What Is the Frauchiger-Renner Theorem?

[Demystifier]

Alice can always just use $\Psi(x,x_A,x_D,t)$.

In that case, according to BM, there is never collapse for Alice.

 

[atyy]

In that case, according to BM, there is never collapse for Alice.

OK, but I don't think there is any contradiction to what I wrote in post #103 which was according to Copenhagen.

 

[David Byrden]

1. The epistemological interpretation seems to be contradicted by Bell's proof.
2. The physical interpretation seems to violate causality (no effects can travel faster than light).

1. Yes, it is.
2. No, it doesn't. Not if you interpret QM in a "many worlds" way, remembering that the "worlds" are local.Before her measurement, Alice is in a "world" where both outcomes are in superposition (or, to look at it another way, no information about either outcome has reached her).
The measurement entangles Alice with the particle's wave function, which immediately splits her into "Alice up" and "Alice down" and they promptly decohere and can never again communicate.
"Alice up" is now guaranteed to find "Bob down" when she makes her way over there, or sends a message, or whatever. Her wave function and "Bob down" are lobes of the same wave function. But nothing has moved faster than light. There is also a "Bob up" who is utterly unreachable by "Alice up" but can meet up with "Alice down".
Whether you wish to consider the alternative Alice and Bob as real and made of meat, or as mere potentialities implicit in the wave function and no more solid than the square root of minus one, is up to you.

 

[Demystifier]

OK, but I don't think there is any contradiction to what I wrote in post #103 which was according to Copenhagen.

What you fail to realize is that there is no Copenhagen interpretation. There are several different interpretations that people call "Copenhagen". You are referring to one particular version of Copenhagen (say the Landau-Lifshitz version), but the FR-Masanes theorem rules out another version of Copenhagen, a version that denies the objective collapse but accepts objective measurement outcomes. The theorem is not applicable to "your" LL version of Copenhagen.

 

[atyy]

What you fail to realize is that there is no Copenhagen interpretation. There are several different interpretations that people call "Copenhagen". You are referring to one particular version of Copenhagen (say the Landau-Lifshitz version), but the FR-Matsas theorem rules out another version of Copenhagen, a version that denies the objective collapse but accepts objective measurement outcomes. The theorem is not applicable to "your" LL version of Copenhagen.

That's fine. But did anyone believe the FR-Masanes version of Copenhagen anyway?

 

[Demystifier]

Not if you interpret QM in a "many worlds" way, remembering that the "worlds" are local.

In many-worlds, the worlds are neither non-local nor local. They are alocal: http://de.arxiv.org/abs/1703.08341

 

[Demystifier]

That's fine. But did anyone believe the FR-Masanes version anyway?

Yes, that's what I would like to know too. It seems to me that @vanhees71 believes something like that. If I'm right, it seems that we finally have a theorem against him.

 

[atyy]

Yes, that's what I would like to know too. It seems to me that @vanhees71 believes something like that. If I'm right, it seems that we finally have a theorem against him.

I thought he converted to BM?

 

[Demystifier]

I thought he converted to BM?

I think BM is now his second best interpretation. Maybe when he learns about the FR-Masanes-Leifer theorem it will become his first.

 

[stevendaryl]

1. Yes, it is.
2. No, it doesn't. Not if you interpret QM in a "many worlds" way, remembering that the "worlds" are local.

Yes, I usually make the exception for MW interpretations. Somewhere someone mentioned the assumption that measurements have unique results.

 

[Demystifier]

@DarMM there is one additional question that I would like to discuss with you. Do we really need the undoing of measurement in the FR-Masanes-Leifer theorem? Or can we achieve the same just by preparing another copy of the system?

Let me explain. The basic common scheme in all these thought experiments is the following:
1. First prepare the system in the state $|\Psi\rangle$.
2. Then perform a measurement described by a unitary operation $U|\Psi\rangle$.
3. After that undo the measurement by acting with $V=U^{-1}$, which gives $VU|\Psi\rangle=|\Psi\rangle$.
4. Finally perform a new measurement $U'|\Psi\rangle$.

But for the sake of proving the theorem, it seems to me that we don't really need the step 3. Instead, we can perform:

3'. Prepare a new copy of the state $|\Psi\rangle$.

After that, 4. refers to this new copy. Note that $|\Psi\rangle$ is a known state, so the no-cloning theorem is not an obstacle for preparing a new copy in the same state.

The only problem I see with this is the following. The state $|\Psi\rangle$ is really something of the form
$$|\Psi\rangle=|\psi\rangle |{\rm detector \;\; ready}\rangle$$
which involves not only a simple state $|\psi\rangle$ of the measured system, but also a complex state $|{\rm detector \;\; ready}\rangle$ of the macroscopic detector. In practice it is very very hard to have a control under all microscopic details of the macroscopic detector, meaning that it is very very hard to prepare two identical copies of $|\Psi\rangle$. Nevertheless, it is not harder than performing the operation $V$, which also requires a control under all microscopic details of the macroscopic detector to ensure that $V$ is exactly the inverse of $U$. So for practical purposes, 3.' is as hard as 3. Yet the advantage of 3.' over 3. is that it is more intuitive conceptually.

So is there any reason why would theorem lose its power if we used 3'. instead of 3.?

 

[atyy]

Some thoughts and questions.

1. Is the claim reasonable that Bohr's version of Copenhagen is refuted, in the light of Haag's statement in his textbook: In Bohr's discussion the time asymmetry appears as obvious. For instance: "The irreversible amplification effects on which the registration of the existence of atomic objects depends reminds us of the essential irreversibility inherent in the very concept of observation" [Bohr 58].

2. In the Healy third argument version of Copenhagen, if we undo the measurement, including the state of the measuring apparatus, then is the record of the measurement outcome lost? In other words, if Alice undoes Carol's "unitary measurement", then is Carol's measurement outcome available to Alice? If it isn't, then this would be another way of saying there is no P(a,b,c,d) for Alice.

3. Does the Healy third argument scenario only make sense in BM? In Copenhagen QM, a measurement must FAPP be irreversible to the observer, and there is no level beyond FAPP. In BM, a measurement is also usually FAPP irreversible. However, since BM has an additional layer, a measurement is reversible in principle. So this is where BM can make predictions where QM is silent, and is a way in which BM differs from QM.

 

[DarMM]

But Dan has a definite outcome - to whom? If Alice acknowledges that Dan has a definite outcome, then Alice must collapse the state. If Alice does not acknowledge that Dan has a definite outcome, then there is no definite outcome for Alice, and there is no P(a,b,c,d) for Alice.

$P(a,b,c,d)$ is the "objective" probability of occurrences of outcomes of the experiment, i.e. simply the frequencies that actually occur. It's not the predicted frequencies of outcomes for any particular observer since nobody in the set up can observe all four of $a,b,c,d$.

Plus this is the sticking point for me, how does Alice know when they've a definite outcome? What condition do you use to switch from a pure state to a mixed state if you're not looking at their laboratory? She models Carol's lab under unitary evolution up to what point? Decoherence?

That's fine. But did anyone believe the FR-Masanes version of Copenhagen anyway?

Brukner, Zeilinger, Healey himself, the older Quantum Bayesians (i.e. not QBism). Anybody who thought QM was purely a probability calculus for objective outcomes.

In the Healy third argument version of Copenhagen, if we undo the measurement, including the state of the measuring apparatus, then is the record of the measurement outcome lost? In other words, if Alice undoes Carol's "unitary measurement", then is Carol's measurement outcome available to Alice? If it isn't, then this would be another way of saying there is no P(a,b,c,d) for Alice.

Alice has no access to $c$ for this reason, but it's not a reason $P(a,b,c,d)$ doesn't exist as it is simply the probability of the outcomes to occur in one run of the experiment, even if nobody has access to all four outcomes.

In Copenhagen QM, a measurement must FAPP be irreversible to the observer, and there is no level beyond FAPP

However is it irreversible to a superobserver with full control over the observer's environment? That's who performs the reversing, not the observer themselves.

 

[DarMM]

Yes, it is.

I genuinely don't understand how the epistemological view of $\psi$ is ruled out by Bell's theorem. I've never seen this expressed in Quantum Foundations papers. The whole motivation of the PBR theorem is to provide constraints on epistemological views when previous theorems didn't, that's what's important about it.

 

[atyy]

$P(a,b,c,d)$ is the "objective" probability of occurrences of outcomes of the experiment, i.e. simply the frequencies that actually occur. It's not the predicted frequencies of outcomes for any particular observer since nobody in the set up can observe all four of $a,b,c,d$.

Plus this is the sticking point for me, how does Alice know when they've a definite outcome? What condition do you use to switch from a pure state to a mixed state if you're not looking at their laboratory? She models Carol's lab under unitary evolution up to what point? Decoherence?

What an observer designates as an objective outcome is subjective and up to the good taste of the observer. This is due to the subjective drawing of the classical/quantum cut. Things on the classical side are objective, and things on the quantum side are not.

If Alice believes Carol has made a measurement, then that means that Alice has granted Carol the same observer status as herself (Alice).

Brukner, Zeilinger, Healey himself, the older Quantum Bayesians (i.e. not QBism). Anybody who thought QM was purely a probability calculus for objective outcomes.

But one must also add that objective outcomes are subjective.

Alice has no access to $c$ for this reason, but it's not a reason $P(a,b,c,d)$ doesn't exist as it is simply the probability of the outcomes to occur in one run of the experiment, even if nobody has access to all four outcomes.

If Alice believes an outcome has occurred, but she has no access to it, then she must collapse her wave function.

However is it irreversible to a superobserver with full control over the observer's environment? That's who performs the reversing, not the observer themselves.

It is traditionally believed that measurements must be irreversible, otherwise there will be a contradiction. The objectivity of an outcome is stored in the mind of the observer. If the observer's mind is reversed, the subjectively designated objective outcome is lost.

A similar view is stated by Peres in his textbook (p376): "Consistency thus requires the measuring process to be irreversible. There are no superobservers in our physical world."

 

[atyy]

Plus this is the sticking point for me, how does Alice know when they've a definite outcome? What condition do you use to switch from a pure state to a mixed state if you're not looking at their laboratory? She models Carol's lab under unitary evolution up to what point? Decoherence?

Another possibility (in addition to Alice treating Carol as classical), is that Alice treats Carol as quantum. This means that Carol's outcomes have no reality for Alice. Only when Alice measures Carol's quantum mind or quantum apparatus, will she get a report of the state of the mind. So if Alice reverses Carol's mind, when she measures the mind, Carol will report to her that she did not make any Carol measurement.

 

[Auto-Didact]

No, not at all clear. As I said we might be using the words differently. Let me give you an example. Classical mechanics of several particles. The ontology of the theory is that there are several particles. That's what exists in the physical world. Their behavior may be described by a function (plus possibly other things as above), but it is meaningless and abuse of language to say that the ontology is the function. If your argument was correct, it would imply that in this example the only ontology is the function. Which is absurd.

It seems to me that by ontology you mean the minimum of mathematical apparatus that is needed to describe the world within a given theory.

It seems to me that this basically has not been answered yet.

Ah, I see what is your problem. Suppose that the ontology is the particle with a well defined position. I think you are fine with that. That position can be described by 3 numbers (x,y,z), but it doesn't mean that those 3 numbers are ontology. The ontology is the position itself, not our mathematical coordinatization of that position. Is it what you are saying?

With all due respect, it is only in discussions on QM foundations, that common academic terms such as 'ontology' and 'epistemology' suddenly become extremely vague, wildly confused and seem to lose all their universally accepted and agreed upon meaning.

Even worse, this is while all the historical QM foundations papers and discussions by Bohr, Einstein et al. did not have this problem at all! Neither were there any such confusions present in the discussions and works of Bohm, Bell nor any of the late 20th century physicists like Aspect, Clauser, Aspelmeyer, Shimony et al.; somewhere between the late 90s and now the situation seems to have changed quite drastically.

It is as if you all agreed to engage in a form of doublethink based on a form of Orwellian newspeak; e.g. similar to a group of people coming together and then secretly agreeing that the well-known word 'triangle' now means 'eat(ing)' and then seriously saying nonsense like "I'm triangle", "I'm having a triangle", "not triangle is not a choice" with a straight face.

Now it may be the case that many of you simply aren't natively English speaking (neither am I), and therefore do not necessarily recognize the ungrammatical nature of 'being ontology'; in that case, 'having (an) ontology' is the correct form. However, it seems to me that this isn't merely an issue of grammar but of semantics.

To clarify, here is an example of a few equivalent statements:

1. time has an ontology
2. time is ontological
3. time has an existence
4. time exists

Well, strictly speaking I think that you right, but that problem can be cured relatively easy. Consider a physical ontological object ~OO~\tilde{O} (in the case above it is a particle with a well defined position in physical space). Let its all mathematically describable properties be described by some mathematical object OOO (in the case above it is the numbers (x,y,z)). When we say that OOO is ontology, it is just an imprecise manner of speak, which really means that ~OO~\tilde{O} is ontology.

So when someone says that the wave function ψψ\psi is ontology, it really means that there is an ontological object ~OO~\tilde{O} such that its all mathematically describable properties can be described by O=ψO=ψO=\psi.

Does it make more sense now?

Quite frankly, no. If anything, it is clearer that a vast oversimplification is being made based on a simple semantic misunderstanding: a physical ontological object is a tautology; there is no such thing as a non-ontological physical object, in the same way there is no such thing in principle as a non-geometrical triangle, a non-mammalian dog or a non-linguistic verb.

To actually consider existing physical objects as non-ontological - i.e. existing physical objects as non-existing - is to either literally engage in doublethink or to mix up the different meanings of the word 'physical' i.e. to mix up 'occurring in nature' with '(speculative) ideas tentatively studied by theoretical physicists'.

This abuse of language is similar to calling the numbers (x,y,z)(x,y,z)(x,y,z) a vector, which really means that the object xex+yey+zezxex+yey+zezx{\bf e}_x+y{\bf e}_y+z{\bf e}_z is a vector. Physicists usually do not have problems with calling (x,y,z)(x,y,z)(x,y,z) a vector, which often annoys mathematicians.

This is a red herring; the abuse of language above with vectors and similar abuses such as viewing covectors and operators such as $\nabla$ or $\partial_{\mu}$ as vectors is logically coherent and therefore completely justified regardless of what mathematicians say; on the other hand, saying that 2+2=5 literally through a novel literal form of mathematical newspeak is not justified because it is demonstrably just nonsense.

 

[DarMM]

All it means is does $\psi$ describe the system as it is independent of the user ($\psi$-ontological) or does it describe the knowledge of an observer ($\psi$-epistemological).

I think any grammatical mistakes are just ours here and don't reflect a wider abuse of terminology in quantum foundations.

 

[DarMM]

Note: I think your position is consistent, I just need its exact form to place it correctly with respect to FR or Masanes. It currently sounds similar or maybe identical to Bub's.

But one must also add that objective outcomes are subjective.

What exactly does this mean?

It is traditionally believed that measurements must be irreversible, otherwise there will be a contradiction

How does one decide what is a measurement to know which of a superobserver's unitaries are irreversible. What prevents a superobserver from obtaining the control over the system necessary to perform reversal

 

[1977ub]

What an observer designates as an objective outcome is subjective and up to the good taste of the observer. This is due to the subjective drawing of the classical/quantum cut. Things on the classical side are objective, and things on the quantum side are not.

If Alice believes Carol has made a measurement, then that means that Alice has granted Carol the same observer status as herself (Alice).
But one must also add that objective outcomes are subjective.
If Alice believes an outcome has occurred, but she has no access to it, then she must collapse her wave function.
It is traditionally believed that measurements must be irreversible, otherwise there will be a contradiction. The objectivity of an outcome is stored in the mind of the observer. If the observer's mind is reversed, the subjectively designated objective outcome is lost.

A similar view is stated by Peres in his textbook (p376): "Consistency thus requires the measuring process to be irreversible. There are no superobservers in our physical world."

For an individual, there is no empirical evidence of other minds. Don't real objective outcomes have irreversibility which is unambiguous? An "observer" writes down a result with pencil on a paper, and this writing begins outgassing into the environment and otherwise affecting the world's particles. Can the measurement truly be "erased" once this happens? Even the measurement in the "mind" of some observer amounts to changes in the brain's particles and layout.

 

[atyy]

Note: I think your position is consistent, I just need its exact form to place it correctly with respect to FR or Masanes. It currently sounds similar or maybe identical to Bub's.

I took a quick look at Bub's https://arxiv.org/abs/1711.01604 and https://arxiv.org/abs/1804.03267. They seem essentially identical with my understanding, which I believe is classic Copenhagen.

What exactly does this mean?

How does one decide what is a measurement to know which of a superobserver's unitaries are irreversible. What prevents a superobserver from obtaining the control over the system necessary to perform reversal

These depend on the observer's common sense notion of "objective outcome" or "reality". If one is using relativistic quantum physics, then this outcome is an event in the sense of classical special relativity, and is an invariant "real happening" at a particular point in spacetime. In the formalism, measurement has a special status. The theory does not say what a measurement is, and that has to be put in from outside by the observer. To help the use of common sense, we usually say something like a measurement is something that produces an outcome that is described by the synonymous terms classical/macroscopic/real/objective/definite/irreversible.

There are no superobservers by hypothesis.

Essentially, these questions are variants of issues accompanying the classic "measurement problem".

 

[atyy]

For an individual, there is no empirical evidence of other minds. Don't real objective outcomes have irreversibility which is unambiguous? An "observer" writes down a result with pencil on a paper, and this writing begins outgassing into the environment and otherwise affecting the world's particles. Can the measurement truly be "erased" once this happens? Even the measurement in the "mind" of some observer amounts to changes in the brain's particles and layout.

Quantum mechanics grants observers/measurements a special status. Quantum mechanics is not a theory of reality, but a theory for observers to predict the probabilities of measurement outcomes.

If you want a theory of reality, you can try Bohmian Mechanics as a possibility.

 

[1977ub]

Quantum mechanics grants observers/measurements a special status.

How do I determine whether a black box which registers and reports measurements gets "observer" status or is merely an inert piece of equipment?

Doesn't QM work the same either way? Is there a set of attributes which something in the world has to have to be granted this special status ?

 

[atyy]

How do I determine whether a black box which registers and reports measurements gets "observer" status or is merely an inert piece of equipment?

Doesn't QM work the same either way? Is there a set of attributes which something in the world has to have to be granted this special status ?

Use your consciousness :)

This is the famous measurement problem. You can find discussions of it here.
http://www.johnboccio.com/research/quantum/notes/bell.pdf
Paul Dirac, The Evolution of the Physicist's Picture of Nature, 1963 (see his comment on the "role of observation in the theory")
Johnny von Neumann, Mathematical Foundations of Quantum Mechanics, 1932

 

[DarMM]

After much reading I think that in fact you are correct @atyy . Masanes result does not provide an argument against Classic Copenhagen.

After reading Bohr and Heisenberg's papers and a good summary of the Copenhagen interpretation in Hans Primas's book "Chemistry, Quantum Mechanics and Reductionism" it is clear that early Copenhagen does not obey the assumptions of Masanes's theorem.

What do obey the assumptions of Masanes's theorem are later Neo-Copenhagen views like Healey, Zeilinger, Brukner and the Quantum Bayesians (not QBism, which is separate). Also perhaps a "naive" Copenhagen where people loosely don't think of the subjective nature of the cut or think measurements are reversible, but this is not really worth consideration.

I need to think about:

1. Bohr and Heisenberg's different views on the Quantum/Classical cut
2. How the subjectivity of the cut relates to the subjectivity of outcomes in QBism
3. Related to the above, it might be that Classic Copenhagen violates two of Masanes's assumptions. Due to the subjectivity of the cut outcomes are subjective, in addition to the violation from measurements being irreversible.
4. Hyperobservers (Superobservers who can also unitarily reverse other observers) are banned, however Superobserver's are not. This does not pose a problem for Classical Copenhagen, but I need a stronger intuitive grasp of why not.

I'll return when I have something thought out enough to be sensible to say on these issues.

However in short, Classical Copenhagen is not affected by the basic form of FR because Superobservers are not a problem to Classical Copenhagen despite an initial naive analysis saying they are. Hyperobservers are banned, so the Masanes version can't even get off the ground.

 

[atyy]

Hyperobservers (Superobservers who can also unitarily reverse other observers) are banned, however Superobserver's are not. This does not pose a problem for Classical Copenhagen, but I need a stronger intuitive grasp of why not.

It's interesting to what extent the cut can be shifted. Hay and Peres looked into this a bit: https://arxiv.org/abs/quant-ph/9712044.

However in short, Classical Copenhagen is not affected by the basic form of FR because Superobservers are not a problem to Classical Copenhagen despite an initial naive analysis saying they are. Hyperobservers are banned, so the Masanes version can't even get off the ground.

Yes, I think what is interesting is where the argument fails in Copenhagen versus BM. In Copenhagen, it can't even get off the ground. In BM, which we have long known differs from Copenhagen in allowing hyperobservers (but I haven't analyzed the consequences of this), the argument goes further along (but still fails). I haven't verified Demystifier's analysis that E(a,b) is not corr(a,b), but I guess that must be it.

What is not intuitively clear to me is that in Copenhagen, after Carol's measurement, the entanglement is broken. To restore entanglement, it is not possible to use LOCC. Is this also true in BM, or does BM allow Alice to use a local unitary to undo Carol's measurement?

 

[Demystifier]

Is this also true in BM, or does BM allow Alice to use a local unitary to undo Carol's measurement?

See my post #116 where I argue that undoing measurements is not essential at all.

 

[atyy]

See my post #116 where I argue that undoing measurements is not essential at all.

Well, if you recreate the state, it involves entanglement of distant objects, so I guess it cannot be done by Alice using LOCC?

 

[A. Neumaier]

a good summary of the Copenhagen interpretation in Hans Primas's book "Chemistry, Quantum Mechanics and Reductionism"

This is a lucid description, but it is of course not ''the'' Copenhagen interpretation but Primas' version of the Copenhagen interpretation - just one of many others!

The Copenhagen interpretation was successful precisely because it was vague enough that many people could agree to something compatible with it...

 

[DarMM]

This is a lucid description, but it is of course not ''the'' Copenhagen interpretation but Primas' version of the Copenhagen interpretation - just one of many others!

Definitely true, even Heisenberg doesn't say quite the same thing as Bohr regarding reversibility. I took it as as good a summary of a sort of "Common Copenhagen" as one can get given Copenhagenists like Omnés seemed to agree with it.

The form of Copenhagen I am fairly sure is safe is Bohr's. I'm not yet so sure about Heisenberg's.

One of the reasons I'll be taking time out of this thread is to do a deep systematic review of all the Copenhagen papers (Pauli, Bohr, Heisenberg etc) and related monographs, especially in relation to the Heisenberg Cut (or Heisenberg Frontier as I've now learned it used to be called!), Subjectivity and Superobservers, so that I can do a proper deep comparison with Masanes's theorem.

 

[vanhees71]

Yes, that's what I would like to know too. It seems to me that @vanhees71 believes something like that. If I'm right, it seems that we finally have a theorem against him.

I'm not sure what to make of this Frauchiger paper since I'm not having enough time to translate their pretty complicated wording into clear formulas, but I've the impression that they make (hidden) assumptions about independence of states from observations which contradict what QT is actually saying (within the minimal interpretation), and this leads to apparent paradoxes. As I said, I'm not sure of that and I'm not able to point my finger at precisely the place where their argument goes wrong, but I've my suspicions with papers which do not provide a clear calculation in terms of bras and kets and rather write many words ;-)).

 

[DarMM]

I'm not sure what to make of this Frauchiger paper since I'm not having enough time to translate their pretty complicated wording into clear formulas, but I've the impression that they make (hidden) assumptions about independence of states from observations which contradict what QT is actually saying (within the minimal interpretation), and this leads to apparent paradoxes. As I said, I'm not sure of that and I'm not able to point my finger at precisely the place where their argument goes wrong, but I've my suspicions with papers which do not provide a clear calculation in terms of bras and kets and rather write many words ;-)).

Read the account in Healey:
https://arxiv.org/abs/1807.00421

First FR itself (section 3) and then Masanes's version (section 4).

 

[vanhees71]

These depend on the observer's common sense notion of "objective outcome" or "reality". If one is using relativistic quantum physics, then this outcome is an event in the sense of classical special relativity, and is an invariant "real happening" at a particular point in spacetime. In the formalism, measurement has a special status. The theory does not say what a measurement is, and that has to be put in from outside by the observer. To help the use of common sense, we usually say something like a measurement is something that produces an outcome that is described by the synonymous terms classical/macroscopic/real/objective/definite/irreversible.

Well, I think there's a lot of confusion in the field of "quantum foundations", because these people tend to be too far from contact with experimentalists. Working as a theorist in a field, which is pretty much in a phenomenological stage (ultrarelatistic heavy-ion collisions, QGP, and all that) with theory using a broad range of techniques to connect the observations finally I've a much more pragmatic view than apparently the quantum fundamentalists have, but of course in some sense they have a point that there is something not completely understood concerning the probabilistic nature of the standard (minimal) interpretation which leads to a desire of some theoretical physicists to either develop another "more deterministic" interpretation of QT (like e.g. Bohm which would imho completely satisfying for everybody if it were possible to make a convincing Bohmian interpretation of relativistic QFT, which I think is still not achieved yet) or seeking for a more comprehensive theory which encompasses QT as a limiting case.

From a pragmatic physicist's point of view, there's nothing lacking with standard QT since there's not a single reproducible observation contradicting it or is even not describable by it. To the contrary all the precision experiments testing the most "counterintuitive" features of standard QT confirm it at uncprecedented levels of significance (only think of all the quantum-optical/AMO Bell tests performed in the recent few years). Looking at this interplay between experiment and theory, in fact defines what quantum physics is, i.e., how QT is a description of the objective world. Physics is a whole! You cannot split it in theorertical and experimental physics, it's a quite sophisticated interplay between both experiments/observations and theory/calculations. The apparent split into two big subfields called experimental and theoretical physics is only due to the complexity of the field so that not even in a small specialized subfield a single physicist is able to do research at the frontier of knowledge in both theory and experiment, but finally one must be able to put theory and experiment together to get a complete picture of the described objective facts about nature.

So it's pretty clear what QT is: The formalism is an abstract description of real-world experiments in the lab (or in the case of astronomy and cosmology observations with instruments in the closer vicinity of the Earth and then extrapolating using the bold assumption that the natural laws are the same everywhere in the universe, known as the cosmological principle). The necessary interpretation of the abstract formalism is successful if it is possible to interpret the measurements and observations in a consistent and objective way and, even more important for the development of physics, being able to think about further developments of new experiments, including the construction of ever more sophisticated and precise measurement apparati to test the theory in even more stringent ways or hoping for a clear contradiction to theories to find an ansatz for discovering better ones. The prime example of this state of affairs is the Standard Model of elementary particle physics, which is describing all established discoveries in the field with high precision, to the dismay of the particle physicists, who know that it cannot be the final answer to all questions, particularly not the very fundamental one as how to include gravity in a consistent way and also on an empirical level concerning the question of whether there's "Dark Matter" and "Dark Energy" with GR being the right macroscopic description of space-time and gravity or whether GR has to be modified. All odds at the moment seem to indicate that GR is right. So there should be a necessity for Dark Matter and more particles than described by the Standard Model.

To put it in other words: Measurements are what experimentalists perform with their ever more sophisticated measurement devices. These deliver quantitative observations to be confronted with our theoretical understanding. As any empirical facts already the observation is based on (a) reproducibility and (b) statistical analysis. So I don't see a problem in having the fundamental theory, QT, as a probabilistic theory to begin with. All our objective empirical knowledge is anyway statistical, and probability theory is the mathematical description and basic foundation of statistics. QT thus is a tool to describe the statistics of measurements with the obviously adequate way to provide the probabilities/probability distributions for the expected statistical outcomes of measurements.

Whenever a physical situation appears to be described by classical physics, it's due to the fact that we look at "macroscopic observables", i.e., averaging over many microscopic degrees of freedom. Then we say an individual experiment has an established result although the observable measured has according to QT not a determined value but only a certain probability for each outcome. E.g., in the double-slit experiment, the particle has not a definite position (at least not in the region, where the detector is located) but all is known about the position is a probability distribution given by QT, and then one can test this probailistic prediction by statistical analysis with real experiments. That an individual single-particle outcome is just a point on the photo-plate is due to the measurment setup. A particle reacts with the material of the photo-plate at some place, and the interactions are local according to the theory, which leads to a local spot on the screen. There's no other way to observe particles (at least for us humans) than letting it interact with a measurement device leading to a macroscopic pointer reading at the apparatus. That's pretty much what Bohr already said in the very beginning but then he and his much more his followers blurred this simple idea by introducing the collapse hypothesis, and that's why many physicists still discuss about the apparent idiosyncratic paradoxes of some "measurement problem" (imho we don't have a measurement problem at all but an astonishing ability to create more and more precise and better and better measurement devices, as the enormous progress particularly in this field of AMO/quantum optics proves) or a "problem about to decide what's real" and seeking for "realistic interpretations of QT". QT in its minimal interpretation is as "realistic" as it gets in describing the world surrounding us and of which after all we are a part.

Of course, all the mathematical theories of physics are epistemological, i.e., they are the description of our knowledge about the "natural laws" as far as we are aware of them (and at least up to now, we know that we don't know these laws completely yet). This is not only true for QT but even already for Newtonian mechanics. In nature there's no fibrebundle. The fibre bundle is the mathematical description of Galilean spacetime as the very fundamental foundation of the theory. It has to be interpreted when applied to real-world observations. The only difference to QT is that it's about a much more coarse grained point of view at the world and correspondingly rough measurement devices like rulers and stop watches in high-school experiments to measure the motion of macroscopic objects like free-falling stones.

 

[DarMM]

Well, I think there's a lot of confusion in the field of "quantum foundations",

Do you have an example of such confusion?

Plus none of this gets away from the fact that if you hold a view with Masanes's four assumptions you have a contradiction, regardless of how well that view works otherwise.

Which of the following do you disagree with:

1. Quantum Mechanics applies objectively to all systems/is universal
2. Single World
3. Superobservers should use superposed states to describe observers, prior to their measurements of them
4. It's possible in principal to reverse measurements

 

[Lord Jestocost]

That's pretty much what Bohr already said in the very beginning but then he and his much more his followers blurred this simple idea by introducing the collapse hypothesis,...

A slight correction regarding Bohr's attitude. Jan Faye writes in https://plato.stanford.edu/entries/qm-copenhagen/ :

"Second, many physicists and philosophers see the reduction of the wave function as an important part of the Copenhagen interpretation. But Bohr never talked about the collapse of the wave packet. Nor did it make sense for him to do so because this would mean that one must understand the wave function as referring to something physically real. Bohr spoke of the mathematical formalism of quantum mechanics, including the state vector or the wave function, as a symbolic representation. Bohr associated the use of a pictorial representation with what can be visualized in space and time. Quantum systems are not vizualizable because their states cannot be tracked down in space and time as can classical systems. The reason is, according to Bohr, that a quantum system has no definite kinematical or dynamical state prior to any measurement. Also the fact that the mathematical formulation of quantum states consists of imaginary numbers tells us that the state vector is not susceptible to a pictorial interpretation (CC, p. 144). Thus, the state vector is symbolic. Here “symbolic” means that the state vector's representational function should not be taken literally but be considered a tool for the calculation of probabilities of observables." [bold emphasis added by LJ]