Another Wigner's Friend Thought Experiment Paper

[StevieTNZ]

https://phys.org/news/2021-05-future.html

The paper in question can be found at https://www.nature.com/articles/s42005-021-00589-1

This thread is for discussion of the said paper.

 

[Demystifier]

It's always nice to see how different interpretations are consistent with the no-go theorems, which the official paper offers. However, the discussion of the Bohmian interpretation is rather superficial and even the cited references (the original two Bohm's papers) are not very useful. For that purpose I recommend the book F. Laloe, Do We Really Undertstand QM, Appendix I.

 

[Kolmo]

These theorems are interesting as a means of exploring how trustworthy or certifiable mesoscopic systems are in quantum circuits, but it's very overblown as something applying to real lab measurements.

 

[msumm21]

Not just in here, but even in the original Wigner's friend experiment, why does Wigner treat the lab as if it's in a pure state? Don't the rules of QM say Wigner should treat the lab as a mixed state? The collapse rule doesn't require "everyone" to measure, only that there be a measurement, right?

 

[PeterDonis]

why does Wigner treat the lab as if it's in a pure state?

Because he has not yet interacted with it, so as far as he is concerned, it is a closed, isolated system, which can be described with a pure state.

Don't the rules of QM say Wigner should treat the lab as a mixed state?

No. They say that the friend should model the part of the lab he has interacted with using a mixed state. But they don't say that about Wigner when he has not yet interacted with the friend or anything else in the lab.

The collapse rule doesn't require "everyone" to measure, only that there be a measurement, right?

The collapse rule does not say anything about mixed states. It says that, once you know the result of a particular measurement, you assign a different pure state to the system that just got measured--the pure state that is the eigenstate of the measured observable corresponding to the observed measurement result.

Mixed states are used when you have an entangled system and you only want to model one particular subsystem of it. But Wigner, before he interacts with the friend or anything else in the lab, is not entangled with them.

 

[msumm21]

Because he has not yet interacted with it, so as far as he is concerned, it is a closed, isolated system, which can be described with a pure state.

No. They say that the friend should model the part of the lab he has interacted with using a mixed state. But they don't say that about Wigner when he has not yet interacted with the friend or anything else in the lab

Can you reconcile this with rule 7 here on this website, which says "each measurement with measuring value $a$ can be regarded as preparation of a new state ... $|a>$?"

Sounds like you're saying Wigner shouldn't apply this rule to the friends measurement.

 

[PeterDonis]

Can you reconcile this with rule 7 here on this website, which says "each measurement with measuring value $a$ can be regarded as preparation of a new state ... $|a>$?"

Sounds like you're saying Wigner shouldn't apply this rule to the friends measurement.

In the "Wigner's friend" scenario, yes, that is exactly what is claimed. It is claimed that what the friend calls a "measurement" is not actually a measurement; only Wigner's interaction with the lab, when it happens, constitutes an actual measurement.

Taking this viewpoint requires that we view everything that happens in the lab involving the friend, before Wigner interacts with it, as reversible--including the friend's actual conscious experience of the measurement result, his memory of it, etc. From Wigner's point of view, the friend and everything else in the lab is just one big quantum system evolving purely by unitary evolution, and what the friend calls a "measurement" is just a unitary interaction that entangles different degrees of freedom inside the friend+lab. In other words, it's no different from putting qubits through various unitary operations in an ordinary quantum experiment. Nobody thinks a measurement occurs when two qubits get entangled inside a lab; the Wigner's friend viewpoint requires that we treat the friend getting entangled with other stuff inside the lab the same way.

Of course, many people do not think this viewpoint is actually valid. (I'm not a fan of it myself.) But there is nothing in the rules of QM as we currently have them that contradicts it. The rules do not tell you what particular things must be considered as measurements. They only tell you what to do if you have decided that some particular thing is a measurement.

 

[StevieTNZ]

Can you reconcile this with rule 7 here on this website, which says "each measurement with measuring value $a$ can be regarded as preparation of a new state ... $|a>$?"

Sounds like you're saying Wigner shouldn't apply this rule to the friends measurement.

Well of course, because no one definite outcome arises from a "measurement" (whatever that may constitute) due to the unitary of the fundamental Schrodinger equation.

 

[PeterDonis]

Of course, many people do not think this viewpoint is actually valid.

Btw, one reason for this is that it seems to contradict the friend's viewpoint--the friend thinks that what he does inside the lab is a measurement, not just an ordinary unitary interaction like two qubits getting entangled. The friend does apply rule 7 once he learns the result of his measurement.

The only way to make this consistent with Wigner taking the viewpoint that, until he interacts with the friend+lab, everything that happens to the friend+lab is just ordinary unitary interactions, is to claim that, even when we humans think a measurement has happened, and irreversible traces have been left (marks on a paper, particular values stored in a computer's memory, particular memories of the person who did the measurement about what the result was), that's not really the case: all that stuff could be reversed by some other entity that could perform the required unitary operations to reverse it. But if we take this viewpoint, then it should also apply to Wigner himself: Wigner has no more basis for claiming that his interaction with the friend+lab is an actual irreversible measurement, than the friend has for claiming that his interaction with the lab is an actual irreversible measurement.

In other words, this viewpoint undermines itself, at least in the opinion of those who make criticisms like the above.

 

[PeterDonis]

Well of course, because no one definite outcome arises from a "measurement" (whatever that may constitute) due to the unitary of the fundamental Schrodinger equation.

This is a different issue from the one I discussed in posts #7 and #9. If we assume that unitary evolution is always true, we get an interpretation like the MWI. But even in the MWI, there is still the question of how we know when a "measurement" has occurred and "worlds" have actually split. The standard answer to this is that "worlds" split in the MWI when decoherence occurs. But in a situation like the friend's situation in the lab in the Wigner's friend scenario, decoherence does occur. So even according to the MWI in its standard form, "worlds" split when the friend makes his measurement in the lab; they don't "wait" to split until Wigner interacts with the friend+lab. So even according to the MWI, unless we take a viewpoint like the one I describe in posts #7 and #9, Wigner cannot treat the friend's interaction with the lab as not being a measurement.

In other words, taking a viewpoint like the one I describe in posts #7 and #9 requires that we claim that decoherence can be reversed by applying the appropriate unitary operations. And if decoherence can be reversed, our justification for viewing anything as an actual "measurement" with actual results--even if all possible results "happen" as occurs in the MWI--goes out the window.

 

[msumm21]

Well of course, because no one definite outcome arises from a "measurement" (whatever that may constitute) due to the unitary of the fundamental Schrodinger equation.

Are you saying the friend's measurement has no outcome? If so, why?

 

[StevieTNZ]

Are you saying the friend's measurement has no outcome? If so, why?

Yes. And quantum mechanics formulation (pure state evolves into pure state and so it continues).

 

[PeterDonis]

Yes. And quantum mechanics formulation (pure state evolves into pure state and so it continues).

You can take this viewpoint, but, as I explained in post #10, you have to then apply it to Wigner himself to be consistent. Which would mean that no measurements anywhere ever have outcomes.

 

[StevieTNZ]

You can take this viewpoint, but, as I explained in post #10, you have to then apply it to Wigner himself to be consistent. Which would mean that no measurements anywhere ever have outcomes.

Except when one becomes conscious of it, in my opinion. See for example the idea in "The Mind Matters" and "Conscious Mind in the Physical World", one universal consciousness we all share.

 

[PeterDonis]

Except when one becomes conscious of it, in my opinion.

Which is fine as a matter of opinion. But, as the rules for this forum will tell you, you cannot state your opinion as fact, or claim that every interpretation of QM must share it. You have to be very, very clear that it's just your opinion and keep it separate from things that aren't your opinion.

See for example the idea in "The Mind Matters" and "Conscious Mind in the Physical World", one universal consciousness we all share.

I am not familiar with either of these references, but I seriously doubt that they are valid references for PF discussion.

 

[msumm21]

Except when one becomes conscious of it, in my opinion. See for example the idea in "The Mind Matters" and "Conscious Mind in the Physical World", one universal consciousness we all share.

I may not be clear, from this quote I think you would conclude that the friend's measurement collapsed the wave function. What are you saying happens when the friend becomes conscious of his measurement result?

I know we may be discussing an opinion or nonstandard interpretation here, can we continue if we're clear that's what we're doing?

 

[StevieTNZ]

I may not be clear, from this quote I think you would conclude that the friend's measurement collapsed the wave function. What are you saying happens when the friend becomes conscious of his measurement result?

I know we may be discussing an opinion or nonstandard interpretation here, can we continue if we're clear that's what we're doing?

In my view, the system continues to evolve as a superposition - the friend is merely aware of one part of the wave function (the definite result). See for example the observable such as that used by GianCarlo Ghirardi in "Sneaking a Look at God's Cards" pages 373-376.

 

[PeterDonis]

In my view, the system continues to evolve as a superposition - the friend is merely aware of one part of the wave function (the definite result).

If you are using just unitary evolution, there is not one result of the friend's measurement. All of the possible results occur (since "just unitary evolution" means you are using the MWI). Each result occurs in its own branch of the wave function, and in that branch, the friend is aware of that result having happened.

If you are using some other interpretation, it would be helpful to have a reference to an actual peer-reviewed paper that describes it. The Ghirardi book you describe is not such a reference.

 

[PeterDonis]

we may be discussing an opinion or nonstandard interpretation here, can we continue if we're clear that's what we're doing?

Opinions and nonstandard interpretations are generally out of bounds for discussion (one can state one's opinion, but discussion of opinions, since it just amounts to an argument that cannot have any resolution, is not allowed). Interpretations, to be discussed, should be backed up with references if they are not common ones or if there is any ambiguity about what they claim.

 

[msumm21]

So in a more standard interpretation, would we agree the friend's measurement collapses the wave function? If so, it seems these Wigner's friend proofs are flawed (in standard QM)? Aren't they dependent on violating rule 7 to make their proof work?

 

[Demystifier]

So in a more standard interpretation, would we agree the friend's measurement collapses the wave function? If so, it seems these Wigner's friend proofs are flawed (in standard QM)? Aren't they dependent on violating rule 7 to make their proof work?

The idea behind the proof is that the collapse depends on the observer. Namely, the friend's measurement collapses the wave function from the point of view of the friend, but not from the point of view of Wigner. (It's not that I endorse this idea, but that's what the idea is.)

 

[msumm21]

The idea behind the proof is that the collapse depends on the observer. Namely, the friend's measurement collapses the wave function from the point of view of the friend, but not from the point of view of Wigner. (It's not that I endorse this idea, but that's what the idea is.)

But this idea that the friend's measurement doesn't collapse the wave function (according to Wigner) is not "standard QM", right? Referring to rule 7 (and presumably QM textbooks?), I see nothing that requires "me" to do the measurement.

 

[Morbert]

But this idea that the friend's measurement doesn't collapse the wave function (according to Wigner) is not "standard QM", right? Referring to rule 7 (and presumably QM textbooks?), I see nothing that requires "me" to do the measurement.

Standard QM would ask you to first specify an observer or equivalently, a set of mutually exclusive alternative possibilities for which probabilities will be computed. Wigner's friend collapses a wavefunction only if it pertains to the set of mutually exclusive alternative possibilities for their observation.

 

[Demystifier]

But this idea that the friend's measurement doesn't collapse the wave function (according to Wigner) is not "standard QM", right? Referring to rule 7 (and presumably QM textbooks?), I see nothing that requires "me" to do the measurement.

Right, it's not standard. But it's negation is also not standard. The standard QM is pretty much agnostic on whether the collapse depends on observer or not.

 

[EPR]

Not knowing whose knowledge(Wigner's or his friend's) collapsed the wavefunction is generally known as the 'Wigner’s friend paradox'.

 

[PeterDonis]

Referring to rule 7 (and presumably QM textbooks?), I see nothing that requires "me" to do the measurement.

I already addressed this; see the last paragraph of post #7.

 

[StevieTNZ]

If you are using some other interpretation, it would be helpful to have a reference to an actual peer-reviewed paper that describes it. The Ghirardi book you describe is not such a reference.

Why the need for restriction to peer reviewed papers? What about books? Are they suddendly off limits?

 

[PeterDonis]

What about books?

Feel free to give a reference to a textbook if you can find one.

Are they suddendly off limits?

Please do not interpret statements overly literally. It should be obvious common sense that I am referring to the general PF rules for valid references. Those rules allow textbooks.

 

[EclogiteFacies]

Are these experiments actually possible with people at the moment though? I always find with these experiments the use of the word Friend is disingenuous.
You guys are not in a superposition, you're composed of billions of uncorrelated degrees of freedom and are constantly interacting with your environment and decohering. Quantum effects wash away.

I feel like it should always be prefaced by saying the lab Wigner's Friend is in represents a completely isolated quantum system separated from any interaction with the environment.

I think these sorts of things when discussed with humans make physicists seem like solipsists or something.