A Realization of a Basic Wigner's Friend Type Experiment

[charters]

Sorry that wasn't to disagree, just as a point of interest.

No worries. By the way, did you ever happen to check out the new Bub paper on Wigner' friend I replied with here a few days/maybe a week ago?

 

[DarMM]

The Copenhagen interpretation is useful because it's simple. But that doesn't mean it's correct. And it should be expected that its usefulness will degrade whenever the precise details of wavefunction collapse (whether effective or real) are important for the behavior of a given system, as in quantum computing.

Copenhagen has collapse as being epistemic, thus there are no details in this sense either effective or real.

So what distinguishes observer A from observer B that permits observer B to avoid collapse where observer A collapses? What physical property is being used to separate them?

Observer B does not avoid collapse. When they apply the theory they must use collapse. When observer A models observer B with QM the statistics they have prior to the measurement of macroscopic observables on observer B are consistent with B having an outcome. However they must use collapse when they finally observe B.

 

[DarMM]

No worries. By the way, did you ever happen to check out the new Bub paper on Wigner' friend I replied with here a few days/maybe a week ago?

Yes I've read it. Any particular aspect you are interested in. To me there are three points of discussion in it.

The consistency of NeoCopenhagen views with Frauchiger-Renner
That MWI violates both the S and C conditions of Frauchiger-Renner
That MWI is empirically wrong

That's also the order of how contentious others may find them coincidentally.

 

[charters]

Yes I've read it. Any particular aspect you are interested in.

Yes, on page 10 he says:

"Since the two Wigners measure commuting observables on separate systems, W[bar] can communicate the outcome‘ok’of her measurement to W, and her prediction that she is certain, given the outcome‘ok,’ that W will obtain ‘fail,’ without ‘collapsing’ the global entangled state. Then in a round in which W obtains the outcome ‘ok’ for his measurement and so is certain that the outcome is ‘ok,’ he is also certain that the outcome of his measurement is not ‘ok.’"

I think this is just wrong. He is letting W-bar know both her own |ok> and F-bar's |tails> at the same time, which is just a complementarity violation. Same error when he just changes the subscript from F-bar to W-bar in the subscripts in (16) to (17). If you keep W-bar and F-bar distinct in the correct way, the whole argument against MWI/representational QT falls apart (and actually instead shows the flaw in his own informational view)

 

[DarMM]

(and actually instead shows the flaw in his own informational view)

I'll have to think about the rest of your post, but regarding this part I think at best it shows Bub's defense of informational views is not correct. Richard Healey has already shown that Informational views can escape Frauchiger-Renner by rejecting intervention insensitivity.

 

[PeterDonis]

quantum dynamics correctly predicts what happens at all scales, without modification.

Only if you include collapse--call it "effective collapse" or whatever you want to allow room for interpretations like the MWI--whenever a measurement occurs. But QM does not tell you when a measurement occurs. QM does not tell you when to include collapse. It just says "do it when it makes sense to do it". That's not a dynamical prediction. It's an admission that there is no dynamical prediction, so the physicist just has to make the best of it.

Decoherence does not help here because decoherence is a continuous process, not an instantaneous change. Decoherence does not tell you "when decoherence has reached point X, include a collapse". It still says "include collapse when it makes sense to include it".

 

[PeterDonis]

a theory which assumes only evolution via $i\hbar{d \over dt}|\Psi(t)\rangle = H|\Psi(t_0)\rangle$ where $H$ is the appropriate Hamiltonian correctly predicts all behavior at both small and large scales

I strongly disagree, for the reasons I gave in my last post.

 

[charters]

I'll have to think about the rest of your post, but regarding this part I think at best it shows Bub's defense of informational views is not correct. Richard Healey has already shown that Informational views can escape Frauchiger-Renner by rejecting intervention insensitivity.

Which Healey paper is this in?

 

[DarMM]

Which Healey paper is this in?

https://arxiv.org/abs/1807.00421
Section 3

 

[DarMM]

Decoherence does not help here because decoherence is a continuous process, not an instantaneous change. Decoherence does not tell you "when decoherence has reached point X, include a collapse". It still says "include collapse when it makes sense to include it".

Precisely, in my language above it causes no inconsistencies if observer A assumes observer B included collapse at any point after B's device has decohered in A's application of quantum theory.

After decoherence A's dynamical model of B is not in contradiction with any presumed application by B of collapse, but nothing says when to apply collapse.

 

[charters]

https://arxiv.org/abs/1807.00421
Section 3

Thanks. I think Healey is correct that F&R don't show a contradiction among the 2 superobservers. But really this was never the issue for informational interpretations, and the whole move of going to 4 players is an unhelpful detour. The issue for informational interpretations is present with just Wigner and Friend, and it is that the superobserver and internal observer disagree about whether the internal (human) observer is a quantum mechanical subsystem or an observer external to quantum theory.

A new paper that explains this very nicely on pg 6-7: http://philsci-archive.pitt.edu/16238/

 

[DarMM]

Thanks. I think Healey is correct that F&R don't show a contradiction among the 2 superobservers. But really this was never the issue for informational interpretations, and the whole move of going to 4 players is an unhelpful detour. The issue for informational interpretations is present with just Wigner and Friend, and it is that the superobserver and internal observer disagree about whether the internal (human) observer is a quantum mechanical subsystem or an observer external to quantum theory.

A new paper that explains this very nicely on pg 6-7: http://philsci-archive.pitt.edu/16238/

I've read her paper before and don't agree with the analysis. The response is covered in many papers and the anaylsis is very similar to one by Deutsch as improved by Brukner's theorem that is the originator of this thread.

Wigner is measuring very different classes of observables to the friend. Thus $|0\rangle_S$ does not allow an inference of $|0\rangle_L$ with $S$ the system the observer is looking at and $L$ their lab.

This inference doesn't even hold in Spekkens toy model or epistemically restricted classical theories, so performing it in QM seems pointless to me.

 

[charters]

This inference

What is your |0>L above? When Wigner simply opens the box to ask Friend his result of the S measurement?

 

[DarMM]

What is your |0>L above? When Wigner simply opens the box to ask Friend his result of the S measurement?

The state of the friend's lab.

 

[charters]

The state of the friend's lab.

I don't understand what the validity of this inference has to do with the issue of the parties disagreeing about whether Friend is a quantum system or an external observer.

 

[DarMM]

I don't understand what the validity of this inference has to do with the issue of the parties disagreeing about whether Friend is a quantum system or an external observer.

Without the inference you don't have the disagreements in probabilities that form the core of the refutations in the rest of the article.

 

[charters]

Without the inference you don't have the disagreements in probabilities that form the core of the refutations in the rest of the article.

But only by conceding Wigner and Friend's state spaces are inequivalent, which is the underlying criticism anyway.

 

[DarMM]

But only by conceding Wigner and Friend's state spaces are inequivalent, which is the underlying criticism anyway.

I don't understand, could you explain?

 

[charters]

I don't understand, could you explain?

For Wigner |x>Lab = |x>S⊗|I see x>Friend. The inference you reject clearly holds, at least for Wigner. If he opens the box on Friend's measurement basis, he can simply go directly inspect the qubit for himself, before or after Friend reports anything verbally, and he will see what Friend saw.

For the inference to not hold, |0>S must be defined in a separate state space from the above (one where F is not a tensor factor but the external measurer).

 

[DarMM]

For Wigner |x>Lab = |x>S⊗|I see x>Friend. The inference you reject clearly holds, at least for Wigner. If he opens the box on Friend's measurement basis, he can simply go directly inspect the qubit for himself, before or after Friend reports anything verbally, and he will see what Friend saw.

Yes of course after he measures the entire state of the lab he might obtain $|0\rangle_L$ but the friend measuring and observing $|0\rangle_S$ does not imply that Wigner will with probability 1 observe $|0\rangle_L$. That's the invalid part preventing the contradictory probabilities.

The argument in the paper doesn't even hold in classical epistemically restricted theories, so why it "must" hold in QM I do not understand.

This is the reason FR and Brukner took the approach of wrapping CHSH and Hardy's paradox in Wigner's friend, because basic Wigner's friend holds no problem for these views. See Chapter 11 of Richard Healey's The Quantum Revolution in Philosophy for a full account.

 

[charters]

Yes of course after he measures the entire state of the lab he might obtain |0⟩L|0⟩L|0\rangle_L but the friend measuring and observing |0⟩S|0⟩S|0\rangle_S does not imply that Wigner will with probability 1 observe |0⟩L|0⟩L|0\rangle_L. That's the invalid part preventing the contradictory probabilities

So you are claiming if Friend gets |0>S, there is still non zero probability Wigners gets |1>L, wherein Friend will tell Wigner "I saw 1"?

Brukner & Baumamn I think agree with my view, insofar as they say Friend can't reason correctly without agreeing to use Wigner's state space: https://arxiv.org/abs/1901.11274.

 

[DarMM]

So you are claiming if Friend gets |0>S, there is still non zero probability Wigners gets |1>L, wherein Friend will tell Wigner "I saw 1"?

Brukner & Baumamn I think agree with my view, insofar as they say Friend can't reason correctly without agreeing to use Wigner's state space: https://arxiv.org/abs/1901.11274.

Yes indeed but all of this can occur in a classical epistemically restricted model with local variables, so there's nothing shocking or unacceptable about it to me.

 

[charters]

Yes indeed but all of this can occur in a classical epistemically restricted model with local variables, so there's nothing shocking or unacceptable about it to me.

It entails the conclusion that Friend who lives in Friend-world is not the same person as Friend who lives in Wigner-world. These two versions of Friend disagree about the outcome of the experiment.

 

[DarMM]

It entails the conclusion that Friend who lives in Friend-world is not the same person as Friend who lives in Wigner-world. These two versions of Friend disagree about the outcome of the experiment.

No it doesn't as the exact same mathematical structures and relations hold in Spekkens toy model without this conclusion.

 

[charters]

No it doesn't as the exact same mathematical structures and relations hold in Spekkens toy model without this conclusion.

Can you explain this or give a source then? All I see is you agreeing to the proposition that Friend can both measure 0 and still tell Wigner he saw 1. I can't understand how you could possibly explain this without having two totally disjoint versions of Friend, in a way more extreme than MWI.

 

[DarMM]

Can you explain this or give a source then? All I see is you agreeing to the proposition that Friend can both measure 0 and still tell Wigner he saw 1. I can't understand how you could possibly explain this without having two totally disjoint versions of Friend, in a way more extreme than MWI.

An explanation within Spekkens model is here:
https://www.physicsforums.com/threa...ncomplete-comments.966033/page-3#post-6152735

 

[charters]

An explanation within Spekkens model is here:
https://www.physicsforums.com/threa...ncomplete-comments.966033/page-3#post-6152735

I don't see how this is relevant. In that post, you say a state |00> is "compatible with the above use of a superposition by the superobserver" |000> + |111>. This was never in issue. The claim you agreed to here is that |00> is further compatible with |111> alone, or that is at least how I read #127.

 

[DarMM]

I don't see how this is relevant. In that post, you say a state |00> is "compatible with the above use of a superposition by the superobserver" |000> + |111>. This was never in issue. The claim you agreed to here is that |00> is further compatible with |111> alone, or that is at least how I read #127.

If you look at my post and know how measurements work in Spekkens model that is also true. If you don't know the details of measurements in Spekkens model I can go through the details here.

 

[charters]

If you look at my post and know how measurements work in Spekkens model that is also true. If you don't know the details of measurements in Spekkens model I can go through the details here.

I reread your post and the measurement subsection the wikipedia entry for Spekkens model, and I can't see how it possibly allows F to get |00> and W to get |111> (absent the solipsist/single user premise Felline discusses). So I'd appreciate the details.

 

[DarMM]

I reread your post and the measurement subsection the wikipedia entry for Spekkens model, and I can't see how it possibly allows F to get |00> and W to get |111> (absent the solipsist/single user premise Felline discusses). So I'd appreciate the details.

No worries I'll try to put it up later today.

 

[DarMM]

I drop normalization here. I think I got confused at one point, so I will start again.

So first do you agree there is no problem with Wigner's measurements in the $\{|000\rangle,|111\rangle\}$ basis and the friend obtaining outcomes like $|00\rangle$?

This is no more a problem than you would have with two states in statistical mechanics being used at two different levels. The statistics match, both obtaining each outcome 50% of the time. One need only reason that each time the friend gets $|00\rangle$ Wigner will get $|000\rangle$ and similarly for $|11\rangle$. Wigners "uncollapsed" probability reflects only his ignorance of what has occured.

It is in fact measurements along other bases like $\{|000\rangle + |111\rangle, |000\rangle - |111\rangle\}$ demonstrating inteference that are the problem.

 

[charters]

One need only reason that each time the friend gets |00⟩|00⟩|00\rangle Wigner will get |000⟩|000⟩|000\rangle and similarly for |11⟩|11⟩|11\rangle. Wigners "uncollapsed" probability reflects only his ignorance of what has occured.

Yes I agree with this of course, but this is not what you were suggesting before. You were saying in a run when F gets |00>, W can get |111> and furthermore that this does not imply a single user/solipsism interpretation. This was what you suggested to undermine Felline's argument. So, to be clear, 1) are you no longer making this claim and 2) if not, where is your disagreement with Felline?

I will also mention I spent some time with Spekkens' original paper yesterday, and I would caution against drawing any conclusions about a QBist or Bub type view based on what works in Spekkens toy model. The latter handles Wigner's Friend in the same manner as a hidden variables approach, and it is still an ontological model at heart. The defective interpretations are these fully non-ontological, informational ones.

 

[DarMM]

Yes I agree with this of course, but this is not what you were suggesting before. You were saying in a run when F gets |00>, W can get |111> and furthermore that this does not imply a single user/solipsism interpretation. This was what you suggested to undermine Felline's argument. So, to be clear, 1) are you no longer making this claim and 2) if not, where is your disagreement with Felline?

As I said I got confused about what we were discussing. I'm aware of how Spekkens model differs.

So as that we can focus the discussion and so that I am not discussing the wrong thing, what to you is the point Felline makes that refutes these views?

 

[charters]

As I said I got confused about what we were discussing. I'm aware of how Spekkens model differs.

So as that we can focus the discussion and so that I am not discussing the wrong thing, what to you is the point Felline makes that refutes these views?

These views being Bub's type of view?

 

[DarMM]

These views being Bub's type of view?

Primarily, although if you wish to include QBism that is fine as well.