The Quantum Mystery of Wigner's Friend - Comments

In summary, the conversation discusses the implications of the second assumption of FR, which states that there is only one outcome for a quantum measurement. This assumption is questioned and it is argued that it excludes the possibility of entangled states. The conversation also touches on the idea of Schrödinger's cat existing in superposition and the concept of de-coherence. The participants also mention other thought experiments and interpretations of quantum mechanics. The paper linked in the signature argues that the implicit assumption that relativistic QFT is fundamental is what leads to these issues and that adopting Bohmian mechanics can provide a solution. Another paper is referenced as a response to FR using BM, but it is noted that the analysis in that paper does not necessarily require adopting
  • #1
RUTA
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Greg Bernhardt submitted a new blog post

Wigner's Friend
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Continue reading the Original Blog Post.
 

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  • #3
"The second assumption of FR is that there is only one outcome for a quantum measurement, so Xena doesn’t measure both heads and tails and send both versions of state s."

Which sounds obviously false to me.
We assume that Schrödinger's cat can indeed exist in superposition of states.
IOW: the cat indeed observes (nee "measures") both possible states of the radioactive atom. And then cat "sends" both signals (in the form of being in superposition itself) to the observing scientist.

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  • #4
nikkkom said:
"The second assumption of FR is that there is only one outcome for a quantum measurement, so Xena doesn’t measure both heads and tails and send both versions of state s."

Which sounds obviously false to me.
We assume that Schrödinger's cat can indeed exist in superposition of states.
IOW: the cat indeed observes (nee "measures") both possible states of the radioactive atom. And then cat "sends" both signals (in the form of being in superposition itself) to the observing scientist.

View attachment 234106
You have grasped the problem exactly -- by saying Xena only measures one outcome in one world, we've excluded the possibility that a future measurement by Zeus in an eigenbasis differing from that for ##\hat{x}## can in any way bear on Wigner's ##\hat{y}## measurement outcome, which we know doesn't work for entangled states. Look at the simple example of polarized light. If the light is polarized in x and I make a measurement in y (intensity would correspond to probability here), I get zero. But, if someone else measures in x + y before I measure in y, then when I do a y measurement I get a non-zero result. Eq. 4 in my Insight (Healey's Eq (13)) is fine as long as Zeus and Wigner only measure ##\hat{x}## and ##\hat{y}##, respectively. That's consistent with classical probability theory. In Hardy's axiomatization of QM, there is only one difference between classical and quantum probability theory -- in quantum theory every Hilbert space basis rotated from the original measurement eigenbasis represents another measurement. That's certainly not true for discrete classical physics (e.g., coin flips). So, if we want classical probability theory modeled via quantum probability theory we're going to have to accept something unusual, e.g., rewriting history or accepting that Wigner's measurement of Yvonne's measurement outcome differs from Yvonne's recorded measurement outcome. Even my solution requires we give up our cherished ant's-eye explanation, which required an entire book to defend. Something we believe to be true is going to have to be sacrificed in whatever new model of objective reality we finally embrace.
 
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  • #5
I'm under the impression that Wigner's Friend is a rip-off of Everett's argument in his PhD thesis for a universal wave function.
 
  • #6
nikkkom said:
We assume that Schrödinger's cat can indeed exist in superposition of statesView attachment 234106

Such as assumption is false. Because it is entangled with the radioactive source it is in a mixed state - not a superposition. It is the essence of de-coherence - which I must emphasize does not solve the measurement problem just shifts the emphasis a bit. There have been many posts explaining this. I have explained it a different way as well - but will just limit my objection to the simple calculation that shows its in a mixed state - not a superposition.

Ruta's reply and reference to Hardy is excellent though IMHO - but not really dependent on being in a superposition.

Thanks
Bill
 
  • #7
John McAndrew said:
I'm under the impression that Wigner's Friend is a rip-off of Everett's argument in his PhD thesis for a universal wave function.
From what I’ve read (I’m no historian) FR is a variation of many other such thought experiments, e.g., Everett and Wigner.

There is an interesting Quanta article on this https://www.quantamagazine.org/frau...where-our-views-of-reality-go-wrong-20181203/

I was surprised that some people believe there will prove to be a limit on the applicability of QM.

Here is Leifer’s closing statement in that article: “It’s likely that we are making some implicit assumption about the way the world has to be that just isn’t true. Once we change that, once we modify that assumption, everything would suddenly fall into place. That’s kind of the hope. Anybody who is skeptical of all interpretations of quantum mechanics must be thinking something like this. Can I tell you what’s a plausible candidate for such an assumption? Well, if I could, I would just be working on that theory.”
 
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  • #8
RUTA said:
Here is Leifer’s closing statement in that article: “It’s likely that we are making some implicit assumption about the way the world has to be that just isn’t true. Once we change that, once we modify that assumption, everything would suddenly fall into place. That’s kind of the hope. Anybody who is skeptical of all interpretations of quantum mechanics must be thinking something like this. Can I tell you what’s a plausible candidate for such an assumption? Well, if I could, I would just be working on that theory.”
In the paper linked in my signature I argue that this implicit assumption (that just isn't true) is that relativistic QFT is fundamental. Once we change that (which is rather natural from condensed-matter point of view), everything in Bohmian mechanics suddenly falls into place.
 
  • #9
Demystifier said:
In the paper linked in my signature I argue that this implicit assumption (that just isn't true) is that relativistic QFT is fundamental. Once we change that (which is rather natural from condensed-matter point of view), everything in Bohmian mechanics suddenly falls into place.

You might like to read this response to FR according to BM: https://dustinlazarovici.com/wp-content/uploads/comment_renner_new.pdf
 
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  • #12
PeterDonis said:
Even though this response is billed as "according to BM", as you say, the key analysis, in particular in section 3, does not appear to me to require adopting BM as an interpretation; it's based solely on observations about quantum states.

I’m not an expert in BM, so I believed them when they said keeping the “empty” components of the wavefunction was necessary in BM and those components are the key for getting the OK-OK piece at the end.
 
  • #13
RUTA said:
I believed them when they said keeping the “empty” components of the wavefunction was necessary in BM and those components are the key for getting the OK-OK piece at the end.

Calling the components "empty" is specific to BM, yes--it means the actual particle trajectories don't lie in those components. But including those components in the wave function up until the final measurement is not specific to BM; that's just standard QM. Basically the point the paper is making is that predicting a nonzero probability for the {ok, ok} result requires interference between two macroscopically different states of the "inside the lab" observers; but the reasoning that leads to the "paradox" that the {ok, ok} result should be impossible given the observations of the "inside the lab" observers is based on the implicit assumption that no such interference takes place. That point is independent of any interpretation of QM, as far as I can see.
 
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  • #14
PeterDonis said:
Calling the components "empty" is specific to BM, yes--it means the actual particle trajectories don't lie in those components. But including those components in the wave function up until the final measurement is not specific to BM; that's just standard QM. Basically the point the paper is making is that predicting a nonzero probability for the {ok, ok} result requires interference between two macroscopically different states of the "inside the lab" observers; but the reasoning that leads to the "paradox" that the {ok, ok} result should be impossible given the observations of the "inside the lab" observers is based on the implicit assumption that no such interference takes place. That point is independent of any interpretation of QM, as far as I can see.

I agree, that equation with the OK-OK piece is not unique to BM. Did you see this paper https://arxiv.org/abs/1710.07212 ? They show that the apparent contradiction in FR is due to conflating inequivalent QM formalisms: standard QM and the relative-state formalism.
 
  • #15
RUTA said:
I agree, that equation with the OK-OK piece is not unique to BM. Did you see this paper https://arxiv.org/abs/1710.07212 ? They show that the apparent contradiction in FR is due to conflating inequivalent QM formalisms: standard QM and the relative-state formalism.
My reading of that and their follow up paper:
https://arxiv.org/abs/1611.01111

Is that QM has three formalisms, Objective collpase, No-collapse (relative state) and subjective collapse. The FR scenario is a situation where the first two give different predictions and the third is inconsistent, unless you restrict QM to being a single user theory.

EDIT: I said follow up, but the date is earlier. Did I make a mistake or was it a retrocausal influence?
 
  • #16
RUTA said:
Did you see this paper https://arxiv.org/abs/1710.07212 ?

DarMM said:
My reading of that and their follow up paper:
https://arxiv.org/abs/1611.01111

Is that QM has three formalisms, Objective collpase, No-collapse (relative state) and subjective collapse.

I hadn't seen either of these papers, I'll take a look. On their face they seem to be claiming that "QM" is not in fact a single theory (i.e., a single mathematical system for making predictions) but at least two (perhaps three if @DarMM is correct). That claims seems like it could stand a lot of examination.
 
  • #17
DarMM said:
My reading of that and their follow up paper:
https://arxiv.org/abs/1611.01111

Is that QM has three formalisms, Objective collpase, No-collapse (relative state) and subjective collapse. The FR scenario is a situation where the first two give different predictions and the third is inconsistent, unless you restrict QM to being a single user theory.

As they say, the difference between interpretations becomes apparent in Wigner’s-friend-type experiments. The reason is that you have assumed the availability of classical information for screened off entites during interference and classical information constitutes “scientific fact.” So, what’s going on “behind the curtain” to account for the QM outcomes is no longer “behind the curtain.”
 
  • #18
PeterDonis said:
perhaps three if @DarMM is correct
Look for the terms "objective collapse", "no-collapse" and "subjective-collapse" in their papers. I find the 2016 paper I linked a bit clearer.
 
  • #19
DarMM said:
Look for the terms "objective collapse", "no-collapse" and "subjective-collapse" in their papers. I find the 2016 paper I linked a bit clearer.

Here is a paragraph in the Conclusion of their 2016 paper:

The Wigner’s-friend experiment can (in principle) discriminate between two competing quantum formalisms describing a measurement — the unitary relative-state formalism and the non-unitary measurement update rule. A specific combination of these two formalisms, together with the assumption regarding possible communication, gives a contradiction. We do, however, not regard a formalism to necessarily imply a particular interpretation like “many worlds” or “collapse.” We believe that the contradiction above does, therefore, not disqualify a particular interpretation of quantum mechanics.
 
  • #20
RUTA said:
Here is a paragraph in the Conclusion of their 2016 paper:
They do consider a third formalism, subjective collapse, as a combination of the first two (though maybe formalism is the wrong word for it).

However the actual way out of the contradiction is rejecting being a multi-user theory as QBism does, then you can use subjective collapse.
 
  • #21
PeterDonis said:
Even though this response is billed as "according to BM", as you say, the key analysis, in particular in section 3, does not appear to me to require adopting BM as an interpretation; it's based solely on observations about quantum states.
I agree. Perhaps the crucial insight in this paper is contained in the following quote:
"The contradiction derived by Frauchiger and Renner simply arises from the fact that one experimentalist, F̄ , uses the wrong quantum state to make predictions about the outcome of a later measurement. Her wrong assumptions may be (subjectively) justified if she doesn’t know about the very special procedure that W̄ carries out on her laboratory. But this – to put it bluntly – is her problem, not a problem with Bohmian mechanics or quantum theory in general."
 
  • #22
Demystifier said:
I agree. Perhaps the crucial insight in this paper is contained in the following quote:
"The contradiction derived by Frauchiger and Renner simply arises from the fact that one experimentalist, F̄ , uses the wrong quantum state to make predictions about the outcome of a later measurement. Her wrong assumptions may be (subjectively) justified if she doesn’t know about the very special procedure that W̄ carries out on her laboratory. But this – to put it bluntly – is her problem, not a problem with Bohmian mechanics or quantum theory in general."

You guys seem to be knowledgeable about BM. Does this response to Wigner's friend in that paper follow for BM as they claim?

the macroscopic quantum measurements performed by [Zeus] and [Wigner] are so invasive that they can change the actual state of the respective laboratory, including the records and memories (brain states) of the experimentalists in it.
 
  • #23
RUTA said:
You guys seem to be knowledgeable about BM. Does this response to Wigner's friend in that paper follow for BM as they claim?
It follows from QM itself, or more precisely from any interpretation of QM (including BM) that takes Schrodinger equation seriously, as an equation valid not only for microscopic systems but also for macroscopic ones.
 
  • #24
Demystifier said:
It follows from QM itself, or more precisely from any interpretation of QM (including BM) that takes Schrodinger equation seriously, as an equation valid not only for microscopic systems but also for macroscopic ones.

I think the subjective formalism (like Healey's) would say no changes to memories or records occur. Rather, people just disagree with each other. Here is a quote from Healey:
So one could argue that whatever Wigner says about his outcome (more carefully, whatever Zeus measures Wigner’s outcome to be) is not a reliable guide to Wigner’s actual outcome. In particular, even if Zeus takes Wigner’s outcome to have been OK (because that’s what he observes it to be in a hypothetical future measurement on W) Wigner’s actual outcome might equally well have been FAIL.

Personally, this strikes me exactly as it did Renner (this excerpt from the Quanta article):
“If you want to maintain the Copenhagen type of view, it seems the best move is towards this perspectival version,” Leifer said. He points out that certain interpretations, such as quantum Bayesianism, or QBism, have already adopted the stance that measurement outcomes are subjective to an observer.

Renner thinks that giving up this assumption entirely would destroy a theory’s ability to be effective as a means for agents to know about each other’s state of knowledge; such a theory could be dismissed as solipsistic. So any theory that moves toward facts being subjective has to re-establish some means of communicating knowledge that satisfies two opposing constraints.
 
  • #25
RUTA said:
I think the subjective formalism (like Healey's) would say no changes to memories or records occur. Rather, people just disagree with each other.
Perhaps, but I would say that such approaches do not take the Schrodinger equation seriously. If the wave function describes only a subjective knowledge of an agent, then all the stuff about which the agent knows nothing (and there is certainly a lot of such stuff) is not described by the Schrodinger equation. In fact it is not described by anything, because in subjective approaches the agent cannot describe something that he does not have a knowledge about. The wave function of the Universe, which makes sense in approaches (like BM and MWI) in which the Schrodinger equation is taken seriously, does not make any sense in subjective approaches.

Is there a wave function of the Moon when the agent knows nothing about the Moon? Subjective approaches say no, other approaches say yes.
 
  • #26
Demystifier said:
Perhaps, but I would say that such approaches do not take the Schrodinger equation seriously. If the wave function describes only a subjective knowledge of an agent, then all the stuff about which the agent knows nothing (and there is certainly a lot of such stuff) is not described by the Schrodinger equation. In fact it is not described by anything, because in subjective approaches the agent cannot describe something that he does not have a knowledge about. The wave function of the Universe, which makes sense in approaches (like BM and MWI) in which the Schrodinger equation is taken seriously, does not make any sense in subjective approaches.

Is there a wave function of the Moon when the agent knows nothing about the Moon? Subjective approaches say no, other approaches say yes.

By "take the SE seriously," you mean wave function realism, right (so-called "psi-ontologists")? The wave function and therefore the SE is still very meaningful for psi-epistemologists (like me), it's just not considered part of objective reality.
 
  • #27
Demystifier said:
Perhaps, but I would say that such approaches do not take the Schrodinger equation seriously
Usually it's taken to be the way to update one's probabilities in the absence of observations.

If you take the Hilbert space and its geometry as epistemic, the Schrodinger equation is quite natural as the evolution of your probabilities as encoded in the wave-function via something like Stone's theorem. Of course the Hamiltonian isn't specified. I've often wondered if QBism considers the form of the Hamiltonian to not be purely epistemic.
 
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  • #28
RUTA said:
By "take the SE seriously," you mean wave function realism, right (so-called "psi-ontologists")?
Perhaps the realism is not the right word. In BM, for instance, the wave function is something analogous to the Hamiltonian in classical mechanics. Would you say that classical Hamiltonian is real?

RUTA said:
The wave function and therefore the SE is still very meaningful for psi-epistemologists (like me), it's just not considered part of objective reality.
But you assume that there is some objective reality, right? Is there anything more specific you can say about that reality? Do you agree that the Bell theorem implies that this reality obeys non-local laws? How do you avoid the conclusion of the PBR theorem that wave function is, in a certain sense, real?
 
  • #29
DarMM said:
I've often wondered if QBism considers the form of the Hamiltonian to not be purely epistemic.
That's a good question.
 
  • #30
Demystifier said:
But you assume that there is some objective reality, right? Is there anything more specific you can say about that reality? Do you agree that the Bell theorem implies that this reality obeys non-local laws? How do you avoid the conclusion of the PBR theorem that wave function is, in a certain sense, real?
Roughly speaking there are two types of ##\psi##-epistemic theories. Adán Cabello calls them type I and type II ##\psi##-epistemic. There is also the names ##\psi##-statistical and ##\psi##-epistemic, which I'll use because I think they're more distinctive, but just note that many use ##\psi##-epistemic to mean both.

In the former the wavefunction is statistical in the sense of classical statistical mechanics, it is in essence a probability distribution over more fundamental degrees of freedom, hidden variables. In ##\psi##-epistemic theories however QM is not the statistical mechanics of some hidden variables. Rather it is just a general theory of inference for classical observables and you can't really recover the underlying reality directly from it, this is because all properties of the wavefunction just reflect generalised inference rules or normative expectations agents should hold. They're not related to "underlying/more fundamental" degrees of freedom.

As a direct contrast ##\psi##-statistical views would say ##(\psi,\phi) \neq 0## means that the two distributions ##\psi## and ##\phi## have some ontic states in common, where as in ##\psi##-epistemic views it just means that an agent who prepared ##\psi## should expect some chance to have a click on a ##\phi## measuring device.

Without going into much detail the PBR theorem essentially eliminates ##\psi##-statistical explanations that don't allow retrocausality or acausality. It says nothing at all about ##\psi##-epistemic views. Similarly retro/acausal ##\psi##-statistical and ##\psi##-epistemic views can escape the nonlocality conclusions of Bell's theorem.
The real force of the Frauchiger-Renner theorem and why it is causing interest in the Foundations community is because it seems to be the first result to say something about ##\psi##-epistemic views.

In the case of ##\psi##-statistical views something very definitive is being said of reality, it would just depend on the particular theory's hidden variables as to what that is.

##\psi##-epistemic views say that you can recover little about the underlying reality as so much of the QM formalism is simply "Agent-Reasoning" based. For example QBism says that the dimension of the Hilbert space (e.g. you need ##d = 3## for spin-##1##) reflects something as it seems to be agent independent. However they'd all basically say you can't really recover reality from QM, a new and very very different theory would be needed. QM will not turn out to be about ignorance of hidden more fundamental degrees of freedom. The most extreme view along these lines would be Bohr and Heisenberg style Copenhagen where the underlying reality has no hope of being recovered.
 
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  • #31
DarMM said:
Roughly speaking there are two types of ##\psi##-epistemic theories. Adán Cabello calls them type I and type II ##\psi##-epistemic. There is also the names ##\psi##-statistical and ##\psi##-epistemic, which I'll use because I think they're more distinctive, but just note that many use ##\psi##-epistemic to mean both.

In the former the wavefunction is statistical in the sense of classical statistical mechanics, it is in essence a probability distribution over more fundamental degrees of freedom, hidden variables. In ##\psi##-epistemic theories however QM is not the statistical mechanics of some hidden variables. Rather it is just a general theory of inference for classical observables and you can't really recover the underlying reality directly from it, this is because all properties of the wavefunction just reflect generalised inference rules or normative expectations agents should hold. They're not related to "underlying/more fundamental" degrees of freedom.

As a direct contrast ##\psi##-statistical views would say ##(\psi,\phi) \neq 0## means that the two distributions ##\psi## and ##\phi## have some ontic states in common, where as in ##\psi##-epistemic views it just means that an agent who prepared ##\psi## should expect some chance to have a click on a ##\phi## measuring device.

Without going into much detail the PBR theorem essentially eliminates ##\psi##-statistical explanations that don't allow retrocausality or acausality. It says nothing at all about ##\psi##-epistemic views. Similarly retro/acausal ##\psi##-statistical and ##\psi##-epistemic views can escape the nonlocality conclusions of Bell's theorem.
The real force of the Frauchiger-Renner theorem and why it is causing interest in the Foundations community is because it seems to be the first result to say something about ##\psi##-epistemic views.

In the case of ##\psi##-statistical views something very definitive is being said of reality, it would just depend on the particular theory's hidden variables as to what that is.

##\psi##-epistemic views say that you can recover little about the underlying reality as so much of the QM formalism is simply "Agent-Reasoning" based. For example QBism says that the dimension of the Hilbert space (e.g. you need ##d = 3## for spin-##1##) reflects something as it seems to be agent independent. However they'd all basically say you can't really recover reality from QM, a new and very very different theory would be needed. QM will not turn out to be about ignorance of hidden more fundamental degrees of freedom. The most extreme view along these lines would be Bohr and Heisenberg style Copenhagen where the underlying reality has no hope of being recovered.

I will assume this answers Demystifier's questions in post #28.

As for my view personally, as I pointed out in many of my Insights and our book, "Beyond the Dynamical Universe," I'm in the psi-statistical camp where QM provides the distributions of momentum-energy transfer between classical objects in spacetime via adynamical global (4D, spatiotemporal) constraints without causal mechanisms or hidden variables. By analogy, it would be like having Fermat's principle of least time for a light ray without any consensus dynamical counterpart (Snell's law). For example, conservation of angular momentum on average supplies a compelling 4D constraint with no consensus dynamical counterpart. So we're simply saying the 4D constraint is fundamental and without controversy while any dynamical counterpart is a matter of personal preference.
 
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  • #32
There seems to be a new experiment done recently that confirm's the wigner's friend hypothesis, published in Februrary 2019. Could anyone provide a simple explanation of what was done in that experiment?
 
  • #34
idea2000 said:
There seems to be a new experiment done recently that confirm's the wigner's friend hypothesis, published in Februrary 2019. Could anyone provide a simple explanation of what was done in that experiment?

You should read the thread Peter supplies in post #33. I have a student working on the calculations now and I will post an overview once the analysis is complete (probably late in the semester, as things are very busy now). As you will see in that thread, consensus is forming that their experiment is not Wigner's friend.
 
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1. What is the Quantum Mystery of Wigner's Friend?

The Quantum Mystery of Wigner's Friend is a thought experiment proposed by physicist Eugene Wigner in 1961 to explore the implications of quantum mechanics on the concept of consciousness and the measurement problem in quantum physics.

2. What is the measurement problem in quantum physics?

The measurement problem in quantum physics refers to the paradoxical nature of the wave-particle duality, where particles can exist in multiple states simultaneously until they are observed or measured, at which point they collapse into a single state. This raises questions about the role of the observer and the nature of reality.

3. How does Wigner's Friend thought experiment address the measurement problem?

In Wigner's Friend thought experiment, a scientist named Wigner performs an experiment on a quantum system while his friend observes from outside the laboratory. The paradox arises when Wigner's friend observes the system and collapses its wave function, but Wigner, who is not aware of the observation, believes the system is still in a superposition of states. This highlights the role of the observer in the measurement process and questions the objective reality of quantum systems.

4. What are some proposed solutions to the Quantum Mystery of Wigner's Friend?

Some proposed solutions to the Quantum Mystery of Wigner's Friend include the many-worlds interpretation, which suggests that all possible outcomes of a measurement exist in parallel universes, and the conscious observer effect, which suggests that consciousness plays a role in the collapse of the wave function.

5. What are the implications of the Quantum Mystery of Wigner's Friend?

The Quantum Mystery of Wigner's Friend raises questions about the nature of reality and the role of consciousness in the measurement process. It challenges our understanding of the fundamental principles of quantum mechanics and has implications for fields such as philosophy, psychology, and neuroscience.

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