A A Realization of a Basic Wigner's Friend Type Experiment

[PeterDonis]

See this recent paper:

https://arxiv.org/abs/1902.05080

 

[PeterDonis]

Here is the previous thread on the Frauchiger-Renner paper, which is referenced in the paper linked in the OP and is relevant to the claims being made:

https://www.physicsforums.com/threads/summary-of-frauchiger-renner.963737/

 

[StevieTNZ]

I will be eager to read posts on this paper, which I was alerted to a few days ago and haven't fully yet read in detail, but can also suggest reading https://arxiv.org/abs/1804.00749 (also official version published, viewable for free, online at Entropy 20, 350, (2018).)

 

[PeterDonis]

can also suggest reading https://arxiv.org/abs/1804.00749

Thanks for spotting this, this paper is another key reference in the paper linked in the OP.

 

[StevieTNZ]

... I was alerted to a few days ago and haven't fully yet read in detail ...

Reading https://www.newscientist.com/articl...-suggests-there-really-are-alternative-facts/ is how I found out about the article.

 

[DarMM]

So in this experiment we have four observers, let's say Alice, Bob, Zeus and Wigner. Alice and Bob measure an entangled Bell pair in the z-basis:
$$
|\pi\rangle = -sin\left(\frac{\theta}{2}\right)|\phi^{+}\rangle + cos\left(\frac{\theta}{2}\right)|\psi^{-}\rangle \\
|\phi^{+}\rangle = \frac{1}{\sqrt{2}}\left(|\uparrow\uparrow\rangle + |\downarrow\downarrow\rangle\right) \\
|\psi^{-}\rangle = \frac{1}{\sqrt{2}}\left(|\uparrow\downarrow\rangle + |\downarrow\uparrow\rangle\right)
$$
which causes them to be in the following state from Wigner and Zeus's perspective:
$$
|\Pi\rangle = -sin\left(\frac{\theta}{2}\right)|\Phi^{+}\rangle + cos\left(\frac{\theta}{2}\right)|\Psi^{-}\rangle \\
|\Phi^{+}\rangle = \frac{1}{\sqrt{2}}\left(|A_{\uparrow}B_{\uparrow}\rangle + |A_{\downarrow}B_{\downarrow}\rangle\right) \\
|\Psi^{-}\rangle = \frac{1}{\sqrt{2}}\left(|A_{\uparrow}B_{\downarrow}\rangle + |A_{\downarrow}B_{\uparrow}\rangle\right)
$$
where the $A_{\downarrow}$ state is the state of Alice's lab had she solely measured an unentangled particle in the $|\downarrow\rangle$ state.

Now Wigner and Zeus perform measurements on Alice and Bob's labs. Wigner for example measuring Alice in either the following basis:
$$\mathcal{Z} = \{|A_{\uparrow}\rangle,|A_{\downarrow}\rangle\}$$
which checks which outcome Alice obtained or he measures Alice in the following basis:
$$\mathcal{X} = \left\{\frac{1}{\sqrt{2}}\left(|A_{\uparrow}\rangle + |A_{\downarrow}\rangle\right), \frac{1}{\sqrt{2}}\left(|A_{\uparrow}\rangle + |A_{\downarrow}\rangle\right)\right\}$$

So essentially by varying these $\mathcal{X}$ and $\mathcal{Z}$ measurements Zeus and Wigner should be able to detect violations of the CHSH inequality.

The contradiction is that from their own perspective Alice and Bob have obtained some definite outcome for their spin measurements, say $z_{1}$ and $z_{2}$. So there is a fact about what Alice and Bob have measured, meaning there is a a definite value Zeus for example will obtain if he measures in the $\mathcal{Z}$ basis. Now if we assume that Zeus and Wigner both measure in the $\mathcal{X}$ basis, we have a set of two $\mathcal{X}$ outcomes and the definite $\mathcal{Z}$ outcomes they would have obtained has they measured in that basis. That's essentially four "elements of reality" or values in each run of the experiment:
$$\left\{\mathcal{Z}_{A},\mathcal{Z}_{B},\mathcal{X}_{A},\mathcal{X}_{B}\right\}$$
Then you'd build a probability distirubition for repeated runs of the experiment $p\left(\mathcal{Z}_{A},\mathcal{Z}_{B},\mathcal{X}_{A},\mathcal{X}_{B}\right)$. However the existence of such a common probability distribution means that one cannot obtain a violation of the Bell inequalities. Hence the contradiction.

This is very similar to Masanes's version of the Frauchiger-Renner argument. Creating a situation where the superobservers should see CHSH violations, but the existence of definite objective outcomes for the observers implies a common probability distribution in contradiction to the CHSH inequalities. This common probability distribution is created by using a "trick" that allows outcomes for all four Bell measurements in a single run. Masanes uses reversibility to obtain this, Brukner uses counterfactuals to include the values $\mathcal{Z}$ would have definitely had had it been measured.

However the problem both have is that this inclusion isn't really valid.

In Masanes case reversing the measurement is either impossible (some versions of Copenhagen) or simply undoes the CHSH correlations he needs (other versions of Copenhagen).

In this experiment's case the problem is that, although given a specific outcome for Alice's measurement then Wigner's measurement of $\mathcal{Z}$ should produce a value in line with that outcome, if Wigner doesn't measure $\mathcal{Z}$ then it has no outcomes and thus shouldn't be included in the list of outcomes. QM does not allow you to include counterfactuals like this.

This is simply an unintuitive aspect of the Quantum/Classical cut. The macroscopic degrees of freedom of a device might show $\uparrow$ as the result of measurement, but that doesn't mean you can consider a measurement of the device's atoms in the $\mathcal{Z}$ basis to already have the value $A_{\uparrow}$. Unless somebody actually performs the measurement there is no such value.

This is actually the same kind of thinking that leads to Many Worlds type observations that my measurement of definite outcomes is in some contradiction with a superobserver's assigning a superposed state to me. All the Frauchiger-Renner type arguments are basically trying to push a contradiction out of this. However there really seems there isn't a contradiction. Somebody can assign a superposed state to me despite my seeing a definitive outcome and it neither implies multiple worlds nor that my outcomes are private/subjective. It's just that the superobserver's state assignment refers to the statistics of superobservables that have yet to be measured.

 

[RUTA]

So essentially by varying these $\mathcal{X}$ and $\mathcal{Z}$ measurements Zeus and Wigner should be able to detect violations of the CHSH inequality.

The contradiction is that from their own perspective Alice and Bob have obtained some definite outcome for their spin measurements, say $z_{1}$ and $z_{2}$. So there is a fact about what Alice and Bob have measured, meaning there is a a definite value Zeus for example will obtain if he measures in the $\mathcal{Z}$ basis. Now if we assume that Zeus and Wigner both measure in the $\mathcal{X}$ basis, we have a set of two $\mathcal{X}$ outcomes and the definite $\mathcal{Z}$ outcomes they would have obtained has they measured in that basis. That's essentially four "elements of reality" or values in each run of the experiment:
$$\left\{\mathcal{Z}_{A},\mathcal{Z}_{B},\mathcal{X}_{A},\mathcal{X}_{B}\right\}$$
Then you'd build a probability distirubition for repeated runs of the experiment $p\left(\mathcal{Z}_{A},\mathcal{Z}_{B},\mathcal{X}_{A},\mathcal{X}_{B}\right)$. However the existence of such a common probability distribution means that one cannot obtain a violation of the Bell inequalities. Hence the contradiction.

This is very similar to Masanes's version of the Frauchiger-Renner argument. Creating a situation where the superobservers should see CHSH violations, but the existence of definite objective outcomes for the observers implies a common probability distribution in contradiction to the CHSH inequalities. This common probability distribution is created by using a "trick" that allows outcomes for all four Bell measurements in a single run. Masanes uses reversibility to obtain this, Brukner uses counterfactuals to include the values $\mathcal{Z}$ would have definitely had had it been measured.

I haven’t reproduced all the calculations yet, I’ve been too busy, but I will definitely do so and report back. Having read the paper, I suspect DarMM is correct, though.

First, this cannot possibly be a true Wigner’s friend experiment, since the friends are not screened off, i.e., they and their labs are interacting extensively with Wigner and Zeus’s labs, so decoherence will certainly render them “classical.” QM behavior does not follow from mere ignorance. Imagine for example that I scatter photons off electrons in a twin-slit experiment and use those photons to create “which-slit” information. If I merely hide the electron detector screen from my view and let my friend watch it, do you believe my friend will see an interference pattern? Of course not, but that’s exactly what they’re doing here.

Second, if DarMM is correct in his analysis I quoted here (again, I suspect he is), then this is really an instantiation of the Quantum Liar Experiment. Read my Insight on that and you’ll see what I mean.

I have a student working with me on this for his senior project. Once we have it all analyzed and properly critiqued, I’ll post something here.

 

[RUTA]

We need to be careful about the names of the participants. In the paper cited in post #1, Alice and Bob replace Wigner and Zeus, so we have Alice and Bob measuring their friends' labs. I attached Fig 2 from their paper.

 

[eloheim]

First, this cannot possibly be a true Wigner’s friend experiment, since the friends are not screened off, i.e., they and their labs are interacting extensively with Wigner and Zeus’s labs, so decoherence will certainly render them “classical.” QM behavior does not follow from mere ignorance. Imagine for example that I scatter photons off electrons in a twin-slit experiment and use those photons to create “which-slit” information. If I merely hide the electron detector screen from my view and let my friend watch it, do you believe my friend will see an interference pattern? Of course not, but that’s exactly what they’re doing here.

I don't think this criticism of the experiment is warranted, from how I take the paper, at least. IMO it spawns from too literal an expectation for the Wigner's friend gedanken experiment aspect of it. It's more like a formal proof of concept, using single photons and a setup where everything can be kept coherent up until the final macroscopic measurement result of each trial is produced. I believe this is separate from any issues it has with assuming counterfactuals.

 

[Demystifier]

All these experiments and no-go theorems are compatible with Bohmian mechanics in which an observer independent reality exists.

 

[RUTA]

I don't think this criticism of the experiment is warranted, from how I take the paper, at least. IMO it spawns from too literal an expectation for the Wigner's friend gedanken experiment aspect of it. It's more like a formal proof of concept, using single photons and a setup where everything can be kept coherent up until the final macroscopic measurement result of each trial is produced. I believe this is separate from any issues it has with assuming counterfactuals.

The criticism is very fair. The experiment is missing the essential point of Wigner's friend, i.e., the friend (measurement and recording device) must be screened off. Indeed, the entire collection of outcomes is compatible with a single, self-consistent (per QM) observer independent reality contrary to their claim. So, this is not even a proof of principle. See the two slides attached from Brukner and Renner's talks, respectively, at the APS March Meeting two weeks ago. Until someone can screen off a measurement, Wigner's friend has not been instantiated in fact or principle.

Again, this experiment is an instantiation of the Quantum Liar Experiment precisely as I explain in this Insight. Alice and Bob are doing measurements X and Y on an entangled system stemming from a particular form of psi (eigen function in Z). Alice and Bob's results in X and Y violate a Bell inequality implying no definite value for Z. But, without psi (in Z), the Bell results couldn't obtain. That's the experiment they should reference.

 

[Demystifier]

Sober comments on the experiment: http://dailynous.com/2019/03/21/phi...ZedZiEogcshXf1R-aPW4WEA-Nmp6vzS0MWJgKWiC5qJxc

In particular, Sean Carroll said the following:
"There is a long tradition in science journalism—and one must admit that the scientists themselves are fully culpable in keeping the tradition alive—of reporting on experiments that (1) verify exactly the predictions of quantum mechanics as they have been understood for decades, and (2) are nevertheless used to claim that a wholesale reimagining of our view of reality is called for. This weird situation comes about because neither journalists nor professional physicists have been taught, nor have they thought deeply about, the foundations of quantum mechanics."

 

[DarMM]

I haven’t reproduced all the calculations yet, I’ve been too busy, but I will definitely do so and report back. Having read the paper, I suspect DarMM is correct, though.

First, this cannot possibly be a true Wigner’s friend experiment, since the friends are not screened off, i.e., they and their labs are interacting extensively with Wigner and Zeus’s labs, so decoherence will certainly render them “classical.” QM behavior does not follow from mere ignorance. Imagine for example that I scatter photons off electrons in a twin-slit experiment and use those photons to create “which-slit” information. If I merely hide the electron detector screen from my view and let my friend watch it, do you believe my friend will see an interference pattern? Of course not, but that’s exactly what they’re doing here.

Second, if DarMM is correct in his analysis I quoted here (again, I suspect he is), then this is really an instantiation of the Quantum Liar Experiment. Read my Insight on that and you’ll see what I mean.

I have a student working with me on this for his senior project. Once we have it all analyzed and properly critiqued, I’ll post something here.

I think the resolution is easy to see in your view.

We have Alice and Bob's friends called $1$ and $2$, large classical systems providing the context for an exchange of angular moment, this angular momentum being represented by the observables $z_1$ and $z_2$. Then we have Alice and Bob, even larger classical systems providing the context for exchanges of quantities related to the atomic structure of Alice and Bob's friends, the superobservables $\left\{\mathcal{X}_i,\mathcal{Z}_i\right\}$.

The contradiction claimed in Brukner's paper (and followed in this experiment) is that Alice and Bob's friends' measurements of $z_1$ and $z_2$ immediately imply a value for the superobservables $\mathcal{Z}_1$ and $\mathcal{Z}_2$, since the result of such a measurement is guaranteed from the $z_1$ and $z_2$ values. Thus if the superobservers measure $\mathcal{X}_1$ and $\mathcal{X}_2$ we have a joint measurement of all four, which ultimately contradicts CHSH.

However in the Relational Blockworld this is instantly avoided. If the superobservers aren't set up correctly to provide the context for $\mathcal{Z}_1, \mathcal{Z}_2$ then those values just don't exist. Yes had the context been set up correctly the $z_1, z_2$ values from the observer context would imply those of the superobserver $\mathcal{Z}_i$ context with certainty. However if you measure $\mathcal{X}_i$ you simply do not provide the context for $\mathcal{Z}_i$ regardless of how determined/fixed it would have been if you had. Thus there is no measurement of all four superobservables, indeed such a context is impossible.

Similar remarks hold for Copenhagen.

In essence yes a Quantum Liar experiment, but thinking the superobserver counterfactual somehow is valid to include because when its context is present it logically follows (i.e. Probability is 1) from an observer fact.

 

[DarMM]

Sober comments on the experiment: http://dailynous.com/2019/03/21/phi...ZedZiEogcshXf1R-aPW4WEA-Nmp6vzS0MWJgKWiC5qJxc

To be fair to the experimenters Brukner and the experiment don't intend to include Bohmian Mechanics or Many-Worlds which are discussed a good bit in that article. Although those interpretations do provide a good refutation of the article title and media headlines. The paper should have been called something like:
"Experimental investigation of Objective Reality in Single World Local Interpretations with Global Unitarity"
since the interpretations in bold are Brukner's target.

These are interpretations that want to retain the validity of QM at all scales (no spontaneous collapse like GRW), have one world and are local. So Copenhagen style interpretations (Consistent Histories, Ignorance Ensemble, Bub/Healey) or retrocausal and acausal ones.

However of course Brukner's theorem doesn't affect such interpretations since it assumes the existence of a superobservable context that is never realized.

Frauchiger-Renner attempts to target such interpretations as well, but doesn't succeed either.

I think it is simply the case that there is no contradiction between locality, one world and an observer being modeled as being in a superposition. All these variations of Wigner's friend keep trying to do this and not succeeding. The outcome for me is more just emphasizing interesting aspects of Copenhagen or acausal views rather than refuting them.

 

[Demystifier]

The paper should have been called something like:
"Experimental investigation of Objective Reality in Single World Local Interpretations with Global Unitarity"

Yes. But then nobody would care.

 

[vanhees71]

All these experiments and no-go theorems are compatible with Bohmian mechanics in which an observer independent reality exists.

Hm, involving photons, I'd be very careful with statements about Bohmian interpretations. Particularly for photons at least I've seen no convincing Bohmian setup yet.

 

[Demystifier]

Hm, involving photons, I'd be very careful with statements about Bohmian interpretations. Particularly for photons at least I've seen no convincing Bohmian setup yet.

How about the version of Bohmian mechanics (linked in my signature below) in which photon does not have a trajectory?

 

[vanhees71]

That sounds promising since indeed photons do not have a trajectory to begin with. I've downloaded your paper to my "to-read folder"...

 

[kimbyd]

This is actually the same kind of thinking that leads to Many Worlds type observations that my measurement of definite outcomes is in some contradiction with a superobserver's assigning a superposed state to me. All the Frauchiger-Renner type arguments are basically trying to push a contradiction out of this. However there really seems there isn't a contradiction. Somebody can assign a superposed state to me despite my seeing a definitive outcome and it neither implies multiple worlds nor that my outcomes are private/subjective. It's just that the superobserver's state assignment refers to the statistics of superobservables that have yet to be measured.

It directly implies multiple worlds.

The multiple worlds interpretation is merely the statement that the wavefunction never truly collapses: it only appears to do so. The "multiple worlds" are merely the different branches of the wavefunction that no longer interact to any significant degree. In this specific situation, the "multiple worlds" comes in in the following way:

Observer A observes a definite effect.
Observer B interprets Observer A as being in a superposition of two states.

As long as you have a quantum mechanical system that behaves in the above way, the many worlds interpretation is intrinsic to that system's description.

Granted, it doesn't necessarily imply one single many-worlds interpretation (there are many), but it does imply many worlds, as the only consistent interpretation of observer B's view of the world is that observer A is in a superposition of "measured state 1" and "measured state 2".

 

[DarMM]

As long as you have a quantum mechanical system that behaves in the above way, the many worlds interpretation is intrinsic to that system's description.

No it isn't. It's only the case if you ascribe an ontic status to the quantum state and add no additional variables. In an epistemic view one observer can have another in superposition without multiple worlds being involved.

 

[kimbyd]

No it isn't. It's only the case if you ascribe an ontic status to the quantum state and add no additional variables. In an epistemic view one observer can have another in superposition without multiple worlds being involved.

The other observer in a superposition is experiencing multiple worlds by definition. That's what the term means.

 

[DarMM]

The other observer in a superposition is experiencing multiple worlds by definition. That's what the term means.

Superposition does not mean "experiencing multiple worlds", it's a form of statistics. What textbook are you getting that definition from?

Superposition doesn't have to mean "being in both states at once". In an epistemic view it simply means you have a chance of being found in either, just as in a state from Kolmogorov probability.

 

[kimbyd]

Superposition does not mean "experiencing multiple worlds", it's a form of statistics. What textbook are you getting that definition from?

Superposition doesn't have to mean "being in both states at once". In an epistemic view it simply means you have a chance of being found in either, just as in a state from Kolmogorov probability.

The observer in a superposition sees a single outcome of their observation, and does not interact with the part of the wavefunction where they see the alternative outcome.

That's textbook multiple worlds. That's all it is.

 

[DarMM]

The observer in a superposition sees a single outcome of their observation, and does not interact with the part of the wavefunction where they see the alternative outcome.

That's textbook multiple worlds. That's all it is.

Well it's the Many World's view of observer superposition, but nothing about observer superposition requires a Many Worlds view, i.e
$$\frac{1}{\sqrt{2}}\left(|A\rangle + |B\rangle\right)$$

Where A and B are observer states, doesn't have to mean the observer actually is in both states, it only means so in many worlds.

 

[microsansfil]

Superposition doesn't have to mean "being in both states at once". In an epistemic view it simply means you have a chance of being found in either, just as in a state from Kolmogorov probability.

How would you interpret the fact that a quantum computer uses the principle of superposition of quantum mechanics to perform an immense number of calculations in parallel ?

/Patrick

 

[DarMM]

How would you interpret the fact that a quantum computer uses the principle of superposition of quantum mechanics to perform an immense number of calculations in parallel ?

/Patrick

By the simple fact that it doesn't. If quantum computers truly made use of all components of the wavefunction as a physical resource, then you'd expect them to be capable of NP-complete problems like the traveling salesman problem, see here for a brief comment on this: https://arxiv.org/abs/1409.1570

In addition quantum computers don't in any sense "do loads of computations in parallel", it's even the subtitle on Scott Aaronson's blog: https://www.scottaaronson.com/blog/

It would be more correct to say they use contextuality as a resource or to paraphrase Jeffrey Bub they have the ability to resolve $A \lor B$ without resolving either $A$ or $B$ individually.

 

[DarMM]

The observer in a superposition sees a single outcome of their observation, and does not interact with the part of the wavefunction where they see the alternative outcome.

That's textbook multiple worlds. That's all it is.

In relation to the post above, the mistake you're making here is viewing the components of the wavefunction as physical. Not all interpretations agree on this and thus there would be no physical alternate outcome part of the wavefunction. The other "branches" of the wavefunction in an epistemic view are as physical as the probability $p(2)$ of getting a roll of $2$ on a dice after you observe a roll of $1$. The $p(2)$ isn't something physical that has "gone somewhere" with its alternate outcome, it's just an epistemic quantity removed after Bayesian updating.

 

[kimbyd]

In relation to the post above, the mistake you're making here is viewing the components of the wavefunction as physical. Not all interpretations agree on this and thus there would be no physical alternate outcome part of the wavefunction. The other "branches" of the wavefunction in an epistemic view are as physical as the probability $p(2)$ of getting a roll of $2$ on a dice after you observe a roll of $1$. The $p(2)$ isn't something physical that has "gone somewhere" with its alternate outcome, it's just an epistemic quantity removed after Bayesian updating.

Sure. But what does physical mean? Both $|A\rangle$ and $|B\rangle$ components of the observer's wavefunction experience those observations. There's no way to distinguish the two anywhere in the math. You can, if you want, say that $|A\rangle$ is physical while $|B\rangle$ is not. Or that the "true" observer is chosen randomly. But that doesn't change the fact that $|B\rangle$ still goes on and has a life and sits down to dinner with her kids. She may be unphysical, but she still has a life.

The interpretations other than many worlds argue one of two things:
1) Only one component is real, and the others are not.
2) The "not real" components cease to exist.

Interpretations like (1) are nonsensical. Interpretations like (2) are what this kind of thing is targeted at.

 

[DarMM]

Sure. But what does physical mean? Both $|A\rangle$ and $|B\rangle$ components of the observer's wavefunction experience those observations. There's no way to distinguish the two anywhere in the math. You can, if you want, say that $|A\rangle$ is physical while $|B\rangle$ is not. Or that the "true" observer is chosen randomly. But that doesn't change the fact that $|B\rangle$ still goes on and has a life and sits down to dinner with her kids. She may be unphysical, but she still has a life.

There's no way to distinguish which one is real between $p(1)$ and $p(2)$ in the probability density function for a dice roll, but still only one of $1$ and $2$ occurs.

You're still implicitly understanding the wavefunction as ontic with statements like $|B\rangle$ goes onto live their life. The point is that in Epistemic views all of $|\psi\rangle$ is not real, it just encodes expectations.

Let me make this very simple. In a dice roll, when I roll a $1$ what "happens" to $p(2)$ in your opinion?

The interpretations other than many worlds argue one of two things:
1) Only one component is real, and the others are not.
2) The "not real" components cease to exist.

Interpretations like (1) are nonsensical. Interpretations like (2) are what this kind of thing is targeted at.

Well there are many more views than this. Bohmian Mechanics for example has all components of the wavefunction as real, but isn't Many Worlds. Also Epistemic views view all of the wavefunction as not real as I explained above.

 

[microsansfil]

.

By the simple fact that it doesn't.
...

Thank you for your answer. It's a question that concerns me as a qubist. I read this viewpoint https://www.aps.org/publications/apsnews/199806/quantumcomputing.cfm

/Patrick

 

[kimbyd]

There's no way to distinguish which one is real between $p(1)$ and $p(2)$ in the probability density function for a dice roll, but still only one of $1$ and $2$ occurs.

You're still implicitly understanding the wavefunction as ontic with statements like $|B\rangle$ goes onto live their life. The point is that in Epistemic views all of $|\psi\rangle$ is not real, it just encodes expectations.

Let me make this very simple. In a dice roll, when I roll a $1$ what "happens" to $p(2)$ in your opinion?

Fundamentally this is a question of whether or not quantum mechanics describes the macro world we inhabit. It very obviously describes the outcomes of a great many experiments to a tremendous degree of accuracy. And the theory predicts that in the macro world we inhabit, the peculiarities of quantum mechanics such as entanglement and superposition just won't be apparent.

So: does quantum mechanics describe not only the small-scale experiments with, say, radioactive materials decaying and being observed with a photomultiplier tube, but also the ways in which my body behaves as I type this post?

By trying to make this real/not real distinction, you're effectively saying that there is a hard separation between the macro world we inhabit and these small-scale experiments. You're saying, "Yeah, this math describes the outcomes of these experiments, but it has no consequences beyond that."

As to your question, it's just too ill-defined for me to answer, and I see no point trying to pull it together to make it make any sense.

Well there are many more views than this. Bohmian Mechanics for example has all components of the wavefunction as real, but isn't Many Worlds. Also Epistemic views view all of the wavefunction as not real as I explained above.

Bohmian mechanics labels one branch as "real" by saying that the particles live there.
https://en.wikipedia.org/wiki/De_Broglie–Bohm_theory#Similarities_with_the_many-worlds_interpretation

This was the interpretation I was describing in that post. The point is that Bohmian mechanics has lots of branches of the wavefunction that are empty and yet continue to evolve according to the relevant wave equation. If you examined those branches, you'd see that they act just like the "real" one that has the particles in it. You could not experimentally determine whether you were in a branch containing particles or not.

 

[DarMM]

Fundamentally this is a question of whether or not quantum mechanics describes the macro world we inhabit.

No it's a description of what the epistemic view of the wavefunction is like. No relation to invoking a macro/micro distinction.

By trying to make this real/not real distinction, you're effectively saying that there is a hard separation between the macro world we inhabit and these small-scale experiments.

Not at all. Consider the macrostate in Statistical Mechanics. That is epistemic and yet supposes no hard micro/macro separation.

As to your question, it's just too ill-defined for me to answer, and I see no point trying to pull it together to make it make any sense.

It's fairly trivial. What happens to components of a probability distribution upon observation of one outcome? The usual answer is they are removed by Bayesian conditioning. Basic probability theory.

Let me make this very basic. In an epistemic view the wavefunction just describes statistics. That statement doesn't require a distinction between the macro and micro worlds.

 

[kimbyd]

But again, the Wigner's Friend experiment is a demonstration that observers can be in a superposition of states. The states are "observed outcome A" and "observed outcome B".

The demonstration of that superposition is evidence that there are different components of the wavefunction who have different experiences.

 

[DarMM]

But again, the Wigner's Friend experiment is a demonstration that observers can be in a superposition of states. The states are "observed outcome A" and "observed outcome B".

The demonstration of that superposition is evidence that there are different components of the wavefunction who have different experiences.

No it isn't, for exactly the reasons I mentioned in post #6. The observer being in superposition can simply be seen as reflecting the epistemic condition of the superobserver regarding superobservables.

 

[PeterDonis]

The demonstration of that superposition is evidence that there are different components of the wavefunction who have different experiences.

This would be true if the "observers" in the actual experiment were things that can have conscious experiences, like humans. But the "observers" in the actual experiment were qubits. So this statement is going way beyond what the actual experiment shows.

 

[vanhees71]

No it isn't, for exactly the reasons I mentioned in post #6. The observer being in superposition can simply be seen as reflecting the epistemic condition of the superobserver regarding superobservables.

It doesn't make sense to say "observers are in superposition". Any vector of a vector space can be written as superposition of other vectors. That's one of the definitions of vectors you learn in the first 5 minutes in the first-semester linear-algebra lecture ;-)). In this sense it's a pretty empty phrase.

 

[DarMM]

It doesn't make sense to say "observers are in superposition". Any vector of a vector space can be written as superposition of other vectors. That's one of the definitions of vectors you learn in the first 5 minutes in the first-semester linear-algebra lecture ;-)). In this sense it's a pretty empty phrase.

It's usually used as a shorthand for the observer being in a superposition of observation states. You'd see it frequently enough in Wigner's friend papers.

 

[platosuniverse]

Fundamentally this is a question of whether or not quantum mechanics describes the macro world we inhabit. It very obviously describes the outcomes of a great many experiments to a tremendous degree of accuracy. And the theory predicts that in the macro world we inhabit, the peculiarities of quantum mechanics such as entanglement and superposition just won't be apparent.

So: does quantum mechanics describe not only the small-scale experiments with, say, radioactive materials decaying and being observed with a photomultiplier tube, but also the ways in which my body behaves as I type this post?

By trying to make this real/not real distinction, you're effectively saying that there is a hard separation between the macro world we inhabit and these small-scale experiments. You're saying, "Yeah, this math describes the outcomes of these experiments, but it has no consequences beyond that."

As to your question, it's just too ill-defined for me to answer, and I see no point trying to pull it together to make it make any sense.Bohmian mechanics labels one branch as "real" by saying that the particles live there.
https://en.wikipedia.org/wiki/De_Broglie–Bohm_theory#Similarities_with_the_many-worlds_interpretation

This was the interpretation I was describing in that post. The point is that Bohmian mechanics has lots of branches of the wavefunction that are empty and yet continue to evolve according to the relevant wave equation. If you examined those branches, you'd see that they act just like the "real" one that has the particles in it. You could not experimentally determine whether you were in a branch containing particles or not.

Excellent post!

This has to be real and a fundamental aspect of reality on a quantum scale. The experiment is pretty clear about this. There's 2 outcomes that can be seen as facts by Wigner and his friend.

First, Wigner's friend measures the polarization of a photon and stores the result. Next, Wigner performs an interference measurement to determine if the measurement and the photon are in a superposition.

You now have two facts coexisting. Wigner's Friend says, it's a fact that I measured the polarization of the photon. Wigner says, it's a fact that my friend didn't measure the polarization of the photon.

This points to several things.

First, Observers on a quantum scale can hold alternative facts so to speak about the same event.

Second, If all is quantum as many Physicist believe, then there's no collapse and this does lend support to a Many Worlds Interpretation. Unless there's something that causes self collapse a la Roger Penrose, there's nothing to stop all of these branches from being real and on a classical level, decoherence just chips away the interference and Observers just appear to be in one singular state or the other.

Third, this has to be a fundamental aspect of reality to prevent FTL communication. If Wigner's friend could perfectly signal Wigner when he is or isn't carrying out a measurement, he could send Wigner information faster than light.

When Wigner's Friend measures the polarization of the photon, Wigner can do an interference measurement and when he doesn't see superposition between the photon and the measurement, he can say my friend carried out a measurement and that's a 1.

When Wigner's Friend doesn't carry out a measurement and Wigner does an interference measurement and sees superposition between the photon and the measurement, that could be an 0.

So this has to be more than just statistics.

 

[DarMM]

And yet you can replicate the Wigner's friend experiment in Spekkens toy model where it is just statistics with an epistemic limit.

 

[StevieTNZ]

A follow up paper which may be of interest: https://arxiv.org/abs/1907.05607

 

[NTHstars]

Doesn't the totality of research indicate locality is likely not going to be an essential part of a successful quantum theory, as evidenced by entanglement? If something has to go between "freedom of choice", "locality", or "observer-independent facts," as they indicate, it seems locality has the largest amount of evidence going against it as being fundamental to all interactions.

 

[vanhees71]

The most successful QT of all times is relativistic QFT with the Standard Model of elementary particle physics, and one of its fundamental building principles is locality, i.e., there are only local interactions in the sense that operators of local observables (like energy, momentum, angular-momentum, charge etc. etc. densities) commute at space-like distances of their arguments.

There's no problem with "freedom of choice" (i.e., you can measure any observable you like) and "observer-independent facts" (i.e., a given state for every observer leads to the same probabilistic outcomes of measuremsnte).

Nevertheless, of course, relativsitic QFT, as any QT, implies the existence of entangled states and long-ranged correlations between parts of a quantum system. By construction, and particularly thanks to the locality of interactions built in from scratch, there cannot be problems with Einstein causality in any model based on this kind of relativsitic QFT.

 

[Demystifier]

There's no problem with "freedom of choice" (i.e., you can measure any observable you like)

Can QFT explain that by deriving it from more fundamental principles of QFT?

 

[vanhees71]

In which sense should this be "derivable"? If you take a concrete QFT, e.g., QED, it tells you which sensible observables there should be, and these observables you can freely choose to measure. Nothing within the theory forbids you to measure any observable you can define within this theory. Whether or not you can realize the more or less precise measurement of a given observable is of course a question of practical realizability, and, of course, whether or not the model or theory describes nature right, is in turn subject to test it with observations. As far as we know QED is a very good theory in the sense that no observation contradicts it yet.

 

[Demystifier]

So it's basically an independent axiom of QT that one can measure any observable one wants. And the meaning of "wants" remains mathematically undefined, but that's not a problem from a practical point of view. Right?

All these no-go theorems about quantum foundations say something about things which go beyond the practical, but since you only care about the practical you dismiss them all with ease.

 

[vanhees71]

Yeah! Physics is about what can be observed and measured in nature, not about philosophical quibbles of individual philosophers. SCNR.

 

[Demystifier]

Yeah! Physics is about what can be observed and measured in nature, not about philosophical quibbles of individual philosophers. SCNR.

So Boltzmann with his atomic theory as a statistical explanation of thermodynamics was a philosopher, not a physicist. Is that right?

 

[vanhees71]

No, why? Given the evidence for atoms in chemistry, it was a good question, whether the hypothesis of the existence of atoms is compatible with the observed behavior of macroscopic matter. Unfortunately for Boltzmann the acceptance of the atomistic structure of matter in the physics community was about to come only too late for him.

 

[Demystifier]

Given the evidence for atoms in chemistry

But by your standards, that evidence was a philosophical quibble. Dalton did not make a new measurable prediction in chemistry, he just proposed an interpretation of known chemical data, according to which matter consists of small invisible and indivisible atoms. How is that more scientific than modern hidden variable theories for quantum mechanics?

 

[vanhees71]

Well, it's way more scientific to assume atoms in Dalton's time than HVs today, simply because the "atom hypothesis" naturally described the observational findings in chemistry, while HVs are simply unnecessary given that we have an accurately working theory that works for all hitherto made observations, which is called QT ;-)). Then the Bernoulli's, Maxwell, Boltzmann et al even made a quantitative theory, called "statistical/kinetic theory of gases" which could explain to a large extent that one can understand the macroscopic matter's behavior via statistics of microscopic degrees of freedom. One should also not forget that there were fundamental unsolved physical problems like the spectrum of black-body radiation, then with the advent of low-temperature technology the heat capacity at low temperatures, the discrete spectra emitted from matter (Kirchhoff and Bunsen) etc. etc. all of which could not satifactorily explained within classical physics, and all of which were only solved with the discovery of QT (partially by old QT but (up to know) "finally" only by modern QT).