How many interpretations of QM do you speak?

[WernerQH]

Thanks for your answer.

Thank you. For the first time I have the impression that somebody is actually trying to understand what I have written.

This clarifies one thing for me, which I always found strange about

Quantum physics, which turns 100 this year, is arguably the most metaphysical of all empirical discoveries. It’s worthy of returning to again and again in life, asking: but how could the world be that way? Is there a different angle that we missed?

Yes, this sounds very familiar. Feynman issued a warning about this (The Character of Physical Law, p. 129) :

Do not keep saying to yourself, if you can possibly avoid it, ´But how can it be like that ?´ because you will get ´down the drain´, into a blind alley from which nobody has yet escaped.

But I do think that the situation is not so hopeless, that there is a "different angle". It´s only one concept, which is not quite new and embarrassingly simple: events. But microscopic, physical events, and not the kind of events that makes mathematicians immediately construct an algebra.

You seem to ask for radical new concepts, but when something like the "original" thermal interpretation with its radical new concepts comes along, you are not interested at all.

Is it really an interpretation? What is new? I can't see any radical new concepts. For me it´s just a new exposition of the well known formalism, with an (for me unpalatable) emphasis on "measurement".

Now category theory doesn't really provide radial new concepts. It is just a different perspective, which allows to see certain new concepts as natural, and see other (often more familiar) concepts as naturally related to such new concepts.

There is a gulf between physicists and mathematicians. I admit having difficulties with abstract concepts, and it´s unclear to me if you aren´t really speaking of mathematical concepts. My impression is that PQP aims at a "high-level" description of quantum processes (really an anthropocentric description, not unlike "measurement"). From the CPTP wiki you mentioned:

Terminologically, quantum channels are completely positive (CP) trace-preserving maps between spaces of operators. In other words, a quantum channel is just a quantum operation viewed not merely as the reduced dynamics of a system but as a pipeline intended to carry quantum information.

You´ve completely lost me here. :-)

In the end, I believe you see your concepts as originating with Julian Schwinger, if I remember correctly.

Yes, the Schwinger-Keldysh formalism is what I used to evaluate the correlation functions mentioned in my previous post. For Schwinger it was probably just a calculational device to introduce the "closed time-path", integrating over a forward and a backwards running time. But I believe that it has also physical significance, that there exist actually two spacetimes "glued together", with time running forward on one and backwards on the other, and events occurring in close pairs on either spacetime. If I´m not mistaken, this is also what plays a significant role in Alain Connes´ non-commutative geometry (https://doi.org/10.1140/epjs/s11734-023-00842-4).

I talk more about QM than about QFT, because I am realistic about my priorities, which don't include really understanding QFT. And here I am not talking about finding its ontology, but about acquiring established understanding of QFT, like why it can predict stuff.

I fully respect your priorities. :-)

With respect to QM, the way it is used for quantum computers and computational complexity is self-contained and independent of QFT. What is also missing from it in that context is hbar. Which brings me to one important property I failed to mention when one forms systems: hbar always stays the same, no matter how we aggregate stuff into systems. For example, a Helium-4 nucleus is a Boson, despite being composed from Fermions, but its quantum properties still depend on the same universal hbar constant.

This hbar thing is also something which gets harder to see when one reduces QM to just Hilbert space.

I don't quite understand the significance you attach to hbar -- for me it just translates "mechanical" quantities to geometry. For example electron mass: $1/m = \rm 1.288 × 10^{-21} sec = 3.86 × 10^{-11} cm$. The wave function should not be put at the centre of quantum theory. Hermann von Helmholtz once wrote that "the final aim of physics is to dissolve itself in mechanics". Nowadays it seems more appropriate to say that the aim of physics, or at least of QFT, is to dissolve itself in geometry and statistics.

 

[PeterDonis]

a description of "the physical phenomena" cannot necessarily be separated from the theory and interpretation. For example, if you are measuring temperature or speed, the meaning of these measurements only makes sense as part of a larger theory.

Yes, but the "interpretation" required here is very minimal: you just need to know the operational meaning of the quantities in the theory, i.e., which of the numbers the theory spits out are the ones I should compare with experimental results, and which experimental results do I compare them with?

the interpretation of quantum theory is still unsettled

Here "interpretation" is being used in a much broader sense. The interpretation of QM in the above minimal sense is not unsettled: there is no disagreement about how to compare the numbers the machinery of QM spits out with experimental results.

The different QM "interpretations" are different stories that people want to tell that go beyond just comparing the theory's numbers with experimental results.

 

[sbrothy]

An interesting point is that even a description of "the physical phenomena" cannot necessarily be separated from the theory and interpretation. For example, if you are measuring temperature or speed, the meaning of these measurements only makes sense as part of a larger theory. Usually the theory governing those concepts is not really disputed. But I think since the interpretation of quantum theory is still unsettled, this is relevant. I think this point was raised in this thread: different quantum theory interpretations use different terms and meanings to describe the "same" phenomena.

Again as I'm a self-taught hack I'm going to formulate this as a question: Would it make sense to say that the measurements and results would also make sense as part of smaller theories, ie.: thermodynamics, electrodynamics etc?

Where does the line go? Up/down? I realize this is probably an impossible question I just wanted to make an academic point.

 

[RUTA]

What is quantum theory about? Is it about quantum objects and measurements performed on them? Or Hilbert spaces? For me the most pressing question is about its ontology, not about a more "coherent" presentation of the formalism.

What do you mean by "a quantum object"?

 

[RUTA]

Let me return to the preconceptions that you mentioned: causality and locality.

We have a strong desire to explain things. The correlations in Bell-type experiments are explained "naturally" by a common source that creates photons with opposite polarizations.

I don't know how to explain these correlations to someone who insists that physics must be causal and local.

If you're willing to accept a principle explanation in lieu of a constructive explanation for polarization-entangled photons, try this one:

 

[Sambuco]

If you're willing to accept a principle explanation in lieu of a constructive explanation for polarization-entangled photons, try this one:

@RUTA, If you don't mind, I have a couple of questions about your work:

1. I do not fully grasp how NPRF + h helps to solve the measurement problem. It seems to me that something more needs to be introduced to achieve this, and that you do so by considering what you called "quantum-classical contextuality".

2. You say that your proposal does not violate locality, statistical independence, intersubjective agreement, or the uniqueness of experimental outcomes. How does your approach address the "local friendliness" no-go theorem? Perhaps I am confusing "intersubjective agreement" with "absoluteness of observed events"?

Lucas.

 

[bhobba]

But it is never explained what a quantum field is physically. Operators?

To be specific, operator-defined distributions as per the Wightman axioms:
https://en.wikipedia.org/wiki/Wightman_axioms

What do you mean physically? What are classical fields physically other than something required to circumvent Wigner's No-Interaction Theorem? We only know how to model them mathematically; we have no idea whether they exist or are just a device to aid calculations. Most (including me) think they exist and the map is not the territory, but that is an assumption.

Art Hobson's book is a realistic interpretation of QFT (it has issues, as has been discussed in other threads) but has the virtue of interpreting not ordinary beginning textbook QM (which we know is wrong, e.g., cannot account for spontaneous emission or even allow an ordinary QM theory of EM fields because Maxwell's equations are relativistic from the start).

My current view on ordinary textbook QM is that it is a useful mathematical model that can be heuristically justified using operators that encode possible outcomes and Gleason's Theorem, but only an approximation to something deeper (QFT) that needs interpreting. Interpreting ordinary QM is only a warm-up exercise. While I recommend Arts book as a good starting point in interpreting QFT, and heuristically dispelling otherwise troublesome concepts like wave-particle duality, a better book is Wallace's The Emergent Multiverse, which, while about the modern version of many worlds, includes much of the detail of decoherent histories (and their extension to QFT).

A particle is an irreducible representation of the Poincaré group
https://en.wikipedia.org/wiki/Particle_physics_and_representation_theory

Thanks
Bill

 

[bhobba]

My account is roughly this:
Expert level: Bohmian mechanics

The following suggests it may not be an interpretation, but a different theory:
https://www.amazon.com.au/Beyond-Quantum-Origin-Meaning-Mechanics/dp/0198853742

Thanks
Bill

 

[bhobba]

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

Some may view it as an interpretation itself. Ensemble, for example, in some sense, is just to shut up and calculate using the frequentist interpretation of probability. In fact, John Baez thinks a lot of interpretations are basically a rehash of the meaning of probability:
https://math.ucr.edu/home/baez/bayes.html
'It turns out that a lot of arguments about the interpretation of quantum theory are at least partially arguments about the meaning of the probability!'

Thanks
Bill

 

[bhobba]

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).

I would add that it's a mathematical model. You can try to understand why the model works or just use it. I suppose it's a matter of taste; if thinking of it as an interpretation and saying, I have a model is enough for me. You do need to know what its parts correspond to.

Thanks
Bill

 

[Lord Jestocost]

Again as I'm a self-taught hack I'm going to formulate this as a question: Would it make sense to say that the measurements and results would also make sense as part of smaller theories, ie.: thermodynamics, electrodynamics etc?

Where does the line go? Up/down? I realize this is probably an impossible question I just wanted to make an academic point.

Physics relies on the scientific method, i.e. of “things”-as-they-appear to us or “things”-as-they-are-measured by us. In case “smaller” theories can be verified experimentally, you are done as an experimentally oriented physicist.

 

[javisot]

What do you mean by "a quantum object"?

A quantum object is a physical-mathematical object. It is a physical object for which we have mathematical descriptions. It differs from other physical-mathematical objects (such as classical objects) in that, to determine its evolution, it is fundamental to take the Uncertainty Principle into account.

But you're RUTA; I realize you know better definitions than this one, and your question was rhetorical. (right?)

 

[WernerQH]

A particle is an irreducible representation of the Poincaré group
https://en.wikipedia.org/wiki/Particle_physics_and_representation_theory

What happens when a particle irreducible representation of the Poincaré group becomes entangled?

 

[WernerQH]

I’m sharing a very recent paper (from last month) that I found quite interesting and that might contribute to the conversation. The proposal involves finding an interpretation-independent formulation of the measurement problem.

I don't think this paper can further the discussion.

There is no way to ‘solve the measurement problem’ simply because it is an empirical fact.

What an outrageous statement! How can the measurement problem be an empirical fact? Even claiming to be interpretation-independent!

we propose to identify the common root of the puzzle in an interpretation-independent way i.e. as a property of the empirical statistics only, before deriving its philosophical consequences

What is meant are interpretations of quantum theory, but of course what can count as empirical depends on the experimenters. They are supposed to know what they are measuring and to interpret their results correctly.

[T]he measurement problem is the fact that quantum matter cannot be described by a Kolmogorovian theory.

It is possible to doubt the applicability of empirical statistics. But an alternative is to question "quantum matter" with its elusive (not to say weird) "properties." I think the common root of the puzzle is a firm metaphysical belief in causality and locality, that there must be some continuous connection between the source and the detectors. Something that has been named particle, wave, quantum object, or field excitation at various times. Experimenters are convinced that they measure the polarization states of photons, in accordance with current theory. But theory casts doubt on the idea that a "measurement" reveals a pre-existing property of a photon. Theory says that in Bell-type experiments each photon individually is in an unpolarized state before detection. The photon concept is contradictory. The experiments can be perfectly described with ordinary
"Kolmogorovian" statistics, if just the incidents at the detectors are registered. Apart from metaphysical preferences there is no need to frame the description of the experiments in terms of travelling "objects". Non-local theories are possible (and I think that QED is one of them) and able to describe the correlations observed between different detectors.

 

[.Scott]

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.)

It's a kind of blessing. And it doesn't lie in the "nature of QM", but in the nature of people.
The notion of "shut up and calculate" has limited use. We want to imagine new candidate QM experiments and new candidate QM applications.
Step one is to see connections to other more familiar things and to draw analogies. Then apply the math and see if the analogy holds. And if it's good to go, try it out.
Our entire intellectual evolution and life-time training is in the world of Newtonian Physics. Our brains deal with survival issues related more to staying warm, well-fed, and shielded against all the familiar threats. We learn to walk before 1 years old. We don't learn to avoid electron interference or quantum state collapse until well after that.
Each of these "interpretations" is a way of looking at QM that contrasts some aspect of it to things that are already very familiar to us.
If we want the kind of "natural interpretation" you describe, the direction to go is less "thorough understanding" and more "thorough education". Perhaps a 3-year-old playing "bind the protein" instead of "Candy Crush".

 

[Sambuco]

What an outrageous statement! How can the measurement problem be an empirical fact? Even claiming to be interpretation-independent!

I think the idea being conveyed is that, ultimately, what underlies what we call the "measurement problem" is a specific statistical pattern, one that is inherent to QM and constitutes a fundamental feature of it. From this perspective, the "problem" arises from the need, on the part of those interpreting the theory, to align it with a set of preconceptions taken for granted in classical theories.

It is possible to doubt the applicability of empirical statistics. But an alternative is to question "quantum matter" with its elusive (not to say weird) "properties."

As I interpret it, Soulas is saying the same thing you are. In fact, that is why I found his paper to be a fresh look on the matter. I think what he is saying is that the "measurement problem" is the problem of trying to maintain a classical ontology, which is compatible only with statistical observations consistent with a Kolmogorovian model, in a world where nature has already told us that this does not work.

I think the common root of the puzzle is a firm metaphysical belief in causality and locality, that there must be some continuous connection between the source and the detectors.

What you have written accurately reflects my own position regarding the measurement problem. I believe that things such as causality are emergent rather than fundamental.

The experiments can be perfectly described with ordinary "Kolmogorovian" statistics, if just the incidents at the detectors are registered. Apart from metaphysical preferences there is no need to frame the description of the experiments in terms of travelling "objects". Non-local theories are possible (and I think that QED is one of them) and able to describe the correlations observed between different detectors.

I also fully agree with this. As I mentioned earlier, I believe Soulas’s paper attempts to demonstrate that if we persist in maintaining preconceptions such as the existence of "permanent entities" (in Schrödinger’s words), we fall into the trap of having to define properties for those entities at every instant in time. In doing so, we realize that the models required to describe the phenomena must be non-Kolmogorovian. In other words, one way to solve the measurement problem is to set aside such preconceptions and accept the only existence of events, without any (ontic) connection between them. By proceeding in this manner, the issue of non-Kolmogorovianity is trivially solved, since it makes no sense to ask where the electron is located between the source and the detector, given that no electron exists apart from the emission and the detection.

Lucas.

 

[WernerQH]

The notion of "shut up and calculate" has limited use. We want to imagine new candidate QM experiments and new candidate QM applications.

Yes.

Step one is to see connections to other more familiar things and to draw analogies. Then apply the math and see if the analogy holds. And if it's good to go, try it out.

Yes.

Our entire intellectual evolution and life-time training is in the world of Newtonian Physics. Our brains deal with survival issues related more to staying warm, well-fed, and shielded against all the familiar threats. We learn to walk before 1 years old. We don't learn to avoid electron interference or quantum state collapse until well after that.

Yes. Our evolution did not force us to deal with velocities close to the speed of light. Relativity still runs counter to our intuitions. But there are few physicists inventing ever new interpretations of electrodynamics. We got used to electric fields and accept them as real.

Each of these "interpretations" is a way of looking at QM that contrasts some aspect of it to things that are already very familiar to us.
If we want the kind of "natural interpretation" you describe, the direction to go is less "thorough understanding" and more "thorough education". Perhaps a 3-year-old playing "bind the protein" instead of "Candy Crush".

I agree that intuition is important, and some interpretations may even have been beneficial for developing our intuition. I'm not proposing "thorough education". On the contrary, my intention is to stop what happened in the past. Physics has suffered from our collective indoctrination with inappropriate concepts. I do believe that "photon" is a concept as problematic as was the "ether". (The latter has been revised and is now called vacuum.)

The problem is apparent if you look at the endless discussions in this forum about "entanglement" or even "entanglement swapping". For me it is obvious that these ideas are based on a misconception.

 

[Paul Colby]

A particle is an irreducible representation of the Poincaré group
https://en.wikipedia.org/wiki/Particle_physics_and_representation_theory

While this is true, my understanding is that representation theory yields just a solution of the reps field equations. The corresponding ladder operators require more work.

 

[RUTA]

@RUTA, If you don't mind, I have a couple of questions about your work:

1. I do not fully grasp how NPRF + h helps to solve the measurement problem. It seems to me that something more needs to be introduced to achieve this, and that you do so by considering what you called "quantum-classical contextuality".

2. You say that your proposal does not violate locality, statistical independence, intersubjective agreement, or the uniqueness of experimental outcomes. How does your approach address the "local friendliness" no-go theorem? Perhaps I am confusing "intersubjective agreement" with "absoluteness of observed events"?

Lucas.

Let me first point out that the quantum reconstruction program is not producing an interpretation of QM, it is producing a reconstruction of the Hilbert space kinematics of QM via empirical and mathematical facts. In doing so, it has advanced QM from an operational theory providing structural explanation to a principle theory (like special relativity) providing principle explanation. If you attempt to use the principle theory of QM to provide constructive explanation, you will likely end up concluding that special relativity is wrong and in contradiction with QM, a' la Maudlin (as shown in my IQSA talk). So, if you believe physics must supply time-evolved constructive explanation via causal mechanisms in the dynamical universe, then you will be led to believe that modern physics is "hideously incoherent." If you can rather accept "all at once" principle explanation via adynamical global constraints in the block universe, modern physics becomes comprehensively coherent and its numerous puzzles, problems, and paradoxes are solved, resolved, or dissolved. And all of this can be achieved phenomenologically, i.e., without having to commit to any particular ontology. I'm working on a book explaining all that with Harald Wiltsche (philosophy) and Timothy McDevitt (math) now.

1. You are correct, the measurement problem (big or small) cannot be solved by a reconstruction of the Hilbert space kinematics, we solve those problems with quantum-classical contextuality. I didn't address that in the talk I linked. See Section 5 of this paper for an answer: https://iopscience.iop.org/article/10.1088/1742-6596/2948/1/012009/pdf. Note: Quantum-classical contextuality can be understood phenomenologically, i.e., without any ontological commitment (forthcoming, ibid).

2. "Absoluteness of Observed Events: The assumption that measurement outcomes are singular, objective, and real facts for all observers, rather than relative or purely subjective." This is true, measurement outcomes are part of the intersubjectively-agreed-upon classical context in quantum-classical contextuality.
"Locality (Local Agency): The requirement that an agent's choice of measurement setting cannot instantly influence events or physical probabilities outside its future light cone (respecting the speed of light)." This is true, the correlations are not explained causally (constructively).
"No-Superdeterminism: The assumption that experimental settings can be chosen independently of the hidden variables or physical state being measured (statistical independence)." Statistical independence holds, there are no hidden variables, the state is defined contextually not intrinsically, and the correlations are not explained causally.

Hope this helps!

 

[WernerQH]

As I interpret it, Soulas is saying the same thing you are. In fact, that is why I found his paper to be a fresh look on the matter. I think what he is saying is that the "measurement problem" is the problem of trying to maintain a classical ontology, which is compatible only with statistical observations consistent with a Kolmogorovian model, in a world where nature has already told us that this does not work.

Okay, perhaps I should also read the parts of the paper that I skipped. :-)

By proceeding in this manner, the issue of non-Kolmogorovianity is trivially solved, since it makes no sense to ask where the electron is located between the source and the detector, given that no electron exists apart from the emission and the detection.

The electron is "tunneling" most of the time (is nowhere). But because of its mass it is forced to reappear incessantly, on the order of $10^{21}$ times per second.

 

[sbrothy]

[...]The notion of "shut up and calculate" has limited use. We want to imagine new candidate QM experiments and new candidate QM applications.[...]

I particularly agree with this sentence. For what it's worth,

 

[jeffn1]

I particularly agree with this sentence. For what it's worth,

I think the other side of the argument is, for practical purposes, "shut up and calculate" has proven extremely useful. I suspect its use has been important in many products we use every day (transistors and microchips, lasers, MRI's, etc.). (Although I will admit my personal bias in favor of a greater "understanding" over "shut up an calculate").

 

[sbrothy]

I would add that it's a mathematical model. You can try to understand why the model works or just use it. I suppose it's a matter of taste; if thinking of it as an interpretation and saying, I have a model is enough for me. You do need to know what its parts correspond to.

Thanks
Bill

True words. Am I wrong if I postulate that physics is full of nooks and crannies where asking why it's like this and not some other way and what does it means is basically a waste of time. It seems QM is special in this way. Although I saw someone in the thread argue that it's a desire of humans to find meaning and patterns where perhaps there are few.

 

[bhobba]

True words. Am I wrong if I postulate that physics is full of nooks and crannies where asking why it's like this and not some other way and what does it means is basically a waste of time. It seems QM is special in this way. Although I saw someone in the thread argue that it's a desire of humans to find meaning and patterns where perhaps there are few.

I think asking why is always valuable. The answer may be that, at least for me, formalism alone is fine.

Thanks
Bill

 

[sbrothy]

I think asking why is always valuable. The answer may be that, at least for me, formalism alone is fine.

Thanks
Bill

I see what you mean. The why is intriguing, but getting the right results are what it's really about.

 

[bhobba]

What happens when a particle irreducible representation of the Poincaré group becomes entangled?

Well, if you use the number operator, you get the number of particles, but they aren't products of single particles.

Thanks
Bill

 

[Sambuco]

But because of its mass it is forced to reappear incessantly, on the order of 1021 times per second.

Why is the electron forced to reappear with such frequency? Is it due to $\frac{\hbar}{mc^2} \sim 10^{-21} \text{ s}$?

Lucas.

 

[Matterwave]

1. they do not satisfy the LTP, as any quantum interference confirms. For example, in a double-slit setup, one can not condition on the particle going through one slit or the other when computing the probabilities, because: p(x) ̸= p(x|right)p(right) + p(x|left)p(left). In this respect, the first implementation of a quantum interference experiment (perhaps Taylor(1909)’s double-slit experiment?) was a landmark in human’s history because, for the first time, it was realised that we live in a non-Kolmogorovian universe;

It is interesting that the first time we realized we don't live in a Kolmogorovian universe was 24 years before Kolmogorov formulated his axioms.