I think you're mixing the issue of how the result at the detectors is calculated (by summing path amplitudes) with the question of what physically happens in the interferometer. RQM doesn't claim that the photon would take both paths, only that accounts of an event can differ for different observers which is a weaker claim. — Andrew M

RQM indeed does not claim anything about what path is taken. Any statement about the path taken (such as it taking one or the other) would be a counterfactual one, and RQM is not a counterfactual interpretation. So perhaps I was in error stating that it takes both paths. Statements about unmeasured things are meaningless in RQM.

Well, maybe you are right. But IMO, it suggests that the only that 'Alice' can know about herself is to consider herself in relation to 'someone else'. — boundless

That seems to work. I considered myself in relation to that alien who came from far away and has yet to observe what's here. To that alien, I am very much in a superposition of lots of states, most of which do not contain a 'me'. So it is a state of superposition of 'here' more than it is a state of superposition of 'me'. I don't need the alien to tell me that such a superposition state exists. He's still in his box, but conveying what he knows is outside, which is very little.

If decoherence has occurred, then Wigner has effectively taken a measurement, and the lab is in one state. If Wigner doesn't know the result, that's just an epistemological problem. The result is fact at that point, known or not. A tossed coin between my hand and arm is not in superposition just because I don't know which side is up.
— noAxioms

Are you sure about this?

IMHO decoherence alone cannot, strictly speaking, give you a definite outcome.

Sure about my statement that decoherence is a measurement? The two are almost synonyms.
Some nucleus in the moon is in superposition of decayed/not-decayed. That decay (or lack of it) affects its environment, so it cannot be contained. The immediately surrounding matter is quickly in a completely different state because of it, so the wave function collapses into a definite outcome for at least that matter in state A or B. That's a measurement taken of the decay event by the surrounding matter. That's decoherence of the atom in superposition, entangling the matter around it into its superposition. Same thing. Within a second or two, that superposition state entangles me as well, even at this distance, and the fact of what happened to that atom becomes a definite outcome to me.

More precisely, it removes the superposition but it is not enough to 'select' a specific 'branch'. That's why MWI supporters like decoherence. Decoherence explains the absence of superposition. But the are still the non-interfering branches.

MWI is misrepresented if it has a concept of branches with identities. There is never a specific branch. The measurement is taken by nearby matter but not yet by something further away, so it is still in superposition from that PoV. That's an RQM description, but MWI never really has distinct worlds. The cat is both dead and alive (same world to Schrodinger, different worlds to the cat). Opening the box entangles Schrodinger with the cat and now there are two of both, at least from their PoV. Each Schrodinger I suppose finds himself entangled with a specific branch, but there is no identity to the branch, only the wave function of some arbitrary system, which is different to different observers.

A quote from Tegmark on the subject: https://arxiv.org/pdf/quant-ph/9709032.pdf
"[MWI (per Everett) does not posit that] at certain magic instances, the the world undergoes some sort of metaphysical “split” into two branches that subsequently never interact."

In my understanding, Consistent Histories instead says that interference disappears due to decoherence and a definite outcome is 'selected' probabilistically via the Born Rule.

Same thing, different spin.

Our disagreement is probably due to my possible misunderstanding of decoherence, then. AFAIK, decoherence can explain the disappearance of superposition, not the 'selection' of a specific branch.

With interpretations where selecting goes on, I suppose that needs explaining. Here are all these possibilities, and only one becomes real and the rest discarded. What makes that choice?
I leave it to them. MWI makes them all real, and RQM doesn't really have selections that happen. A measurement isn't really done by any observer who is but an event with only a history, but not the ability to 'select'. I cannot measure the photon, but I can have already measured it, so no 'selection' is ever done. At least that's how I interpret RQM.

Bob's knowledge of the paper means nothing: The device may have randomly declined to take a measurement and emit a blank paper. Bob can tell if it happened by measuring superposition or not. So the device taking the measurement, and not Bob's knowledge of that action is what collapses the wave function.
— noAxioms

But in that case this is not a relevant information for Bob.
— boundless

Bob's knowledge of the action can be obtained without consulting the device that did the action, so that information passed on by the device is not relevant. Bob has independent access to this information already.

Ok, thank you again! I believe that now I see your point. So, there is at best a 'probability' of finding the Moon but not 'the Moon'.

It might be purple and tiny, but if there's one and there are locals living on what it orbits, then I suppose to them it would also be 'the moon' just like it is for us. There's a probability for finding that purple one and a probability of finding me.

If at any time I take any measure at all of the alien's approaching ship, then there is a 100% chance that the alien that steps out will find me. This is unremarkable. From a MWI perspective, the alien, upon opening his door will 'split' into every possible world that could be found and all those worlds would see the alien. That is decoherence of the state of 'here' from massive superposition to something concrete.

So this means that the Moon is a possible outcome of the 'measurement'. The same goes for an electron, an atom and so on. 'Measurements' are random process but at the same time they can give only a class of result.

I cannot measure the moon right now and not find it, so that limits my possible class of results, sure. The alien measuring the same thing will likely get no-solar-system here since he never measured one in the first place like I did. My measurement collapsed a much simpler wave function that has almost zero possibility of no-moon.

If one accepts counterfactual definiteness, this is explained by the fact that we, indeed, find something already there. If not, it is still undeniable that there are regularities. So, I wonder how we can explain them, if we can (unfortunately, this leads us to metaphysics...).

Example? I measure the moon twice and find it both times? Be freaky to get a different result. But I find it because I has already measured it prior, so its existence to me is about as defined as it can be.

For RQM, the path travelled is only counterfactually indefinite for the observer outside the interferometer. — Andrew M

The interferometer seems to be the only measurement taken (the only observer). Anything outside that is only noting what was measured by that device. The photon definitely takes both paths relative to the interferometer because it takes no measurements until the paths join up again.

It says nothing about what the full-silvered mirrors within the interferometer might measure (the result of which gets subsequently erased by the final beam splitter).

Those mirrors don't take any measurement since they retain zero state from the light that reflects from them. If they did take a measurement, the photon would take only one path and not interfere with itself at the final beam splitter. That would effectively be putting a detector on each of the slits in the double slit setup. No interference is observed in such a case.

That's your brain interpreting it that way. The reflection very much still appears to raise the arm on the same side, but appears to have switched front to back.
— noAxioms

Yes. Physically it's a front/back reflection. And it can also potentially be perceived as a 3D object that is half rotated around the up/down axis and reflected left/right.

The perception as a rotation is an illusion made possible by our near bilateral symmetry. But my wife in the mirror looks sort of unfamiliar to me, and my cat is unrecognizable. I don't look that way to myself because I rarely see myself, only my reflection.
Not sure why this aspect of the discussion is relevant to the topic. We seem to have gotten off track. I was expressing my dissatisfaction at the analogy, as I have with my own.

The first section says:

"... O cannot have a full description of the interaction of S with himself (O), because his information is correlation information and there is no meaning in being correlated with oneself. If we include the observer into the system, then the evolution is still unitary, but we are now dealing with the description of a different observer." — boundless

Under any 'consciousness causes collapse' or other anthropocentric take, the friend, being conscious, cannot be in superposition. In any other interpretation, the friend very much can be. — noAxioms

My comment is applicable to your reply. Wigner's friend is is superposition in relation to Wigner. The friend measuring himself sees no such thing and cannot detect his own interference with himself in the other state. In other words, Alice (the friend) is in superposition of having measured vertical and horizontal polarization. Bob (Wigner) sees this and can see Alice interfere with herself (per the OP) yet Alice cannot detect this self-interference. Perhaps that's what they mean by inability to self-measure. Alice needs Bob to tell her she's in this superposition of states.

I grouped this with your quote above because it mentions dealing with the description of a different observer. In the consciousness-causes-collapse interpretation, maybe the consciousness is that introduced second observer.

There is, however a problem here. If Wigner asks his friend before entering in the lab if he sees a definite result, it appears that at this point Wigner already knows that everything in the lab is in a definite state.

No he doesn't. The friend in superposition would also indicate that. Wigner does not learn from that answer that the lab is in a definite state. This is of course assuming that the friend (and the rest of the lab) is very capable of keeping the result a secret, which is why Alice is never a human in such experiments.

So, superposition is now destroyed.

Not if Wigner is unaffected by the actual measurement result, and not the mere taking of it. It is not the case of the classic unseen coin.

The problem is that Wigner still does not know in which state the lab is (including is friend).

If decoherence has occurred, then Wigner has effectively taken a measurement, and the lab is in one state. If Wigner doesn't know the result, that's just an epistemological problem. The result is fact at that point, known or not. A tossed coin between my hand and arm is not in superposition just because I don't know which side is up.

Let's consider the RQM explanation. O = 'Wigner's friend'. O'= 'Wigner'. If O' 'asks' O if the state of S is definite (let's say spin 'up' or spin 'down'), and O answers 'yes', it seems that now according to O, S is in a definite state, let's say 'up', but at the same time for O' the state is definite but it is either 'up' or 'down'. In MWI terms, for O' there are still two non-interfering branches. Yet, when O' enters, there must be an agreement between them.

I disagree with all of this, assuming O can keep a secret, which only certain lab instruments can do. With actual humans, O' and O need not communicate at all. O's measurement affects O' at nearly light speed because no lab is a Schrodinger's box.
Decoherence can be temporarily prevented with distance, but then O and O' cannot communicate. This has been demonstrated with entangled pairs.

Just thought of this: Per time symmetry, is there such a thing as radioactive un-decay, and would such an event constitute an end to a causal chain? If not, I don't think the decay can constitute an uncaused event.
— noAxioms
I do not think that it is reversible, hence I'd say that time symmetry is broken!

I'd bet otherwise, but what do I know? They create some exotic new element in a particle accelerator somewhere. Isn't that un-decay of a sort? Perhaps not. The exotic nucleus decays before it can even acquire some electrons and write home to its mommy that it has grown up and become an atom. I digress. The thing decays into different pieces than the pieces that that they probably smashed together to make it. If it can be the same pieces, that's un-decay in my book.

Bob's knowledge of the paper means nothing: The device may have randomly declined to take a measurement and emit a blank paper. Bob can tell if it happened by measuring superposition or not. So the device taking the measurement, and not Bob's knowledge of that action is what collapses the wave function.
— noAxioms

But in that case this is not a relevant information for Bob.

Exactly. Wigner learning that his friend took the measurement is not relevant information. What's relevant is being affected by the result of that measurement (and not even the knowledge of that result). Being affected by it puts him in the causal chain of that measurement and entangles Wigner (Bob) with the state of the thing measured.
This is what happens in the OP, where the fact that the measurement is done is simply not relevant information to the other observer, and thus the other observer still can measure superposition.

So, to him the state is still undefined (even if he does not believe that...knowledge is not belief). If, instead, the measurement apparatus works perfectly, he really knows that the state is definite (but we fall in the aforementioned problem, where according to Bob, there are two possible states of 'Alice').

What? All this assumes perfect lab equipment. Bob knows the measurement was done (by something else), and yet that irrelevant information does not change the superposition state of the thing measured to Bob. He doesn't need to know or believe anything. He can measure the superposition of the thing directly.


Concerning the counterfactual existence of the moon:

For the alien not to measure the moon, he'd have to put the moon (and everything else) in Schrodinger's box, which is best achieved by making a ship that is one, inside out. Zero information can penetrate from outside to inside the box. He opens the box randomly at some location which happens to be here, and there is some vast wave function of what he might find here that collapses quite improbably to us and our moon. Far more likely it collapses to empty space. Depend on from what distance he came, but it would have to be from over 5 billion light years away because the moon (or the whole solar system) needs to be unmeasured from the start. He'd have to come from a helluva longer distance to find no galaxy here. How far must I travel now to find a place where I have zero information about what is there? A lot further than the event horizon. It cannot be done. We see stuff that is 22 BLY away, which is not possible to reach ever. But the moon is young enough that it can be done.
— noAxioms

I agree that the Moon and everything else are in the Schrodinger's box. But this means that in some sense there is 'something' that corresponds to the Moon in the perspective of the alien. When the alien 'opens the box', the Moon 'collapses' in a definite state according to him.

It most very likely does not. Our moon, or us for that matter, are unlikely things to find in a random sample of totally unknown space. This location (which is known from inside the box due to inertial calculations) is in total superposition of anything that might have evolved from the known state of this area say 8 billion years ago. There wasn't even a galaxy here, but with really good instruments, perhaps it could be computed that there would be. So he's probably not going to pop into totally empty space like he would if he came from even further away.

But this seems to imply that the Moon in some sense 'exists' before the measurement.

Intuitive but not so if the principle of counterfactual definiteness is wrong. Think of it from a MWI perspective. The moon exists in that interpretation, but only in a tiny percentage of possible worlds that might stem from the state (past light cone) of where our alien shut himself in that ship 8 billion light years away. Most of those worlds have no moon, and far fewer have humans. He's not at all likely to witness either of them, but it is hard to imagine finding humans and no moon.

Before that there is no moon to be nonexistent. He can equally declare torrid-planet Vulcan to not exist. In both cases, he's just making stuff up.
— noAxioms
I see. But I would say that the comparison is apt for Vulcan and the definite state of the Moon, not the Moon.

Why any difference? OK, I don't think the torrid planet is going to happen naturally, but perhaps the Vulcans that live there find it convenient for some reason, so they made it that way. It could happen.

Ok. So, you endorse some kind of 'panpsychism' or 'panexperientalism'? (see: https://en.wikipedia.org/wiki/Panpsychism#Panexperientialism)

What? Where'd you get that? More the opposite. Living things are just arrangements of atoms just like pens. There's nothing experiential required to collapse a wave function.

Kind of hard not to observe an electron. Its state might be hidden if put in a box, but we put it there so we know its there. If not in the box, it interacts with other things and that makes it exist. I cannot escape that interaction.
— noAxioms

Well, yes. So, it must 'exist' even if unobserved!.
...
Then maybe we are in agreement! The unobserved electron is not really non-existent.

The interaction is observation. I did not describe an unobserved electron in that bit you quoted. So the unobserved electron is not really unobserved in those examples.

More precisely, it does not exist in a definite state. So, before measurement it simply does not make sense to talk about electrons in definite states.

Right. Even after observation, the state is only somewhat more definite. Never totally definite, as per Heisenberg.

I don't see why it needs to posit indeterminism or not.
— noAxioms

IMO, because results of observations are random.

OK, that sort of determinism. MWI is deterministic because the entire universal wave function is one completely deterministic thing. Consistent histories is not, but I don't know it well enough to say why. With RQM, it sort of depends on how you word things. Observations appear random in every interpretation, so none is deterministic in any sort of subjective way.

Prior to that measurement, there is no collapse, so the wave function puts the photon on both paths, not on 'either' path.
— noAxioms

The problem is that the above doesn't really make sense on a single-world interpretation, at least in my view. What I'm interested in is whether there is a way for the photon to travel on only one path but yet have physical interference still occur. And also while avoiding hidden variables. — Andrew M

It makes perfect sense. The photon cannot take one path, unmeasured. That would be the counterfactual definiteness that any local interpretation denies.

One thought here is the idea of reflection. When Alice looks in the mirror and raises her left arm, it appears in the mirror that she is raising her right arm.

That's your brain interpreting it that way. The reflection very much still appears to raise the arm on the same side, but appears to have switched front to back. This doesn't seem to change the point you're making.

Now Alice's action and her reflection are both the results of local physical processes. Bob, observing from afar, might receive light signals directly from Alice and indirectly via the mirror that end up superimposing for him, so it seems that Alice is raising both her left and right arms at the same time.
That would be a physical interference effect.

I don't see that as superimposing or interference. Bob perhaps sees two images facing in different directions, but not one image facing in both directions. Bob cannot tell which is the reflection, and technically they're both images, modified by mirrors and lenses and such, so none of those images is Alice.

I didn't think my analogy was very good either. I like that we're both trying. I suspect there isn't a classic analogy.

From some (most) perspectives, there is no 'it' to be an electron. To consider an electron is to have already assumed a perspective where it exists. It is a measurement already taken.
— noAxioms

Sorry, I can understand this but only in part. But I am just not sure that this is satisfactory. I mean, according to RQM, measurement involves an interaction. Are you saying that according to RQM the measurement process 'creates' the 'electron' 'out of nothing'? (this is actually a problem that can be raised to any form of CI) — boundless

Suppose you wanted to measure the diameter of a pizza. The way to do that is to put the pizza in front of you and hold a tape measure up to it, but the act of putting the pizza in front of you already performed the pizza/not-pizza measurement. I could in theory walk into a dark room, hold out the tape, have the lights turned on and hope by improbable chance that a pizza appears directly under that tape, but that is not likely to happen if I had zero knowledge of the presence of a pizza in that dark room.

So the electron is like that. Maybe I shot it and want to measure where it goes, or any other property of it, but to do that, I'm not taking a measurement of a random volume of space and hoping an electron appears in it. I probably already have a specific one in mind, meaning the measurement of its mere existence has already been done.

As far as I understand it, the main difference between MWI and CH is that the universal wave-function is unreal and that CH is indeterministic (as you pointed out). My problem is with the first part, in fact. I think I'll have to re-read that article.

I have more of a problem with MWI having a real wave function because it makes for a weighted reality of each of the worlds. One world seems to exist more than the other, but existence seems not to be anything but a True/False state. How cat X exist twice as hard as Y? So maybe CH resolves that problem for me.
I don't see why it needs to posit indeterminism or not.

I believe that in CH is more or less like MWI + Born Rule. In MWI, all results are equally 'real'.

Equally weighted then, but why do we find ourselves in a world where far more 'likely' collapses occur than the 'unlikely' ones? If all results are equally real, why are the probabilities of measuring those results unequal?

Well, yeah that is one of the problems of MWI.

In the case of CH, it seems that 'histories' have non-equal weight and you consider only the 'history' that you observe as 'real'. Not sure that this is really tenable, though.

Sounds like RQM. My history is real to me. My future is not, thus 'I' am defined as this endpoint event along with its history, plus an arbitrary designation of which events are 'me' and which ones are 'other stuff'. There's nothing in physics to make that designation.

Information preservation seems to prevent multiple histories from culminating in the same state

Ah, ok! I see. S can have a 'partial knowledge' of its own state, but not complete. This makes sense. — boundless

The article says that an external system can take this more 'full measurement' of some system, but not any system containing the system being measured. I don't understand this since Heisenberg's uncertainty says that even an external system can't do that. So they must mean something else when discussing the sort of information they expect from this 'self measurement'.

But, I still believe instead that here for Wigner also his friend is in superposition. That's why I referred to Bitbol's take as a sort of middle ground between CI and RQM.

Under any 'consciousness causes collapse' or other anthropocentric take, the friend, being conscious, cannot be in superposition. In any other interpretation, the friend very much can be.
The view cannot be falsified given that they've never put anything more complex than a barely visible dust mote in local superposition. Until they can put a human in a box, the interpretation remains valid.

Radioactive decay might be an example of a new causal chain. Depends on interpretation.
— noAxioms

Just thought of this: Per time symmetry, is there such a thing as radioactive un-decay, and would such an event constitute an end to a causal chain? If not, I don't think the decay can constitute an uncaused event.

According to RQM all physical systems are good 'end points'. Bitbol, however, notes that 'our' knowledge is always made from 'our' perspective. So, for 'us' there is a special 'end point': us. According to him, this reflects an intrinsic limitation of our knowledge: we cannot know, in principle, what is 'seen' from other perspective. The 'starting point' for our knowledge is lived experience. We cannot totally 'disregard' this and, therefore, we should not speak of how the world is 'for an experimental apparatus'. On the other hand, we can have knowledge of other human (at least) perspectives because they are qualitatively (very) akin to ours.

I have a hard time figuring out what Bitbol finds special about us. I am special to me, but everything has a relationship like that with itself. Another human ('us' but not 'me') measuring something gives me no more or less knowledge of that measurement than a dot on an unseen paper, and the wave function collapses either way. Wigner's friend just doesn't change that, so there is no 'us', just 'me', which is solipsism if you posit any QM significance to that.
Sorry. You can see I have little patience for anthropocentrism. I'm biased all to hell.

As you say, however, at the level of practice this does not make any difference. After all, in RQM it is still true that for 'our' perspective, 'we' are 'special'.

I see no such thing in any way that QM changed.

Without a description of what constitutes that registration act then, I have no idea what he's talking about.
— noAxioms
I believe that the registration act involves 'something' that is able to store information. E.g., a computer that stores '0' ('1') for spin 'up' ('down') when the spin of an electron is measured. But anyway, if one follows Rovelli in saying that everything can store information in that way, I believe that Wheeler's CI becomes RQM.

I was going to make Rovelli's point. If something changes state at all, it stores that information in its changed state. A prism is an example of something that sort of interacts with a photon without a state change (storage of the information). The photon is absorbed and immediately a new one is emitted in the same direction, leaving no state change to the prism and a change to the photon for the tiny delay in its journey. The prism does not store the information, and thus does not collapse the wave function of the photon. Objects that do change state don't seem to need to perform an act of registration to be affected like that, so that wording is still a bit unclear to me.

If CI is taken as an epistemological interpretation, I suppose this is true in a way, but it seems trivial to falsify. Alice (a simple device) measures the spin on one of an entangled pair, and prints the result of that measurement on a little paper that ejects face down on the table. Human Bob sees the paper but has gained no knowledge of the result, only knowledge that the measurement has taken place. He now measures the other particle of that pair and is is no longer in superposition. His lack of knowledge did not prevent the collapse of the wave function of his sample The device (Alice) has no knowledge since it retains no state of the incident after finishing its little printout. Does the unseen paper have the knowledge? It certainly contains the (hidden) information, but does it have 'knowledge'? If it does, why that anthropocentric choice of words?
— noAxioms

Another excellent point. This in fact seems to go against Bitbol's interpretation. Maybe the 'counter-objection' is that when Bob sees the piece of paper at that point he knows that there is no superposition and, therefore, the situation is now more or less like the unseen coin. I don't know if this makes sense, though.

Bob's knowledge of the paper means nothing: The device may have randomly declined to take a measurement and emit a blank paper. Bob can tell if it happened by measuring superposition or not. So the device taking the measurement, and not Bob's knowledge of that action is what collapses the wave function.

I agree that claiming that the piece of paper has knowledge is an anthropocentric choice.

Yes, I agree that everybody on this Earth measures the Moon and, therefore, we all agree on its existence. But let's consider an alien who never measured it. Suppose that he visits our Solar System and 'measures' the Moon. So, at this point he agrees with us about the existence of the Moon. But before that, is it right to say that for him the Moon is simply non-existent?

For the alien not to measure the moon, he'd have to put the moon (and everything else) in Schrodinger's box, which is best achieved by making a ship that is one, inside out. Zero information can penetrate from outside to inside the box. He opens the box randomly at some location which happens to be here, and there is some vast wave function of what he might find here that collapses quite improbably to us and our moon. Far more likely it collapses to empty space. Depend on from what distance he came, but it would have to be from over 5 billion light years away because the moon (or the whole solar system) needs to be unmeasured from the start. He'd have to come from a helluva longer distance to find no galaxy here. How far must I travel now to find a place where I have zero information about what is there? A lot further than the event horizon. It cannot be done. We see stuff that is 22 BLY away, which is not possible to reach ever. But the moon is young enough that it can be done.

But before that, is it right to say that for him the Moon is simply non-existent?

Before that there is no moon to be nonexistent. He can equally declare torrid-planet Vulcan to not exist. In both cases, he's just making stuff up.

Oddly, this point might be made by Bitbol himself. Since we cannot define S but it is very easy to define our own 'perspective', then it is not very natural to define the the perspective of a pen.

I don't follow. The perspective of the pen seems the same as that of a human being there. The pen just pays a lot less attention. I honestly give humans or any living thing no special regard in this topic.

I think there are interactions between two electrons (like both being part of the same Helium) that ambiguates which one is which when one of the two changes state (like exits the atom). So yes, an electron departed, but it wasn't necessarily a particular electron. This is not a measurement thing. Of course there is no way to measure which one stayed and which one left. I'm just saying that metaphysically, it seems that electrons don't have identities, and thus act like a distinct 'system S'.
— noAxioms
I think I agree!

Cool. Few agree with that. It is controversial.

But I am not sure that we can say that the unobserved electron does not exist.

Kind of hard not to observe an electron. Its state might be hidden if put in a box, but we put it there so we know its there. If not in the box, it interacts with other things and that makes it exist. I cannot escape that interaction.

In other words, let O be an 'observer'. What is the ontological status of unobserved systems according to O?

An unobserved system is in superposition of possible states that follow from the last observed state of the system. The real trick is how to go about not observing it for any length of time. Hence our alien showing up in an inside-out Schrodinger's space ship.

In which case, how does this address the 'observer problem'? The observer might occupy a particular reference frame, but without an observer, then what is being measured/observed? — Wayfarer

I think this was addressed to me. An observer occupies all reference frames, but is stationary in only one of them. Without an actual observer doing an act of observing, relative velocities of rocks and table lamps and such are still defined, just not noticed in any conscious way. You seem to continue the classic definition of 'observer' here rather than the one being described by, Rovelli was it?

Just a curiosity...

Interestingly, it seems that RQM agrees with the Consistent Histories interpretation about the lack of a 'single history'

Well, I really find this interpretation (at least as it is presented there) somewhat difficult to understand. So, I still have not formed an idea about it. In fact, I somehow have a problem to really distinguish it from MWI.

What do you think of Consistent Histories? — boundless

Hard to understand is my current state of affairs. I don't get a clear statement about how it differs from the others.

From the SEP article on Consistent Histories, section 11.4:

"... This cannot lead to inconsistencies, a proposition being both true and false, because of the single framework rule. But it is contrary to a deeply rooted faith or intuition, shared by philosophers, physicists, and the proverbial man in the street, that at any point in time there is one and only one state of the universe which is “true”, and with which every true statement about the world must be consistent."

The man on the street, sure, but to philosophers and physicists who know their QM, that faith in there being an actual state of affairs is not very deeply rooted at all. Those roots were discarded a century ago. There are interpretations that support it and interpretations that do not. That this interpretation lists itself among the latter is hardly contrary to established belief. Yes, it seems a lot like MWI and I'd like to see a thing that spells out where the two differ in metaphysics, and not just terminology.

From a quick glance at the wiki chart, both have universal wave function, but only MWI considers that wave function to be real. MWI is deterministic and CHI is not, but it isn't clear from the linked article why not.

I have a problem with worlds of non-equal weights in MWI, and I'm not sure if the interpretation addresses that, or if CHI has a similar problem.

Consider an electron. According to RQM, its state is 'perspective'-dependent. But all of them are in agreement that it is an 'electron'. — boundless

From some (most) perspectives, there is no 'it' to be an electron. To consider an electron is to have already assumed a perspective where it exists. It is a measurement already taken.

I have a hard time keeping up with you guys :)

Here an article by Bitbol about the Wigner's friend scenario:
An excerpt:
" ... In the same way as we had to reduce the probability function of the whole measurement chain, we now have to reduce its wave function. There is a big difference between the two cases, however: whereas the ordinary probability functions could be considered as the expression of our ignorance about what effectively occurred in the lab., this is not true for the wave functions of quantum mechanics." — boundless

This isn't the QM I know. There is no 'big difference' between the two cases. The cat is both dead and alive (superposition), not either dead or alive (result of an unobserved coin toss). The latter probability function is said expression of ignorace about how the coin toss turned out, but the probability function of the cat is not ignorance. It really is dead and alive until measured, else it could not interfere with itself. Of course, being a classic object, it is not easily going to interfere with itself, but quantum states very much do, so it seems to be a mistake to qualify the probability function as a mere expression of ignorance.

Maybe the colleague (Wigner's friend) changed that (being a conscious being that the cat apparently isn't), and that explains this assertion above. This I suppose cannot be disproved since there is no way to prevent the decoherence of something as classic as a human, but it also goes for the cat of course. They seem to allow the inconsistency of having a cat in superposition but not a lab assistant.

"The wave function of the measurement chain just provides a probabilistic prediction of the result of a possible (uncontrollable) interaction between it and something else."

What does the measurement chain have to do with it? I shoot a photon through the slits and it makes a mark on the screen. That's a measurement even though it might be a minute before the operator bothers to look and learn of the result. The result is not being kept hidden in a box to prevent decoherence. If consciousness causes collapse, it doesn't imply that knowledge causes it. The conscious guy is in the room, ignorant of the result of the measurement, but nevertheless present. The collapse occurs because of that presence, no? If it is awareness, then I should be able to measure a thing but not take note of the result, and find the thing still in superposition later on. But that doesn't happen. It seems a falsification for the 'conscious knowledge' causing collapse idea.

I'm just trying to shoot holes in what I think might be being described in these quotes.
More of that quote:

"...
Let me show how this indexical “solution” (or “dissolution”) of the measurement problem of quantum mechanics works. It is based on the far-reaching, and already documented, difference between a property and an observable. A property of an object is defined in the absolute, whereas an observable refers to a possible relation between the object and something else. Each time one wishes to make the relation explicit, one has to introduce the “something else” into the field of description. But then, the characterization of this something else is itself relative to a possible interaction with a third element, and so on and so forth. Isn't there a natural end to this chain of relations? Yes, there is. WE are this end. Of course, I am not trying to say that WE are unique or privileged beings in nature (this would be collective solipsism of an absurd sort), but only that WE are privileged beings for US!"

So what? Only the table lamp is the privileged object for the table lamp. We are no more a natural end to the chain of reactions than is the table lamp. The reactions continue right on through us to affect other things. I have a hard time thinking of any causal chain that is initiated (rather than propagated) and cannot think of any that can end ever (per conservation of information). Radioactive decay might be an example of a new causal chain. Depends on interpretation.

"As soon as we establish a relation with an element of the measurement chain, this element acquires a determination relative to US. Nothing has thus to be changed in the physical description, since determinations of the measurement chain are still relative to something. But everything is different for US, since the determinations of the measurement chain are now relative to US. And a relation of which WE are one term is something quite peculiar, even if it is only peculiar ... from OUR point of view."

OK, but I can substitute "table lamp" for "US" everywhere in that bit and it still holds. If "WE" are not special, why bring US up all the time like that?

Oddly, I think that Bitbol's view seems a mixture of CI and RQM, but in fact I think it is still a form of CI. In usual formulations of CI, there is only a 'classical world'. Yet, Bitbol's point is that Wigner still has to consider his friend in superposition.

I didn't get that from reading that bit.

I don't formally know what Wheeler means by 'registration'.
— noAxioms

I see. Me too, actually. So do not trust my word :wink:

OK, so take caution when the term is used below:

Does CI explicitly define measurement far enough to classify it in general as a 'registered device'? I thought it left measurement fairly undefined, allowing all sorts of interpretations on the spectrum from interaction through information processing and full on has-a-soul.
— noAxioms

Yes, I agree with you on the ambiguity of CI. But some proposals to resolve it, involve that registering devices 'collapse' the wave-function. If true, I believe you need the physical interaction and the act of registration.

Without a description of what constitutes that registration act then, I have no idea what he's talking about.

According to Bitbol, collapse is due to an increase of knowledge. It is not a physical process. The wave-function is regarded as a tool."

If CI is taken as an epistemological interpretation, I suppose this is true in a way, but it seems trivial to falsify. Alice (a simple device) measures the spin on one of an entangled pair, and prints the result of that measurement on a little paper that ejects face down on the table. Human Bob sees the paper but has gained no knowledge of the result, only knowledge that the measurement has taken place. He now measures the other particle of that pair and is is no longer in superposition. His lack of knowledge did not prevent the collapse of the wave function of his sample The device (Alice) has no knowledge since it retains no state of the incident after finishing its little printout. Does the unseen paper have the knowledge? It certainly contains the (hidden) information, but does it have 'knowledge'? If it does, why that anthropocentric choice of words?

Oh good, then you might take a stab at my questions, even though they're geared towards a Wigner sort of setup. Are you saying that consciousness is not physical?
— noAxioms

Well, yes and I also believe in free-will.

Well, so do I, but my definition might be different than yours, so it isn't saying much. I will to be on the other side of these jail bars, so physics, deterministic or otherwise, very much does get in my way on that account, but few consider such restriction to be an example of lack of free will.

[But I am a bit unwilling to explain why I think so here... :smile:.]Agree, this would be too far off topic. There are always other threads about such things.

If there's not another topic to discuss it, then why not here? I'm in no way against inter-subjective agreements. I just don't think that makes things objective. We both see the same moon, sort of, but the moon's existence is still relative. It totally doesn't exist to something that has taken no measurement of it, but it also doesn't stop existing to us if everybody looks away for a moment. It cannot be un-measured.
— noAxioms

Ok...Yes, I agree that inter-subjectivity and objectivity in the usual sense are different. What moral relativism denies, however, is that there are no universal ethical truths.

Double negative. It denies that there are universal ethical truths. I'm sure you meant that.

I find it somewhat irrational, actually. In a sense, it is somewhat dogmatic: 'perspectivism' does not entail that it is impossible to find something that is considered 'good' for everyone.

Actually, I find that any particular rule is probably meaningless for most situations. 'Torture of babies is bad' has no application to the vast majority of things where 'torture' and 'babies' are both meaningless or at least poorly defined. Note my use of 'things' instead of the already restrictive word 'everybody' which detracts from the universality of the rule. A universal rule should not only apply to things that are member of 'everybody'.

I believe that, indeed, there are actions that can be considered good/bad by everyone. And I do not believe that rejecting 'objectivity' in the usual sense denies that.

Same here. I don't necessarily deny objective morality, just playing devil's advocate.

Regarding the example of the Moon, I am a bit perplexed however. If the Moon is completely non-existent for anybody that has not measured it, how can I say that measurement is an interaction?

Don't understand your, um, misunderstanding. How can it exist unmeasured? I've not measured the 2nd even larger moon, so it doesn't exist, and that seems not a problem to you. Nobody can be on this Earth (can exist relative to me say) and not have measured the moon. We have an inter-subjective agreement about that. Similarly, if one has not measured the moon (or has measured that bigger one), then that person cannot establish any inter-subjective relationship with me. It would be a contradiction.

S always exists to itself it would seem. The live cat measures a live cat, never the dead one, let alone no-cat. I don't see how that is problematic.
— noAxioms

Intuitively I would agree with you! But look at the link I gave to Wayfarer of the SEP article on RQM :smile:

The article says a complete measurement of ones self cannot be taken, not that you can't measure yourself. My pants size is a measurement of myself. :sad:

You need to define S. This collection of atoms currently has an arbitrary box drawn around them and is designated as S or 'noAxioms' for the moment, even though many of those atoms come and go continuously. You may have a different idea if you have a different philosophy of mind and identity. For me, at best, my identity is the stuff in that abstract box drawn around a bunch of matter near a certain event, and the entire worldline that led to that event. That definition only works because I cannot subjectively split or merge. It wouldn't work for an amoeba, starfish, or a candle flame.
— noAxioms

Ok thanks, I see. But it seems that this kind of definition of 'S' (or whatever physical object you have in mind) presupposes that can be used to all perspectives.

I don't have a definition of S. It seems arbitrary. It is fairly easy to do for a human, but less so for other things. The ease of defining human X (and equating that definition to the same X at some other time) allows the existence of a legal identity of X. You can't do that with amoebas and flames.

Take for instance even an electron. Its state might be different for the various 'perspectives'. But for all physical systems it is indeed an 'electron'.

I think there are interactions between two electrons (like both being part of the same Helium) that ambiguates which one is which when one of the two changes state (like exits the atom). So yes, an electron departed, but it wasn't necessarily a particular electron. This is not a measurement thing. Of course there is no way to measure which one stayed and which one left. I'm just saying that metaphysically, it seems that electrons don't have identities, and thus act like a distinct 'system S'.

So, you need something invariant, after all. Can we still have invariants if we do not have something perspective-independent? I am struggling with this point and I am not arrived to a solution.

Not sure if we're talking about the same thing here. Most of my relation with the electron is 'it is there', and relative to some other observer, it isn't there, or anywhere, just like the moon. To any two intersubective observers, they should agree on the electron being there or not, even if viewed from different frames. Isn't 'it is there' an invariant between said intersubjective perspectives? One says it is northbound and the other says eastbound, but both say it is there. That relation is not invariant between perspectives that are not intersubjective.

I would love to talk philosophy with a sentient amoeba.
— noAxioms

Well, me too! But as Wittgenstein remarked, we might not be able to understand a lion, let alone an amoeba :sad:

We should if it is sentient. I just want to talk to an alien that reproduces by suddenly falling into two halves. There is so much stuff that we find intuitive only because we don't do that, and talking to such a being would drive out all those inbred biases in not just us, but the amoeba as well. It would learn from us. The flame is even worse, because two flames can merge into one. Even the amoeba can't do that. We may well be able to talk to the flame some day soon because any AI entity (say an intelligent virtual being) would benefit from working that way


Appreciate the good discussion. Been a while for me.

Anyway, note that there is a problem of self-reference even in Relational approach(es). I suggest you to read section four of the SEP article about 'Relational Quantum Mechanics': https://plato.stanford.edu/entries/qm-relational/#SelRefSelMea . It is a discussion about the possibility of self-measurement. (Maybe Andrew M and @noAxioms will find it intriguing too!) — boundless

OK, that's what you meant by the self reference issue. Theorem aside, it seems quite intuitive that an aparatus cannot measure all of its own states since the taking of that measurement changes those states. This seems a little analogous to Heisenberg uncertainty where the more you know about one aspect of yourself, the less the can be known about some other aspect.
None of this means I cannot collapse my own state in sufficient hindsight, which is sort of what I meant with my first reply to this issue.

I hope you realize that an observer, without something observing is contradictory. — Metaphysician Undercover

Not the way Rovelli is defining the word. I personally dislike such usage because it encourages exactly such misinterpretations.

So if I claim the table lamp takes a measurement of the air heating it, I'm not laying claim that it has subjective experience.
— noAxioms

A measurement is a comparison, between something observed, and a scale (an ideal). A table lamp does not measure.

It is warmer than the same lamp measuring colder air. The air temperature has had an effect on the lamp. One system has effected the 2nd, and that's the second definition of measurement that @boundless gave.

I think I understand what a reference frame is. But I don't understand how an inanimate object can be regarded as equivalent to 'an observer'. — Wayfarer

He's not saying that the lamp is taking note of the hand motion. He's just saying that the motion is relative to the motion of the lamp, be it noticed or not. The word 'observer' is being used in place of reference frame.

An inertial reference frame doesn't even need an origin. 20 things can all have identical velocity in some inertial frame and some other thing will have identical velocity relative to any of them, so in this sense they're all the same 'observer' as Rovelli uses the term. It doesn't work for accelerating frames, so 20 things can all be different parts (let's say rivets of a ship) of some rigid accelerating object and those 20 parts will have different velocities in any frame in which they are moving. So an 'observer' in an accelerating situation like that needs to be a specific point like one of those 20 objects or any other coordinate in that accelerating frame.

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.
— noAxioms

I recall that you accept Tegmark's mathematical universe hypothesis, so that would seem to follow. — Andrew M

Sort of. Tegmark takes the MWI position that the structure (the universal wave function) exists, and thus he has to explain its 'being'. I don't buy into Platonic existence of numbers, and to both of us, the universal wave function is a mathematical thing, so same category.

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.
— noAxioms

OK, but that seems like a nonphysical answer.

It is very physical if you consider the physical universe to be a mathematical thing. The answer is still lousy, but I find it to be a very physical answer.

Just to follow up a bit further, suppose we have an equal path-length Mach-Zehnder interferometer that sends every emitted photon to detector 1.

Now since RQM respects locality, it raises a question of what is physically going on in the interferometer prior to detection of the photon. MWI says there is amplitude for a photon on each path which results in subsequent interference at the final beam splitter. But it seems that RQM can't explain it that way since it raises the prospect of branches.

Does RQM have branches? That is more an MWI thing. Prior to that measurement, there is no collapse, so the wave function puts the photon on both paths, not on 'either' path. The photon in this state can interfere with itself on both paths (to detector 1 and to detector 2) after the final beam splitter resulting in one wave function that yields almost all of the probability of measurement on detector 1 and none on detector 2.
That's not so much a description of RQM as it is a description of QM. I don't know where you might suggest locality comes into play. Nothing is being affected faster than light or anything, unlike what apparently goes on in some delayed choice experiments.

Actually I read your [boundless's] account of Rovelli's 'observer' again, and there's something about it I can't buy. It says "I use the word “observer” in the sense in which it is conventionally used in Galilean relativity when we say that an object has a velocity “with respect to a certain observer”. The observer can be any physical object having a definite state of motion'. — Wayfarer

That sounds like a reference frame, which A) doesn't require an object stationary in that frame to define it, and B) has nothing to do with what causes a wave function to collapse. Rovelli says this in the context of equating that 'observer' to the mechanism that collapses/splits a wave function? I think not the latter. An observer is a point of view, but not necessarily something at that point observing, and not something that has an effect.. Just a point from which a description can be made. It does imply a partially collapsed state, so it isn't really defined without the table lamp, stationary or otherwise.

I’ll spell out my bottom line - that all measurement or observation has a subjective aspect, i.e. it is undertaken by a subject, and the subject is (obviously) never disclosed in the act of observation, because observation is always of objects or at least of the objective domain.

I kind of agree with this terminology being ambiguous, which is why I hesitate to use the word observation. Wave function collapse has presumably been going on long before any object has been capable of being this sort of 'observer'. So if I claim the table lamp takes a measurement of the air heating it, I'm not laying claim that it has subjective experience.

Surely you have a spin on the whole 'why is there something' debate, else you'd have to face the subjectivism yourself.
— noAxioms

I think there's little understanding of the idea of 'first cause' nowadays. Because science has habituated us to thinking of causality in terms of material and efficient causation, then we can only conceive of 'first cause' as something at the temporal beginning of the sequence. But I think the original notion of the first cause is of a different order or kind to material and efficient causes.

Is that an answer? It is a of a different order or kind. OK, I can agree with that, but it isn't a specific answer. Indeed, I think it is a category error to treat the universe as an object, which, like all objects, doesn't exist, comes into being, and then exists (for a duration). 'Exists' means something different than how we use it for objects. But what does it mean actually? I find it to hold no distinction. An existing object is distinct from a non-existing one, but that is the object category. It seems not to apply to a universe category. That's just me and my relativist mindset. I'm sure you see a distinction, but you haven't really identified what 'order or kind' we're talking about here.

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.
— noAxioms
I agree with this. Maybe Wigner himself however thought that all conscious observers are 'classical', i.e. according to him in the thought experiment Wigner's friend causes the collapse also with the respect to Wigner. — boundless

Don't think there is such a cut with Wigner's interpretation. Consciousness causes collapse, period. If the cat is conscious, then it can't be in superposition of dead/alive. If not, then it can. There is no way to disprove this since there is no way to isolate a human in Schrodinger's box except by distance. I cannot take a human Alice and measure superposition on her. In principle I can, but there is just no practical way to prevent decoherence of a human. Alice is just not going to interfere with herself, even though I thought of a way for her to do it.

Bingo! That's IMO the billion dollar question for all versions of CI. Classicality is both the result of collapse and the necessary condition of it.

Does CI explicitly define measurement far enough to classify it in general as a 'registered device'? I thought it left measurement fairly undefined, allowing all sorts of interpretations on the spectrum from interaction through information processing and full on has-a-soul.

Note, though, that if you accept the versions of CI where consciousness has a special role and you do not have a physicalist theory of consciousness the argument you are making does not really apply. Why? Because, consciousness is not a result of a physical process. That's the point that, for instance, Bitbol makes (also Wayfarer makes this point in my understanding).

Yes, that's the full-on definition. It is outside methodological naturalism, but not outside science. Wigner concluded that the interpretation led to solipsism partly because other consciousnesses will collapse waves functions differently, and that puts each of these consciousnesses in different physical worlds. If that's not the view, then you don't exist until I collapse you, and that again makes for solipsism.

That definition of 'measurement' is hardly confined in RQM.
— noAxioms

I am not completely sure. Take Wheeler's view for instance. While not giving a special role to consciousness, he nevertheless gives a special role to the act of registration. So, it seems that in CI 'measurement' is at least given by both the interaction and the act of registration. Does this make sense to you?

Not really. I don't formally know what Wheeler means by 'registration'. I tried to look it up but found the term only fairly well buried in papers beyond my ability to absorb. I used the word above, but only to echo the notion of a system that meets some unstated qualification of processing the information of measurement.

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').

What I like has little to do with it. I'm interested in what works: is self consistent.

Yes, Bitbol does not consider a dust mote to be an observer. In my understanding, Bitbol thinks that only conscious beings can be observers.

Well it works I suppose. If consciousness is not a classical physical thing, then there's no chicken/egg problem. Still, how does it manage to collapse a state of total superposition into a state with matter present, let alone matter that can host consciousness? Collapse seems not to be in any way a function of will. I cannot will a measurement of vertical polarization, yet I would seem to need to do that to find my physical host.

FWIW, I am not a 'physicalist' myself but I am not convinced that consciousness has a special role in QM.

Oh good, then you might take a stab at my questions, even though they're geared towards a Wigner sort of setup. Are you saying that consciousness is not physical?

Anyway, I am not noAxioms, but I believe that his point was not 'nihilistic'.
— boundless
My post above sort of indicates that while I don't mean to be nihilistic, I'm not sure the view doesn't lead necessarily to that camp.
— noAxioms

Oh, I see. I believe that it does not necessary lead to nihilism or relativism. After all, there is still the possibility of inter-subjective agreement on ethical matters (so in some sense we can still talk about 'objective ethics') in a similar way that there is inter-subjective agreement on scientific matters. [Maybe this is too off-topic, though]

If there's not another topic to discuss it, then why not here? I'm in no way against inter-subjective agreements. I just don't think that makes things objective. We both see the same moon, sort of, but the moon's existence is still relative. It totally doesn't exist to something that has taken no measurement of it, but it also doesn't stop existing to us if everybody looks away for a moment. It cannot be un-measured.

A system with states still in complete superposition is still an existing system. There was no description of S existing in relation to R. Just 'there is a system S', which sounds more like MWI and not RQM.
— noAxioms

So S defines a perspective and everything (i.e. all its properties) about it is defined in relation to other systems? Is there something about S that is not defined in relation to other systems?

Another question, maybe problematic: How is S in relation to itself?

S can define a perspective, yes. S is perhaps a collapsed state at some event say. Doesn't matter what the system is. The collapsed state here might be the two of us looking at a moon, and might be a moonless Earth without either of us, or it might be empty space between galaxies. In MWI speak, those are all existing worlds, and the we only exist in the first one, so the moon exists relative to that S and not to other potential states for the same collapsed S at that event.

S always exists to itself it would seem. The live cat measures a live cat, never the dead one, let alone no-cat. I don't see how that is problematic.

Since I am not really an event, I'm also not a defined measured state S. Relative to parts of me, other parts are in superposition. This does not in any way impede my classic functionality, but it does mean that lacking a defined state, I lack a defined state and thus a defined identity, which was never a problem to my view, so I don't care. But one has to be careful when trying to pinpoint an intersubjectivity between two systems. So each of us measures a past state of the other, not a current one. These are nitpicks since the parts in superposition are trivial differences.

S does have intrinsic properties. I'm just balking at the suggestion that there is S, unqualified.
— noAxioms

I'd agree with you if by 'intrinsic properties' you mean properties that distinguish S from other systems.

You need to define S. This collection of atoms currently has an arbitrary box drawn around them and is designated as S or 'noAxioms' for the moment, even though many of those atoms come and go continuously. You may have a different idea if you have a different philosophy of mind and identity. For me, at best, my identity is the stuff in that abstract box drawn around a bunch of matter near a certain event, and the entire worldline that led to that event. That definition only works because I cannot subjectively split or merge. It wouldn't work for an amoeba, starfish, or a candle flame.
I would love to talk philosophy with a sentient amoeba.

In this case, we would have that both S is associated with a perspective and everything about it is defined in relation to other objects. I am not sure if this position is fully consistent but it is very fascinating! :smile:

Maybe it isn't consistent. Hence the appreciated cage rattling.

Basically, according to RQM you can only define a state of a system S in relation to something else. In other words, when you speak of the state of S you always need to specify the 'perspective' according to which you are making such a description. You can't avoid that.

So, RQM seems to imply that while there are 'individual' systems like S, their states are meaningfully defined only in relation to something else. — boundless

It is meaningful to say that there are individual systems like S? That sounds an awful lot like counterfactual definiteness. A system with states still in complete superposition is still an existing system. There was no description of S existing in relation to R. Just 'there is a system S', which sounds more like MWI and not RQM.
I tend to go whole-hog on the implications of the interpretation, so I found less reason than you did to counter the suggestion of a sort of nihilism.

But if S does not have any intrinsic properties, what does S, S?

S does have intrinsic properties. I'm just balking at the suggestion that there is S, unqualified.

I do not believe that RQM per se goes as far as denying that we can speak of individual objects, i.e. a 'no-thing' view, so to speak.

? I would say we can only speak of objects. There is no 'no thing' view. That would be the objective view, things that exist without the relation, or with only an objective relation.

The price to pay for that is subjectivism/relativism/nihilism. — Wayfarer

Existential nihilism perhaps. Life still has meaning, even if that meaning isn't objective. Even a typical theist doesn't really consider say morality to be objective. Don't kill because God says it's wrong. 'Don't do it because I said so' is relative to the thing saying so, not objective.
This is as opposed to objective morality where God tells you not to do wrong things, and a good god doesn't do them either. The morality is a higher authority than God if it is objective, which makes for a weaker god if it answers to this higher objectivity.

Yes, it seems something along the lines of those 3 views is a price paid. If a view you find distasteful is more sound than one you'd like to be the case, does comfort trump logic?
Surely you have a spin on the whole 'why is there something' debate, else you'd have to face the subjectivism yourself.
Is time a property of the universe or does the universe, like any other object, exist in time? Only with the latter does it make sense for it to have come into being, in which case one has to posit a way that came about. Is there a first cause? RQM doesn't posit this 'being', so it doesn't have to answer to the conundrum.

Anyway, I am not noAxioms, but I believe that his point was not 'nihilistic'. — boundless

My post above sort of indicates that while I don't mean to be nihilistic, I'm not sure the view doesn't lead necessarily to that camp.

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.