nonrelativistic QM. It is perfectly all right to have an interpretation of a theory that has a domain of application, we do it all the time (e.g., the "force of gravity" again). The overarching point is, we should not take any of our theories so seriously that we think the "best" version is "the truth" that we are trying to interpret. (Again, the true skeptic sees the flaw in that thinking.)
vanhees71 said:
The minimalism I like is, that as a natural scientist one should look at what's observed/observable and not at some philosophical fictions, which never can be objectively tested. The latter are simply not within the realm of the natural sciences, which may be unsatisfactory for some philosophically inclined, who think there's more to be known about Nature than what's objectively accessible to observations, but these are questions more in the direction of a personal world view or religion.
But again, I would bring up "the force of gravity." Your statement here condemns the use of that notion categorically! I think it is perfectly fine to explain your own philosophical preferences, which is essentially what you are doing, and to recognize that those preferences have a context that is pragmatic for your purposes, but it is difficult to apply them as objective principles over all of science. How would you respond to the characterization of Poincare covariance, and Occam's Razor itself, as exactly the kind of "fiction" about which you object? You hold that a minimal interpretation is absent of any "religion," but is it not already a kind of religion of its own?
 
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  • #96
Ken G said:
This by itself is not really an objection however, because there are lots of other examples in physics where we calculate things that we don't need to calculate because we don't measure them. A classic example is the "force of gravity", taught to essentially every single physics major in the world. Not only don't we measure that, our best theory of gravity says it is actually impossible to measure it, but this does not prevent every first year physics course from going on about it! The reason is clear, it is regarded as a nice fiction, which means the term "fiction" cannot be categorically perjorative. The true skeptic regards all of physics as similarly a "nice fiction," and expects every single current physics law to be regarded as incorrect at some point in the future.
Of course, we need the "force of gravity" (in Newtonian mechanics) to calculate the motions of planets, stars, moons etc. E.g., you can derive Kepler's Laws from it. That's well observed (indeed Kepler figured them out by analyzing observations).
Ken G said:
I would agree that we don't want our interpretations of some theory to impose limitations the theory itself does not impose. Still, even if it is true that BM cannot be made to interpret relativistic QM (a point that I believe @Demystifier has provided counterargument to, ironically in the spirit of saying that BM might not be able to match the "fictions" of relativity, like Poincare invariance, but can be made to match the observables), one could still correctly say that BM interprets nonrelativistic QM. It is perfectly all right to have an interpretation of a theory that has a domain of application, we do it all the time (e.g., the "force of gravity" again). The overarching point is, we should not take any of our theories so seriously that we think the "best" version is "the truth" that we are trying to interpret. (Again, the true skeptic sees the flaw in that thinking.)

But again, I would bring up "the force of gravity." Your statement here condemns the use of that notion categorically! I think it is perfectly fine to explain your own philosophical preferences, which is essentially what you are doing, and to recognize that those preferences have a context that is pragmatic for your purposes, but it is difficult to apply them as objective principles over all of science. How would you respond to the characterization of Poincare covariance, and Occam's Razor itself, as exactly the kind of "fiction" about which you object? You hold that a minimal interpretation is absent of any "religion," but is it not already a kind of religion of its own?
I don't know, how you come to this conclusion about gravitation or Poincare covariance. The letter is key to our most fundamental understanding of physics and thus for sure no superfluous metaphysical addition to our fundamental theories but to the contrary a key element of it. It's, of course, also based on precise observations of Nature.
 
  • #97
vanhees71 said:
Of course, we need the "force of gravity" (in Newtonian mechanics) to calculate the motions of planets, stars, moons etc. E.g., you can derive Kepler's Laws from it. That's well observed (indeed Kepler figured them out by analyzing observations).
We don't need it. We have other ways that never refer to it at all, even within Newtonian mechanics (like Hamiltonian or Lagrangian approaches). That's what I mean, the force of gravity is an unobservable fiction that doesn't exist in a minimalist interpretation of Newtonian mechanics, and never needs to be calculated, so why would you?
vanhees71 said:
I don't know, how you come to this conclusion about gravitation or Poincare covariance. The letter is key to our most fundamental understanding of physics and thus for sure no superfluous metaphysical addition to our fundamental theories but to the contrary a key element of it. It's, of course, also based on precise observations of Nature.
Poincare covariance is a property of the equations, nothing that can be observed. If @Demystifier is correct in his claim that the observations of relativistic quantum mechanics can be correctly predicted using forms of BM that do not obey Poincare covariance, then that is the extent to which your aversion to including elements in a theory that cannot be observed applies to Poincare covariance as well. You put it like, " as a natural scientist one should look at what's observed/observable." So this raises the questions, how does one observe Poincare covariance? How does one observe the force of gravity? If you are actually saying one does not have to observe something, but it is convenient to invoke it when one makes predictions that can be observed, then you are in the subjective realm of "what is convenient" and not in the objective realm of "what can be observed."
 
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  • #98
How else do you calculate planetary motion in Newtonian mechanics, if not assuming a force law. It was one of Newton's many great achievements to have found the universal law of gravitation, being the discovery of one of the fundamental interactions in Nature, as we call it today.

Of course you can clarify the question, which spacetime model is correct, by observation. That's, in fact, how it turned out that Galilei-Newton spacetime is an approximate description with a limited realm of applicability, while Einstein-Minkowski spacetime (and it's localized form in General Relativity) has been found to describe all observations correctly. This implies the Poincare invariance of all observables and thus a theory that's not obeying Poincare invariance is at best an approximation.
 
  • #99
vanhees71 said:
How else do you calculate planetary motion in Newtonian mechanics, if not assuming a force law. It was one of Newton's many great achievements to have found the universal law of gravitation, being the discovery of one of the fundamental interactions in Nature, as we call it today.
I added the edit (sorry I tend to do that, in hopes the answer isn't read immediately!) that I am talking about Hamiltonian or Lagrangian approaches. What I mean is, let's say Hamilton came along 100 years before Newton, and demonstrated his approach to calculating the motions of planets. Huge success, all of physics seems to now be deterministically accessible. Then Newton comes along and says, "hey you can understand all this if you invoke the existence of forces, including the force of gravity." Hamilton's followers accuse him of inventing philosophical fictions that can't be observed!

Or, what if Einstein in 1600 presents his approach to gravity in the form of an unknown parameter c that is too large to be observed. He also predicts the motion of the planets correctly to lowest order in v/c squared, not only without invoking any force of gravity, but with a theory whose form respects the equivalence principle (as well as Poincare covariance). If Newton then came along later and showed his theory that does not invoke a c parameter but is not Poincare covariant, would people see that as an improvement because it doesn't need an unknown large c parameter, or a fiction because it doesn't obey the key principle of Poincare covariance? Would they point out that Newton's theory is invoking a force that is expressly impossible to observe when there is an equivalence principle that is already regarded as a key symmetry, even though c is still unknown? Would that not still be the case today, if it happened that c was so much larger than it actually is that we still haven't been able to observe it? (I realize this is a hypothetical question because we have observed c so we know Einstein's theory is better than Newton's, but I'm framing this within the context of predictions that are the same in both theories at v scales where the differences are unobservable.)

Or consider Lagrange's approach that all of classical mechanics is described by minimizing action, such that the minimal principle is all processes have an action and that's it. No forces at all, they're all philosophical fictions! Throw in a Feynman like path integral interpretation to the reason that action is minimized, classical motions are now wavelike long before quantum mechanics. Even Huygens had a successful interpretation of Newtonian mechanics, is that the minimal way to look at it because it invokes a concept of interference of propagating signals rather than unobservable definite trajectories? I mean, what if wave mechanics was understood prior to particle mechanics, and then particle mechanics was seen as a duality based on some unobservably small and unknown h parameter, akin to the unobservably large c parameter in Einstein's gravity metric. Are those minimal interpretations because they don't need to invoke some unobservable fiction like particles following definite trajectories that are never actually observed (because one never observes a definite trajectory)? What if statistical explanations had been proposed long before determinism was a thing, would not determinism have to be regarded as a philosophical fiction if Newton suggested it later on?
vanhees71 said:
Of course you can clarify the question, which spacetime model is correct, by observation. That's, in fact, how it turned out that Galilei-Newton spacetime is an approximate description with a limited realm of applicability, while Einstein-Minkowski spacetime (and it's localized form in General Relativity) has been found to describe all observations correctly. This implies the Poincare invariance of all observables and thus a theory that's not obeying Poincare invariance is at best an approximation.
Well here my position is contingent on @Demystifier's claim that the observations of relativistic quantum mechanics can also be predicted correctly with a form of BM that is not Poincare covariant. I don't know if that is actually true or not, so my logic is if it is true, then BM must be regarded as a successful interpretation of relativistic quantum mechanics by your own reasoning, and if it is not true, then we must fall back to the position that BM is a successful interpretation of nonrelativistic quantum mechanics, like you are arguing that the force of gravity is a successful interpretation of Newtonian mechanics. At issue is, if interpretation X successfully interprets theory Y, and if theory Y passes all the same observational tests as theory Y', then interpretation X also successfully interprets theory Y'. If one adds the further requirement that interpretation X is not successful unless it is minimal, then the Newtonian force of gravity is not a successful interpretation of Newtonian mechanics because the Hamiltonian, or Lagrangian, or Einsteinian with untested c parameter, or Huygensian with untested h parameter, approaches have no such object in them as anything but an unnecessary add on, like Bohmian trajectories.
 
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  • #100
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