How many interpretations of QM do you speak?

[Demystifier]

For me, different interpretations of QM are like different languages, namely, different ways to speak about the same quantum formalism and phenomena. When someone speaks about QM in terms of one interpretation, usually there is a way to translate it to another interpretation. The more interpretations you know, the richer your understanding of QM is. Which interpretations are you familiar with, and at which level?

My account is roughly this:
Expert level: Bohmian mechanics
Fluent: many worlds, statistical ensemble, instrumental, Copenhagen (various dialects), collapse by consciousness
Good: QBism, relational, Nelson (stochastic)
Basic: dynamical collapse, consistent histories, quantum logic

 

[Matterwave]

Half a step above basic: Instrumentalist / Copenhagen / Ballentine style Ensemble

I was mostly taught this during my undergrad/grad program many years ago. Ballentine is my reference for basic QM formalism.

Basic:

Bohmian
I watch a lot of Tim Maudlin on YouTube and read posts here by @Demystifier haha.

MWI
I've read the basic development in Everett's dissertation and I watch Sean Carroll talk about it sometimes.

I know they exist:
Consistent Histories, Qbism, Relational.

Not exactly interpretations, but I know it exists:
GRW

 

[jeffn1]

Sorry if this is slightly off the question (if so, can move to other thread), but how how does Art Hobson's view fit in this? Is it fair to say that he claims (non-local) quantum fields and decoherence explain everything (maybe a bit too simple). But (continuing), he does not offer an interpretation with enough details to be considered an "interpretation"? Is there an existing full blown interpretation that would tend to encompass his view? What is closest? [Sorry, I suppose I am being a bit needy here.]

 

[gentzen]

…, but how how does Art Hobson's view fit in this? Is it fair to say that he claims (non-local) quantum fields and decoherence explain everything (maybe a bit too simple). But (continuing), he does not offer an interpretation with enough details to be considered an "interpretation"? Is there an existing full blown interpretation that would tend to encompass his view? What is closest?

The closest is orthodox QM. The belief „there is nothing special here to see“ is a typical orthodox position. It is not Copenhagen, because Copenhagen emphasises the role of observers or of a macroscopic measurement apparatus. A typical representant of that school is van Kampen.

 

[PeterDonis]

The closest is orthodox QM.

By "orthodox QM", do you basically mean "shut up and calculate"? In other words, once you've done the math and made the predictions and seen that they are confirmed by experiment, there's nothing more you can do?

 

[Matterwave]

By "orthodox QM", do you basically mean "shut up and calculate"? In other words, once you've done the math and made the predictions and seen that they are confirmed by experiment, there's nothing more you can do?

This is roughly my view of Orthodox QM. Maybe sprinkled with a little bit of Copenhagen. And some hand waving to get rid of consciousness causes collapse.

 

[gentzen]

By "orthodox QM", do you basically mean "shut up and calculate"?

No, rather „let me calculate and explain“. But even this is too short, because it is not just „calculate“, but also to analyse which effects will be important and which can be neglected. On the other hand, „calculate“ is not a bad characterization, because one explains and talks about things where one can still calculate, not about some hypothetical Hilbert space of the entire universe.

 

[PeterDonis]

rather „let me calculate and explain“.

What is left to explain once you've done the calculations, made the predictions, and confirmed them by experiment?

it is not just „calculate“, but also to analyse which effects will be important and which can be neglected.

Yes, this is understood when one is calculating to make predictions--once the prediction is accurate enough (i.e., as accurate as the expected experimental error), there's no point in continuing calculations any further.

On the other hand, „calculate“ is not a bad characterization, because one explains and talks about things where one can still calculate, not about some hypothetical Hilbert space of the entire universe.

Yes, this is also understood since of course we are calculating predictions we can compare with experiment, so there's no point in calculating something that one can never compare with experiment.

 

[Morbert]

I'm comfortable navigating literature around instrumentalist, consistent/decoherent histories, many-worlds, and the unistochastic formalism.

I have interest in getting familiar with the ETH and Bohrification approaches but I need to swot up on my C*-algebra.

 

[javisot]

"Shut up and calculate" = choose an interpretation, master it, and calculate , or "Shut up and calculate" = calculate without interpretations?

I never understood the idea of doing QM without interpretations. There is a single set of experimental results known as QM. It is a strange set of results, which allows each author to generate an interpretation distinguishable from the others, yet they all end up explaining the same set of results. I do not understand how an author can fully explain this set of experimental results without ever needing to use an interpretation.

 

[PeterDonis]

"Shut up and calculate" = choose an interpretation, master it, and calculate , or "Shut up and calculate" = calculate without interpretations?

Calculating is just math. No interpretation is necessary to do the math. Different interpretations might point you in the direction of different formulations of the math, but all of them are equivalent, so you can use whichever one you like to do the calculations, and picking one that a particular interpretation points you to still does not mean you're using that particular interpretation to do the calculation.

I do not understand how an author can fully explain this set of experimental results without ever needing to use an interpretation.

That depends on what you consider to be an explanation. Some physicists would say that the calculation--the math itself--and the predictions it makes, and the fact that those predictions match experiment, is the explanation, or at least all the explanation you can get. That's basically what "shut up and calculate" amounts to.

 

[javisot]

No interpretation is necessary to do the math.

This is strictly true in the context of QM?

 

[PeterDonis]

This is strictly true in the context of QM?

It's true for any theory of physics. Math is math. You don't need an interpretation to do math. You just do it.

You do need an understanding of what quantities in the math correspond to measurement results, so you can compare the math with experiment. But that's part of the basic theory. It's not an interpretation, since it's the same regardless of what additional "interpretation" you might try to adopt (and nothing forces you to adopt any additional "interpretation" at all).

 

[javisot]

It's true for any theory of physics. Math is math. You don't need an interpretation to do math. You just do it.

Okay, I can do that. Present a QM problem to me, and I’ll solve it. I assume you’ll present the problem in a certain way, using specific terminology.

I will solve the problem using the same terminology you used to present it.

In the scenario I’m describing, at what point are we doing QM without interpreting?

 

[PeterDonis]

Okay, I can do that. Present a QM problem to me, and I’ll solve it. I assume you’ll present the problem in a certain way, using specific terminology.

I'll present it using math.

I will solve the problem using the same terminology you used to present it.

Meaning math, yes.

In the scenario I’m describing, at what point are we doing QM without interpreting?

The whole scenario.

 

[Matterwave]

"Shut up and calculate" = choose an interpretation, master it, and calculate , or "Shut up and calculate" = calculate without interpretations?

Usually the latter. You do enough problems from textbooks, standard references, and research papers so you know "given this description of an experiment, this is the math I should calculate".

I do not understand how an author can fully explain this set of experimental results without ever needing to use an interpretation.

The math works perfectly well.

But even among folks who are "orthodox", I think people fall onto a spectrum.

1. Those who haven't thought much about it. The math works. They can do their job. Or maybe they looked but didn't find any interpretation appealing.
2. Those who looked deeply and understand deeply but since there's no experimental way to distinguish between interpretations, they remain agnostic.
3. The hardcore ones who think there really isn't anything deeper than the math + experimental procedure. Reality is described by the math, and that's enough. Questions like "what is there really?" are category errors.

And probably many more~

There's also the issue of course of even getting a consensus on what counts as an interpretation in the first place.

 

[gentzen]

What is left to explain once you've done the calculations, made the predictions, and confirmed them by experiment?

That is not how my solid state physics textbooks (or the papers) work. For example, there is the Lindhard dielectric function and the Mermin dielectric function. The Lindhard DF occurs as example for linear response theory, as example for the random phase approximation, and as example for self-consistent approximations. After it is derived, one looks at various predictions, like Friedel oscillations and other expected (like Thomas-Fermi screening) and unexpected effects. And then one tries to explain why it works so well, despite ignoring most details of the electronic structure of the solid.
The Mermin dielectric function is a generalization of the Lindhard DF, which includes finite lifetime/damping. The paper which introduced it is only two pages long, but I find its explanations a bit too high level. OK, I remember that previous attemps to introduce damping failed to be self-consistent, and that the Mermin DF is the simplest way to introduce damping while remaining self-consistent. When textbooks talk about the Mermin DF, they also explain which physical aspects are still not modeled by it.

 

[PeterDonis]

And then one tries to explain why it works so well, despite ignoring most details of the electronic structure of the solid.

Wouldn't the obvious explanation be that the details of electronic structure that the model ignores have negligible effect on the things the model is predicting? After all, that's why the model ignores them, correct?

In any case, "explain" is a broad term, but here we're talking specifically about the kind of "explanation" that an interpretation of QM provides. That kind of explanation is not the kind you're talking about here. Why not? Because all QM interpretations make the same predictions. So the "explanations" the interpretations give can't possibly be explanations of why the predictions are accurate in a particular regime--because they're all mutually contradictory, but they all refer to the same predictions! I don't think solid state physicists would accept such a variety of "explanations" for why a particular model works well despite ignoring a lot of details. They would want an explanation along the lines I gave above--which of course could be tested by more accurate experiments that verify that yes, indeed, the details of the electronic structure are indeed negligible in the regime where the current model works well.

QM interpretations, however, cannot be tested by experiment--because they all make the same predictions. The "explanations" that QM interpretations give are "just-so-stories" about what is going on "in reality", not explanations about why the theory itself works.

 

[Sambuco]

Which interpretations are you familiar with, and at which level?

I think I am fluent in:
- Relational (Rovelli's, but also the related Brukner's information-based and Zurek's existential interpretation).
- Bohmian mechanics.
- QBism.

I don't have as much vocabulary as I'd like in:
- Many worlds.
- Objective collapse (GRW or CSL).
- Consistent histories (@Morbert, Where should I start?).
- Ensemble interpretation (Ballentine).

I know only the basics of:
- Nelson's stochastic mechanics.
- Transactional interpretation (Cramer and Kastner).
- Two-state vector formalism (Aharonov and co-workers).
- Modal interpretations (Van Fraassen, Lombardi).
- Grangier's Context-Systems-Modalities.

Lucas.

 

[marcusl]

I’m a lurker in these interpretation threads—I like the discussions even though I often don’t understand a lot of what’s being discussed. My undergrad text was Merzbacher, which I didn’t like because it kept circling back to the same topics instead of dealing with them once and for all. Grad text was Gordon Baym, which I liked it but found challenging. (I was an experimentalist for sure.) Both texts put me into the Copenhagen interpretation, right? I never remember any discussion of interpretations, in fact. Maybe because it was a long time ago and Schiff had passed away but still influenced teaching in the department.

 

[WernerQH]

The more interpretations you know, the richer your understanding of QM is.

The plethora of QM interpretations is not a blessing but a curse. It's a symptom, indicating that we haven't yet found a natural interpretation that the vast majority of physicists agrees on. After a century one would expect QM to be thoroughly understood, and debates on its interpretation to subside. (I don't share the view that this situation must be permanent because it lies in the "nature" of QM.)

I believe there is a historical precedent: Maxwell's electrodynamics. For more than four decades it was not fully understood. The mathematical equations were there (and highly successful), but people kept trying to understand what this theory actually describes. For them an interpretation of Maxwell's equations meant constructing a mechanical model for the "ether". Today the ether has been replaced by the electromagnetic field, and we have understood that the concept of an ether was fraught with metaphysical preconceptions. I'm convinced that the photon concept (and quantum theory in general) is similarly burdened with metaphysical preconceptions that we haven't yet clearly identified.

 

[Demystifier]

The plethora of QM interpretations is not a blessing but a curse.

This makes my analogy even stronger, because some would say the same for the plethora of human languages.

 

[jeffn1]

The plethora of QM interpretations is not a blessing but a curse. It's a symptom, indicating that we haven't yet found a natural interpretation that the vast majority of physicists agrees on. After a century one would expect QM to be thoroughly understood, and debates on its interpretation to subside. (I don't share the view that this situation must be permanent because it lies in the "nature" of QM.)

I believe there is a historical precedent: Maxwell's electrodynamics. For more than four decades it was not fully understood. The mathematical equations were there (and highly successful), but people kept trying to understand what this theory actually describes. For them an interpretation of Maxwell's equations meant constructing a mechanical model for the "ether". Today the ether has been replaced by the electromagnetic field, and we have understood that the concept of an ether was fraught with metaphysical preconceptions. I'm convinced that the photon concept (and quantum theory in general) is similarly burdened with metaphysical preconceptions that we haven't yet clearly identified.

Would this put you in the Art Hobson camp? ;)

 

[WernerQH]

Would this put you in the Art Hobson camp? ;)

No, not at all. It is often claimed that the concept of wave particle duality has become obsolete, and that the modern view of quantum fields explains everything. But it is never explained what a quantum field is physically. Operators? The wave/particle dualism is still there: when the wave aspects are important, there is talk about the field, when localized events are of interest, one talks about excitations of the field. Field is, after all, a classical concept.

 

[javisot]

So, in this analogy between QM interpretations and languages, practicing QM without interpretations is equivalent to communicating through noises and limb movements.

 

[Sambuco]

Would this put you in the Art Hobson camp? ;)

I must say that, after reading several of his articles (though not his book), I do not hold a very favorable view of Hobson’s proposal. As he himself acknowledges, his idea does not solve the measurement problem, and thus it does not constitute an interpretation of QM. In addition, the exposition of his idea is somewhat vague, and sometimes even contradictory.

For example, he argues for a view in which quantum fields are fundamental, whereas particles are not. However, he also says: "it's neutral on the interpretations (e.g. many worlds) and modifications (e.g. hidden variables, objective collapse theories) designed to resolve the measurement problem." How can both things be true? How can the idea that quantum fields are fundamental be compatible with Bohmian mechanics, which posits particles as the primitive ontology for non-relativistic QM? That type of contradiction appears several times throughout his writings.

Another one: He states that quantum fields are "real." Leaving aside the fact that quantum fields are not classical, but rather operator-valued fields, does he mean they are ontic? If so, this would be compatible with a many-worlds interpretation of QFT, where the wave functional is the fundamental entity. But then he posits an instantaneous collapse to explain the measurement process. So, what is the dynamical equation for the "real" fields? Nothing is clear.

Lucas.

 

[gentzen]

How can the idea that quantum fields are fundamental be compatible with Bohmian mechanics, which posits particles as the primitive ontology for non-relativistic QM?

Bohmian mechanics is flexible about the particles vs fields issue. So Art Hobson is not wrong in this respect.

 

[jeffn1]

"it's neutral on the interpretations (e.g. many worlds) and modifications (e.g. hidden variables, objective collapse theories) designed to resolve the measurement problem." How can both things be true? How can the idea that quantum fields are fundamental be compatible with Bohmian mechanics, which posits particles as the primitive ontology for non-relativistic QM? That type of contradiction appears several times throughout his writings.


Lucas.

He is very critical of consciousness-based theories and, at least at some points, the Many Worlds interpretation. I guess his view is that quantum fields are as fundamental as we currently know in our current science, and other issues (details?) still need to be worked out (consistent with this framework). (But, I am a bit above my pay grade here).

I think he does view quantum fields as "ontic". I think his criticism of Many Worlds Theory is that the consequence of the theory is so fantastical (e.g., there are "gazillions" of versions of me existing in alternatives universes (branches)) it should be considered less plausible than other interpretations. He seems to say the fact that the math works, in and of itself, is insufficient.

 

[Sambuco]

Bohmian mechanics is flexible about the particles vs fields issue.

I know, but then what does "hidden variables" mean? The term is used in NRQM to refer to whatever is added to the wave function. However, according to Hobson, the wave function is the non-relativistic limit of the quantum field, which is what is "real." Of course, one could interpret this to mean that his view is consistent with a Bohmian-like interpretation of QFT, where the quantum field takes on well-defined values at every time while being "guided" by the wave functional. But we cannot know for sure, because he is not precise.

It is unclear exactly what problem Hobson is attempting to solve, given that the conceptual issues in QM are linked to the measurement problem and, by extension, to the question of the theory's ontology. However, he himself acknowledges that his proposal neither solves the measurement problem nor posits a specific ontology. Consequently, the claim that fields are more fundamental than particles lacks any real implications.

Lucas.

 

[gentzen]

However, according to Hobson, the wave function is the non-relativistic limit of the quantum field, which is what is "real."

No, the quantum field and the wave function are not related like this for Hobson. The non-relativistic wave function is not considered as "real" by Hobson.

It is unclear exactly what problem Hobson is attempting to solve, given that the conceptual issues in QM are linked to the measurement problem and, by extension, to the question of the theory's ontology.

Yes, I also failed to figure out which concrete problem Hobson believes to have solved. But he thinks it is easy, once you realize that particles are not "real", and that the theory's ontology is fields all the way down. Or maybe I no longer remember correctly.