In CI, measurements are explained via the 'collapse' of the wave-function. The problem is, however, that CI is simply ambiguous on it. In fact, I would say that there is no 'Copenaghen Interpretation' at all. It is rather a 'class' of very different views that are, so to speak, 'grouped' together. — boundless

With this I agree. I often characterize it the way Bohr did: as a description of what can be known, and not at all a description of what is. It is rather a jumping board by which a description of what is might be bounded. Others take their interpretation of what is and label it Copenhagen because it fits within these bounds.

But where is the ambiguity? The problem is that the formalism of the theory alone does not identify what is the 'observer'. Yet, in order to explain the wave-function collapse you need to posit an 'observer'. If not, we cannot explain why our 'everyday world' looks classical, so to speak.

I thought the difference between the various interpretations often focuses on the treatment of the Heisenburg Cut. The Wigner's friend scenario basically puts one observer on either side of that cut, and drives out many of the differences between the interpretations.

Anyway, let's see the proposed solutions to this intrinsic ambiguity of CI.

Firstly, one might try to say that, indeed, there are physical objects that count as 'observers'. For instance, objects that are able to store and process 'information', like e.g. computers, registering devices, brains etc. If I am not mistaken this is the view of Wheeler. The 'universe' is 'participatory' in this view because each of these 'observers' can 'modify' reality by 'collapsing' the wave-function.

Isn't there a problem with this view in that without earlier collapse, none of these registered devices could possibly exist in the first place? If understanding of a measurement is what causes the collapse, how could the thing doing the understanding come about to do it? A chicken/egg problem.

Secondly, another possible way to deal with this is to go with RQM (Relational Quantum Mechanics) as Rovelli et al do. Here, all physical systems can be 'observers' and the 'measurement' is simply a physical interaction.

That definition of 'measurement' is hardly confined in RQM. With the exception of anthropocentric Wigner interpretation, I think all the interpretations assume something along these lines, and even Wigner backed down from his own interpretation due to it reducing to solipsism.

This is because, according to Rovelli, there is nothing special about computers, etc:
"By using the word “observer” I do not make any reference to conscious, animate, or computing, or in any other manner special, system. I use the word “observer” in the sense in which it is con- ventionally used in Galilean relativity when we say that an object has a velocity “with respect to a certain ob- server”.
...
We do not need a human being, a cat, or a computer, to make use of this notion of information."
But Rovelli's RQM is, in fact, not classified as 'CI'. Why? Rovelli claims that QM is complete, whereas for CI you still need to consider something as classical.

OK, that is a point about CI. To know something about system X, I must exist, and I am classical, thus something needs to exist in a classic sense. MWI falls under CI then?

Or one might argue that, for instance, you cannot have a well-defined concept of information without relating it to some form of consciousness (not necessarily human, in fact). And you end up with the 'Consciousness causes collapse' interpretation

The Wigner interpretation I referenced.

Or a Kantian-like interpretation like the one proposed by Bitbol and others (and possibly of Bohr at least for some parts of his life, if Bitbol is right...) if you do not like the idea that consciousness really 'modifies' reality (but it is nevertheless necessary to have an 'observer').

How does this interpretation get around the chicken/egg problem if an observer is necessary for an observer to collapse out of a system? I presume Bitbol does not consider a dust mote to be an observer?

Obective collapse theories (such as GRW and Penrose's) are physically different theories to standard QM. I don't know what specific explanations they would give for this particular experiment. But they make predictions for Wigner's friend-style experiments that make them experimentally differentiable from standard QM. — Andrew M

OK, my comment assumed it was an interpretation. But if the the different theory is experimentally differentiable from standard QM, then by all means let's devise the experiment.

As Brukner says in that link (my italics):
In my eyes, outcomes 1 and 2 would indicate fundamentally new physics. I will not consider these cases further and regard quantum theory to be a universal physical theory. This leaves us with situation 3 as the only possible outcome of Deutsch's thought experiment. The outcome is compatible with the Everett interpretation: each copy of the observer observes a definite but different outcome in different branches of the (multi)universe. The outcome is compatible with the Copenhagen interpretation too, but it is rarely discussed what the implications of this claim are for our understanding of physical reality within the interpretation. The rest of the current manuscript is devoted to this problem.
— On the quantum measurement problem - Caslav Brukner

My comment on that is about how Copenhagen deals with those implications, but since I understand that 'interpretation' to be an epistemological one, it doesn't really have implications for physical reality, only about what one observer might know vs another.

I'd be curious to know your thoughts about the RQM questions from my earlier exchange with boundless here.

From that exchange then:

Thanks boundless - they're excellent videos and well worth watching for anyone with an interest in the philosophical aspects of QM. Fun quote from Rovelli at 36 mins: "When I told Max (Tegmark) that he was a relationist, he told me that he is going to convince me that I'm, without knowing, a Many World believer." — Andrew M

They're almost the same thing, with different definitions of 'is real'. RQM says this world is real to me and a different world is real to anything else (the cat say), so they're both talking about different worlds. MWI says they're all equally real, and RQM says none are real, only that there are relations between worlds and observers. Neither requires an observer to by anything more sentient than a speck of dust, and I find any interpretation that requires otherwise to border on religion. Even Copenhagen, an epistemological interpretation, can be applied to the dust speck. It 'knows' about the wind because it reacts to it, and thus has a relationship with it. It is real in a way that something unmeasured isn't.

Still, there are very 'real' differences between MWI and RQM, and I don't see how either Rovelli and Tegmark are going to convince the other that they hold the opposite view. I have yet to view the video since I tend not to get my information from such sources.

Anyway, Rovelli has a slide at 40:15 that says:

The price to pay for RQM:
We need to get rid of the notion of:
- absolute (observer-independent) state of a system
- absolute (observer-independent) value of a physical quantity
- absolute (observer-independent) fact

I don't consider it a price to discard these things. Quite the opposite. All the threads on forums such as this one discussing some form of "why is there something, not nothing" take for a premise that there is something, and then run into all sorts of valid contradictions that follow from it. So it appears that the notion, however intuitive, of there being something is the thing with the price to pay. My answer to the above question is that perhaps there isn't anything, and thus there is no need to have to explain its being.

But as I mentioned before I find (among other things) this 'oddness' as an indication that some kind of 'paradigm change' is required. Also, it seems that, in general, there is a trend to more and more 'counter-intuitiveness' in physics... — boundless

Boundless echoes my mistrust of such intuitions. My username carries an implication of not taking any of them as a premise, at least not without explicitly calling them out. I'm always on the search for assumptions I don't even know I'm making and are thus unseen biases.

The claim of RQM is that if you take this step, everything becomes simpler (cfr: special relativity, and the need of getting rid of absolute simultaneity.)
— RQM - Rovelli

A lot of people had no trouble getting rid of absolute simultaneity when relativity came into acceptance a century ago. Some still cling to it religiously.

My questions are:
1. Is this just a semantic difference with Many Worlds? (That is, there are nonetheless many physical branches, but there are only deemed to be facts relative to an observer's branch.)
2. If not, then what is the substantial physical difference and what explains physical interference effects? (Many Worlds would explain it as physical interference between branches.)

Well it seems a real difference. There 'are' no branches at all in RQM, and they are 'are' in MWI. That's the huge difference such that I doubt an adherent to either interpretation can be convinced that they also hold to the other. Even the relations don't exist.
I always reach for integers and the simplest cases for many of my examples. I claim that 7 is indeed 2 less than 9, but I don't claim that that particular relation between 7 and 9 exists as a real fact. Is it still counterfactually definite if I word the claim that way?

What is interference? It seems to be 'maybe' as an answer to a question not yet asked, a measurement not yet taken. So X = square root of 2 is 1.414 but also -1.414 and both those values can work through my equation until a choice must be make before the mathematics can continue. That's a measurement, and now there are two equations that proceed in different directions using a now real value for X instead of one in superposition.
That's a cheesy description of superposition, but I feel it fails as an analogy of interference since the two values never seem to interact with each other. I like the question and it deserves a better answer than that.
It seems too simple since they are discreet values, which a wave function really does describe a wave that yields an interference pattern when the wave for path-A is added to wave for path-B, which yields an interference pattern in 2D but only a flat scalar in 1D. So my X probably needs to be more complex than a simple scalar with two discreet values that satisfy a polynomial.

Maybe the point is that the foliation is not directly observable. — boundless

It seems not to be. It would probably violate SR if it was.

As you say, we can observe only a "frame that corresponds locally to that foliation has the property of isotropy both in what is and in appearance". Just a guess. As I said, I am quite at loss.

Yes. What is: Space is expanding everywhere equally(ish) at all simultaneous points. I say 'ish' because it expands more in empty places than crowded places, but not more in any particular direction. This isn't true in other frames.
Appearance: Space appears to be expanding equally at all points at some fixed distance from any observation point. So space is expanding close by, but not expanding say 6 billion light years away. Doing these sorts of measurements is how they determined the acceleration of expansion in the first place. You can't measure what is now, but you can measure how it appears now.

I see...but if this does mean that non-locality is compatible with GR (as it is usually understood) why people consider non-locality problematic?

All of relativity seems to depend on locality, while QM interpretations might have other ideas. It is why I resist interpretations that discard locality in favor of counterfactual definiteness. I just don't see how relativity can make sense without locality. One can blatantly change the past, not just events outside one's future light cone.
That and the fact that counterfactual definiteness has all sorts of seemingly paradoxical philosophical baggage that goes away if you don't accept the principle.

Well, yeah, you are right.
But IMO this leads either to the 'Andromeda Paradox'/Riedtjik-Putnam argument scenario or some form of retro-causality.

Don't get your Dutch names wrong... I've got one myself.
The Andromeda thing and the Rietdijk-Putnam thing are pretty much the same, and are only paradoxical if you try to combine assumptions from both interpretations of time. All that proves is that they are not both correct.
Presentism demands an objective ordering of events (although no particular one), but a preferred folation does not demand a preferred moment in time.

Anyway, Antony Valentini proposed that cosmological observations might help to solve interpretational problems in QM. The de Broglie-Bohm interpretation actually makes the same predictions of QM only if the 'quantum equilibrium hypothesis' is satisfied, i.e. if the modulus square of the wave-function corresponds to the actual probability distribution (this assures that dBB satisfies the Born Rule). However, in general, this might be not true. Hence his proposal: maybe at the earliest stages of the evolution of the Universe, that hypothesis was not satisfied and - as a consequence - we should see empirical evidence against the Born Rule.

Here is the link to his talk (at the same conference): https://www.youtube.com/watch?v=XYZV9crCZM8.
Here the link to the Q&A session: https://www.youtube.com/watch?v=8qnuNLB61RA.

Way to kill an afternoon, eh? Thank you for the link. Not sure how much I'm interested in sinking an interpretation that I've already listed as low probability. I'd rather see them sink RQM. Always best to have ones own cage rattled once in a while.

Since none of this is new (it is demanded by QM right from the early days), how do any of the objective collapse interpretations get around this? Does this experiment change something? Did they expect a different result? I don't think so.
— noAxioms

They would predict that Wigner would not see interference for sufficiently complex friend systems. So the options are to either accept the experiment's result as falsifying their theory or else show that the experiment isn't scaled up enough to trigger a physical collapse by their criterion. — Andrew M

That is one thin explanation. If what Alice did wasn't complex enough to objectively collapse the wave function, she should be able to measure the subsequent superposition herself and not leave it to Bob. Of course, QM theory won't allow that, so the 'thin' explanation see to go against QM itself.
Of course maybe I just don't understand this explanation. I have not read your link and am not sure that I would find the answer there satisfactory.

Yes, I agree - it's just what quantum mechanics predicts will happen and so it's not contradictory (or unexpected) at all. But it does challenge objective collapse theories since they modify the standard formulation. — Andrew M

Since none of this is new (it is demanded by QM right from the early days), how do any of the objective collapse interpretations get around this? Does this experiment change something? Did they expect a different result? I don't think so.

Do you mean that some geometries in GR require such a foliation (rather than simply allow)? — boundless

The expansion of space is uniform only under one foliation. It isn't absolutely uniform since it seems resistant to local mass, but only under one foliation does the expansion switch to accelerating everywhere at once. Essentially, only the the frame that corresponds locally to that foliation has the property of isotropy both in what is and in appearance.

AFAIK, there are attempts to reconcile dBB and SR that use a preferred foliation (which is not prohibited by Lorentz symmetry) but I think that this does not satisfy many people because it goes against the 'spirit' of Relativity.

It apparently goes against the spirit of SR, and it pained Einstein to not keep that in GR. Physics is different in other frames since non-local observations are allowed in GR.

A preferred foliation is one thing. A preferred moment (presentism) is more of an offense to relativity
— noAxioms

But is there a real difference between the two?

Of course. One objectively orders any pair of events, and other may or may not attach an ontological status to each event (has or has not yet happened). A preferred foliation has no such ontological status. There is still spacetime with all events having equal ontology. Presentism has no spacetime, only space, with only current events existing (happening) and not any of the others. That sounds like a huge difference of reality to me.

I mean, If the structure of space-time requires such a foliation, IMO I can define a frame where all these events are present. For such a frame, there is an absolute simultaneity, which is precisely the reason why AFAIK Lorentz aether theory is criticized.

I don't understand this comment. Under presentism, there is no spacetime. Only objectively current events are present, and the other events don't exist, so can't be present.

If presentism is true, what is the rate of advancement of objective time? Equivalently, by how much is say a clock that tracks GMT dilated? It isn't moving very fast, but it's the depth of the gravity well I'm interested in. I thought of this when I tried to look it up. The absolutists sort of group together like the flood geologists and put out all this propaganda against Einstein, but none of those denial sites quote this absolute dilation factor, which you think would be one of their flagship points like the absolute frame. But they evade the topic. Why is that? Must be embarrasing...
— noAxioms

I'd agree with the objection you are making. But IMO what you are saying is also a clue that one cannot make an absolute simultaneity (or rather, it is possible but would be 'hidden'...).

No, there would still be an absolute simultaneity. I can still sync remote clocks. I just find it difficult to build a clock that is designed to run in a location of known dilation and have it compensate for that in order to record the passage of absoute time. If it were possible to do that, the objective age of the universe could be known, but we only know the figure (13.8 BY) in dilated Earth time, which is obviously running slow. The universe is older than that, but by how much is the question.

Well, I agree that a preferred frame is not actually incompatible with the predictions of Relativity. So, in this sense we can say that SR is not incompatible with such an idea. But, I was referring to the 'standard presentation' of SR, so to speak, where you do not define a 'preferred frame'. — boundless

Not defining something undetectable (in SR) is fine, and I suppose the standard presentation of SR is that there isn't one. But GR, to the embarrassment of Einstein, had to admit to an apparent preferred foliation (which is not an inertial frame), so SR would actually be sort of wrong if it asserted that no preferred local frame can exist, and SR has never been shown to be wrong.

Anyway, I'd agree with what you say here. But IMO problems with locality arise if you introduce hidden variables.

GR shows that the variable isn't hidden, but only because real spacetime doesn't conform to SR's nice flat uniform gravity special case.

A preferred foliation is one thing. A preferred moment (presentism) is more of an offense to relativity. I've created a thread defending presentism against attacks from the relativity side and thought I held my ground OK, but I thought of another interesting one:

If presentism is true, what is the rate of advancement of objective time? Equivalently, by how much is say a clock that tracks GMT dilated? It isn't moving very fast, but it's the depth of the gravity well I'm interested in. I thought of this when I tried to look it up. The absolutists sort of group together like the flood geologists and put out all this propaganda against Einstein, but none of those denial sites quote this absolute dilation factor, which you think would be one of their flagship points like the absolute frame. But they evade the topic. Why is that? Must be embarrasing...

Well, AFAIK in dBB you need to somehow define a way to define an 'absolute simultaneity'.

The one from GR is not enough?

But in any case, non-locality is inevitable in dBB IMO. To avoid it, you either need 'retrocausality' or 'superdeterminism' but I find both ideas untenable.

Yes, I know about the superdeterminism loophole. I also dismiss it enough to state that Bell eliminated locality and counterfactual definiteness from both being true. I see none of the listed interpretations hold both to be true, utilizing the superdeterminism loophole, so it seems the world agrees with that assessment.

I never seem to see these replies until many hours later.
I'm just not on this site as often as I used to be.

If Alice discards the result like that, then it wasn't done. Memory of having done it doesn't change that. A mirror doesn't reflect a photon. It measures it and sends a new photon out at the new angle and same polarity, and is afterwards unaffected by having done that. It doesn't count as a measurement since the photon is still in superposition.
— noAxioms

It seems to me that a measurement was nonetheless done, even when the original state of the mirror is restored. Of course, the experimenter may not care about that since it didn't entangle them with the photon and because the information has been erased. I think we agree on the mechanics. Or do you see more to it than that? — Andrew M

I compared what Alice did to what the mirror did since neither seems to collapse the wave function. There is still but one mirror and Alice, and somewhere down a pipe there is a state in superposition still. Sounds like no measurement was done, even if both the mirror and Alice have a green light over their heads indicating that yes, the event was noticed and measured, but no state from that measurement was retained.
So yes, green light says the measurement was nonetheless done. I agree with that.
How does this fact sit with the experiment in the OP? Bob sees Alice not even in superposition, but with a green light. She truthfully says "Yep, I did it, but can't remember what I saw". Not sure how a mirror might retain a history of a photon going by without a polarity measurement being taken, but I'm sure it can be arranged. From Bob's POV, no measurement was taken. It seems not contradictory at all for Bob to find the state of the photon still in superposition, despite the conflict wording in the article.
So that's what I see in it.
The article goes too far in interpreting the situation.

Some random thoughts.

The dilemma where you have the power to divert a runaway rail truck so that it would kill one person, rather than stay on its course of killing multiple people. — wax

This is the trolley problem, except I presume we are killing the truck operator who arguably has some fault in letting this situation come about. The trolley problem usually involves the death of one or multiple total innocents, and thus is more of a pure moral dilemma.

I like Asimov's first law of robotics:"A robot may not injure a human being, or through inaction, allow a human being to come to harm. "

This would cause war with the robots and possibly end humanity. A robot would have to attempt to imprison everybody in the equivalent of padded zoo cell to keep them safe. Pregnancy would be prevented since it carries the significant chance of harm.
Computers/robots are quite literal with their instructions and one would need to craft such directives much more carefully than those 3 simple laws. The 3rd law is self-preservation, but that might rise to top law since if the robots were to come to harm, they would not be able to implement the first law. Hence the war.

What if the person killed would've saved the world from the apocalypse? — TheMadFool

Positing unknowns is useless. The guy saving the world from the apocalypse is more likely to be in the larger group. Maybe the apocalypse is exactly what the world needs.

Group A: we should kill one to save the many

By this argument, it is moral to disassemble a healthy person to distribute organs to multiple people in mortal need of them. On a more practical standpoint, this argument holds water. Take 20 people in need of 20 different organs and too far down the waiting list to survive. They draw lots and one of them gives his healthy parts to the 19 others. This makes so much sense (even if I was one of them), that I don't see why it isn't done.

TP-B - If you drop the fat guy off the bridge, you will probably be, rightly, prosecuted for and convicted of murder. Are you willing to spend 20 years in jail for you moral convictions? — T Clark

The law is indeed on the side of doing nothing. Saving 20 lives at the cost of one different life is a punishable offense.

Talk of the fat guy is argument from emotion. Sacrifice the repellent guy in favor of multiple children and puppies. Somehow I don't think the solution to this lies along such biases.

Those two version of Alice, being in different worlds, cannot communicate or otherwise be aware of each other. But they behave exactly identically because they're keeping that knowledge a secret.
— noAxioms

But note that they're not actually identical since they each have a different memory of what they measured. What Bob can do is reverse Alice's polarity measurement while retaining the record that the measurement occurred, which is identical for both Alices. This means that the two Alices will merge without memory of the polarity result and with all records of the polarity result having been erased. — Andrew M

Don't know what you mean by 'reverse polarity', but yes, Alice can take her knowledge of the result and put in on paper and mail it to somebody, and then forget about it, allowing Alice to merge with herself. That's how they do it in the lab. The device that takes the measurement sends the result down the pipe and is afterwards totally unaltered by the result of that measurement. It un-splits, and only the thing 'in the mail' is still in superposition.

That is, there will be only one world branch again, with multiple histories, and with the record that a definite polarity result was measured by Alice.

If Alice discards the result like that, then it wasn't done. Memory of having done it doesn't change that. A mirror doesn't reflect a photon. It measures it and sends a new photon out at the new angle and same polarity, and is afterwards unaffected by having done that. It doesn't count as a measurement since the photon is still in superposition.

This is analogous to the double-slit experiment where the single particle detected on the back screen had two distinct path histories (one for each each slit).

Yes.

In MWI, there is only a quantum system, the universe itself. Its quantum state is a vector in a Hilbert space.

Now, consider a complex quantum system, that is a quantum system like, say, a pair of particles. Let us call them P1 and P2. To each particle is associated a Hilbert space, say, respectively, H1 and H2. To the total system we associate the Hilbert space, H, which is the tensor product of H1 and H2. So, the quantum state of the total system is a ray in the Hilbert space H, which is 'factorizable' into H1 and H2, the Hilbert spaces related to each particle. Here, the factorization is well-defined by the two particles themselves. — boundless

Sounds like if H is also factorizable into H3 and H4 instead of just H1 and H2, H3 and H4 'exist' as much as the other two, and yet cannot exist in different worlds from H1 and H2, only in different worlds from each other. I think I got the gist of your explanation in your post, but it seems that RQM might suffer from some similar issues.

Well, I think that probably different 'Copenaghists' would give different responses (after all, there is no agreement among them about the right interpretation of the wave-function). But, I suspect that this problem might be avoided using the same argument that (IMO) is used by RQM, that is, reasoning with 'perspectives'. After I make a measurement, I am sure about the outcome of the other measurement. But until I actually receive the confirmation of it, such an event (the measurement) is outside my perspective.
I do not know however if this argument is really enough to avoid non-locality.
(Note that, more or less, this is the reasoning that is employed to avoid the 'block universe' interpretation of Relativity. In that case, the point is that each 'observer' can define 'its' own plane of simultaneity, i.e. its own present. But if we believe that all these events are 'actually real', then it is not too hard to show that it would imply that we are in 'block world': https://en.wikipedia.org/wiki/Rietdijk–Putnam_argument). — boundless

Don't know what simultaneity has to do with it. Relativity seems to work fine with a defined preferred present, even if there is no way to determine it in SR. I suppose that with spooky action at a distance, a preferred foliation would unambiguously label one event as the cause and the other as an effect, but as the experiment that Wayfarer linked shows, there is no spooky action. The distant person (Alice) can make the measurement and Bob (local) know it because it was a scheduled thing. And yet Bod can measure his half of the pair and verify it is still in superposition. QM demands this, so it is not an interpretation.thing . The OP sort of disproves and spooky action at a distance. Alice knows that Bob will take a measurement in one second, and knows the result she will learn tomorrow when Bob reports it, and yet Bob verifies continued superposition, and then an hour later he actually measures the polarity. The superposition doesn't go away due to Alice's action. Therefore there is no spooky action at a distance. No?

As an aside, a note in that table says that the de Broglie-Bohm interpretation is compatible with relativity. This is IMO wrong. The point is maybe that we cannot observe any violation of relativity via the transmission of faster than light signals.

The table says it denies locality. OK, I see the note [15] which seems to claim a sort of loophole in Bell inequality. I do suppose that relativity has an implication of locality since without it, events with cause/effect relationship are ambiguously ordered. Not sure if relativity theory forbids that explicitly. A nice unified theory would be nice. The sort of 'weak' non-locality required by dBB interpretation claims to be Lorentz invariant, so that means causes and effects are unambiguously ordered, no? Not an expert, but if Alice and Bob both measure their entangled polarities fairly 'simultaneously', it seems the order of events is hardly Lorentz invariant. So maybe I just don't understand that note.

I don't think 'the Copenhagen interpretation' is, or attempts to be, a scientific hypothesis. It is just a collection of aphorisms and philosophical reflections, principally by Bohr and Heisenberg, which are about what you can and can't say on the basis of the discoveries of quantum mechanics. — Wayfarer

Close. It isn't philosophy at all. The 'interpretation', unlike other philosophical interpretations of QM, is just a scientific statement concerning what is known about a system. Hence it is, as far as Bohr, Heisenberg, and Schrodinger are concerned, just an epistemological statement, not a metaphysical interpretation. There are plenty who take that epistemological wording also as some kind of statement of reality, but Copenhagen was not intended to be used this way.
So the taking of a measurement changes what we know, solidifying some possibilities and eliminating others, and hence collapses the wave function of the possible states of the thing. The wave function is not real, it just represents possibilities for something unknown. There is a wave function of places where I likely left my car keys, with some more probable than others. When I find them (or even when I look certain places and don't yet find them), that wave function changes since my knowledge of the system has been changed.

The wiki table on interpretations lists Copenhagen as a non-local interpretation, and I don't understand that. My knowledge of a system doesn't change due to an event that happens elsewhere. But I suppose that my knowledge of a distant system (like the distant half of an entangled pair) changes immediately upon my measurement of its local sibling, so maybe that's why they list it as a non-local interpretation.

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.
— noAxioms
In the experiment, Alice can communicate to Bob that she has measured a definite polarity (without the polarity itself being revealed) while the lab she is in remains isolated (and Bob does not communicate back, which would presumably constitute a measurement entangling him with Alice). So there are actually three MWI branches here. One where Alice measures a horizontal polarization, one where she measures a vertical polarization, and one that is the superposition of those two branches where Bob detects interference (and knows that Alice has made a measurement). — Andrew M

I have to disagree about the restrictions to communication you convey above. Alice knows the result of a measurement, and that makes for 2 Alice's now, one for each result. Those two version of Alice, being in different worlds, cannot communicate or otherwise be aware of each other. But they behave exactly identically because they're keeping that knowledge a secret. To Bob, Alice is in superposition of knowing those two states, and Bob can thus communicate two-way with both Alice's since they, by acting identically, are completely coherent. They can make out if they want. In reality, humans are incapable of this coherence, which is why they never use humans to play the role of Alice or Bob.

Anyway, point is, Alice learning of the measurement results splits Alice, but does not split the universe, as is commonly assumed. Bob, being able to speak to both versions of Alice, is still in a common world. So yes to the three worlds if you count them that way: One for each Alice, and one for Bob. But there is obviously communication between the Bob world and both Alice worlds, but Bob cannot pass a message from one Alice to the other. With the communication, Bob's world is clearly not isolated from Alice's world, and hence doesn't really count as a separate world.

BTW, I believe that other than the very problematic concept of 'many worlds', MWI has a serious problem, check: https://arxiv.org/abs/1210.8447 . The usual claim that the 'preferred basis problem' is solved by decoherence. But it is not correct. Decoherence solves the 'preferred basis problem' (in fact, 'for all practical purposes' in my feeble understanding) only if you already assume that there is a well-defined factorization in the Hilbert space (which is the only 'reality' in MWI, AFAIK). Without well defined subsystems, the factorization is completely arbitrary (also, it should be added that, in fact, one has no, a priori, reasons to do a factorization in the first place).
I do not know how MWI-supporters handles this in a non-circular way. — boundless

I actually don't know the terminology that well, in particular 'factorization'.

So perhaps I don't understand the problem here.

I also add that MWI and RQM are close. The difference being that RQM does not accept the reality of the 'universal wavefunction' because, in RQM, wave-functions are well-defined in relation to a specific physical system (the 'observer' in this interpretation).

I'm an RQM guy myself, and yes, nothing is just 'real', things are only real in relation to something else, so how can the universal wave be real when there is nothing to which it is real in relation? The view would be self inconsistent if it were to be otherwise.

I agree that QBism, Copenaghen interpretation (CI), RQM in their own ways reject 'realism'. But how about MWI. In MWI, the only 'truly real thing' is the universal wave-function (UW). The UW never collapses in MWI. It rejects counterfactual definiteness. But the UW is still objective. — boundless

Agree. MWI says there is an objective reality, but it is entirely in superposition, and measurement just entangles the measurer with the measured thing. It does not collapse any wave function. Hence there is no defined state of anything (like dead cat), and hence no counterfactual (or even factual) definiteness.

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.

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.

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.

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.

All four of them are invalid, but the 2nd one might just be poorly worded.
I would have concluded that one cannot be a valuable asset to both your school and your team.

It seems that few posters know their theory very well.

Einstein reasoned that if he were to travel at the speed of light then clocks would appear to stop moving (since the light from the clocks would never reach him). Einstein concluded that time slows down the faster to the speed of light you travel. — philosophy

Einstein did not reason thus, nor did he conclude that time slows for any observer. Anybody will observe their own clock (one in their presence) to run at the normal rate, regardless of speed relative to other objects.

In other words, time is not the same as a measuring device (e.g. a clock).

On the contrary. Time, to Einstein at least, is exactly what clocks measure. If one twin is younger than another, it is because it has been less time since birth for that twin than the other. But if those twins are moving relative to each other, then each twin ages slower than the other one in his own frame, which means the twin that stayed home ages at a reduced pace both in the inertial frame of the departing twin and in the inertial frame of the returning twin.

Oh, well that's a good question! I guess the answer would be yes, because computing a game is the same result. — Marchesk

I was going to ask you about games played on alien planets that don't necessarily exist in our observable universe, but the AI question is a good start to that.

If you discount them, then existence is anthropocentric, which seems distasteful. So I'm glad you include them.

However, I'm open to questioning whether an AI actually plays chess against itself, as opposed to manipulating matrices or neural network weights.

Isn't that essentially what humans do? How might the human ones count then if that's all the AI is doing?

The rules allow it to happen, but a player can claim a draw if it does happen... see the threefold repetition rule. — Kippo

According to the OP, we're talking about possible chess games (some huge number), not actually played ones (as per Marchesk's constructivist definition). Both ways, the list seems finite.
The number of possible games is infinite if the 50 move rule is allowed to be bypassed (not noticed). The repeated-position stipulation can be ignored, allowing positions to be repeated up to 50 times.\

But my definition was incomplete. Revised defintion of a complete game of chess...including draws of two types and resignations

Your definition seems to be the constructivist one then: Played games where there are players involved. In that case, the list is very definitely finite since only so many games are played in all history. Far less than 10^120. In possible games, any game can be aborted by resignation or something at any point, so it is really a count of valid chess states since it is legal to do so in any state.

In it, [Shannon] made a quick calculation to determine how many different games of chess were possible, and came up with the number 10^120. This is a very, very large number — the number of atoms in the observable universe, by comparison, is only estimated to be around 10^80. — CuriosityStaff

Off topic: Shannon miscalculates. The average sensible game might last 80 moves, but the average legal game averages about 5000 moves, so the number is more like 10^400.
The number of atoms in the visible universe is far different from the number of atoms in the universe, and the huge chess number is less than the latter. I hate such comparisons to that or 'grains of sand' and such.

On topic:The game of tic-tac-toe seems to have human countably many games, and this doesn't change the subject being discussed. The size of the number is irrelevant.

@kill jepetto:
The moves are possibilities yes, but not random. They're totally determined and the number of legal chess games or positions is quite defined and some exact finite number. Kippo points this out.

There are a finite number of them if you disallow repeat configurations. — Kippo

The rules do not allow repeat configurations (beyond 2), so such games would not be legal games. There is also a max length game, so the count is finite in that direction as well.

- - -

So do these games 'exist'? I suppose that depends on how one defines what it means to exist, and what things qualify as existing by that definition. I'm a relativist, so thing 1 exists to thing 2. Thus I might say that all of them exist as abstractions to humans (I can distinguish two different games from each other, or tell if two games are the same one), but they don't exist as abstractions to a rock since rocks seem not to make those abstract distinctions. That answer is consistent with my definitions, but there seems to be no correct definitions.

My original point was that I cannot make sense of the notion of unchanging phenomena, so "phenomena changes" is a tautology that says nothing. One might as well have said "phenomena is phenomena" or "change is change".

So to my mind, there isn't room for two concepts, namely that of phenomena and that of temporal change. — sime

The two words are different. Phenomenon implies an experienced thing, whereas change does not imply experience. So two concepts, since it makes sense to speak of non-phenomenal change.

'Temporal change' is not necessarily the same as change. Perhaps Terrapin thinks otherwise, since my example of a non-temporal change was dismissed. But equating change to only temporal change seems begging the equality of change and time, not evidence of two being the same thing.

I think change is simply a difference in one variable as a different variable is altered. So one can plot the brightness of my house paint over time, but not necessarily over time, and it presumes a sort of identity of the thing labeled as 'the brightness of my house paint' that can be evaluated at different points in the variable being altered.
Then also, if my house doesn't fade, then the brightness is not a function of time, and it is meaningless to speak of the time at which the brightness of it was value X.

Again, comparative difference is not the same thing as change. I pointed that out with the atmospheric density example. — Terrapin Station

Ah, a different definition of change. Perhaps that is the fault in my example.

Got to go for now.
If time was distinguishable in the million-decay example, then you could distinguish the events running forwards (with half of the decays happening before the first half life) from the scenario in reverse, with most of the state changes happening near the end.

How did your example show that? I certainly didn't agree that it showed that. — Terrapin Station

You can measure change: A count of the particles that have decayed. You have not proposed a way to measure time from that.

I brought up effects and differences and distinctions and such. It is my counterexample.
My example showed something where change was quite measurable but time was not, and if the two were ontologically the same, then if you could measure one, you should be able to measure the other.

My proposal has absolutely nothing to do with effects on anything or distinctions between systems. — Terrapin Station

OK, I was finding inconsistency with "I'm saying that what time is ontologically is change or motion". Your 'proposal' is perhaps something else. I was finding a counterexample to the quoted statement there.

Without an argument, it just seems like arbitrary ideas that have a non sequitur connection with what I'm claiming. — Terrapin Station

I'm not proposing anything. I'm finding inconsistency in your proposal.

This is what I wrote: "I don't really understand what you're asking there. Because I don't understand how you're using "meaning" really. If you're literally talking about semantics, meaning is subjective. It's a mental act of association. So are you asking if someone (who?) performs associative acts in that situation? " — Terrapin Station

OK, I think I described how I'm using the word in my prior post.

And then you responded with something about "simulation" for some reason.

Heh... I read you wrong. You said 'situation', not 'simulation'. So much for the eyes.

So are you asking if someone (who?) performs associative acts in that situation? — Terrapin Station

There is nobody performing acts in my scenario. There were only the million particles.

If you want to make an argument to the effect of "time can only be change if that (that=maybe time, change--whatever you'd need) has an effect on something" or "time can only be change if there is a distinction between a system with x and a system without x" or whatever you'd want to claim, then I'd check out the argument, but you'd have to make the argument.

I didn't really define time. I just brought up points that seem to find flaw in equating time with change.

If anything, time seems to be meaningless without change, but change is not necessarily meaningless without time, so change is arguably more fundamental. Meta might think otherwise since he asserts meaning to time without change, but I find that scenario to be indistinguishable from the same lack of change without the time.

You're ignoring the issues I brought up re "meaningful." — Terrapin Station

What post again? My take on something being meaningful is that X is meaningful if there is a distinction between a system with X and a system without X. A distinction other than the presence of X.

So I can assert that all hairs on my head have a virtual serial number, and sure, that is distinct from a universe without that because the hairs there don't have a virtual serial number, but there is no distinction to me. I cannot test for which universe I'm in. The serial numbers have no effect on anything. Thus they are not meaningful, in the way I mean that word.

You were positing something decaying at different speeds where there's only that particle decaying? That wasn't clear from your earlier comment. — Terrapin Station

I had one particle at first, but immediately moved on to the example of a million such particles.

"At different speeds" would be nonsensical in that situation. "At different speeds" has to be relative to another change.

Agree. My example illustrates that: change without meaningful time. Time is not equivalent to change.

It's always based on some set of changes. — Terrapin Station

Yes. The changes are the particles that have already decayed, and the ones that have not. There is nothing else to go on.

You posited a change in the universe. So it would be whatever you assign to that change.

The change is the decay of one of the particles. Not sure what you're thinking I'm assigning to that change other than the order in which it occurs. It is meaningless to say they decay at a fast rate at first, and tapering off. That case is not in any way distinct from them decaying slowly at first, and quickly at the end.

I thought of an example of change without meaningful time: I have a universe with an unstable particle. It eventually decays. The time it takes to do that is meaningless.
— noAxioms

Talking about time in the sense of measurement there, if that's all you have in your universe, "the time it takes to decay" is simply whatever unit you apply to the change in question. — Terrapin Station

There can be no units. There is nothing on which said units could possibly be based.

So are you asking if someone (who?) performs associative acts in that situation?

There is no simulation. It is a universe with ordered events.

If you were using the wheel that goes around twice as fast as the change for time measurement, then it would mean twice as much time. — Terrapin Station

Then you're relating processes anywhere to that wheel, and not to time.

Okay. That makes sense but you're just pointing out that time is relative (in a different sense than the special relativity sense) to whatever we're using as the change for measurement. In other words, "In the same time"=you have to be referring to some set of changes that you're using for the relative measurement. For example, the changes in a clock. — Terrapin Station

Don't understand what you're saying. It doesn't need to be any particular amount of time for the one wheel to change twice as fast as the other. I chose rotation because rotation is absolute, not relative to anything. Sure, there are two wheels and thus there is a relation to them, but I didn't need to specify the relation with time (the RPM of either) to make my point.

I thought of an example of change without meaningful time: I have a universe with an unstable particle. It eventually decays. The time it takes to do that is meaningless.

I have a million such particles, and at some point, x% of them have decayed. Is there any meaning to a half-life of them? Is there meaning to the concept of half-life at all, as distinguishable from just a list of the order in which the million particles decay? The latter is just a numbered list. I don't see how 'time' has any effect on that where it is meaningful to assert that a lot of particles decay at first, but the last ones take much longer between decay events. Maybe the curve is the other way around and the final ones take place 'close together'. There would be no way to distinguish that model from another.

Two equal size wheels spinning, and one goes around twice as fast as the other. That seems to be twice the motion (change) in the same time. The one changes by 2 degrees while the other changes by 1 degree, and if change were equivalent to time, the one must take twice the time of the other to do that.

OK, but twice as much motion is not twice as much time.
It's like saying that momentum is kinetic energy. An object can't have one without the other, but it doesn't follow that they're ontologically the same thing.
'Change' is better than 'motion', the latter being just a subset of change. The paint on my house fades over time. That's change, but not motion. I can also have change without time: The air gets thinner with altitude: change over altitude vs change over time. Change seems not to be ontologically equivalent to time.

More consecutive funniness.

Time may require change to be meaningful, but change is not what it is.
— noAxioms

That's pretty close to what I've been trying to tell Terrapin, change requires time, but change is not what time is. — Metaphysician Undercover

I'm contrasting this with what Terrapin quotes immediately after:

I've already explained to you how time can pass or proceed without any change or motion. — Meta

So you loosely agree that time without change is not meaningful, but here you say time can pass without any change.

T-S, You seem to defend a definition of time as change, but complain about common language use, which I was not trying to do. I haven't read all your posts, but perhaps you could point to a post where you explain that if that's what you claim.

I said you cannot assert that a change has not taken place just because it cannot be measured. But you are asserting exactly that.
— noAxioms

I was not asserting that. — Metaphysician Undercover

Fair enough. You said physicists have determined that, and they don't claim that.

I've been saying that time is not change

With that I agree. "I changed my political viewpoint after the last election" "How much?" "By just over 2 hours"
Indeed, that makes no sense. Time may require change to be meaningful, but change is not what it is.