No one is saying that Wigner has to be entangled. — Kenosha Kid

I'm not commenting on the paper; I'm commenting on the notion that branches are universal. I don't think I have any comments on the paper at this time.

ETA: Or perhaps I'm just misinterpreting what you mean when you say the branches in MWI are universal? What I mean to clarify is that "universe branching" is "subject relative"; what looks like different "universes" to the cat isn't necessarily different "universes" to Schrodinger.

What I mean to clarify is that "universe branching" is "subject relative"; what looks like different universes to the cat isn't necessarily different universes to Schrodinger. — InPitzotl

Okay you're talking about subjectivities across branches, yes? That's not what's meant by observer-dependence here: it's not about different versions of you across different branches seeing different facts, rather that different observers will disagree within a branch/unbranched universe.

The equivalent in MWI if what's happening here is that friend measures cat, friend term in wavefunction branches, Wigner entangles with friend, but Wigner can still access both branches. This is a nonsense. We could modify the concept of entanglement to ensure that it can only be said to occur when Wigner makes an actual measurement, but then we're back in magical consciousness territory, contrary to the point you made earlier that observers are just examples of physical systems.

The equivalent in MWI if what's happening here is that friend measures cat, friend term in wavefunction branches, Wigner entangles with friend, but Wigner can still access both branches. — Kenosha Kid

I agree that this is nonsense, but what you said that I responded to was this:

When Wigner's friend measured the cat, the universal wavefunction would split then universally. — Kenosha Kid

...and there's certainly nothing universal happening in the sense that Wigner entangles with his friend when his friend entangles with the cat.

there's certainly nothing universal happening — InPitzotl

In MWI there is a single, universal, objectively real wavefunction. Any branching is universal: it is a branch in the universal wavefunction.

Any branching is universal: it is a branch in the universal wavefunction. — Kenosha Kid

...but you apparently agree the branches are not universal in the sense that Wigner branches when Wigner's friend branches:

After the friend measures:

(B) | not measured > X ( | measured live > | alive > + | measured dead > | dead > )/root(2) — Kenosha Kid

No one is saying that Wigner has to be entangled. — Kenosha Kid

Right?

There's a part of your post in which you briefly stopped speaking of entanglement and spoke of observation. That's the part I referred to in my previous reply to you. — Kenosha Kid

Does not every observation, as an interaction, cause entanglement? (Leaving room still for partial observation; see below).

What we see instead is evidence of observer-dependent collapse: Wigner knows that collapse has occurred for the friend, but for Wigner the friend is still in superposition as evidenced by interference effects between the alive and dead terms (collapse has not occurred for Wigner). — Kenosha Kid

Is this while Wigner and his friend are already entangled? Because this sounds like the same relationship the friend has to the cat before opening the box: he knows there is a cat for whom the particle’s wavefunction has collapsed inside the box, but to him the cat is still in superposition of having experienced different kinds of particle collapse.

I’m getting the sense that this new evidence is of the possibility of SOME information from inside the box being communicated to the friend without it being enough of the right information to collapse the wavefunction; likewise the friend can communicate some info to Wigner without collapsing the wavefunction from Wigner’s perspective. Is that accurate? If so, would it also be accurate to say that the particle is not entangled with the friend or Wigner in those respective cases? Basically, Wigner can observe anything about his friend that can’t imply anything about the collapse of the particle’s wavefunction, and keep those aspects of the system (friend, cat, particle, etc) superposed from his perspective so long as he does so?

If that is what’s going on here, that’s as expected by my interpretation, which I’ve (perhaps erroneously) been calling MW.

I've long wondered about what does the MWI branch into?

If so, why is it difficult for you Albert to grasp that absent an observation, the moon's existence is a question mark, a very big question mark! — TheMadFool

The moon has a gravitational influence on Earth, such as with tides and keeping the Earth from wobbling too much, which keeps seasonal variation from being extreme. We can ask the same thing about the stars and galaxies out there, but inertia is based on all the mass in the universe resisting our change in acceleration.

Point is that there are all sorts of indirect observations being made even when we don't realize it. Nocturnal animals make use of the moon and starlight, even while we're asleep. The moon is part of the world we observe. Asking whether it exists when we're not looking is to ignore the larger context of the world itself the moon is part of. It can't just not exist and the world we observe remain the same. It's like how the ground holds us up even when we're not consciously aware of it.

That's the problem with asking whether things exist when we're not observing them. The things we do observe depend on the things we're not observing at the time.

How is it that the very method you critique derived the evidence you're using to substantiate that critique? — Isaac

What I'm critiquing is not science but scientism.

Behind the Blind Spot sits the belief that physical reality has absolute primacy in human knowledge, a view that can be called scientific materialism. In philosophical terms, it combines scientific objectivism (science tells us about the real, mind-independent world) and physicalism (science tells us that physical reality is all there is). Elementary particles, moments in time, genes, the brain – all these things are assumed to be fundamentally real. By contrast, experience, awareness and consciousness are taken to be secondary. The scientific task becomes about figuring out how to reduce them to something physical, such as the behaviour of neural networks, the architecture of computational systems, or some measure of information.

Indeed, scientists and philosophers have begun to see through it. The collapse of the purportedly observer-independent nature of fundamental physical particles is a big part of that.

Effectively that registration reduces the number of possibilities to unity for you (or it), while staying distributed for me (or some other device). So something else is going on here. — Kenosha Kid

Doesn’t it suggest that perspective is fundamental? Intuitively, you would think that perspective is a product of the evolved brain, a recent layer of the pre-existing reality, but this suggests that perspective is actually a constituent.

That's the problem with asking whether things exist when we're not observing them. The things we do observe depend on the things we're not observing at the time. — Marchesk

Once, in Karl Popper's living-room, I asked him why he rejected [Kantian idealism], whereupon he banged his hand against the radiator by which we were standing and said: 'When I come downstairs in the morning I take it for granted that this radiator has been here all night'. — Bryan Magee, Philosophy of Schopenhauer

but you apparently agree the branches are not universal in the sense that Wigner branches when Wigner's friend branches: — InPitzotl

But that's not MWI, that would be some pretty meaningless compromise. The whole point of MWI is a single, real, universal wavefunction.

In remember that quote exactly! Wonderful book. But man that was quite a silly thing for Popper to argue. It takes so much for granted, which is a common mistake or so it seems to me.

But that's not MWI, — Kenosha Kid

Nobody is saying it's MWI.

What I'm trying to unwind is what you could mean by this:

that is, once entangled, there can be no interference between the live and dead terms apparent to Wigner. What the recent experiments show is that, even after Wigner's entanglement, those interference effects persist, and Wigner remains as per (B). It is only when Wigner _knows_ his friend's measurement outcome that he himself branches, i.e. the wavefunction is epistemic, not ontic. — Kenosha Kid

Unwinding, (B) is this:

(B) | not measured > X ( | measured live > | alive > + | measured dead > | dead > )/root(2) — Kenosha Kid

...for clarity I've underlined Wigner's state and bolded Wigner's friend's states.

Given this, the above paragraph makes no sense:

• At B, Wigner's friend is entangled with the cat('s life); but Wigner is not entangled
• At C, Wigner's friend and Wigner are entangled with the cat('s life).

To speak of Wigner being entangled (with the cat's life) without speaking of (C) has no meaning to me.

So what do you mean by it? It sounds like you're describing some state where Wigner's entangled with the cat('s life) yet still at (B).

But man that was quite a silly thing for Popper to argue. It takes so much for granted, which is a common mistake or so it seems to me. — Manuel

After we came out of the church, we stood talking for some time together of Bishop Berkeley's ingenious sophistry to prove the non-existence of matter, and that every thing in the universe is merely ideal. I observed, that though we are satisfied his doctrine is not true, it is impossible to refute it. I never shall forget the alacrity with which Johnson answered, striking his foot with mighty force against a large stone, till he rebounded from it, "I refute it thus."

I've discussed this many times on forums, and the consensus usually is, Johnson was right. (That said, I'm not a Berkeleyian idealist, but I won't divert the thread on that point.)

A bit surprising to see that even here. Which is why it is important to state what you (or anyone) take idealism to entail.

But no serious idealist would ever say that a rock is not solid or that I can move an apple by thinking about it or whatever craziness they may say.

In either case for QM, I know you think observation is crucial for the collapse. I personally think that despite what QM discovers, idealism, materialism, objectivism or anything else remain intact. Anyone who already believes in one of these views will simply accommodate the experiment to whatever metaphysical views they already have.

Probably not the best way to go about it, but I think it's true at this point in time.

I know you think observation is crucial for the collapse — Manuel

Not only my opinion. The question I ask is, if the many worlds conjecture is a solution, what is the problem? What would its fanatics advocates, like David Deutsche, be obliged to admit if it was shown it could not be real?

I know. I believe the view that human beings are specifically relevant for collapse is a minority one, though this by no means implies it is wrong at all.

I think they're trying to maintain the intuition of determinism. They don't like that current QM is probabilistic is my guess. I don't know what they'd do, but since there's no way to test it, arguments as to its implausibility is going to change minds. They know Many Worlds is pretty wild.

Unless we build a much larger collider, we seem to be stuck. And even that might not change the results.

Unless we build a much larger collider, we seem to be stuck. And even that might not change the results. — Manuel

The bigger the equipment, the bigger the questions.

Curious how one of the things 'the scientific view' does is, in the first place, to deprecate h. sapiens to being 'another species', 'chemical scum', in Hawking's charming phrase, and then on the other, to declare, nevertheless, that h. faber, scientific man, is the arbiter of reality.

It's a delicate balance imo. Not that reality isn't shaped to a very large extent by us, I agree with you on that point. But to think of ourselves as bigger than we are is a problem. Much misery came our way when we thought we were the center of the universe or that the sun went around us.

We've done remarkable things to discover all these new facts about the universe. But I think we ought to keep in mind our animal part, while appreciating our amazing intellectual capabilities. So no, I don't think we are "merely" another species, but I also don't think we should be too proud, as we burn our planet to ash.

But to think of ourselves as bigger than we are is a problem. — Manuel

'Size' has nothing to do with it. What we bring is perspective.

And knowledge, and explicitness and order, etc. ;)

Many Worlds is fully compatible with Wigner's Friend. It's just a situation where worlds not only can split but also merge again under the right conditions.
— Andrew M

That would be different to many worlds in itself. If you have to add a thing (merging criteria) that's a new theory. — Kenosha Kid

For MWI, merging as well as splitting is entailed by the unitary dynamics. From the friend's perspective he has, for all practical purposes, performed an irreversible measurement - he measures spin-up in his world while his doppelganger measures spin-down in another world. But from Wigner's perspective, the friend (and his measurement) is simply in superposition (i.e., within Wigner's single world) and he can always apply a unitary transformation that reverses the friends' measurements, thus merging the friends' worlds back into one.

Copenhagen was originally epistemological, yes. Iirc Bohr himself went the ontological route in the end (I didn't know this until someone here found a relevant quote, should be able to dig it out if need be). — Kenosha Kid

I would be interested in the quote if you can locate it.

But anyway there's a bunch of ontological Copenhagenists out there. — Kenosha Kid

Strictly-speaking, their position is more like an objective collapse theory (like GRW or the Penrose interpretation) which necessarily differs from the standard unitary formalism. In those particular objective collapse theories, superposition applies to microscopic objects only (which can include the microscopic Wigner's friend experiments that have been performed). Whereas the standard unitary formalism applies also to macroscopic objects.

Agreed, if 'reality' is left ambiguous between a unique realist objective ontology and many relativist subjective appearances. Philosophical disagreement and repeated failed attempts to discover some missing factor to make everything orthodox make all objectivist attempts suspect from the start. — magritte

This is where specific interpretations attempt to fill in the gaps. Those interpretations will remain with us until we have more definitive experiments.

I'd be interested in hearing both your thoughts on what kind of relativism this is. — Kenosha Kid

I think Rovelli's relational interpretation is helpful here. It provides a clean abstraction around the idea of reference frames that covers all the issues raised by Wigner's friend:

The essential idea behind RQM is that different observers may give different accurate accounts of the same system. For example, to one observer, a system is in a single, "collapsed" eigenstate. To a second observer, the same system is in a superposition of two or more states and the first observer is in a correlated superposition of two or more states. RQM argues that this is a complete picture of the world because the notion of "state" is always relative to some observer. There is no privileged, "real" account. — Relational quantum mechanics

As I see it, the main difference between MWI and RQM is that RQM is silent on what a superposition consists of (since it is a more abstract interpretation). Note that what is now called MWI was originally called the relative state formulation by Hugh Everett, and the two interpretations share that relational (or relative) aspect.

Since the observer-dependence of collapse in these Wigner's friend experiments is essentially a disagreement between observers in their own frames as to whether something is in superposition or not, something like this might be the answer.

https://www.nature.com/articles/s41467-018-08155-0 — Kenosha Kid

I entirely agree. In fact, I see we had a brief discussion on this a year back!

Nobody is saying it's MWI. — InPitzotl

Your initial objection was to my comment that MWI doesn't hold up any more if we accept the observer-dependence interpretation of the Wigner's friend experiments. If you agree that what you're proposing is not MWI, how is this progressing your counter-argument?

for clarity I've underlined Wigner's state and bolded Wigner's friend's states. — InPitzotl

prior to entanglement. But as I've told you, Wigner and his friend are not unentangled immediately prior to Wigner's measurement of his friend.

Of course, in practice, Wigner and his friend ought to have been entangled even at this stage, since their actions are correlated and entanglement _is_ correlation.

So what do you mean by it? — InPitzotl

The OP is asking for your views about the purported observer-dependence of facts (collapse/branching) in light of the recent strengthening of the validity of non-destructive measurement techniques.

What the Wigner's friend experiment show is that, after entanglement, after the friend has made his measurement, but before Wigner has made his measurement, when we would expect something like (C), Wigner is still seeing the world as per (B). Wigner is entangled with the lab insofar as communication and coordination about the experiment is ongoing between he and his friend, but Wigner still sees the lab in a superposition: the lab has branched for the friend, but both branches are evident to Wigner, contrary to MWI.

If you agree that what you're proposing is not MWI, — Kenosha Kid

Proposing? What the heck are you talking about? I just described what's not a thing, and had thought you were agreeing it wasn't a thing. But given you think I'm proposing something that isn't MWI, I would say that clearly you're confused.

But as I've told you, Wigner and his friend are not unentangled immediately prior to Wigner's measurement of his friend. — Kenosha Kid

This is a handwaved concept of entanglement. Wigner and Wigner's friend have lots of states that are entangled. What constitutes being "unentangled" to you?

What the Wigner's friend experiment show is that, after entanglement, after the friend has made his measurement, but before Wigner has made his measurement, when we would expect something like (C), — Kenosha Kid

No. We would expect (B). Wigner's friend entangling with the cat's life does not instantaneously make Wigner entangled with the cat's life. If Wigner hasn't done any measurements, there's no reason for Wigner to be entangled.

Wigner is entangled with the lab insofar as communication and coordination about the experiment is ongoing between he and his friend, — Kenosha Kid

I've no idea what "entangled with the lab" means, but it sounds like a fuzzy red herring. Surely Joe, who does a simple double slit experiment (or quantum erasure experiment, if that's what's confusing you), is just as entangled with his lab as Wigner is with his.

Wigner still sees the lab in a superposition: the lab has branched for the friend, but both branches are evident to Wigner, contrary to MWI. — Kenosha Kid

MWI has no objection whatsoever to the universal wavefunction being in a state like (B). Incidentally, (B) implies that Wigner's friend has branched. That live cat in (B) does not know what hydrocyanic acid smells like. But Wigner himself is not branched in (B). The branch is "universal" in the sense that it's a branch in the universal wavefunction (it's right there, in (B)). But it's not "universal" in the sense that Wigner's friend branching implies Wigner branched (he clearly hasn't). Having a person not branch with respect to a wavefunction that is in superposition is not a problem; how else will Joe see an interference pattern?

For MWI, merging as well as splitting is entailed by the unitary dynamics. From the friend's perspective he has, for all practical purposes, performed an irreversible measurement - he measures spin-up in his world while his doppelganger measures spin-down in another world. But from Wigner's perspective, the friend (and his measurement) is simply in superposition (i.e., within Wigner's single world) and he can always apply a unitary transformation that reverses the friends' measurements, thus merging the friends' worlds back into one. — Andrew M

What you're describing art the start is two unentangled systems (B). That is not what the experiment is describing, in which Wigner and his friend are correlated (should be (C), but isn't).

As I said above, the alternative is to insist that entanglement doesn't occur just when two systems exchange information, but when an observer makes a measurement, which is not justified by the experimental setup.

As for merging, in this experiment Wigner is simply performing an interferometer experiment. Also since, in his frame, the lab is still in superposition, there's nothing to be undone. From the friend's point of view, there's nothing Wigner can do to find a different outcome: in the alive branch the cat is alive and this is what Wigner must find; in the dead branch the cat is dead and this is what Wigner must find.

I would be interested in the quote if you can locate it. — Andrew M

Will do.

I think Rovelli's relational interpretation is helpful here. It provides a clean abstraction around the idea of reference frames that covers all the issues raised by Wigner's friend: — Andrew M

Yes, I think relational comes out strong.

I entirely agree. In fact, I see we had a brief discussion on this a year back! — Andrew M

Well remembered!

This is a handwaved concept of entanglement. Wigner and Wigner's friend have lots of states that are entangled. What constitutes being "unentangled" to you? — InPitzotl

It's not a point of view. Can you at least look it up on Wikipedia or something?

What the Wigner's friend experiment show is that, after entanglement, after the friend has made his measurement, but before Wigner has made his measurement, when we would expect something like (C),
— Kenosha Kid
No. We would expect (B). — InPitzotl

"after entanglement" here refers to Wigner's entanglement with the lab, not his friend's entanglement with the cat. Again you would understand it if you just bothered to read the paper containing the experiment rather than taking this course of adopting a strong position out of ignorance and me having to repeatedly point out where you're being irrelevant.

I've long wondered about what does the MWI branch into? — Shawn

I imagine (because my math sucks!) the branches are like folds in an origami and the worlds of MWI are worldlines of particle-waves, that is, the many unfolded/enfolded shapes of the sheet. We 'inhabit' the edge of an unfolded shape (or worldline) that doesn't branch "into" (something else) but only branches away from other worldlines.

... I've asked this of others in the past and not quite got an answer, what differentiation do you make between a maximally pluralistic objectivity (e.g. one objectivity per reference frame) and relativism (as in frame-dependence, not subjectivity, as would apply to an atom, a device, or a point in spacetime)? — Kenosha Kid

I think of it this way: the latter concerns 'variable/configurable observer-observations of one (the only) worldline' (e.g. one elephant and the many blind men) whereas the former concerns 'any one-worldline measurement of N-possible worldlines' (e.g. locked inside a completely enclosed carnival ride and looking out at the carnival through a peephole). So tell me what's wrong with this pluralism/relativism conception or why it doesn't work for interpreting fundamental physics.

P.S. I should add that observer-dependent facts is only one possible interpretation, even if it is the one that gets preferential treatment in paper titles.

The other options are non-locality (FTL communication) and super-determinism (backward or nonlocal causality). — Kenosha Kid

They all probably work after a fashion (like epicycles & the aether) but I'll stick with the most parsimonious observer-invariant (my anti-idealist, anti-antirealist bias) interpretation of QM we've got which for me (as far as I know) is the MWI.

What you're describing art the start is two unentangled systems (B). That is not what the experiment is describing, in which Wigner and his friend are correlated (should be (C), but isn't).

As I said above, the alternative is to insist that entanglement doesn't occur just when two systems exchange information, but when an observer makes a measurement, which is not justified by the experimental setup. — Kenosha Kid

I'm not sure whether we're referring to the same experiment. I'm thinking of David Deutsch’s version of the Wigner’s friend experiment which Caslav Brukner describes in A no-go theorem for observer-independent facts (one of the papers you linked to earlier).

From that paper:

The novelty of Deutsch’s proposal [9] lies in the possibility for Wigner to acquire direct
knowledge on whether the friend has observed a definite outcome upon her measurement or not without revealing what outcome she has observed. The friend could open the laboratory in a manner which allowed communication (e.g. a specific message written on a piece of paper) to be passed outside to Wigner, keeping all other degrees of freedom fully isolated. (italics mine)

So the upshot is that the friend has made a definite measurement and reported that she has done so to Wigner, without telling him what the result was. At the same time the lab remains in superposition for Wigner, per your (B). Thus the fact about the friend's measurement result is reference-frame dependent (at least, that's the option I choose from Brukner's no-go theorem). That's prior to entanglement between Wigner and the lab, per your (C). At that point there will be no interference effects.

What I'm critiquing is not science but scientism. — Wayfarer

You clearly said that the scientific method as incapable of accommodating the subjective. The method, not the philosophy sometimes associated with it. You then went on to say that science derived directly from that method proved this.

You've basically disproven your point. Science is perfectly capable of exploring the intrinsic subjectivity of experience. There are dozens of theories about reality which include, or even hinge upon, the idea that we subjectively create aspects of it. It's probably the leading theory in neuroscience at the moment, for example.

If you're critiquing, not the method, but the philosophy - scientism, then the same critique should apply to all philosophies which attempt to acquire the 'view from nowhere'. Platonism, for example, which posits that there exist forms like numbers and essences which are true independent of the human minds which grasp them - the view from nowhere. Where is your critique for these approaches?

Science is perfectly capable of exploring the intrinsic subjectivity of experience. — Isaac

Do you recommend Daniel Dennett's approach to the subject?

Do you recommend Daniel Dennett's approach to the subject? — Wayfarer

To the extent that I'm aware of it, yes. I was in broad agreement with his article 'Quining Qualia' for example. I think it made some very good points about the subjectivity of experience. Again, it seem the opponents of Dennett's position who are wanting the 'view from nowhere', where 'the blue quale' is a thing that we can call into existence as a group of humans and talk about as if we all knew what it really means.

Accepting reality as being that which we subjectively model from hidden external states forces us to discard the objective reality of things like qualia, number, consciousness, purpose, God... All these things must only be models, better or worse for their utility.