Putting this together with your earlier comment that you are not attached to locality, it sounds like you have an affinity to the 'non-local hidden variables' school, of which David Bohm's 'pilot wave' interpretation of QM is perhaps the best-known. In most other popular interpretations, the imprecision about location is not just epistemological. — andrewk

I believe that the de Broglie-Bohm (dBB) theory should be given more attention. I am not a dBB-supporter but I believe that it is a valid alternative. Interestingly, there are different views about the ontology of dBB. Some like Bohm himself in his original work consider the contribution of the wave-function to the motion of the particle in a similar way to an additional force, given by the Quantum Potential (the link is to Wikipedia article on it).
To my knowledge, most dBB-supporters however do not like this formulation and prefer a first-order formulation (i.e. without second order temporal derivatives, i.e. without accelerations and forces) - The SEP article on 'Bohmian mechanics' has a section that explains why the 'quantum potential' formulation is criticized. But even here there is no consensus about the ontology. Some take the wave-function as a physically real field. Others do not and prefer a 'nomological' approach (but even here in the 'nomological' camp there are different views: check this article https://arxiv.org/pdf/1406.1371.pdf. I commented on it and quoted an excerpt in my previous post).
A somewhat related discussion is found in this thread in physics forums: https://www.physicsforums.com/threads/the-reality-of-configuration-space.554543/ (note that it is not only about dBB, strictly speaking...)

As I said elsewhere, however, I find dBB somewhat 'bizzarre' in its ontology. I am inclined to believe that this shows that some radical 'paradigm change' is necessary. But I nevertheless believe that it is a very interesting theory.

I like Bohm. I have his book 'Quantum Theory' which is interesting because it was written before the modern Dirac notation for QM with 'bras' and 'kets' became standard. — andrewk

I like him too very much. He really had fascinating ideas during all his career. He really wanted to understand things in depth.

I unfortunately do not have this book (but I believe reviews are generally positive).

FWIW Bohm was quite a mystic, and had a famous series of public discussions with Krishnamurti about physics and spirituality. — andrewk

Yeah, that's correct. Unfortunately, this is a reason why people are averse of his (especially later) work. Personally, I disagree with them. Even if one is uninterested in spirituality, his later ideas are IMO intriguing.

I have a rough time with this distinction. Something not real can still be used to describe a real thing. It just isn't the actual thing. — noAxioms

I believe that if you say that something 'not real' describes 'a real thing' you're just re-asserting a realist/representational view. The 'unreal'/'non-representational' view of the wave-function advocated by Rovelli, Bitbol etc is that the wave-function does not describe anything. It is just a tool.

This is very compatible with the view that collapse is due to an increase of knowledge (i.e. an 'epistemic', not 'ontic' view). But, of course, if the wave-function is not representational in any sense, it is difficult to explain why QM predictions are very good, for instance. In fact, a non-representational view of the wave-function implies that we cannot know anything about the unmeasured objects and the 'collapse' mechanism.

In other words, 'real' and 'representational' should be taken as synonyms (or very close to that) - the point is that there is a biunivocal correspondence between mathematical formalism and reality. In MWI all outcomes occur because you consider the wave-function as real. This is true for the objective collapse theories. In CI there is a spectrum. In dBB theory too this is a controversial issue, actually.

For instance, many dBB-supporters do not consider the wave-function as a 'real field' but as 'nomological'. The point is that the wave-function is a 3N-dimensional function, while particles or (normal) field configurations live in the 3-dimensional space. This paper explains some possible views of this 'nomological' camp: https://arxiv.org/pdf/1406.1371.pdf. The authors distinguish between two possible meaning of 'nomological'. The first is called 'Humeanism' - it is actually a reductive approach: there is no real 'reason' why particles move in the way that move. The wave-function is just a useful tool to 'describe' (or, maybe better, to calculate) the evolving configuration of particles (at least as I understand it) - it is more or less 'non-representational'. This is somewhat ironic because dBB is usually chosen to understand what's going on. Indeed, another 'nomological' view is dispositional, quoting from the article (p.16):

On this view, the universal wave-function Ψt of the system of particles at a given time is a mathematical object that represents the disposition to move in a certain manner at that time. This disposition is a holistic property of all the particles in the universe together – that is, a relational property that takes all the particles as relata. It induces a certain temporal development of the particle configuration, that development being its manifestation. In other words, given a spatial configuration of the particles (actual or counterfactual) and the disposition of motion at a time as represented by the wave-function as input, the Bohmian law of motion yields the velocities of the particles at that time as output.

This is entirely different from Humeanism. Now the wave-functions is not indeed a 'physical field'. Yet it represents some physical property. So I would say that the wave-function is indeed 'representational'. It represents a physical, real property - it's not just a tool like in Humeanism. The point is that in Humeanism, the 'wave-function' does not add anything to ontology whereas in dispositionalism, it is related to a physically real disposition.

The point in bringing this distinction between two different 'sub-interpretations' of dBB was simply to explain better what I mean by 'real'. If you think that the wave-function has some ontological meaning, then the wave-function is real/representational. If not, it is just a tool of some sorts.

Edit: a somewhat related discussion on physics forums can be found here: https://www.physicsforums.com/threads/the-reality-of-configuration-space.554543/

That doesn't follow from either interpretation of the wave function. It seems to require an additional postulate. — noAxioms

Agreed!

He was shunned by the physics community after his PHD and went into the defense industry instead, but was asked to present his work 5 years after the paper was published. Somewhere around that time DeWitt coined the MWI term from Everett's original "relative state formulation" which sounds an awful lot like RQM. — noAxioms

Interesting. BTW, I knew that Everett's original views are sometimes distinguished from DeWitt's et al interpretation of them. Anyway, the IEP article on 'Everettian interpretations' lists also some Relational interpretations. So, maybe Everett was really a 'relationalist' :wink:

That works given a postulate of such selection going on. My statement was an opinion, not an assertion. — noAxioms

I fully agree.

It seems that some objective collapse interpretations might fit the bill:

On the other hand, it is shown that dynamical collapse models, of the type originally proposed by Ghirardi-Rimini-Weber, can be re-interpreted as set selection criteria within a quantum histories framework, in which context they appear as candidate solutions to the set selection problem.
- Quantum Histories - Adrian Kent — Andrew M

Thanks, interesting. It makes sense.

Schrodinger himself took a 'wave-only' view during his life (he also took a 'non-representational' view for some years). I wonder if he did endorse that view (at least for some years). Maybe, however, he was more close to the 'usual' GRW-like approach.

There is certainly no one version of probably any of the interpretations, but there are probably some fundamental features that characterize each. Take that away and it isn't really a different version of something (like RQM), but rather a whole different interpretation. So sure, all outcomes occur, but they don't all occur to a given X (or anything else). They very much occur (are real) to things Y that interact with (measure) them. — noAxioms

Ok, I see. For 'Wigner's friend' only one outcome occurs. For 'Wigner' there is still a superposition (of both his friend and the physical system).

Tegmark is kind of funny this way. An MWI person might refer to 'universe' as the one universal wave function and all these resulting worlds, but Tegmark often uses 'multiverse' splitting into universes so that it falls under his type-3 multiverse. But other times he speaks of worlds and one universe. — noAxioms

Yeah, in fact I was familiar with his 'multiverse' terminology.

So worlds don't actually split off, but different terms simply become sufficiently decoherent for their interaction to become negligible. — noAxioms

Agreed. Now that you mention it, I remember reading a discussion in physicsforums where this issue came. And I was surprised in reading that in principle the various branches could still interfere after the 'split'. So, I actually forgot this.

Everything in that paper seems to apply to RQM since it seems to separate out the needless metaphysical assertions piled on top of the one postulate. Section III-C seems to offer a choice between effectively MWI and RQM, making RQM a valid offshoot of MWI physics. — noAxioms

Interesting. I think I'll read the paper, then :smile:

Yes, there is a splitting in this sense. What I meant is that for each reference, there is no splitting. In your example of the polarization, if 'I' observe a horizontal polarization the observation of a vertical polarization does not occur in RQM (for 'me'). In MWI, it does. — boundless

Sounds good. — noAxioms

Excellent!

The issue here is how to interpret the wave-function. If you interpret it as 'real' (or 'representational'), then you are right, there is no selection and the 'other branches' are still 'real' after the measurement.

If, instead, the wave-function is not treated as real/representational (as I think Rovelli does), it does not give a description of reality. There is a single outcome given by a probabilistic law. Maybe in this case, the word 'selection' is apt and it creates only confusion. The process of the 'collapse' here is treated like in CI (where it is an 'axiom' and is not explained e.g. in terms of decoherence).

Note that what is said in my quote above is compatible with both interpretations of the wave-function!

It isn't stated in MWI because it doesn't need it. All worlds are real, so none of them is in need of selection over the other. It's only when you have a metaphysical selecting (dice throwing as Einstein disdained) that such a postulate is introduced. — noAxioms

Agreed!

Everett was forced to reign in his views in order to gain acceptance.
...
His thinking was too out-of-box for the time. — noAxioms

I see. His proposal was certainly revolutionary (regardless whether one agrees with him or not).

some persistence over time seems required. — boundless

I think information preservation principle gives the persistence needed. If I measure the photon and then the big boot hits me from the sky (Python-style) before I can pass on the findings, was the measurement done? Information preservation says yes, the boot doesn't erase that. — noAxioms

Interesting idea! Maybe you are right!

Regarding empty space, I am not sure if it can be an 'observer' in RQM. — boundless

Agree. Nothing to collapse a wave function. — noAxioms

Yep. This is certainly the case for QM.

What about QFT? In QFT, the 'vacuum state' is not really 'void'. So, maybe quantum fields can be used as 'events'? (hope this makes sense) — boundless

Maybe so. Not up on QFT enough to comment with anything but the ignorance PoV. — noAxioms

Yeah, I too do not know very much about QFT.

Anyway, I do not believe that this affect my point. I would say that the point where the screen is hit is 'selected randomly'. — boundless

I say it isn't selected at all. — noAxioms

If one sees the wave-function as real then there is no selection, I agree. Otherwise, there is a 'selection'.

Such an interpretation would seem to propose counterfactual definiteness. Somewhere off to the side, some measurement is taken by not-me and causes some state to be real and the other results not.

From the Rovelli bit you quoted in the post above: — noAxioms

Even beyond its foundational role in relativistic field theories, locality constitutes, therefore, the base of the relational methodology: an observer cannot, and must not, account for events involving systems located out of its causal neighborhood (or light-cone).10
— Rovelli

An interpretation that such selecting of reality is going on outside of some privileged light cone is doing exactly this: accounting for events involving systems located out of its light cone. As such, the interpretation bears little resemblance to local interpretations like MWI or CH as I understand it.
Per you post above, it seemed that Hawking and Weinberg posited something along these lines, so I wonder what they'd say to my point here. — noAxioms

Very good point!

If true, I wonder if this is the reason why in CH, the universal wave-function is considered unreal.

As I said previously, I am sorry but I think I'll answer much more slowly in the following days :sad:

Most directly, Bob in the OP is able to directly measure superposition even after Alice has taken a measurement. — noAxioms

Well, yes, with this I agree.

If your interpretation says that a single outcome occurs. — noAxioms

Again, I agree. But this is IMO the position of RQM. On the other hand, I do not believe that there is only one 'version' of RQM. In fact, I believe that there are different relational approaches to QM. So, maybe some versions accept that all outcomes occur.

I never used the word 'universe' in what you quoted since it means such different things to different people. — noAxioms

Yeah, but when MWI-supporters deny that there is a 'splitting', they actually do not usually deny that there is a splitting that others have in mind IMO (of course, there are exceptions).

There is splitting of a sort in RQM also. To me, the photon is polarized vertical. Relative to another me, the photon is horizontal. Relative to a 3rd reference, there's not even a me or a photon. Sounds like those are separate worlds, some connected more than others. — noAxioms

But this is a different issue, IMO. Yes, there is a splitting in this sense. What I meant is that for each reference, there is no splitting. In your example of the polarization, if 'I' observe a horizontal polarization the observation of a vertical polarization does not occur in RQM (for 'me'). In MWI, it does.

That very much goes against the Everett postulate. It is a different interpretation and should have a different name. — noAxioms

I agree. Furthermore, I do not see a real justification for treating other 'worlds' as 'unreal'.

OK, I see better now what CH proposes that is unique. You'd think they'd put that in plain language in the introduction somewhere. How is what Hawking and Weinberg push different from the CH view then? Why add a 2nd postulate when the first one perfectly predicts the experience we have? — noAxioms

IMO, the difference is that in the MWI 'version' of Hawking and Weinberg the axiom is not stated (in fact, my 'proposed interpretation' consisted to add this axiom). Also, the wave-function in CH is not real (I wonder what this actually means in practice).

That's why I like RQM which is the main Everett postulate: "All isolated systems evolve according to the
Schrodinger equation" without MWI's secondary metaphysical postulate that said equation is real. The latter postulate makes no change to the evolution of the equation and thus 'what happens', and thus isn't needed to explain what the experience would be. — noAxioms

Ok!

I'm not sure that Everett did. It was a physics interpretation of QM, not necessarily making any metaphysical assertion. MWI as we know it might have been built on Everett's work, but I don't believe he called it 'MWI'. — noAxioms

Agreed! I believe that there is some controversy on Everett's own views.

I figured. That one is hard to explain. RQM says such and such is real to a second thing, say 'me', but 'me' needs definition.
...
That was a very eternalist way of describing things, but I cannot think of a way to do it in presentist terms. I consider 'observers' (that to which a reality relates) to be events in RQM, and consequently almost anything can be such an event. Humans are not special at all. Not sure if empty space is a valid event since there is nothing there to take a measurement of anything. It has to be something capable of being affected by state. — noAxioms

Well, you have a good point here about presentism. Maybe it does not lead to 'eternalism' but some persistence over time seems required. Regarding empty space, I am not sure if it can be an 'observer' in RQM.

What about QFT? In QFT, the 'vacuum state' is not really 'void'. So, maybe quantum fields can be used as 'events'? (hope this makes sense)

I never observe an electron passing through a slit. If I do, it goes through one and doesn't interfere with itself. So I don't get this scenario. What I have observed is where the electron hit the screen, or the pattern from many such electrons. At no point does any local interpretation of QM interpret the electron taking one path to get there. I think pilot wave theory might assert it, but they've really shot that one to hell when they put a partition between the two slits — noAxioms

Correct! I would have said that, sorry. Anyway, I do not believe that this affect my point. I would say that the point where the screen is hit is 'selected randomly'.

Take a point exactly 50 billion LY north. There is a nearly pure wavefunction describing what exists there, and one set of solutions to that wave function is finding a moon like ours nearby, or just an electron, or whatever. If there was a way for 'me' to just suddenly teleport and take a measure of that point, under MWI, I (a whole multitude of 'I', however many it takes) would measure every one of these possibilities. Under RQM, each of these possibilities would be real to the 'me' that appears there. Same story, but different wording. Both views also say that to an observer on that moon or at any of the other possible states there, I'd probably not suddenly appear in front of them. My appearance there is as unlikely as is theirs to me. — noAxioms

Ok, I see your point but I am not sure that this is right. Or rather: if you explain 'measurement' in terms of decoherence, then you are right. No selection happens. So, each 'outcome' happens.

For me this is problematic and, in fact, this is one of the main reasons - if not the main reason - why I do not accept MWI. But that's just me.

,

Just a curiosity: has anyone ever suggested an interpretation where the 'universal wavefunction' is real (like in MWI) and a single branch is 'selected' by a probabilistic rule (as in Consistent Histories as I understand it)?

This would be similar to the 'unreal' interpretation of MWI referenced in the Wikipedia article about MWI where only one branch is 'real' and the others are not. The only difference is that here there is an explicit axiom of a probabilistic selection.

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. — noAxioms

This is actually my understanding of Rovelli's own view.

Also, Rovelli makes a similar point in this article: https://arxiv.org/pdf/quant-ph/0604064.pdf ('Relational EPR')...for locality (at the end of page 3):

Even beyond its foundational role in relativistic field theories, locality constitutes, therefore, the base of the relational methodology: an observer cannot, and must not, account for events involving systems located out of its causal neighborhood (or light-cone).10

[Footnote 10] We can take this observation as an echo in fundamental physics of the celebrated: “7. Whereof one cannot speak, thereof one must be silent” [from Wittgenstein's Tractatus]

Right, that's the point, there are epistemic issues with "observations" no matter how you define the term. Sometimes the "observer" might be focused so as to miss many possibly relevant factors. In a human observer, this is one's attention. The person might observe with eyes and not ears, or vise versa, and miss some relevant information. In the case of an observing machine, its capabilities are limited by the intent of the design. — Metaphysician Undercover

This is a very interesting point. Maybe this is also the point of @Wafarer, in fact (regardless of 'idealism'). I am not sure that this applies to RQM where measurement is understood simply as a physical interaction, in fact. But this is IMO relevant for CI.

What do you mean when you use 'know' in this sense? What is the (range of appropriate) physical interpretation(s) of it, if it has one? — fdrake

Well, I agree with was said by Andrew M.

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. — noAxioms

Ok, I think we agree on this :smile:

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. — noAxioms

I agree with what you said here. But I am not sure that it solves the 'measurement problem' completely. It explains why we do not observe superposition. AND it explains why a single outcome is observed.
BUT you need an additional assumption in order to explain why only one outcome actually occurs.

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." — noAxioms

Not sure why you think you are disagreeing with me. If by 'universe' in MWI you mean the 'universal wavefunction' then I agree with you and Tegmark. But this is not normally what one means by 'world' or even 'universe', so, in fact, I think that one can definitely say that there is splitting. And this is for me a reason to not accept MWI.

Well, I know that some supporters of MWI consider only one branch as 'real' and other branches as 'unreal' ( e.g. apparently Hawking and Weinberg are supporters of this 'flavor' of MWI, see the Wikipedia article on MWI). But unfortunately, MWI cannot IMO justify that without an additional axiom (as in Consistent Histories, where there is the axiom that a 'history' is selected probabilistically. In dBB one 'branch' is occupied by particles).

My point is that this version of MWI simply does not explain why the 'other branches' are 'unreal'.

On the other hand, Deutch, DeWitt etc consider all branches as physical, 'real'. So it is not just me that understands MWI in that way.

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? — noAxioms

In dBB you have particles. In CI it is left unexplained, I think. In Consistent Histories it is an additional axiom in my understanding (and, therefore, it is left unexplained IMO).

I leave it to them. MWI makes them all real, — noAxioms

My point was exactly this.

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. — noAxioms

I do not understand this. Take the double-slit experiment. When you observe that an electron passes through slit A then you either explain this observation of a single outcome via a selection or you accept that the 'other history' is equally true as in MWI. So, I believe that both CI and RQM leave it unexplained.

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. — noAxioms

Ok, agreed!

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. — noAxioms

Ok, I see.

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. — noAxioms

I agree again. My point however is that there is no explanation why there is a probability of finding a Moon, an electron or whatever in the first place. If one accepts hidden-variables this is of course explained.

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. — noAxioms

Right, you find the Moon again because you already measured it. As you say it is not that remarkable. It is more remarkable that you find an electron probabilistically when you perform a measurement without hidden variables.

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. — noAxioms

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'. I am still leaning to this 'stronger' implication :smile:

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. — noAxioms

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. — noAxioms

Ok, I apparently almost agree! (see later)

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. 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.

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

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. — noAxioms

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.

On the other hand, if no decoherence occurs and O 'tells' O' that 'he' (O) sees a definite result then for O' there is still superposition.

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. — noAxioms

Ok, I see your point. You're probably right, actually!

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

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. — noAxioms

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'). — boundless

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. — noAxioms

:up:

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.. — boundless

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.. — noAxioms

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

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. — noAxioms

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. — noAxioms

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'.

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.

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...).

Maybe, reasoning in terms of potentiality/actuality can help here (e.g. see the quotation of Shimony here) at least in terms suggested by Andrew M here (but maybe his take is not really different from your model :smile: ...).

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. — noAxioms

Ok, sorry!

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

Agreed! Good point! (I have a tendency to forget it, for some reasons...)

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. — noAxioms

Agreed (except in the case of some hidden variables interpretations but I think that was implicit...)

Thanks for clearing up the issue of the interference. It makes more sense now.

Yes. This seems to align with Wittgenstein's private language argument. Our language develops via interactions with other people and things in the world. By which we come to learn things about ourselves as well. — Andrew M

Very interesting point :up:

As for why that should make a difference, my thought is that there are many possible spacetime paths between the present moment for O' and the measurement event for O. Similar to the Andromeda paradox, perhaps the time of the event for O can potentially be in the future of O' (until fixed in the past of O' by an interaction). — Andrew M

Ok, I see. Much confusion about this arises probably from an unconscious tendency to think in terms of a 'singular history' (i.e. a fixed present for everyone...), so to speak. But that's precisely what both Relativity (if one does not want to endorse the idea of a 'block universe') and RQM question. It is, however, simply very difficult (or impossible?) to 'overcome' that tendency...

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. — noAxioms

Ok, I see.

I actually made that comment following this remark of yours:

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. — noAxioms

My point was that a 'panexperientialist' might agree with this remark. I believe that for some forms of 'panexperientialism', there are degrees of 'sentience' and ours is just more complex. This is more or less the point of views like psycho-physical parallelism.

Anyway, sorry for the misunderstanding! I will answer in detail to your post later.

(BTW, unfortunately, I will soon have less time available so I will probably leave the discussion...)

I have somewhat of a taste for Whitehead's notion of pan-experientialism; the idea that experience or relation appertains to all entities and is thus the 'substance' of reality. Another way Whitehead expresses this is with his notion of concrescence. So, it might be better to say that experience rather than observation collapses the wave function. Experience can be a very broad term even in ordinary usage: as when we say things like "The cliff face experienced the erosive effects of the wind and rain". — Janus

I am too fascinated by panpsychism, pan-experientialism etc. But it too does have problems IMO.

In case you are interested, for a criticism of panpsychism, I suggest this article by Bitbol: https://www.academia.edu/36160525/BEYOND_PANPSYCHISM_THE_RADICALITY_OF_PHENOMENOLOGY. It is a bit long but very interesting IMO. Anyway, criticism to panpsychism can be found at pages 7-13.

(He also criticizes emergentism, at pages 6-7)

But a discussion on consciousness maybe is off-topic...

In my opinion Wheeler's view is a bit ambiguous. At times he suggests some form of 'panpsychism'. In other places, he seems to suggest that an 'observer' can be a sufficiently complex physical object. By 'sufficient complex physical object', I mean that such an object must be able to store and process information. And maybe, he considers that these objects are somehow sentient.
But IMO, he does not give a 'special role' to human consciousness (or animal consciousness...).

Personally, I prefer either Bitbol's approach, where you can define perspectives to sentient beings, or Rovelli's approach where you can define a perspective to everything (and might relate Rovelli's view to a form of panpsychism).

Yes, it alludes to the self-referential problems associated with predicting what oneself is going to do in the future. Fortunately, you can usually just choose! — Andrew M

Well, I am not persuaded that it says just that. As I said to noAxioms in my previous post, it seems that the only way for O to have 'information' about 'himself' (or better 'itself', to avoid anthropomorphic language as RQM does) it must consider 'himself'/'itself' as an object to another system. To me this somewhat makes sense.

Yes. Whereas, in my view, potential just means that the value has not been actualized yet for the observer (which would require a local interaction).

It's like the problem of non-referring sentences. The sentence "The King of France is wise" has a potential use but not an actual use until the appropriate physical conditions occur (i.e., a King of France is installed). As a consequence, you have to be careful about the logic applied to such statements. — Andrew M

Ok.

Let me ask a question that I posed to noAxioms. According to RQM, the state of S (let's say that S is an electron) is observed dependent. To be more precise, S can have a definite state, e.g. spin 'up', for O but not for O'. For O' it is still in a superposition. Now O' can ask O if 'it' 'sees' S in a definite state. O answers 'yes'. So, now it seems that according to O' the state of S 'collapsed' to a definite result. O' does not know which one, however. It seems that, at this point, for O, S has spin 'up' but for O', the spin can be either 'up' or 'down'. So, it seems that there are two 'branches' (using MWI language). But when O' 'opens the box' (or 'enters the room', as Wigner does in the Wigner's friend scenario), O' must agree with O according to RQM. But why? How is this justified in RQM? I mean: how the 'disappearance' of the 'other branch' is justified under RQM?

(edit: I am not sure that there is no more interference for O' when O says that he sees a definite state to O')

My own view is that there is a universal quantum state that is invariant, but RQM seems to reject that. Though perhaps another invariant is that we are all human beings with similar physical structures so we should always be able to agree that there are electrons and on the form of an electron. — Andrew M

That's my problem exactly. It seems that there is an ambiguity about the 'unobserved' objects in RQM.
Considering that the electron 'exists' but not in an undefined state IMO solves at least part of this issue.

Basically the same as you. I think almost all of these views can end up looking like Many Worlds when you dig into them. It makes sense in a way since they all depend on unitary QM. Though I think RQM would say that a history can be indefinite rather than there being multiple histories. — Andrew M

Ok, I agree!

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'. — noAxioms

After some reflection, I am not convinced by this explanation.

The first section says:

Dalla Chiara shows that the duality in the description of state evolution, encoded in the ordinary (i.e. von Neumann's) approach to the measurement problem, can be given a purely logical interpretation: “If the apparatus observer O is an object of the theory, then O cannot realize the reduction of the wave function. This is possible only to another O′, which is ‘external’ with respect to the universe of the theory. In other words, any apparatus, as a particular physical system, can be an object of the theory. Nevertheless, any apparatus which realizes the reduction of the wave function is necessarily only a metatheoretical object ” (Dalla Chiara 1977, p. 340).
...
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.

In other words, from its own perspective, O is 'meta-theoretical', i.e. QM can describe anything but not everything. It must be applied in a particular context or 'perspective'. The only way for observers to know about themselves is to consider themselves in relation to others. This makes somewhat sense to me, actually.

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

I agree that this is the usual understanding. But in Bitbol's case, it seems that he faithfully follows the remark: "QM can describe anything but not everything". In other words, Wigner can apply QM even to his friend. The 'measurement' for Wigner takes place when he actually enters in the lab.

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. So, superposition is now destroyed. At this point, it seems it is not very different from the classical case of an unseen coin. The problem is that Wigner still does not know in which state the lab is (including is friend). But it seems that when he enters Wigner and his friend must be in agreement! So for them, S is in the 'up' state.

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.

But what happened to the 'other branch' in O' (or Wigner's in Bitbol's interpretation)?


(edit: I am not sure that there is no more interference for O' (or 'Wigner) when O (or Wigner's friend) says that he sees a definite state to O')

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!

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. 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').

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

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.

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. — noAxioms

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. — noAxioms

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. — noAxioms

Ok. So, you endorse some kind of 'panpsychism' or 'panexperientalism'? (see: https://en.wikipedia.org/wiki/Panpsychism#Panexperientialism)
Actually, I agree with you that it seems that Wheeler's take is somewhat ambiguous. More precisely, it seems somewhat arbitrary to think that registering devices are 'special'. I can understand taking sentient beings (including non-humans ones if any) as special. But honestly, it seems that Wheeler's model is somewhat an artificial way to avoid either some form of RQM and giving a special status to conscious observers.

I believe that what Bitbol's find fascinating is that QM seems to force the idea that knowledge is perspectival. This is also what Rovelli finds fascinating of QM. The difference is that according to Rovelli, you can define a perspective for everything, whereas for Bitbol this is not true.

Note, however, that Bitbol's take seems in some sense 'skeptical'. Since we 'directly' know only how the world appears to us, we cannot really 'know' how the world appears to a non-sentient object (if such a thing is meaningful). So, if Bitbol can be charged with some form of anthropocentrism, I believe that his kind of anthropocentrism is not the usually criticized one: it is not due to some wishful thinking that we are 'special' but, rather, it is due to an epistemic reason, namely that we 'directly' know the world as it appears to us. In other words, Bitbol (and in a similar way d'Espagnat) does not deny a mind-independent reality (whether independent of 'my' mind or 'our' minds), but he says that it is inaccessible.

Cool. Few agree with that. It is controversial. — noAxioms

Well, first electrons are identical particles according to QM (if we do not accept hidden variables). Furthermore, QFT seems to give some support to this kind of view: electrons are just excitations of a field. So, it seems justifiable to think that an electron does really have an identity.

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!

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. — noAxioms

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. — noAxioms

I see. Then maybe we are in agreement! The unobserved electron is not really non-existent. 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.

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. — noAxioms

Ok, I see. This for me shows that it is paradoxical to talk about an universal wave-function (whether an epistemic tool or a real entity).

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

IMO, because results of observations are random.

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? — noAxioms

Agreed! That's definitely a problem for MWI.

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 — noAxioms

Yes, I agree. It does not seems very different from RQM.

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)

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

Ok, same here. 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.

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. — noAxioms

Agreed!

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. — noAxioms

I believe that in CH is more or less like MWI + Born Rule. In MWI, all results are equally 'real'. In CH, apparently there is the additional axiom that a particular result is 'selected' with a probability given by the Born Rule. I just do not get the unreality of the universal wave-function. I can appreciate the philosophical reason behind considering it 'unreal' (i.e. do not consider unrealized experimental results as 'real') but IMO this makes somewhat questionable its introduction in the first place.

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. — noAxioms

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.

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? I guess I will have to find time to go back and look at those resources that boundless provided in more detail. — Wayfarer

Well, according to Rovelli there is no 'observer problem' in the sense of CI. According to his interpretation, 'observer' is actually an excessively anthropomorphic expression to denote whatever physical system. Rovelli tries to do that in order to overcome the ambiguity present in CI about what an 'observer' is (in CI it is strictly speaking unspecified...).

But maybe even Rovelli's solution is problematic. After all (as it came up in my discussion with noAxioms) defining a 'physical system' is arbitrary. Let's take the example of the pen on my table. It can be argued that I can define my 'physical system' as both the pen and the 'pen + my table'. But also the 'pen tip' can define a 'physical system'. In other words, it might be argued that there is an undesirable explosion of the number of 'perspectives'. (Maybe it is more like an 'inelegance' rather than a problem, however...)

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. — noAxioms

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

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

No worries!

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. — noAxioms

Well, I think that you actually pointed to the 'big difference': quantum probabilities are not explainable in terms of ignorance :smile:

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. — noAxioms

I see. Maybe I misunderstood.

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.

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. — noAxioms

This is a very good point and I'll think about it :smile:

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. — noAxioms

Yes, that's the reasoning of RQM. 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.

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'. And our knowledge is perspectival. In a 'Kantian-like' philosophy it is impossible to know 'things as they are' because phenomena (in our immediate experience) are conditioned by a-priori categories of our mind. So, for Bitbol, we can only be (almost) sure about our perspective.

Note however, that I do not believe that accepting a 'Kantian-like' philosophy requires that you have to choose a determinate interpretation of QM. (After all, Kant himself did not know QM)

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.

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.

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

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. — noAxioms

Agreed! I agree that 'free will' is not 'complete free-will'. But in my opinion, it is not just an 'effective free will' a la what's is required in Bell's theorem to avoid superdeterminism, for instance. I believe that we have some autonomy - our actions cannot be (entirely) explained via physical processes (after all, I believe that neither determinism nor randomness nor some combination of the two are compatible with a 'real free will'). But as you say, it is O.T. :smile:

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. — boundless

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

Yes, sorry.

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'. — noAxioms

Ok. I think I see what you mean. Ethical statements are contextual. If there were no human beings (or more generally 'sentient beings' if one accepts that there are non-human sentient beings and those beings have some kind of 'ethics'), there would probably be no ethical truths.

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

Ok!

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. — noAxioms

Yes, I agree that everybody on this Earth measure 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?

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: — noAxioms

LOL. You are right! :smile:

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. — noAxioms

Ok, I see. Good point!

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 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!

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. — noAxioms

Right, I agree.

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

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

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 — noAxioms

:up:

Yeah, I actually agree with you that a (partial) understanding of other sentient beings. I was a bit joking. Wittgenstein was far too extreme. Language, culture etc are not such an insurmountable barrier.

Appreciate the good discussion. Been a while for me. — noAxioms

Ditto :wink:

,

Just a curiosity...

Interestingly, it seems that RQM agrees with the Consistent Histories interpretation about the lack of a 'single history'. From the SEP article on Consistent Histories, section 11.4:

In a probabilistic theory the limiting cases of a probability equal to 1 or 0 are equivalent to asserting that the corresponding proposition (e.g., “the system has property P”) is, respectively, true or false. In the histories approach probabilities are linked to frameworks, and for this reason the notions of “true” and “false” are also framework dependent. 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. In Sec. 27.3 of Griffiths (2002a) this belief is referred to as unicity, and it is what must be abandoned if the histories interpretation of quantum theory is on the right track.

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?

Yes, it seems she is simply talking about a logical consequence there. If the cafe closes then no-one can eat there. But her characterizing a logical consequence as an instantaneous and acausal effect, or a form of global non-locality, is a category mistake. — Andrew M

Mmm, I see!

Also, both she and Shimony are in fact clear that 'potentialities' for them are in some sense non-local.

Fair enough! As it happens, I also find Aristotle's form/matter distinction useful for considering information. A physical system, being substantial, can be abstracted in terms of its matter and form (or state). Which provides a natural isomorphism between a physical system and a point in state space. — Andrew M

Agreed!

Let me, however, ask you the same question that I asked to noAxioms.

Consider an electron. According to RQM, its state is 'perspective'-dependent. But all of them are in agreement that it is an 'electron'. So, is 'out there' something identifiable as an 'electron' even when is not measured? What I am saying is that it seems we need after all something invariant, equal to all perspectives. Is this compatible with RQM? :smile:

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. — noAxioms

Yes, you are right. That's one difference between Wigner's original proposal and Bitbol's take (other than a less 'ontological' take on what the 'collapse' is) and possibly also QBism (but I admit that I do not understand it very well). Here an article by Bitbol about the Wigner's friend scenario: https://core.ac.uk/download/pdf/148348264.pdf. An excerpt:

Now, what happens if WE open the door of the lab. to see what happened in it? As previously, we see a well-defined chain of characters on the screen of the computer, we see that the cat is either dead or alive, and we hear a colleague saying that he had already seen (well before our arrival) what we are now discovering. And as previously, we must modify suddenly our tool of probabilistic prediction in order to account for our certainty of finding everything in the same configuration, if we come back in the lab. some time later. 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. 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. And this manifests itself through the persistent presence of a term W (for “wavering” or “wavelike effects”), in the probabilities calculated from the wave function.
...
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! 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.

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. In other words, just like in the case of Rovelli, in Bitbol's take of CI, there is no single 'history', so to speak. As Bitbol points out in the other article I linked before (i.e. this: http://www.bourbaphy.fr/bitbol.pdf):

This is not to deny that quantum mechanics, as one of the most accomplished realizations of the ideal of universal description pursued by the natural sciences, could indeed describe any phenomena.
Yet, in doing so, it could not avoid leaving the preconditions for description outside its scope. As a well-known article about the measurement problem of quantum mechanics puts it: the quantum theory can describe anything, but not everything [Peres1982][Fuchs2000].

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.

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. — noAxioms

Yes, that's my concern too. Regarding consciousness, I believe that science can give us some knowledge of it. But it cannot give us the full story.

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. — noAxioms

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

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'). — boundless

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

Me too. And unfortunately, I do not know if it is really self-consistent (honestly, I do have a similar concern for RQM).

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. — noAxioms

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.
Of course, it becomes more problematic if a minimalist take on the wave-function is untenable.

Regarding your question how is possible that matter can host a 'non-physical mind', I just do not know how to answer. And that's one of the reason why giving a special role to consciousness in a scientific theory is for me problematic.

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. [But I am a bit unwilling to explain why I think so here... :smile: Anyway, in brief, for example, I believe that this is a consequence to both the so-called 'hard problem of consciousness' and my acceptance of free-will (because I do not believe that free-will, if is real as I think it is, can be explained in purely physical terms...).]

At the same time even if I have this belief, I am not so sure that giving consciousness a so special role in a scientific theory is a very good idea. But, oddly, the fact that this has been done in various ways fascinates me.

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. 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. Also, it has absurd consequences: 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. (I referred to a discussion on ethics when I referred to 'off-topic')

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?

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:

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. — noAxioms

Ok!

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.

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'. 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.

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:

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

Thanks!

Actually I read your 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

I agree that 'observer' is a poor choice of words in Rovelli's interpretation. That's why in some of my former posts I preferred the more general word 'perspective'.

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!)

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. As Bitbol says, the inability to recognise the ubiquitous presence of the subject is precisely the blind spot of modern science. Philosophers (including Kant and Schopenhauer) understand this but many scientists do not, because it’s not, by definition, a scientific issue. But the fact that it becomes a subject of discussion here is because scientists are perhaps operating at ‘the limit of objectivity’. — Wayfarer

Well, I think that I agree in part with this. But as you say, it's not by definition a scientific issue :smile:
So, I do not think that your reasoning here is really affected by what interpretation of QM is 'right'. (After all, Kant and Schopenhauer did not knew QM)
At the same time, what I find fascinating is that with the advent of QM and with the introduction of interpretations like the Von Neumann-Wheeler a.k.a 'consciousness causes collapse', Bitbol's (and d'Espagnat) etc and also Many Minds, this issue has become of interest of scientists themselves.

I agree that knowledge is perspectival and since we are conscious, it is at least in some but very deep sense, 'subjective'. But at the same time, my fear is that, if we embrace this kind of reasoning completely, we end up to deny 'reality' completely.

In terms of the noted parable of the blind men and an elephant (here the Wikipedia article), the risk is to deny the 'elephant' instead of denying the 'perspectival' (in this case 'subjective') character of knowledge. In other words, my fear is that we fall into the other extreme, so to speak :smile:

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. — noAxioms

Yeah! Bohr's IMO was not an ontological position. And yet, 'Copenaghen Interpretation' is often used to denote one.

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. — boundless

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.

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. — boundless

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. — noAxioms

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.

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).

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. — boundless

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?

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? — noAxioms

I agree on the first part. Regarding the question, instead, I would say that MWI is CI plus universal wave-function.

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 — boundless

The Wigner interpretation I referenced. — noAxioms

Yep!

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'). — boundless

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? — noAxioms

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

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

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]

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. — 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 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.

? 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. — noAxioms

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:

Thank you :blush:

You're welcome.

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

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.

In other words, RQM threatens the view that physical objects have some intrinsic properties. If true, the very concept of 'object' becomes problematic for it seems difficult to speak of objects that do not have any intrinsic properties. But if S does not have any intrinsic properties, what does S, S?

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. After all, perspectives here are defined in relation to these objects. But it certainly comes close to this idea: ultimately there are no 'objects' (which should not be taken as saying that 'there is no reality'). Rather objects are useful abstractions or approximations :smile:

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). — boundless

I am definitely very ignorant about Information Theory, but I believe that the status of 'Shannon information' is somewhat controversial. The point is that there is a disagreement e.g. whether it is objective or subjective etc.

On this issue, maybe some might find interesting this paper by Basil Hiley (who collaborated with Bohm for many time): https://pdfs.semanticscholar.org/bb05/2740074f3e6b19f685315c9ddb994b563e29.pdf.

Thanks for the links! I had a quick skim. I find Rovelli's approach more natural than either of those. Bitbol's approach seems overly metaphysical and Kastner's approach is non-local. — Andrew M

You're definitely welcome.

I can understand the unease with Bitbol's approach. But note that consciousness in his interpretation does not 'do' anything, in fact. It does not affect physical reality. It simply define the 'perspective' of the 'observer'. In a way analogous to Kant, Bitbol in fact IMO says that the 'quantum world' is indeterminate. But it is not a denial of it and neither he claims that it is 'modified' by consciousness. But as I said, I can hear your unease because I share it (even if I do like Kantian-like philosophies).

Regarding Kaster's approach I am not sure to call it 'non-local'. In the paper, Kastner et al explain (page 5):

As one of us (SK) has observed (Kauffman 2016, Chapter 7), we might plan to meet tomorrow for coffee at the Downtown Coffee Shop. But suppose that, unbeknownst to us, while we are making these plans, the coffee shop (actually) closes. Instantaneously and acausally, it is no longer possible for us (or for anyone no matter where they happen to live) to have coffee at the Downtown Coffee Shop tomorrow. What is possible has been globally and acausally altered by a new actual (token of res extensa).6 In order for this to occur, no relativity-violating signal had to be sent; no physical law had to be violated. We simply allow that actual events can instantaneously and acausally affect what is next possible (given certain logical presuppositions, to be discussed presently) which, in turn, influences what can next become actual, and so on. In this way, there is an acausal ‘gap’ between res extensa and res potentia in their mutual interplay, that corresponds to a form of global nonlocality.

[Footnote 6]: While ‘acausal’ in the classical sense of efficient causality (wherein one actual state causally influences another actual state), in the quantum mechanical sense of causality wherein potentia are treated as ontologically significant, the actualized state is understood to ‘causally’ alter the probability distribution by which the next ‘possible’ state is defined. For further discussion of this distinction between classical efficient causality and quantum mechanical causality, see Epperson (2004, 92-93; 2013, 105-6). On the other hand, under certain circumstances and at the relativistic level, where decay probabilities are taken into account, the relation between an actualized state and the next QP state may itself be indeterministic (see, e.g. Kastner 2012, Section 3.4 and Chapter6).

Shimony's take, instead, seems definitely 'non-local'. But maybe he meant something like Kastner et all above (as did maybe other proponents of an 'Aristotelian-like' reading of CI).

First, as with Rovelli, I think that quantum mechanics is local. Second, as with Aristotle, potentialities don't "do" anything, only actual systems do. — Andrew M

I feel I am in agreement!

Instead, the term "potential" provides a natural way for Wigner and his friend to describe the scenario from their own perspective and also to describe the scenario from the other's perspective.

So when the friend (Alice) measures spin up, that actualizes (i.e., realizes) the particle's spin potential for her. But she also knows that both the spin and her subsequent measurement of the spin are only potentials for Wigner until Wigner measures the friend's system in that basis.

The actual/potential terminology combined with RQM's relationalism provides an ordinary language abstraction over the underlying mechanics. That abstraction preserves locality, factual definiteness, freedom of choice and, crucially, a referent within the universe that provides a view from somewhere (i.e., the system's reference frame). — Andrew M

:up:

To be more complete, in fact I lean towards RQM and CI. The problem I have with RQM is that 'information' maybe is not something well-defined in relation to all physical systems. But as I said in my previous post, this is a quite controversial point. If 'information' is something that can be defined in relation to all physical system, then RQM is IMO the best choice.

If not, maybe something like Bitbol's interpretation (with maybe some elements of 'actuality/potentiality' dualism) would be best.

I am simply undecided.

There is an ambiguity in 'Copenaghen Interpretation' (CI) that creates endless debates like this one :smile:

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.

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.

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.

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. This is because, according to Rovelli, there is nothing special about computers, etc:

In order to prevent the reader from channeling his/her thoughts in the wrong direction, let me anticipate a few terminological remarks. 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”. The observer can be any physical object having a definite state of motion. For instance, I say that my hand moves at a velocity v with respect to the lamp on my table. Velocity is a relational notion (in Galilean as well as in special relativistic physics), and thus it is al- ways (explicitly or implicitly) referred to something; it is traditional to denote this something as the observer, but it is important in the following discussion to keep in mind that the observer can be a table lamp. Also, I use information theory in its information-theory mean- ing (Shannon): information is a measure of the number of states in which a system can be –or in which several systems whose states are physically constrained (corre- lated) can be. Thus, a pen on my table has information because it points in this or that direction. We do not need a human being, a cat, or a computer, to make use of this notion of information.

(Source: https://arxiv.org/pdf/quant-ph/9609002.pdf ; emphasis mine)
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.

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

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').

Of course, if one is really pragmatic one can simply choose to ignore the problem (but maybe this is not satisfactory for someone philosophically inclined). Finally, one can choose other interpretations of QM.

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

Same impression!

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. — noAxioms

Ok, thanks! I think I'll revise GR and cosmology. I admittedly do not know very much about both.

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. — noAxioms

:up: I agree! I would add the 'no signaling' theorem.

Don't get your Dutch names wrong... I've got one myself. — noAxioms

Ops! Sorry!

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. — noAxioms

Right :smile:

Presentism demands an objective ordering of events (although no particular one), but a preferred folation does not demand a preferred moment in time. — noAxioms

Maybe dBB requires a preferred moment in time and not just a preferred foliation, then! (alternatively, a preferred foliation without a preferred moment in time and retro-causality.)

Way to kill an afternoon, eh? — noAxioms

LOL, yeah!

Thank you for the link. — noAxioms

You're welcome :wink:

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. — noAxioms

I see! That's perfectly reasonable :smile:

Actually, I admit that dBB (in some forms*) is my third favorite interpretation. After all, it is still somewhat 'odd' but at the same time counterfactual definiteness is very intuitive. 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...Hence, I lean towards RQM and Copenaghen.

*(I do not like the purely 'nomological' view where there is no physical explanation of the movements of the particles - I prefer the 'dispositionalist' version. Also, I think that I am in the minority but I find the 'quantum potential' formulation interesting - after all, the 'classical limit' becomes quite intuitive if the contribution of the wave-function is seen as additional force.)

Thank you very much for the transcription!

So we can see Rovelli's reasoning in the above exchange. For Alice on Andromeda, Carlo on Earth only potentially exists until a local interaction (say, a telescopic observation at light speed) brings him into her present (and then past). Similarly, for Bob the superintelligent being in the future, Carlo is only potentially at the conference until a local interaction decoheres the superposition (say, Bob talks to Carlo). — Andrew M

:up:

A further thought here is that I think this allows a representational interpretation of the wave function for RQM in terms of what is actual and potential for any given observer. What is locally entangled with an observer is actual (the past and present, measurements and interactions), what is not is potential (the future, spacelike separated regions, superpositions). — Andrew M

Yeah, I'd agree. Both the representionalist and the non-representionalist views are possible. Rovelli himself wrote against a 'realistic' interpretation of the wave-function: https://arxiv.org/abs/1508.05533.

Actually, this interpretation of the wave-function is also held by some Copenaghists. For instance, Abner Shminoy wrote in the older version of the SEP on Bell's Theorem:

There may indeed be “peaceful coexistence” between Quantum nonlocality and Relativistic locality, but it may have less to do with signaling than with the ontology of the quantum state. Heisenberg's view of the mode of reality of the quantum state was briefly mentioned in Section 2 — that it is potentiality as contrasted with actuality. This distinction is successful in making a number of features of quantum mechanics intuitively plausible — indefiniteness of properties, complementarity, indeterminacy of measurement outcomes, and objective probability. But now something can be added, at least as a conjecture: that the domain governed by Relativistic locality is the domain of actuality, while potentialities have careers in space-time (if that word is appropriate) which modify and even violate the restrictions that space-time structure imposes upon actual events. The peculiar kind of causality exhibited when measurements at stations with space-like separation are correlated is a symptom of the slipperiness of the space-time behavior of potentialities. This is the point of view tentatively espoused by the present writer, but admittedly without full understanding. What is crucially missing is a rational account of the relation between potentialities and actualities — just how the wave function probabilistically controls the occurrence of outcomes. In other words, a real understanding of the position tentatively espoused depends upon a solution to another great problem in the foundations of quantum mechanics − the problem of reduction of the wave packet.

The link is to the section 'Philosophical Comments' of the article - Shimony lists other possible positions.

There are different takes. For IMHO a very interesting Neo-Kantian non-representionalist reading (among the 'Copenaghists'), check this article of Michel Bitbol (I already quoted it in this thread - I quote it again here for convenience): http://www.bourbaphy.fr/bitbol.pdf (according to him, Bohr's epistemology was close to Kant's views...). Or, if one prefers the video of the talk: https://www.youtube.com/watch?v=pYRLapWBqJY.

Another instance of interpretation of the wave-function in terms of potentiality-actuality can be found in this paper by Kastner et al: https://arxiv.org/abs/1709.03595.

I actually thought about it but, unfortunately, I did not arrive at a satisfying conclusion.

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. — noAxioms

Ok, that's right. Maybe the point is that the foliation is not directly observable. 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.

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. — boundless

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. — noAxioms

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?

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. — noAxioms

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

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'...). — boundless

No, there would still be an absolute simultaneity... — noAxioms

OK, I also agree with you on this.

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.

I suggest these videos also to @Andrew M.

According to the 'no signaling' theorem, if predictions of QM are satisfied there cannot be any direct evidence of violation of locality. Hence, it just seems that there is a sort of 'conspiracy' if there are non-local influence. This is actually one of the reasons I do not think dBB is true. But, interestingly, according to Valentini, this might be a clue that dBB is, instead, right: the fact that the world seems 'local' is due to the fact that the 'quantum equilibrium hypotesis' is true. IMO, his proposal however makes perfect sense in the light of dBB.

[Another reason for which I do not accept dBB is its 'strange' ontology. dBB is, in fact, characterized by both particles and the wave-function. In my understanding, particles have no role except moving in a way 'dictated' by the wave-function*. The wave-function seems to do all the job. Furthermore, the wave-function is not a field in real space but lives in the 3N-dimensional 'configuration space' where N is the number of particles. This prompted some proponents to adopt a 'nomological' view of the wave-function. But IMO this is still odd: laws are generally understood to not be dynamical objects and if the wave-function is merely a law, it just seems that there is no reason why particles move in that way. In fact, some think that the wave-function is better understood as representing dispositions: https://arxiv.org/abs/1406.1371.

*This is not true if one introduces the 'quantum potential' AFAIK. But even in this case, you still have a weird ontology where you have to make sense of particles living in our usual 3-dimensional world and an equally real field that lives in a 3N-dimensional world. And also some dBB supporters are critical of the 'quantum potential' formulation, see: https://plato.stanford.edu/entries/qm-bohm/#QuanPote

Also, I am a bit puzzled by the asymmetry between position and other observables. QM is perfectly 'symmetrical' with them, i.e. it treats them equally. For dBB, instead, position is somewhat 'special'. The counter-argument that is found in the SEP article on 'Bohmian mechanics' does not convince me. ]



Good find! I'll read it :smile:

Thank you very much!

Actually in the article I linked, there is in it a link to a paper by Wigner himself: https://jawarchive.files.wordpress.com/2012/03/informationquantumphysics.pdf.

I did some further research on Wheeler. I found this article: https://plus.maths.org/content/it-bit - it quotes a paper by Wheeler himself.

He certainly IMHO thought that 'information' (which he believes to be 'immaterial') plays a central role and is more 'fundamental' than matter.
But he did not seem to give a 'special role' to human observers (he seemed to have a more general idea of observer).

I am still not sure about the 'participatory anthropic principle' thing.

You might be interested in the following article that addresses this issue:

But it didn't take physicists long to realise that while the Wheeler-DeWitt equation solved one significant problem, it introduced another. The new problem was that time played no role in this equation. In effect, it says that nothing ever happens in the universe, a prediction that is clearly at odds with the observational evidence.
— Quantum Experiment Shows How Time ‘Emerges’ from Entanglement — Andrew M

I forgot to say that I actually read the article and I found it very intriguing :smile:

Thanks for the very informative answer! I need some time to think about all of this :smile:

I do not understand your point, actually.

On one hand, I agree that pop-science is not always reliable. On the other hand, in this case, it seems that you are saying that John Wheeler never proposed the idea of the 'participatory anthropic principle' (or 'participatory universe') even if there are a lot of sources that claim otherwise. Am I right?
Or you're suggesting that is their interpretation of the 'participatory anthropic principle' problematic? If so, what did he really mean?

Thanks in advance!

Nice find with the PF article and I fully agree with it. I was going to mention the Andromeda paradox and the idea of potentiality in relation to it in my previous post. So we seem to thinking along similar lines here. — Andrew M

Great. Interestingly, I discovered that the same point is made by Carlo Rovelli to defend his 'relational' view, see: https://www.youtube.com/watch?v=vbYeAaCloiM. At 4:55, Valentini makes the same question that he made in the other video (namely that different observers might disagree about what happens) and at 53:52 Rovelli answers by citing the Andromeda Paradox - so we are in good company :wink: . It is a very good discussion, BTW (other than Rovelli and Valentini, also Saunders and Wallace (and others) participate in the discussion). This might also be of interest to @noAxioms.

It also reminds me of Aristotle's future sea battle example where he contrasts potential and actual: — Andrew M

Very interesting, thanks!
In fact, some time ago I read a philosophy of science paper that tried to use the potential-actual distinction in SR in order to avoid situations like Andromeda Paradox. Unfortunately, I do not remember neither the title nor the author :sad:

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

Unfortunately, I know very little about objective collapse theories. Anyway, I agree with you. It would be interesting to see how these theories deal with this experiment.

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. — noAxioms

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

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.

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? 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.

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'...).

The one from GR is not enough? — noAxioms

I don't know!

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. — noAxioms

:up:

BTW, I think that the idea of a 'special role' of consciousness in QM is not really so rare among physicists themselves.

See the article on Wikipedia about 'Von_Neumann-Wigner interpretation' (there is however a nice discussion about Von Neumann's ideas on physics forums: https://www.physicsforums.com/threads/where-does-von-neumann-say-that-consciousness-causes-collapse.884128/.)

As I already mentioned, a separate (but somewhat analogous) strand is 'Many-minds interpretation' (a form of MWI where the 'splitting' occurs in the minds of the observers). See also the 'Everett plus minds' section in the article 'Everettian Interpretations of Quantum Mechanics' in the Internet Encyclopedia of Philosophy.
[Edit: after some reflection, I am now not so sure that 'Many minds' can be said to be a form of 'idealism'. Still, mind does have a 'special' role. Sorry for the late edit!]

Note that I do have some reservations about these ideas myself (well, maybe I fall in the 'transcendental realist' camp according to Kant...). But I find very interesting that these idealistic or quasi-idealistic ideas now are taken seriously among physicists (and philosophers of physics) themselves. I believe that is something that is worth of serious attention.

This pre-print by d'Espagnat might also be of interest: https://arxiv.org/pdf/1101.4545.pdf.

P.S.

A 'strong correlationist' might say that it appears that ionic bonding would exist before the advent of sentient beings. I believe that Schopenhauer is a 'stronger correlationist' than Kant, because he says explicitly that you cannot even think about the universe where no sentient being exist and the previous story of the universe is actually related to the opening of the 'first eye' (i.e. the appearance of the first conscious being) and he also believed that the 'thing in itself' was singular. — boundless

For those interested in Schopenhauer's views, the passage I referred to is quoted here: https://partiallyexaminedlife.com/2015/05/01/schopenhauers-idealism-how-time-began-with-the-first-eye-opening/

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? — noAxioms

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'.

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

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. — noAxioms

Well, AFAIK in dBB you need to somehow define a way to define an 'absolute simultaneity'. Lorentz invariance is not the real problem. In fact, I read that you can define some form of 'absolute simultaneity' and at the same time not violate Lorentz invariance. But IMO that's a bit agains the 'spirit' of Relativity, so to speak (I am not saying that this is necessarily bad, of course...).

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.

Thanks - that would be my reading as well. — Andrew M

:up:

As I see it, the decompositions that are of interest are those that are robust to interactions with the environment. So the ordinary objects of our experience, by virtue of being persistent and observable, are robust. That physical structure has emerged through an evolutionary process (as underpinned by decoherence), it's not a priori.

In other words, as you say above and Rovelli mentions in his talk, we start from the structure that we observe in our experience and work from there. It's not a Platonic endeavor. Now RQM is not solipsistic. It generalizes from individuals, to humans, to things, and ultimately to all systems and composites of systems that can interact. I think that MWI just takes that one step further and sees the universe itself as a system with a reference frame and a quantum state that can be described. So you don't need an excursion through arbitrary decompositions to take that final step. — Andrew M

I see! Yeah, you are right. Schwindt's paper only refutes the idea behind something like 'pure MWI', that is Hilbert space without any structure is the only reality [edit: I meant a version of MWI where you start from the 'universal wavefunction' alone].
If one introduces a substructure (as dictated by experience) and does not have any problem with that, then you are right it seems that those problems do not apply.

But on the idea that nothing happens in the Everettian universe, I think that is true in one sense. If one person is pulling on a rope from one end and someone else is pulling with equal force from the other end then there is a high-level abstract sense in which nothing is happening. But there's obviously a lot going on at lower levels. If the universe is itself in superposition then, similarly, in that frame of reference, nothing is happening - there's no time, no dynamics, etc. But it doesn't follow that under the hood, in the reference frames of subsystems, that nothing is happening. — Andrew M

Interesting analogy, thanks! I'll read the paper.

Maybe a related idea here is to regard values in non-interacting systems in terms of potential. So, for Wigner, interference indicates that the friend has made an actual measurement in their reference frame but the measurement only has a potential value for Wigner until an interaction actualizes it for him (in accordance with the principle of locality).

That is distinct from a hidden variable theory that supposes that the friend has made an actual measurement that is merely unknown to Wigner, with the Bell inequality issues that that would entail. — Andrew M

Yeah, that's a nice way IMO to avoid issues with relativity.

And BTW, as I said to NoAxioms a similar problem arises in Relativity, if one wants to avoid the 'block universe idea' as suggested by Rietdijk-Putnam argument(here's the link to the Wikipedia article). There is a very nice 'insight article' on Physics Forums that gives a counter-argument (which is reminiscent of the reasoning on which, for instance, RQM is based): https://www.physicsforums.com/insights/block-universe-refuting-common-argument/.