I Quantum mechanics is not weird, unless presented as such

[A. Neumaier]

If you make two mutually exclusive predictions and then test one of them the other one becomes counterfactual statement.

No. In this case you made two predictions, tested one of them, and remain silent about the validity of the other prediction. I see nothing counterfactual in this.

It becomes only counterfactual if you start arguing after the test ''If I would have done...'' - but such arguments must also be cut out since they are not testable. Testable are only statements about the future.

Counterfactual statement may be useful for planning an experiment but have no place in an objective analysis of the results.

 

[lightarrow]

Who is Alfred?

Sorry. Arnold.

--
BlueRay

 

[zonde]

Counterfactual statement may be useful for planning an experiment but have no place in an objective analysis of the results.

Right

No. In this case you made two predictions, tested one of them, and remain silent about the other prediction. I see nothing counterfactual in this.

I remain silent about factual truth or falsity of that other untested statement.

It becomes only counterfactual if you start arguing after the test ''If I would have done...'' - but such arguments must also be cut out since they are not testable. Testable are only statements about the future.

Counterfactual statements can't be tested, that's right.
But "what if" type of reasoning is irreplaceable tool in consistency checks (thought experiments). Say if I intend to subject physical configuration to external parameter "a" theory makes prediction "A" but if I intend to subject physical configuration to external parameter "b" theory makes prediction "B". Now I want to check that description of physical configuration from first case is consistent with description of physical configuration from second case. For this I need "what if" type of reasoning. And consistency is a valid requirement for a theory.
So we can analyze a theory from perspective of consistency. Say we can add something to the theory or remove something from it and look if it remains consistent.

 

[A. Neumaier]

Say we can add something to the theory or remove something from it and look if it remains consistent.

But this means that you make different predictions of the same situation with two different models. There is no need to use counterfactual language, which is inappropriate in science.

I remain silent about factual truth or falsity of that other untested statement.

Yes, and this removes any counterfactuality. it is just an irrelevant prediction, so it doesn't need to be discussed - and in science, it shouldn't.

 

[zonde]

A. Neumaier,
But science requires that theories are consistent. So it has to include tools to check this consistency.
Edit: I mean self-consistent.

 

[A. Neumaier]

So it has to include tools to check this consistency.

The scientific tools for this are mathematical deductions (where logic guarantees consistency) and the comparisons of experiments with predictions that assume only what was actually prepared.

Counterfactual comparisons are logically meaningless since they neither apply to reality (where the link has to be made in ordinary language) nor have the formal stringency of a mathematical argument (where one can decide consistency objectively). They are one of the traditional backdoors that allow weirdness to creep into an otherwise rational and beautiful theory.

 

[ddd123]

But this means that you make different predictions of the same situation with two different models. There is no need to use counterfactual language, which is inappropriate in science.

Yes, and this removes any counterfactuality. it is just an irrelevant prediction, so it doesn't need to be discussed - and in science, it shouldn't.

Sorry for my naughty thinking, but it seems you're saying that just because counterfactuals are a big part of the weirdness, not because of a rock-solid reason. In another universe without QM you could be saying counterfactuals are absolutely needed for consistency for whatever reason. Though making theories not-weird is not science's purpose, the purpose is to seek the truth about nature. If weirdness is the truth (not saying it is, but we don't know that it isn't either) it wouldn't be a good idea to sweep it under the rug.

 

[A. Neumaier]

In another universe without QM

You use counterfactual reasoning to justify counterfactual reasoning. Isn't this already weird? (As I argued earlier, there is only one universe to which we have access.)

 

[ddd123]

Because, if anything, this way of thinking has been a propeller for science, like Einstein's Gedanken experiments (of which, EPR was intended as a reductio, so that's already a purposefully impossible envisioning, though in reality and not in purpose it ended up being true). If we declare we've solved, say, the measurement problem and sit down, while our resolution isn't fully satisfactory but we've just deluded ourselves into thinking it is, we're stymieing possible groundbreaking ideation in the future.

 

[A. Neumaier]

Einstein's Gedanken experiments

Good gedanken experiments like Einsteins are not counterfactual. Making simplifying assumptions in a situation where better models do not exist or are intractable is not meant counterfactual, but is the usual state of affairs where we substitute the always unknown true model of reality by an approximate model. It becomes counterfactual only if you change the model to try to make a point about reality.

If you change the model to make a point about different predictions there is nothing counterfactual: It is legitimate to say that two models predict X and Y, experiment shows X, hence the model predicting Y is ruled out. This is correct science, no counterfactual language is involved,

 

[A. Neumaier]

making theories not-weird is not science's purpose

The purpose of science is to make Nature understandable and predictable. The less weird something is presented the more understanding it conveys. Something completely rational has no trace of weirdness. To the extent that science does not strive for complete rationality it is only pseudo-science.

If weirdness is the truth

Weirdness is always in the view of Nature, not a truth about Nature. Nature simply is. Our judgments about it are artifacts of our culture that allow us to speak of it.

 

[stevendaryl]

The purpose of science is to make Nature understandable and predictable. The less weird something is presented the more understanding it conveys. Something completely rational has no trace of weirdness. To the extent that science does not strive for complete rationality it is only pseudo-science.

I think that there is a distinction between eliminating the weirdness by (1) investigating it and explaining it, and by (2) choosing a way of presenting it that hides the weirdness.

 

[zonde]

Good gedanken experiments like Einsteins are not counterfactual.

Let's check this statement.
EPR:
"Suppose now that the quantity A is measured and it is found that it has the value a_k.
...
If, instead of this, we had chosen another quantity, say B, ..."

And if we examine Galileo's thought experiment there too is "what if" type of reasoning (small and big stones fall separately and what if we unite them):
"Salviati. If then we take two bodies whose natural speeds are different, it is clear that on uniting the two, the more rapid one will be partly retarded by the slower, and the slower will be somewhat hastened by the swifter. Do you not agree with me in this opinion?
Simplicio. You are unquestionably right.
Salviati. But if this is true, and if a large stone moves with a speed of, say, eight while a smaller moves with a speed of four, then when they are united, the system will move with a speed less than eight; but the two stones when tied together make a stone larger than that which before moved with a speed of eight. Hence the heavier body moves with less speed than the lighter; an effect which is contrary to your supposition. Thus you see how, from your assumption that the heavier body moves more rapidly than ' the lighter one, I infer that the heavier body moves more slowly."

 

[ddd123]

I am most likely wrong since I'm not at all an expert, but it seems to me that removing counterfactuals has a superdeterministic flavor. Like, "what if I had done" is related with a theory's generality, so eliminating counterfactuals seems to imply that there's some dependence on initial conditions.

 

[A. Neumaier]

there is a distinction between eliminating the weirdness by (1) investigating it and explaining it, and by (2) choosing a way of presenting it that hides the weirdness.

Yes, there is a big difference. The first gives rise to endless debates without any final conclusion (as witnessed for many years by the discussions on this forum) and eliminates nothing but inflates it; the second eliminates it and puts an end to the spook.

Weirdness is always misunderstanding, since things that are understood are no longer weird.

 

[A. Neumaier]

Galileo's thought experiment

There is no subjunctive in the piece from Galileo's dialogue that you quoted. No counterfactual reasoning, but a proof by contradiction. Assuming a particular model (''your supposition''), its predictions are analyzed, and it is found that nonsense results. Conclusion: Model disproved - it has nothing to say about Nature, which is consistent since it exists.

So, yes, you are allowed to do as much counterfactual reasoning as you wish - but do not think it tells anything about Nature. Hence it is no physics. It is philosophy (pre-physics) used to weed out unscientific notions. Only what remains after all counterfactual reasoning is over deserves to be called science.

 

[A. Neumaier]

here's some dependence on initial conditions

In physics, everything depends on initial conditions (unless it is time invariant). Theories are general precisely because they separate the contingent initial conditions that depend on the situation from the dynamical laws that show how these initial conditions propagate in time, thereby giving rise to causality.

 

[A. Neumaier]

(my boldfacing in the quotations)

I think that there is a distinction between eliminating the weirdness by (1) investigating it and explaining it, and by (2) choosing a way of presenting it that hides the weirdness.

The first gives rise to endless debates without any final conclusion (as witnessed for many years by the discussion on this forum) and eliminates nothing but inflates it

Let me back this up with some statistics: Of the 25 PF Quantum Physics threads with >=250 replies, all but one (The holographic universe) are about investigating and explaining quantum weirdness. The oldest of these 24 threads started on May 12, 2005, the youngest on Jun 17, 2015. (Verify this here!) 23 of the 24 threads are now closed, even those started in 2015, probably for the reasons I indicated. (This thread soon reaches the 250; I wonder what its fate will be...)

The problem that I have with QM is that it is so unclear what its semantics are. Is the wave function a description of the state of the world, or is it a description of our knowledge about the world? Or somehow both? Neither alternative really fits all the facts comfortably.

And apparently the 24 threads didn't help in any way to eliminate the weirdness. Too many and too contradictory investigations and explanations, presumably. Whereas fixing the semantics (which means cleaning up the language by presenting QM in a rational way) would make it clear and settle the issue once and for all. I have a clear semantics and no problem with fitting all facts comfortably, and I tried to communicate it here.

 

[nrqed]

It is rather meaningless do discuss about whether QM is weird or not. It is completely subjective. One cannot tell someone that they are "wrong" if they think that QM is weird, and vice versa. One can of course look at the math and decide that no, QM is not weird (and that SR, GR, Cosmology, etc are not weird either) and that is all fine but it is not a "better" or "superior" point of view than the point of view of someone finding these topics weird and this latter person is therefore "wrong" in some sense. One can argue that because one does not find it weird, one has a deeper and better understanding that someone who finds it weird but that is completely unwarranted, in my humble opinion (I guess that one point of view is that someone who does not find a physical theory weird is smarter because he/she understands it "better" but I don't believe this is a valid argument). So this type of discussion belongs more to the general discussion forum than to the QM forum. But of course, I am probably wrong :-)

 

[A. Neumaier]

[...] whether QM is weird or not. It is completely subjective.

Not more than the collapse of the wave function, which is responsible for some of the weirdness.

because one does not find it weird, one has a deeper and better understanding that someone who finds it weird

This does of course not follow. But in my personal experience, whenever I found something weird, it was often the case that after gaining understanding the weirdness disappears. With a regularity that I take weirdness as a sure sign of poor understanding. But not conversely - for example, lack of weirdness may also be caused by lack of understanding.

 

[stevendaryl]

Let me back this up with some statistics: Of the 25 PF Quantum Physics threads with >=250 replies, all but one (The holographic universe) are about investigating and explaining quantum weirdness.

Well, I think that shows that it is the number one most important topic in physics, when importance is ranked by "stuff that I don't understand, but want to understand". And it also shows that many people feel that it hasn't been adequately answered.

I think that the skew towards that topic is going to be different in a discussion forum than it will in an actual journal.

 

[A. Neumaier]

that shows that it is the number one most important topic in physics

Why then are all but one of these threads closed? Because they neither ''eliminated the weirdness by (1) investigating it and explaining it'', as you proposed, nor pointed to journal literature that did it.

 

[stevendaryl]

Another comment about "quantum weirdness": It's true that "weird" is subjective. However, the list of respectable physicists who thought that there was something not completely understood about the foundations of QM is pretty long and impressive: Einstein, Bell, Bohm, Feynman (well, he said that nobody really understands QM, but he might have been being flippant), Everett, DeWitt, whoever it is who worked on stochastic QM, the "consistent histories" guys, Penrose^*, T'Hooft, etc. There are dozens of alternative interpretations of QM: Bayesian and Many-Worlds and time-symmetric and superdeterministic interpretations and stochastic interpretations and explicitly nonlocal interpretations and on and on. This work is being done by professionals, not amateurs who learned about physics from Deepak Chopra. I don't think you can blame all the dissatisfaction on QM being presented in a sloppy way. If the foundations of QM were really firm, and understood, I don't think you would have all this ferment. Just for contrast, very soon after the introduction of SR, there were basically no professional physicists who felt the need to work on the foundations of relativity. There may have been some dissenters, but a tiny number, and they were not mainstream physicists.

^*Penrose certainly doesn't believe that the foundations of QM have been settled, because he has suggested that gravity might be the reason for wave function collapse. I think that's pretty speculative and I don't take it very seriously, but the fact that Penrose would even venture such a speculation means that he doesn't consider the status of the wave function and collapse and so forth to be settled.

 

[Andrew Wright]

Perhaps the threads on quantum weirdness get closed because each interpretation presented is possible but unprovable.

 

[A. Neumaier]

I don't think you can blame all the dissatisfaction on QM being presented in a sloppy way. If the foundations of QM were really firm, and understood, I don't think you would have all this ferment.

I was also dissatisfied, and spent a lot of time on foundations - even to the point of writing research papers about it. But this was before I really understood...

Thinking in terms of particles (for historical reasons - almost everyone does it!) is the biggest part of it; sloppiness is responsible for much of the remainder.

I haven't seen any serious attempt that bases the foundations on quantum fields instead on particles. The main reason is that those active in quantum field theory don't need the foundations and don't care about it - ''shut up and calculate'' is the road to success in QFT. But they do everything they do in terms of quantum fields - particles live only in the language. When I realized this, and that shifting the emphasis in the same way in the foundations makes many things simple that looked before formidable, it was a revelation for me, and I systematically explored these new foundations.

But seeing how little respect one earns by putting forward a new interpretation, I had decided not to publish anything. My time is far better spent making quantum mechanics intelligible by pointing out how close its formalism is to classical mechanics when presented correctly. This is independent of foundational problems, and already gives an enormous insight - insight that I'd have liked to read in textbooks when I began studying quantum mechanics but had to acquire the hard way. The results are in my book, which will be on the market in 2017. The application to entanglement, which I regard primarily as hype, will most likely not be part of it.

 

[zonde]

So, yes, you are allowed to do as much counterfactual reasoning as you wish - but do not think it tells anything about Nature.

I have no illusions that by counterfactual reasoning I could find out something about Nature. As I see counterfactual reasoning can only be used for analysis of models of Nature.

 

[A. Neumaier]

counterfactual reasoning can only be used for analysis of models of Nature.

Only to debunk them, and throw them away. But quantum mechanics is supposed to last, hence counterfactual reasoning cannot add anything to the understanding of quantum mechanics.

 

[zonde]

Only to debunk them, and throw them away. But quantum mechanics is supposed to last, hence counterfactual reasoning cannot add anything to the understanding of quantum mechanics.

Quantum mechanics has a lot of different approaches and it is quite certain that some (most) of them are not going to last.

 

[zonde]

Thinking in terms of particles (for historical reasons - almost everyone does it!) is the biggest part of it

Dropping particle idea is not going to resolve entanglement as the only thing you need from particles is paired detector clicks for non-locality weirdness to be there.
Well, actually there is another possibility if detection are not perfectly paired for idealized case. If fields approach can introduce new loophole that no one has thought about then it can save locality. But then it definitely would be a new theory as it would have to make different prediction.

 

[stevendaryl]

Dropping particle idea is not going to resolve entanglement as the only thing you need from particles is paired detector clicks for non-locality weirdness to be there.
Well, actually there is another possibility if detection are not perfectly paired for idealized case. If fields approach can introduce new loophole that no one has thought about then it can save locality. But then it definitely would be a new theory as it would have to make different prediction.

I certainly understand that the concept of a "particle" is a fuzzy one in QFT, and it is only in special cases that it makes sense to talk about particles as distinct entities. But to me, it seems that the spin-1/2 EPR experiment is pretty clear-cut. You don't have to mention particles at all to describe what's weird about it.

 

[A. Neumaier]

Quantum mechanics has a lot of different approaches and it is quite certain that some (most) of them are not going to last.

Like a tree has many leaves, and most of them do not last. I am talking about the tree, not the leaves.

 

[A. Neumaier]

But to me, it seems that the spin-1/2 EPR experiment is pretty clear-cut. You don't have to mention particles at all to describe what's weird about it.

Can you please summarize the details relevant for displaying its weirdness without mentioning particles, so that I can see what you mean?

 

[Feeble Wonk]

Thinking in terms of particles (for historical reasons - almost everyone does it!) is the biggest part of it

I haven't seen any serious attempt that bases the foundations on quantum fields instead on particles. The main reason is that those active in quantum field theory don't need the foundations and don't care about it - ''shut up and calculate'' is the road to success in QFT. But they do everything they do in terms of quantum fields - particles live only in the language.

I'm hesitant to do so, but I'm compelled to point out that to us curious laypeople your statement here, in itself, confers a strong degree of "weirdness". Aside from QT's seemingly odd violations of locality, the strangeness of quantum entanglement, etc., the fact that matter should not really be thought of as material is definitely weird to us. It leaves us wondering what physical existence is actually composed of. If not material, what is the ontological nature of the quantum field?
I don't really expect you to be able to answer that for me. I'm simply suggesting that the weird quotient is a very subjective thing. To those of us looking for answers from the professional physicists, you being able to balance the formula is not sufficient. We want you to be able to tell us what the numbers mean. We want you to be able to tell us what physical existence "is". When the mental image is counterintuitive, it seems "weird" to us.

 

[stevendaryl]

Can you please summarize the details relevant for displaying its weirdness without mentioning particles, so that I can see what you mean?

Well, this is ground that was already covered by Bell in his essays about EPR, but since I don't know of an online reference, I'll try to reproduce it:

Abstractly, you have a situation like that in this diagram. Suppose that you have two devices. Each device has a pointer that can be used to choose among a number of settings (3 in the picture). Each device has two LEDs that can light up--one red and one blue. Alice has one of the devices, and Bob has another.

You have a source of some unknown kind of signal that periodically sends a pair of signals, one to Bob's device and the other to Alice's device. We're not going to get into what the signals are, whether they are light signals, or messages on pieces of paper, or particles, or whatever. But the abstract behavior is this:

1. Each time the source sends its signals, exactly one of Alice's LEDs light up, and exactly one of Bob's LED's light up.
2. If Alice and Bob choose the same pointer settings, then they always get opposite results (if Alice's red light glows, then Bob's blue light glows, and vice-versa)
3. Alice's lights, when examined without considering Bob's, seem completely random: regardless of the setting, she gets red 50% of the time, blue 50% of the time.
4. Bob's lights are similarly random when considered alone.
5. If Alice and Bob choose different settings, they get the same results 75% of the time and opposite results 25% of the time.

Without mentioning particles at all, there is a mystery as to how there can be perfect anti-correlation between Alice's and Bob's results. The most straight-forward way to try to understand it would be to suppose that the unknown signals sent from the source box somehow encodes the instructions for their results. For example, it could be literal instructions saying "If Alice chooses setting 1, then light up her blue light. If Bob chooses setting 1, then light up his red light." But that sort of predetermined result is precluded by the statistics: There is no way to generate such instructions that reproduces the statistics described in lines 3 through 5.

 

[ddd123]

That's what I meant about counterfactuals. Sure, if you prevent me from saying "but if Alice had chosen that other setting" I can't voice my astonishment about that result. But that's all removing counterfactuals accomplishes, shutting me up, not making things more clear.

 

[dextercioby]

[...]
I haven't seen any serious attempt that bases the foundations on quantum fields instead on particles. The main reason is that those active in quantum field theory don't need the foundations and don't care about it - ''shut up and calculate'' is the road to success in QFT. [...]

Nein, ist leider nicht so. Quantum mechanics will always be perceived as quantized classical mechanics of point particles in which fields are extraneous. Think of the free quantum (Galilean) moving massive dot. How do you describe it in QM? Yes, you take the x, p. and H = p² /2 from classical mechanics and use the Dirac quantization rule (which has it limitations, but it's not an issue). The same goes for the H-atom and all other elementary models. They all start with a classical description.
What are the classical field theories (as opposed to the standard particle view of classical mechanics)? Electromagnetism, Continuum mechanics and Gravitation theory (GR). Apply a quantization scheme to them: you get QED which is marred by the particle interpretation of it and that's it. There's no quantum continuum mechanics (no quantum elasticity theory for example), no valid quantum theory of the gravitational field. And if there were, you'd still have the particle interpretation attached to these quantum fields.

 

[strangerep]

[...] my book, which will be on the market in 2017. The application to entanglement, which I regard primarily as hype, will most likely not be part of it.

Istm, the entanglement swapping experiments, in which an entangled pair is produced without a "common cause", enhances the case for a quantum field picture of the world, rather than detracting from it.

 

[A. Neumaier]

that matter should not really be thought of as material is definitely weird to us

But this is not a fault of quantum mechanics, but one of classical mechanics. In classical mechanics, space is empty except for a huge number of point particles, which is extremely weird. In quantum field theory, space is filled at every position with quantum fields, and the electron field acts as a very strong glue that keeps macroscopic matter together. There is nothing weird in the quantum description, unless you insist on a particle picture.

 

[A. Neumaier]

We want you to be able to tell us what physical existence "is".

Physical existence is having a density in space-time. This density tells the spatial-temporal extent of the object. In quantum mechanics the density is encoded in the density matrix.

 

[A. Neumaier]

Abstractly, you have a situation like that in this diagram.

Thanks a lot for your clear description, which I really appreciate.

I haven't seen anything particle-free like this in the literature. Therefore ''investigating and explaining quantum weirdness'' for this precise setting will take me some time. In order to have some insurance that I don't waste my time, I'd first like to know your answer to the following question:

Suppose that I can convince you that your scenario (without any later change to the setting), once all hidden features implies by the use of classical language, is not significantly more weird than a similar classical situation. Would you then agree that I have explained quantum weirdness in a satisfactory way?

If not, which features would a satisfactory explanation of quantum weirdness (and hence satisfactory foundations for quantum theory) need to have to convince you, in this particular case? For if no amount of ''investigating and explaining quantum weirdness'' would satisfy you, it would be futile for me to spend time on your scenario.

 

[A. Neumaier]

Quantum mechanics will always be perceived

What has been the case in the past need not be so in the future. In the past, the perception of many fields of physics hasn't been invariant in time, and what holds for the past is likely to hold for the future.

Think of the free quantum (Galilean) moving massive dot.

In a future where quantum mechanics is interpreted in terms of fields, the free point particle will be viewed as a very idealized toy example illustrating certain features of quantum mechanics. Not more.

 

[A. Neumaier]

the case for a quantum field picture of the world

But this case is made (informally) here in this thread, not in my book.

My book has a far more modest goal - to show how close quantum mechanics can be to classical mechanics (both formally and in its interpretation) without losing the slightest substance of the quantum description, but removing much (not all) of its weirdness. (I am planning another book, on quantum field theory, but this is not yet ready for discussion.)

 

[Feeble Wonk]

Physical existence is having a density in space-time. This density tells the spatial-temporal extent of the object. In quantum mechanics the density is encoded in the density matrix.

Density of...? Would you say that physical existence, ontologically, is an expression of spatial-temporal information?

 

[A. Neumaier]

Density of...?

Density of mass, energy, charge, or more complicated stuff. It is all encoded in the density matrix characterizing a physical state.

Would you say that physical existence, ontologically, is an expression of spatial-temporal information?

No. But without an expression of spatial-temporal information nothing can exist in any physically meaningful sense.
(Diverse other sorts of ontological existence that entities such as ''the theorem of Pythagoras'', ''the hero of the book The Lord of the Rings'', or ''the current emperor of Rome'' may have are not warranted in our context.)

 

[Feeble Wonk]

(Diverse other sorts of ontological existence that entities such as ''the theorem of Pythagoras'', ''the hero of the book The Lord of the Rings'', or ''the current emperor of Rome'' may have are not warranted in our context.)

I'm sorry to be dull, but I don't understand what this means.

 

[zonde]

Suppose that I can convince you that your scenario (without any later change to the setting), once all hidden features implies by the use of classical language, is not significantly more weird than a similar classical situation. Would you then agree that I have explained quantum weirdness in a satisfactory way?

I'm not stevendaryl and I'm not sure if it matter but I say yes to your question with one correction to stevendaryl's explanation: statistics in point 5. are given for 3 settings case (as in picture).

 

[A. Neumaier]

I don't understand what this means.

Well, ontology is the theory of existence. To clarify which sort of existence I am talking about I gave four examples of entities that may be considered to exist in some sense (how could we talk about things that don't exist in any sense?) but where the concept of existence is not the one appropriate for physics.

 

[Feeble Wonk]

Well, ontology is the theory of existence, and I gave four examples of entities that may be considered to exist in some sense (how could we talk about things that don't exist in any sense?) but where the concept of existence is not that of physics.

I see. I suppose my question was more in regard to the physical ontology. In the absence of "material" existence, we are left with your description of a quantum field imbuing all of space-time with "mass, energy, charge, or more complicated stuff" as you said, which is "all encoded in the density matrix characterizing a physical state"... which is a probabilistic expression. It leaves me unclear as to the physical ontology other than its information content.
But, I won't press the issue. I suspect it leads to metaphysical/philosophical discussion that will get the thread closed.
I'm simply pointing out that to those that expect the mathematical description to be describing something ontologically "real", in a physical sense, this seems "weird".

 

[A. Neumaier]

"all encoded in the density matrix characterizing a physical state"... which is a probabilistic expression.

No. For a 2-level system, the density matrix is a matrix expressible in terms of four definite real numbers, which is not so different from a classical phase space position that takes 6 real numbers for its description. There are are also classical observables with matrix shape, such as the inertia tensor of a rigid body. The density matrix is analogous.

For more complex quantum systems, the number of definite real numbers needed to fix the state is bigger (or even infinite), but this is also the case in classical complex objects or fields. Thus the ontology of physical reality is as real as one can have it in a formal model of reality.

Response probabilities can be determined from the density matrix, but one can also determine response probabilities from classical chaotic systems. This therefore has nothing to do with the underlying ontology.

 

[A. Neumaier]

In the absence of "material" existence

Why is material existence absent when there is a mass density? Classically, in classical elasticity theory (which governs the behavior of all solids of our ordinary experience) and hydrodynamics (which governs the behavior of all liquids and gases of our ordinary experience), all you have about material existence is the mass density - unless you go into the microscopic domain where classical descriptions are not applicable.