Undergrad What Is the Frauchiger-Renner Theorem?

[Demystifier]

Basically you're finding fault with the proof because it results in a contradiction about p(a,b,c,d) and saying some assumptions must be wrong. However as I said, yes indeed, that's the point of the proof, one of the premises must be wrong. However they are all premises of Classic Copenhagen.

I agree with everything you said in the great and very clear post above, except that I am not convinced that the bold sentence quoted above is true. What exactly is "Classic Copenhagen"? Is there a standard QM textbook in which it is unambiguously clear they all really are the premises? I ask this because it still looks to me as if the theorem (either FR or the Masanes version of it) rules out an "interpretation" that nobody seriously believed in the first place.

 

[Demystifier]

Also Masanes version doesn't use any of this, so even if this objection were valid it says nothing about Masanes version covered in Healey's paper.

In addition, the Masanes version is technically much simpler, so it is much easier to see the forest for the trees.

 

[DarMM]

I agree with everything you said in the great and very clear post above, except that I am not convinced that the bold sentence quoted above is true. What exactly is "Classic Copenhagen"? Is there a standard QM textbook in which it is unambiguously clear they all really are the premises? I ask this because it still looks to me as if the theorem (either FR or the Masanes version of it) rules out an "interpretation" that nobody seriously believed in the first place.

By Classic Copenhagen I mean roughly what Bohr, Heisenberg, etc thought. They're not consistent with each other or even themselves at different times, so in truth there is no "Copenhagen interpretation", but I mean roughly an interpretation following the list below which most of them would have held. Wasn't sure what to call, especially since there is no single Copenhagen interpretation, should have been clearer.

Before I go digging into Bohr's papers and those of the old Copenhagen group, I would just like to narrow the search for what you want. Which do you think Bohr for example didn't hold:

1. Observations are objective events, not agent experiences
2. There are no hidden variables, i.e. QM is complete
3. QM may be universally applied to any system (note this is not the same as saying there is no Heisenberg cut)
4. The wavefunction is not an ontic object

From my reading of him he said all four. I just want to know which one you think he didn't say, or what form of Copenhagen do you have in mind that denies one of these four statements and which one? Haag for example says something I believe in Local Quantum Physics, though I have to check that when I'm at my books.

To clarify does it need to be a textbook that says it, most textbooks are very operational and say nothing about the reality of $\psi$ for example. Would Omnés book "The Interpretation of Quantum Theory" count even though its not a "textbook" designed to teach QM?

Again most of the Foundations community would be wrong on this, they all seem to say it invalidates Classic Copenhagen, see Leifer's lectures I linked to before.

 

[DarMM]

In addition, the Masanes version is technically much simpler, so it is much easier to see the forest for the trees.

I 100% agree on this!

 

[atyy]

By Classic Copenhagen I mean roughly what Bohr, Heisenberg, etc thought. They're not consistent with each other or even themselves at different times, so in truth there is no "Copenhagen interpretation", but I mean roughly an interpretation following the list below which most of them would have held. Wasn't sure what to call, especially since there is no single Copenhagen interpretation, should have been clearer.

Before I go digging into Bohr's papers and those of the old Copenhagen group, I would just like to narrow the search for what you want. Which do you think Bohr for example didn't hold:

1. Observations are objective events, not agent experiences
2. There are no hidden variables, i.e. QM is complete
3. QM may be universally applied to any system (note this is not the same as saying there is no Heisenberg cut)
4. The wavefunction is not an ontic object

From my reading of him he said all four. I just want to know which one you think he didn't say, or what form of Copenhagen do you have in mind that denies one of these four statements?

To clarify does it need to be a textbook that says it, most textbooks are very operational and say nothing about the reality of $\psi$ for example. Would Omnés book "The Interpretation of Quantum Theory" count even though its not a "textbook" designed to teach QM?

Again most of the Foundations community would be wrong on this, they all seem to say it invalidates Classic Copenhagen, see Leifer's lectures I linked to before.

So what is wrong with Scott Aronson's objection?

 

[DarMM]

So what is wrong with Scott Aronson's objection?

He doesn't deal with Masanes's version.

 

[atyy]

He doesn't deal with Masanes's version.

OK, so basically the first 2 versions of the theorem are wrong?

For te Masanes version, I would count myself a believer in classic Copenhagen (ie, QM is formally complete), but also believe in hidden variables - I dare say many older physicists take this position - Dirac, Messiah, Bell, possibly even Landau & Lifshitz - because it has generally been assumed that Copenhagen has a measurement problem, the most important problem in foundations - not these horrible fake problems that Frauchiger and Renner solve.

 

[DarMM]

OK, so basically the first 2 versions of the theorem are wrong?

Not wrong, but the set of interpretations eliminated by them is almost null and I believe nobody held them outside of those who don't think much about Foundations. I don't think they have much content for serious interpretations.

For te Masanes version, I would count myself a believer in classical Copenhagen (ie, QM is formally complete), but also believe in hidden variables - I dare say many older physicists take this position - Dirac, Messiah, Bell, possibly even Landau & Lifshitz - because it has generally been assumed that Copenhagen has a measurement problem, the most important problem in foundations - not these horrible fake problems that Frauchiger and Renner solve.

What "fake problems" are you talking about?

You start off talking about Masanes's version and then switch back to Frauchiger-Renner. Are the fake problems with Masanes or the original Frauchiger-Renner.

As I said, like Healey I believe that the original FR papers have little force, but like most of the foundations community I think Masanes's version is a serious result. If Masanes's version has "fake problems" I'd like to hear them.

 

[atyy]

Not wrong, but the set of interpretations eliminated by them is almost null and I believe nobody held them outside of those who don't think much about Foundations. I don't think they have much content for serious interpretations.

Well, those are equivalent FAPP (as stevendaryl said, one can be charitable). Anyway, I'm glad I haven't spent much time with the first 2 FR versions.

What "fake problems" are you talking about?

You start off talking about Masanes's version and then switch back to Frauchiger-Renner. Are the fake problems with Masanes or the original Frauchiger-Renner.

As I said, like Healey I believe that the original FR papers have little force, but like most of the foundations community I think Masanes's version is a serious result. If Masanes's version has "fake problems" I'd like to hear them.

Fake problems in the sense that they are, as you say, completely irrelevant. Given the bad track record of this theorem, I'm going to wait before investing time in the Masanes version. However, I would like to know how that accounts for the view that QM is only formally complete, but not de facto complete because of the measurement problem. I think many would count this a variety of classic Copenhagen - Dirac, Messiah, Bell, possibly L&L. Demystifier himself is, I think, a proponent of this version of classic Copenhagen: https://www.physicsforums.com/insights/stopped-worrying-learned-love-orthodox-quantum-mechanics/.

 

[DarMM]

Given the bad track record of this theorem, I'm going to wait before investing time in the Masanes version.

Just to be clear Masanes's version is almost as old as the FR result itself. The FR result had problems recognised almost immediately, Masanes has yet to have any pointed out.

I don't think it can be judged as poor based on the track recorded of a related theorem. They're not the same result.

Although I understand your weariness.

The rest of your post requires a longer answer which I'll only have time for over the weekend. To help though what do you mean by "formally complete". That it isn't self-contradictory, even though it leaves somethings unexplained like measurement?

Also do you take a perspectival view of Copenhagen, i.e. the measurement outcomes aren't fully objective but associated with the agent observing them, relational to some degree.

 

[atyy]

The rest of your post requires a longer answer which I'll only have time for over the weekend. To help though what do you mean by "formally complete". That it isn't self-contradictory, even though it leaves somethings unexplained like measurement?

Formally complete, in the sense that it is (or can be) consistent after one has defined one classical/quantum cut. However it is not complete from the larger point of that the observer has a special status in quantum mechanics. Here one views QM as a complete theory that is emergent from a more complete theory, just as Newtonian mechanics is a complete theory emergent from more complete theories like special relativity and quantum mechanics.

Also do you take a perspectival view of Copenhagen, i.e. the measurement outcomes aren't fully objective but associated with the agent observing them, relational to some degree.

Within QM, outcomes are fully objective to all agents on the classical side of the classical/quantum cut. I'm not sure what you mean by perspectival, but we have to make only one classical/quantum cut - and things on the other side of the cut cannot be granted the status of agent or observation outcome.

 

[DarMM]

To be clearer then, in a Wigner's friend type set up, do Wigner and the friend have the same cut or different ones?

 

[atyy]

To be clearer then, in a Wigner's friend type set up, do Wigner and the friend have the same cut or different ones?

Wigner is classical, and the friend is quantum.

 

[DarMM]

From Wigner's perspective I get that, but do you think the friend should model themselves as quantum as well?

 

[atyy]

From Wigner's perspective I get that, but do you think the friend should model themselves as quantum as well?

The friend is not an observer and cannot model himself.

Edit: The question of whether the cut can be consistently shifted is interesting. However, I don't think it is in formal QM, since making the cut itself already requires subjectivity. However, I do like Hay and Peres's https://arxiv.org/abs/quant-ph/9712044. I think the question more generally requires a more complete theory like Bohmian Mechanics to be correctly dealt with.

 

[DarMM]

Okay you would be saying the presence of Wigner's measuring device alters the context of the situation and means the friend could not be placed on the classical side of the cut, even by himself.

The highly unusual confined nature of the friend and the contextual effects of Wigner's device means the friend evolves nonclassically?

I think this is legitimate, as an issue I always have with these set ups is what has to actually be done to get a Wigner's friend scenario is very extreme.

I'll need to think a bit.

 

[atyy]

Okay you would be saying the presence of Wigner's measuring device alters the context of the situation and means the friend could not be placed on the classical side of the cut, even by himself.

The highly unusual confined nature of the friend and the contextual effects of Wigner's device means the friend evolves nonclassically?

I think this is legitimate, as an issue I always have with these set ups is what has to actually be done to get a Wigner's friend scenario is very extreme.

I'll need to think a bit.

Yes, the friend is just the same as a Schroedinger's cat or a qubit.

 

[Demystifier]

Before I go digging into Bohr's papers and those of the old Copenhagen group, I would just like to narrow the search for what you want. Which do you think Bohr for example didn't hold:

1. Observations are objective events, not agent experiences
2. There are no hidden variables, i.e. QM is complete
3. QM may be universally applied to any system (note this is not the same as saying there is no Heisenberg cut)
4. The wavefunction is not an ontic object

My problem with this list is that 2 and 4 contradict 1 in a rather trivial way. 2 implies that there is no ontology which is not $\psi$. 4 says that there is no ontology which is $\psi$. Hence 2 and 4 together say that there is no any ontology at all. Yet 1 says that there is some ontology. A contradiction!

 

[DarMM]

My problem with this list is that 2 and 4 contradict 1 in a rather trivial way. 2 implies that there is no ontology which is not $\psi$. 4 says that there is no ontology which is $\psi$. Hence 2 and 4 together say that there is no any ontology at all. Yet 1 says that there is some ontology. A contradiction!

Well I would say (2.) means that your ontology doesn't involve a world that admits an objective description with mathematical terms. It might be non-mathematical, or it might be mathematical but completely reltional. Bohr thought the former, Schrödinger thought the latter.

Regardless it seems to me pretty clear that many believed 1-4. I understand why you think they are daft, I'm not disagreeing with you on this, but they're there in Bohr, Heisenberg and others writings. As silly as one might find them, there was no proof they were self-contradictory until Masanes.

 

[DarMM]

Basically @Demystifier and @atyy , the "problem" is that both of you already hold non-self-contradictory positions* that Masanes's proof doesn't affect and consider the positions it does refute as obviously wrong. However there was no clear proof they were wrong until now and they are the Copenhagen of Bohr and others of early QM.

*e.g. @atyy you have a similar view to Bub, QM must be used from the perspective of a final ultimate classical user and rejects the idea that the Wigner's friend set up has the friend capable of being considered on the classical side, even by himself. I think it's a very clear resolution and you seem to consider it obvious, but note that Bub had to publish a paper about it, it's not a widely known position.

 

[DarMM]

Yes, the friend is just the same as a Schroedinger's cat or a qubit.

Okay sorry for all the questions! I think this is the last one, what about the situation makes this true for you explicitly? Basically do you think that Wigner completely confining his friend to a totally sealed lab and the physical presence of Wigner's equipment causes quantum effects to propagate up to the macroscopic scale for the friend invalidating him considering himself or his equipment as classical?

I can imagine this, Wigner's device would be pretty "extreme" equipment whatever it is.

 

[martinbn]

My problem with this list is that 2 and 4 contradict 1 in a rather trivial way. 2 implies that there is no ontology which is not ψ\psi. 4 says that there is no ontology which is ψ\psi. Hence 2 and 4 together say that there is no any ontology at all. Yet 1 says that there is some ontology. A contradiction!

I don't understand this. How does 2. imply that there is no ontology other than $\psi$? In fact, in my opinion, it is an extreme abuse of language to say that $\psi$ is ontology. What does it even mean?

 

[DarMM]

What does it even mean?

It's true in Many-Worlds and Bohmian mechanics, the state space of ontic objects $\Lambda$ has the form $\Lambda = \mathcal{A} \times \mathcal{H}$ with $\mathcal{H}$ the quantum Hilbert space. So the ontology has $\psi$ as an element.

 

[martinbn]

It's be true in Many-Worlds and Bohmian mechanics, the state space of ontic objects $\Lambda$ has the form $\Lambda = \mathcal{A} \times \mathcal{H}$ with $\mathcal{H}$ the quantum Hilbert space. So the ontology has $\psi$ as an element.

May be I don't know what the word means. But for me "ontology" is related to existence/being. So something like an element in a Hilbert space cannot have ontology. It makes no sense to say that it exists. What exists is the particles/fields, not $\psi$.

 

[Demystifier]

I don't understand this. How does 2. imply that there is no ontology other than $\psi$?

Let me try to answer this step by step. (2) says that "There are no hidden variables, i.e. QM is complete". It really means that in the mathematical description of the world, we don't need anything else except $\psi$. I guess you agree so far.

Now consider ontology. It's a philosophical term, but its meaning is not essential here. All what we explicitly need to prove what I want to prove is the following assumption:
Assumption 1: Ontology can be described mathematically and we need it to describe the world.
Then, from (2) and Assumption 1, it follows that there is no ontology which is not $\psi$. Is it clear now?

So if you want to avoid my conclusion, you must deny Assumption 1, i.e. you must hold that either ontology cannot be described mathematically, or that we don't need ontology to describe the world. The first option looks like mysticism to me, which is logically legitimate but not scientific in spirit. The second option implies that, in order to describe the world, we don't need to believe that the Moon exists when nobody observes it, which is also logically legitimate, but contradicts common sense.

 

[Demystifier]

May be I don't know what the word means. But for me "ontology" is related to existence/being. So something like an element in a Hilbert space cannot have ontology. It makes no sense to say that it exists.

I would basically agree with that.

What exists is the particles/fields, not $\psi$.

A priori, it is not obvious that particles or fields exist. What is obvious is that things such as the Moon exist. But the Moon must be made of something more elementary, which could perhaps be particles, fields, strings, or something else. In my "Bohmian mechanics for instrumentalists" I explain why particles, as objects with well defined positions at all time, are the most natural possibility.

 

[martinbn]

Let me try to answer this step by step. (2) says that "There are no hidden variables, i.e. QM is complete". It really means that in the mathematical description of the world, we don't need anything else except ψ\psi. I guess you agree so far.

For the purpose of this conversation, yes, I agree. Strictly speaking there may be more things needed, say equations, a choice of Hilbert space representation, specific operators, boundary conditions etc.

Now consider ontology. It's a philosophical term, but its meaning is not essential here. All what we explicitly need to prove what I want to prove is the following assumption:
Assumption 1: Ontology can be described mathematically and we need it to describe the world.
Then, from (2) and Assumption 1, it follows that there is no ontology which is not ψ\psi. Is it clear now?

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.

 

[Demystifier]

Their behavior may be described by a function (plus possible 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.

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?

Well, strictly speaking I think that you right, but that problem can be cured relatively easy. Consider a physical ontological object $\tilde{O}$ (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 $O$ (in the case above it is the numbers (x,y,z)). When we say that $O$ is ontology, it is just an imprecise manner of speak, which really means that $\tilde{O}$ is ontology.

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

Does it make more sense now?

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

 

[DarMM]

May be I don't know what the word means. But for me "ontology" is related to existence/being. So something like an element in a Hilbert space cannot have ontology. It makes no sense to say that it exists. What exists is the particles/fields, not $\psi$.

@Demystifier has already stated this, but basically in a $\psi$-ontic view $\psi$ directly describes the physical properties of the objects of the theory. In a $\psi$-epistemic view, $\psi$ instead describes an agent's knowledge of those properties.

 

[atyy]

Okay sorry for all the questions! I think this is the last one, what about the situation makes this true for you explicitly? Basically do you think that Wigner completely confining his friend to a totally sealed lab and the physical presence of Wigner's equipment causes quantum effects to propagate up to the macroscopic scale for the friend invalidating him considering himself or his equipment as classical?

I can imagine this, Wigner's device would be pretty "extreme" equipment whatever it is.

Well, the friend can model himself as classical, but then Wigner will also model the friend as classical. Basically, we only allow one observer - Wigner. Wigner can subjectively choose where to put the classical/quantum boundary (ie. whether it includes the friend or not). However, what Wigner regards as an objective measurement outcome differs depending on whether the friend is classical or quantum. If Wigner regards the friend as classical, then the quantum state used by Wigner will collapse when the friend makes the measurement (ie. the friend's measuring apparatus is also Wigner's measuring apparatus).