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

[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).

 

[DarMM]

To the person who emailed regarding this:

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

It's quite a good paper and no surprise really, it's a more rigorous version of Brukner's argument. Obviously Bohmian Mechanics would violate locality and Many-Worlds and retrocausal views take place outside the framework of the theorem. So the only interesting case is Copenhagen style views which seem to be required to reject "objectivity of facts".

Indeed they do, but it's not in a very surprising or strong way. The theorem essentially says that we have Alice and Bob (the superobservers) and Charlie and Diana (the observers). We have the outcomes $c$ and $d$ from Charlie and Diana performing observations from an entangled pair. Then we have outcomes $a$ and $b$ from Alice and Bob performing superobservations upon the entire atomic structure of Charlie and Diana's labs.

The theorem is basically that $a,b,c,d$ do not occur in a common sample space. Or put another way after Charlie and Diana's measurements there is no "fact of the matter" for how Alice and Bob's measurements will pan out.

This is no surprise for Charlie and Diana's observations are only on the entangled system and don't necessarily imply values for superobservables concerning every single atom in their lab.

What's interesting is that the "Local Friendliness" correlations are strictly wider than Bell's. Meaning there are theories which are fundamentally random and with no hidden variables, but where observations do imply the values of superobservations. Performing a measurement on an atomic system does restrict quite strongly the outcomes of some measurements involving every atom in your lab. This theorem shows QM is not such a theory.

Of course one can just reject the existence of superobservers.

 

[kimbyd]

One thing that irked me about that paper is that in this discussion they made a play at talking about how this wasn't really testable because observers (i.e. humans) tend to have highly-collapsed wavefunctions, and that this could be worked around with "strong AI" that has negative ethical implications.

I think that's pretty absurd. The concept of a quantum observer does not in any way require any sort of intelligence. All that is required is an experimental apparatus that reports a result. So you don't need anything complicated at all: all you need is a microscopic apparatus that emits some signal if it encounters some state or another. Then the experimental apparatus itself is the "observer" and the person running the experiment is the "super-observer". Intelligence isn't at all required to report or record the result of a measurement.

 

[StevieTNZ]

One thing that irked me about that paper is that in this discussion they made a play at talking about how this wasn't really testable because observers (i.e. humans) tend to have highly-collapsed wavefunctions, and that this could be worked around with "strong AI" that has negative ethical implications.

I think that's pretty absurd. The concept of a quantum observer does not in any way require any sort of intelligence. All that is required is an experimental apparatus that reports a result. So you don't need anything complicated at all: all you need is a microscopic apparatus that emits some signal if it encounters some state or another. Then the experimental apparatus itself is the "observer" and the person running the experiment is the "super-observer". Intelligence isn't at all required to report or record the result of a measurement.

That's one interpretation. One interpretation is consciousness causes wave function collapse - as far as I'm aware that hasn't been experimentally refuted.

 

[charters]

I think that's pretty absurd. The concept of a quantum observer does not in any way require any sort of intelligence

The ultimate issue here is quantum time evolution is unitary but collapse is non-unitary. This means that in an experiment with multiple putative observers acting in sequence, such as Wigner's Friend, which measuring system you arbitrarily call "observer" changes experimental predictions/statistics. On its face, this is bad news for any measurement induced collapse interpretation. But, if only one such measuring system is a conscious being, there's an easy way to resolve the tension - simply say that only the conscious being is really a collapsing observer. The reason to introduce multiple conscious beings is to close this loophole.

But note as well that if the Friend is not a person, just a machine, if you model this as a collapsing measurement, then the superobserver isn't able to use unitary quantum theory to make general predictions (i.e., the superobserver cannot unitarily restore the state of closed off lab system), undermining the universality of QT.

 

[charters]

So the only interesting case is Copenhagen style views which seem to be required to reject "objectivity of facts".

Indeed they do, but it's not in a very surprising or strong way

If Charlie and Diana believe themselves to belong to a special class of classical observers, who are not described by a quantum state (only a classical one) and can effect non-unitary changes on quantum states, they would be pretty surprised by the idea that Alice and Bob can act on them/their lab with unitary quantum operations. There's a pretty widespread account of Copenhagen that treats humans exactly this way.

 

[DarMM]

If Charlie and Diana believe themselves to belong to a special class of classical observers, who are not described by a quantum state (only a classical one) and can effect non-unitary changes on quantum states, they would be pretty surprised by the idea that Alice and Bob can act on them/their lab with unitary quantum operations. There's a pretty widespread account of Copenhagen that treats humans exactly this way.

Not all though. I've attempted a more detailed classification of Copenhagen views over in the QBism thread, but it's difficult to repeat something like this when one wants to use the shorthand "Copenhagen". There are forms of Copenhagen that reject superobservers like Peres. I assume that's what you are referring to.

 

[charters]

Not all though. I've attempted a more detailed classification of Copenhagen views over in the QBism thread, but it's difficult to repeat something like this when one wants to use the shorthand "Copenhagen". There are forms of Copenhagen that reject superobservers like Peres. I assume that's what you are referring to.

I am just saying that most people who've had some basic but not too deep exposure to QM and the idea of "collapse" would in fact be surprised to discover that a superobserver could in principle act on an internal observer unitarily, and put them in interfering superpositions, as if they were a quantum state. They would be surprised to learn that what one person sees as collapse, another sees as just entanglement - that the where and when of collapse is not an objective fact.

 

[kimbyd]

That's one interpretation. One interpretation is consciousness causes wave function collapse - as far as I'm aware that hasn't been experimentally refuted.

Pretty sure it's refuted by the measured fact that collapse measurably occurs for systems far less complex than humans, without results being ever observed by humans.

If some want to claim that that's not "real" collapse and "real" collapse only occurs due to consciousness, then that's a pretty extraordinary claim requiring extraordinary evidence.

 

[kimbyd]

For why I say that collapse has been experimentally measured to occur for less complex systems, here's an example:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.77.4887

They manually collapse the wavefunction by increasing the intensity of the light source which interacts with the effective dual-slit system they're using. The light is never observed, or even measured. But it causes the wavefunction to collapse nevertheless.

 

[charters]

For why I say that collapse has been experimentally measured to occur for less complex systems, here's an example:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.77.4887

They manually collapse the wavefunction by increasing the intensity of the light source which interacts with the effective dual-slit system they're using. The light is never observed, or even measured. But it causes the wavefunction to collapse nevertheless.

This only shows interference is increasingly suppressed as the atom-photon entanglement gets stronger. It does not tell us anything about if or when a nonunitary collapse occurs. Loss of interference (explained by decoherence) is not the same thing as collapse to a definite outcome/eigenstate (not explained by decoherence).