The key piece I took away from the article is the following:

The experiment produces an unambiguous result. It turns out that both realities can coexist even though they produce irreconcilable outcomes, just as Wigner predicted.

So, they can co-exist as long as there is something that can be agreed about between observers. So, we might be talking about the same thing despite having differing views about it.

@Terrapin Station What does this say about extra-mental facts?

So, they can co-exist as long as there is something that can be agreed about between observers. So, we might be talking about the same thing despite having differing views about it. — Wallows

In that case, you have drawn the exact opposite conclusion to what the experiment suggests.

If it said something as quotidian as you think it said, there would be no point in doing the experiment, and certainly nothing to write a story about.

The same paragraph makes the point that the two observers see two different things, which are irreconcilable. It is those observations which are at issue.

In that case, you have drawn the exact opposite conclusion to what the experiment suggests. — Wayfarer

Yes; but, there can be no modality without the observer effect on a macroscopic scale concerning n>1 observers, such as has been demonstrated here? So, what's the big fuss about?

IN case you post this on PF, please send a link. I stopped posting there indefinitely.

I think the bigger import of this story is that wavefunction collapse fails under such an interpretation of results.

Physicists have long suspected that quantum mechanics allows two observers to observe different, conflicting realities. — Wayfarer

I don't believe in objective reality so this is a bit moot for me. But I'm very interested in QM so I feel the need to comment.

I think it's overstating to say that Wigner and his friend experience conflicting realities. Rather, the friend just has more information than Wigner. The difference can be interpreted as purely epistemological. Wigner knows that his friend knows which way the spin goes, but Wigner doesn't know which way. So Wigner models the lab as a superposition while the friend does not.

I can't see anything new in Wigner's thought experiment. To me it sounds the same as observing that, if we put another cat in the box with Schrodinger's first cat, but the two are separated by a transparent, airtight wall, so that the second cat doesn't get poisoned if the vial breaks, then the second cat (Wigner's friend) knows whether the first cat is alive or dead, but Schrodinger (Wigner) does not.

I can't see what there is to experiment on. I suppose I'll have to read the paper to find out, but I don't know when I'll get time to do that..

Wigner knows that his friend knows which way the spin goes, but Wigner doesn't know which way. So Wigner models the lab as a superposition while the friend does not. — andrewk

As I said to W., if it were that simple then it wouldn’t rate a comment.

I think to resort to Schodinger’s famous simile, it’s as if Bob observes a live cat, and Alice a dead one - and they’re both right.

I think to resort to Schodinger’s famous simile, it’s as if Bob observes a live cat, and Alice a dead one - and they’re both right. — Wayfarer

Is that what the paper does - create a situation where both observers have the same level of detail in their knowledge, and the details contradict each other? That would be a much stronger result than anything contemplated in Wigner's thought experiment, which seems to rely on a belief in 'objective collapse' to have any interest at all..

@andrewk You’re too smart for us. I would like to hear your thoughts after reading the paper.

My interpretation of the article was that person one measured and the wave function collapsed. Person two didn’t measure, so to him the wave function didn’t collapse. Two separate realities.

Physicists have long suspected that quantum mechanics allows two observers to observe different, conflicting realities. Now they’ve performed the first experiment that proves it by experimental realisation of what was previously a thought-experiment called ‘Wigner’s Friend’. — Wayfarer

Good to to see the experiment done. But as you might expect, the main interpretations already have standard answers for this. For example, Many Worlds has those observers in different world branches, RQM allows observers to have different accounts of the event as long as they don't compare their results, and Copenhagen would not regard the friend's interaction as a measurement (for Wigner).

I think it's overstating to say that Wigner and his friend experience conflicting realities. Rather, the friend just has more information than Wigner. The difference can be interpreted as purely epistemological. Wigner knows that his friend knows which way the spin goes, but Wigner doesn't know which way. So Wigner models the lab as a superposition while the friend does not. — andrewk

That would be a hidden variables theory (and thus non-local).

What troubles me about this is that, on my understanding of QM, wave function collapse is non-measurable. It is a matter of interpretation, of ontology, not something that can be measured - so strictly speaking it isn't even part of QM. So either this experiment implies less than the MIT pop summary says it does, or I am going to have to radically revise my understanding of QM.

I've downloaded the full paper from arxiv and will see what I can make of it.

I've downloaded the full paper from arxiv and will see what I can make of it. — andrewk

Cool. Perhaps @fdrake would like to give his thoughts as well?

@Dfpolis? Thoughts?

Is that what the paper does - create a situation where both observers have the same level of detail in their knowledge, and the details contradict each other? — andrewk

That is what the MIT abstract says that it does:

Wigner can...perform an experiment to determine whether this superposition [in respect of a particular particle] exists or not. This is a kind of interference experiment showing that the photon and the measurement are indeed in a superposition.

From Wigner’s point of view, this is a fact— the superposition exists. And this fact suggests that a measurement cannot have taken place.

But this is in stark contrast to the point of view of the friend, who has indeed measured the photon’s polarization and recorded it. The friend can even call Wigner and say the measurement has been done (provided the outcome is not revealed).

So the two realities are at odds with each other. “This calls into question the objective status of the facts established by the two observers,” say Proietti and co.

——

Many Worlds has those observers in different world branches, — Andrew M

I don’t regard that as an explanation so much as a cop-out.

But Proietti and co’s result suggests that objective reality does not exist. In other words, the experiment suggests that one or more of the assumptions—the idea that there is a reality we can agree on, the idea that we have freedom of choice, or the idea of locality—must be wrong.

So much for freedom of choice.

I don’t regard that as an explanation so much as a cop-out. — Wayfarer

Right so perhaps you could correct your title to

Quantum experiment is interpretable in a way that I'd already decided was true before the experiment was even done.

Many Worlds has those observers in different world branches,
— Andrew M

I don’t regard that as an explanation so much as a cop-out. — Wayfarer

It takes a theory to beat a theory.

But Proietti and co’s result suggests that objective reality does not exist. In other words, the experiment suggests that one or more of the assumptions—the idea that there is a reality we can agree on, the idea that we have freedom of choice, or the idea of locality—must be wrong.

So much for freedom of choice. — Banno

Freedom of choice refers to an experimenter being able to choose what experiment to perform. All mainstream interpretations accept that assumption.

The assumption of reality, as defined in Bell test experiments, just means counterfactual definiteness. Most interpretations, including Many Worlds and Copenhagen, reject counterfactual definiteness. Bohmian Mechanics accepts it.

The wording of the paper seems to be a argument against counterfactual definiteness (an objective reality). I'm all for that since I don't think there is such a thing, and Bell's theorem demonstrated long ago that you can't have both that and locality.
Anyway, Bohmian mechanics seems to be the interpretation that asserts counterfactual definiteness. There are other interpretations that do, but not very mainstream ones. It would be interesting to describe the experiment from that perspective.

Wigner knows that his friend knows which way the spin goes, but Wigner doesn't know which way. So Wigner models the lab as a superposition while the friend does not.
— andrewk

As I said to W., if it were that simple then it wouldn’t rate a comment.

I think to resort to Schodinger’s famous simile, it’s as if Bob observes a live cat, and Alice a dead one - and they’re both right. — Wayfarer

It isn't a live and dead cat, a blatant contradiction which cannot arise. Bob observes the cat and knows if it is dead or alive. Alice measures the cat still in superposition. That's very different than Alice measuring a dead cat and Bob a live one.

It is more complicated than what AndrewK says, one knowing the answer and the other not. The real conflict is that Bob knows the one answer, and Alice knows there are still two answers.
This sort of thing has been going on long ago. I can entangle a pair of particles. Bob measures one and Alice (far away) measures the 2nd one to still be in superposition. The difference here is they seem to be doing to the same object, instead of leveraging entanglement.

I don't believe in objective reality so this is a bit moot for me. — andrewk

The wording of the paper seems to be a argument against counterfactual definiteness (an objective reality). I'm all for that since I don't think there is such a thing, and Bell's theorem demonstrated long ago that you can't have both that and locality. — noAxioms

If you reject "objective reality", is there any interpretation other than Many Worlds which is acceptable?

That is what the MIT abstract says that it does:

Wigner can...perform an experiment to determine whether this superposition [in respect of a particular particle] exists or not. This is a kind of interference experiment showing that the photon and the measurement are indeed in a superposition. — Wayfarer

That the photon's state is in superposition. The other measurement is not in superposition with the photon. I suppose you can word it that the result of that known measurement is in superposition.
Anyway, yes, they can take the photon and do an experiment on it where it interferes with itself, without measuring the state. If they have learned of the result of the measurement taken by the other, this superposition is not observed.

From Wigner’s point of view, this is a fact— the superposition exists. And this fact suggests that a measurement cannot have taken place.

Yes, the superposition exists. The suggestion that a measurement cannot have taken place is false. The article does suggest this, but QM rules do not under any interpretation. From the beginning, Schrodinger's cat is in superposition despite the measurement obviously having taken place.

But this is in stark contrast to the point of view of the friend, who has indeed measured the photon’s polarization and recorded it. The friend can even call Wigner and say the measurement has been done (provided the outcome is not revealed).

So the two realities are at odds with each other. “This calls into question the objective status of the facts established by the two observers,” say Proietti and co.

Are they at odds? Schrodinger's reality is not at odds with that of the cat, and never has been. You can put a human in the box watching the cat if your interpretation insists that humans are special, but I assure you that none of the measurements mentioned by that article were made by humans. Humans learn of the results (of probably thousands of runs) only well after the fact.

Many Worlds has those observers in different world branches,
— Andrew M

I don’t regard that as an explanation so much as a cop-out.

No interpretation is a cop out, but MWI cannot have those observers in different world branches since they communicate. Alice knows the polarity and tells Bob that she does. Bob knows that the particle is still in superposition and tells Alice so. That cannot happen if the two are in different branches.

If you reject "objective reality", is there any interpretation other than Many Worlds which is acceptable? — Metaphysician Undercover

Most interpretations reject it. You take away Bohmian mechanics and Stochastic and Transactional interpretations, the latter two being interpretations with which I am not familiar. But all the ones you hear about (Copenhagen, MWI, Consistent histories, objective collapse, Wigner, QBism and Relational) all reject an objective reality. I have a personal preference for Relational, but I don't assert the other ones must be wrong.

If most interpretations reject objective reality, then how is the article referred to in the op saying anything new?

If most interpretations reject objective reality, then how is the article referred to in the op saying anything new? — Metaphysician Undercover

They're not. They're spinning it as something new. But if they've actually disproven the principle of counterfactual definiteness like the wording of the article implies (but does not actually state), then I'd like to hear from the side of those that assert it, like a Bohmian guy interpreting the results. I don't know enough about the interpretation to know how they interpret a superposition state.

First, before even reading the article, it's worth noting that among the many ways that I'm a relativist is that I'm a "perspectivalist." I don't actually like that term, because it suggests that I'm necessarily talking about the perspectives of persons when that's only a subset of it. I think that things are relative to "points of reference" (human perspectives, in particular places at particular times, being one set of points of reference) . . . although I don't like that term, either, because for one I don't want to suggest realism about points, and also people think of "frame of reference" in the physics sense, which is a more limited idea than my view. I haven't thought of/don't know a better term to use for it yet, though, so I use "perspectivalism."

At any rate, on to the article:

So first, this is partially because I don't know enough about how it is achieved, perhaps, but I've always been skeptical of the notion that we conduct experiments where we know with any degree of certainty that we're looking at a single photon, electron, etc. at a time. For one, obviously we can't check such things with our unaided senses. We have to rely on what machines are telling us is the case, and they can only tell us what we've constructed them to tell us, in whatever manner we've devised for them to indirectly tell us something.

The problem is both a control issue--how do we really know that we're only releasing a single photon, electron, etc.? And a knowledge issue--how do we know for sure that (a) we do finally have a correct model of subatomic structure (it turned out to be the case not too long ago that we didn't have the correct model), and (b) we do have a complete model, so that we know with any certainty that there aren't other things going on--other sorts of phenomena that we're simply not aware of yet?

Aside from that, what this experiment is actually doing is taking a pair of supposedly entangled photons (I say supposedly because I'm not sure how we're observationally confirming that that's what we have) and splitting them so that Bob observes m re his photon, x, and Alice observes n re her photon, y. Theory has it that x and y should have a specific relationship, and m and n are not consistent with the relationship x and y are supposed to have.

So the first obvious question is this: if we're observing x and y to have a different relationship than they're supposed to have (and this is supposing that we're observing both x and y, which from my scan of the experiment (the actual paper is here, by the way: https://arxiv.org/pdf/1902.05080v1.pdf), on the one side we're actually observing the classic "interference pattern," we're not really observing the properties of a single photon), then the theoretical notion that x and y would be incompatible with respect to m and n is simply wrong, and insisting that it's correct is an example of theory worship.

The bottom line is that Bob and Alice are observing two different things in response to two different things (we're talking about two different photons). The supposed problem arises because of what theory proposes about what they should be observing about their two different things. Curiously, we interpret this as the theory being correct rather than noting that theory doesn't gel with what we actually observe, thus we are going to need to revise our theory at some point.

Thank you for the thoughtful post. I agree about your point on theory worship.

Thanks to calling my attention to this thread. I note that the cited article does not fully describe the experiment, and says in one paragraph that the experimenter is Caslav Brukner, and in the next that the experiment is the work of Proietti and co. Thus, one must not rely on the article too heavily.

As Mark Twain said of reports of his death, the conclusion that reality is inconsistent is greatly exaggerated. Let us begin with a simple observation. Assuming that the experiment has been adequately described, there is no dispute over the observed facts. Everyone reading of the experiment will agree that the observations on each side are exactly as reported. There is no evidence supporting the claim that reality is self-contradictory -- because no one has observed that what is, is not.. The contradiction lies in the conclusions drawn from the two sides of the experiment. As these conclusions are based on interpretive assumptions made about self-consistent facts, it is these assumptions, and not reality, that is drawn into question.

As I have not yet read the original work, but only the cited MIT Technology Report article, I can offer no detailed analysis of the experiment -- as I have for experiments allegedly supporting the "Delayed Choice" and "Quantum Erasers" myths. (https://www.youtube.com/watch?v=F-PAjJcRVCs). I suspect that my response to the detailed experiment would rest on the fact that quantum observations do not reveal the prior state of the "observed" system, but the interaction of that system with the experimental apparatus. Thus, in Wigner's Friend experiments, neither Wigner nor his friend obtain unambiguous information about the prior state of the system. Rather, each obtains information about the interaction of that system with their (different) local measurement apparatus.

In response to a recent question on my quantum erasure video, I outlined by views on quantum entanglement, which may be relevant here:

Let us begin by saying what quantum entanglement is not. It is not any kind of causality or spooky, instantaneous action at a distance. How do we know this? Because, theoretically, entanglement is a consequence of relativistic quantum theory, and relativity precludes this sort of interaction.

Yes, I know that there is no information transmitted faster than the speed of light in entanglement experiments, but that is not way relativity precludes spooky action at a distance. Imagine a EPRB-type experiment with two observers, A and B, equidistant from the entangling event. In our frame of reference A and B detect the spin simultaneously, so, if action at a distance were involved, it is indeterminate whether A's detection event is acting on B's, or B's on A's.

However, that is not the worst of it -- for if we consider the problem in a frame of reference in which A is moving toward the initial event, then A will detect the spin first and, if action at a distance were involved, necessarily, the detection event at A would have to act on that at B. If we consider the experiment in a frame in which B is moving toward the origin, the reverse is true. Thus, neither can be acting on the other and there is no sort of action at a distance.

So, what is going on here? Two factors are neglected by the usual analysis: (1) Detection dynamics and (2) transtemporal symmetry.

First, the result of a spin observation is not the spin of the quantum prior to observation. Consider a spin-0 quantum that decays into two quanta with spin. Let the EPRB detectors be set at right angles. Then, no matter what spins are detected, the sum of the detected spins cannot be zero! So, the detected spins are not initial spin (which was zero). This would seem to violate conservation of angular momentum, but not if we consider the detectors as well as the observed system. Obviously, the extra spin comes from the detectors. Thus, the detectors must be considered as well as the observed system, and the observed spin is not the the prior spin of the system, but the result of the interaction of the system with the detectors.

There is no time limit on quantum entanglement, so, we must acknowledge that EPRB detectors are not isolated and independent, but synchronized and entangled -- and the material in them has been entangled since the Big Bang. Thus, part of the answer Aspect-type experiments is to apply the idea of quantum entanglement on a cosmic, rather than a local, scale.

Second, none of the analyses I've seen consider transtemporal symmetry. Every case of entanglement involves some conservation law. The original EPR paper involved conservation of momentum. EPRB and Aspect-type experiments involve conservation of angular momentum. By Noether's theorem, all conservation laws reflect dynamic symmetries. Conservation of momentum reflects translational invariance and conservation of angular momentum reflects rotational symmetry. This suggests a deeper reflection on symmetry.

When we consider translational and rotational symmetry in different relativistic frames of reference, we wind up connecting points at different times, because the points that are symmetric in different frames have different times.

The most relevant application of transtemporal symmetry involves the Pauli exchange principle. In non-relativistic quantum theory, when we exchange the spatial coordinates of two Fermions (such as electrons), the multi-Fermion wave function changes sign. In the relativistic formulation, we must consider the Fermions not only at the same time, but each Fermion at its own time (this is Dirac's multi-time formulation). That means that world wave function, the joint wave function of every similar Fermion, has symmetries that link it not only at a given time, but at all times since the Big Bang.

This confirms what I said earlier about the non-independence of detectors in Aspect-type experiments. The detector wave functions are related and constrained by a transtemporal symmetry extending through all space-time. So, entanglement does not involve action at a distance, but transtemporal symmetry.

As bulk matter (such as cats and quantum detectors) is held together by nonlinear electron-electron interactions, the superposition principle does not apply to them. So, Schroedinger's cat is either alive or dead and never both.

The citation for the actual experiment is: https://arxiv.org/abs/1902.05080