Regarding the 'parallel' idealism-realism debate, I think that physics - and science in general - is, well, silent on that issue as well.

For instance, the concept of 'reference frames' is central to the theory. In fact, many physical quantities have undefined values before a reference frame is specified, i.e. this means that they are not intrinsic properties of physical objects but only relational ones. Reference frames are associated to physical objects but are actually abstract concepts of the theory - they more or less correspond to 'perspectives' where a given object is at rest. And all measurements happen in a given frame of references.


Anyway, even in classical, newtonian, mechanics the 'physical world as it is, independent from any perspective' is quite different from 'the observed reality', which is always associated to a given frame of reference.

Related to the above, I would ask: what constitues a 'perspective'/'reference frames' in physics?
And how is the physical world independent of all perspectives? Can we describe it?

Nope. Enzymes are large mechanical structures. Decohered and classical for all intents and purposes. But they can dip their toe into the quantum realm, exploiting tunneling to jump chemical thresholds. — apokrisis

Hi, I read your linked post and I enjoyed it. But still I don't understand how 'classicality' 'comes to be' in your view.

Let's consider a less crude version of the Schrodinger's cat experiment, where the cat is either awake or asleep. Decoherence IMO can only remove interference, not superposition, hence the cat is still, if we take the quantum formalism literally, awake and asleep at the same time. However, decoherence gives the appearance of classicality because it says that it is observed in a definite state.

Let's say that I observe the 'asleep' cat. In your view, what happened to the 'awake' cat?

In MWI, the superposition is always preserverd and each definite state is 'actualized' in a branch of the wavefunction (but observers have access only to one). The 'branching' can be seen as due to decoherence.
In epistemic Copenaghen-ish views, QM is not to be interpreted literally and the collapse is just an update of knowledge (these views do not make any ontological commitments to what 'happens' before the observation: the state of the 'unobserved' cat is beyond the range of descriptions).
In (non-local) hidden variables interpretations, the cat is always in an unique state and it is determined by these hidden variables
In spontanueos collapse theories (these are actually not interpretations as they make different predictions from QM), at a certain scale wave-functions collapse and give an unique definite outcome
In RQM, anything can be an observer and any interaction is a measurement. But, again, like in epistemic views the 'unobserved' cat is in an indefinite state.
And so on.

Spontaneous collapse theories - (Edit: or maybe some version of MWI) - IMO seem to me the most compatible to your views.

I make this same point all the time. :up: — apokrisis

:up: Glad to hear that! I think that, what is common to all physical theories is that they are predictive tools with an extraordinary range of practical applications. Physics per se does not give us any ontology IMO. I also think that many debates about topics like free will, reductionism* etc are due to an unneeded ontological interpretation of physical theories. In fact, I think that the most consistent form of 'realism' with all physical theories is actually what Bernard d'Espagnat called 'open realism' in his "On Physics and Philosophy" (p.28, emphasis in the original):

Realism (Open). This is - indeed quite 'open' - view that there is something the existence of which does not hinge on thought
Is this 'something' the set of all objects, of all the atoms, of events, God, the Platonic Ideas, still something else? Open realism is mute on this.
...
It just says 'something,' in the widest possible sense of the world.

Of course this does not mean that is the only viable form of realism.

*I do believe, in fact, that even newtonian mechanics is not really 'reductionistic'. Conservation laws seems to me properties of a whole isolated system, not reducible to the properties of its parts. Of course, if one does not make any ontological commitments, newtonian mechanics is neither reductionistic nor holistic.

I posted this general story here - https://thephilosophyforum.com/discussion/comment/679203 — apokrisis

Thanks for the link. I'll ask more questions after reading it :)

What you often hear from idealists (Kastrup and Hoffman are good examples) is that materialism and a physical world is debunked and quantum physics tells us reality comes into being by the act of observation. Therefore idealism is a more reasonable and parsimonious explanation for our experience. I've often thought that the arguments in favour for idealism are actually more arguments against old school materialism than any great championing of an 'it's all consciousness' style metaphysics. — Tom Storm

Note, however that Hoffman does not really say that. He says, more or less that QM suggests that 'physical reality as it appears to us' comes into being by the act of observation. And he says that this is consistent with his 'interface theory of perception', i.e. the view that perception gives us a simplified interface of the 'external reality' which is useful for our survival.

Unfortunately, this is then mixed up with his 'conscious realism', i.e. his Berkeleyan-like view that conscious agents are the only reality. To his credit, he never say that the 'interface theory of perception'necessarily implies 'conscious realism'.

Note that his 'interface theory of perception' is a kind of epistemological idealism (which is actually compatible with the existence of a mind-independent reality). His 'conscious realism' is of course a form of ontological idealism. Unfortunately, sometimes his two theses are mixed up.

Hi apokrisis, all,

I have outlined many times how biosemiosis now adds the epistemic cut to the business of quantum interpretation. As a mechanism, a modelling relation, even our enzymes and respiratory chains are actually doing that - preparing states of coherence with the intention of collapsing them and so ratcheting the entropy flows of a Cosmos guided by the telos of the newly NASA-rediscovered concept of dissipative structure. — apokrisis

I think that the problem of this view is that it does not explain how those 'complex' objects/processes like 'enzymes' or 'respiratory chains' arose in the first place. Was everything in superposition back then? What happens when these kind of object 'cause' a collapse?

I think that all positions of what consitutes an 'observer' are susceptible to this kind of objection and it is a serious objection if QM is interpreted ontologically. I think that it is best to interpret it epistemically: wave-function, Born Rule etc are all useful concepts, computing techniques etc that enable us to make correct predictions. In this view the 'collapse' is just an update of knowledge and QM is silent about what 'happened' before the appearances of 'observers' (and it is also silent on what constitutes an 'observer'). QM is IMO best seen as a theory that does not make any ontological commitments: it does not give a 'picture' of 'how reality is'* and it does not say to us what an 'observer' is but is still very useful to make predictions, applications and so on. (My favorite interpretation is QBism, althought I think that, unfortunately sometimes its proponents seem to present it as an ontological interpretation of QM, perhaps unwittingly... especially when they talk about 'participatory realism')

As an aside, I do not think that 'decoherence' alone is enough to solve the measurement problem. It does explain (at least 'for all practical purposes') an appearance of a classical world, but it does not IMO explain why we observe a single outcome in quantum experiments (MWI supporters like decoherence because it explains why 'branches' of the wave-function separate. But MWI makes the additional assumption that all outcomes are actually observed even if they are inaccessible to us)

*I actually think that we tend to do that also in classical physics. For instance, an ontological interpretation of the concept of 'force' in newtonian mechanics is clearly inappropriate but it is an useful conceptual fiction that helps us to make predictions, build things etc (like, say, the concept of 'sunrise' or 'sunset'). The same IMO holds for QM.

https://erlichdallc.wordpress.com/2019/04/11/my-current-understanding-in-a-nutshell-of-everetts-relative-state-formulation-of-qm/ — i aM

Nice!

Isn't a core idea of SR "relativity of simultaneity", i.e. simultaneity of events is entirely dependent on the reference frame of the observer? — i aM

I believe that it depends on how you define SR. In the usual definition, both preferred foliation of spacetime and retro-causality are incompatible with SR.

Yet, I think that they are both compatible with Lorentz symmetry.

By my understanding, in PWT the pilot wave controls the velocity of the particle. And that velocity depends not only on the position of the particle, but also the positions of all the particles it is entangled with; and that information is all available instantaneously to the pilot wave which controls the velocity of the particle. I don't see how that can be reconciled with SR. — i aM

Well, there have been attempts to reconcile SR and PWT. To my knowledge, they involve the use of preferred foliations of space-time or retro-causality. Of course, this is different from saying that PWT can be reconciled with SR in its standard formulation, but the point is that apparently it can be reconciled with Lorentz symmetry.

I was however referring to this way of reconciling SR and PWT via this defense of presentism proposed by NoAxioms:

I have done an advocatus diaboli thread defending the compatibility of relativity and presentism, so I maintain that they're not incompatible. SR says that the preferred frame cannot be determined given the special case after which it is named. But inability to detect such a frame does not mean that there isn't a special one. Presentism doesn't even require it to be a inertial frame, and no presentist that knows their physics seems to assert that it corresponds to such a frame. The foliation is always bent, which has the interesting paradoxical implication that no two stationary observers are simultaneous in each other's inertial frames. I find that hilarious, but not paradoxical. — noAxioms

Yes. With locality, which is essentially saying no FTL. — noAxioms

Ok! Fine, then we agree :smile:

Pilot wave is a form of Bohmian mechanics: Pro counterfactual definiteness (objective state) and denial of locality. So I wonder how they interpret spooky action at a distance using pilot waves. I don't know the official line on that. They certainly cannot reproduce spooky action using a classic pilot wave setup like they use for double slit. — noAxioms

Considering that the 'wave-function' is a 3N-dimensional (N being the number of particles) object and the theory predicts that the 'influence' is instantaneous, I do not think that there can be an explanatory dynamical model like - say - the one that uses mediators.

(As an aside, I prefer using the term 'de Broglie-Bohm theory' or 'pilot-wave theory' for various reasons. Among these, 'Bohmian mechanics' is actually the preferred term of a specific sub-school of the dBB-supporters and also downplays the role of de Broglie. It gained success and maybe it is the most used term in literature... :smile: YMMV )

I simply meant that without the selection postulate, it seems that RQM implies the splitting.

Anyway, I agree with you. RQM seems simply silent on this point. — boundless

Maybe it's embarrassed. :yikes: — Wayfarer

Well, possibly! :razz: — boundless

This is why I resist describing RQM under presentist terms. If time is external to the structure that is the universe, then such selection is an objective act relative to this realm under which time exists, and it isn't really RQM anymore if such an objective action takes place.

With time being part of the structure, no event/state (something to which a relation can be made) 'flows' to a different event, necessitating such a selection. Thus there is no selection postulate.
This isn't an embarrassment, just an implication of a relative interpretation. — noAxioms

Ok, I can see the problem! Even the presentism you referred before as compatible with SR seems to imply a unique, well-defined state of the universe (the 'unicity' referred in the SEP article on Consistent Histories) :smile:

(as I said it was not exactly what I had in mind. But I think I'll leave it at least for now...)

The Andromeda Paradox is about the ambiguity of what time it is elsewhere, not about the state being definite. The former is a frame dependent thing and the latter is a statement of superposition of something unmeasured. I think you meant the former but your wording suggested the latter. — noAxioms

Yeah, sorry!

Totally agree. Two observers at the same place but different frames might disagree about what is going on at Andromeda, but they'll agree entirely about what has been measured. The light cone from that location is a frame independent thing. — NoAxioms

Yes! In Relativity the ordering of events in every light cone is an invariant (unless one accepts tachyons or any FTL influence). — boundless

I didn't say that. I said the set of events in a given light cone is frame independent. The ordering of those events is still quite frame dependent. — noAxioms

I am not sure I am following you. In fact, I just am saying that the cause precedes the effect in all reference frames without FTL. Isn't it right? :smile:

Interesting corollary for a presentist, who by definition cannot observe any existing thing. In 8 minutes, the thing I observe will not be the present state of the sun. It will be an observation of something nonexistent.
— noAxioms

Yep! Presentism is somewhat problematic in Relativity. I would say that 'global presentism' is simply incompatible with relativity of simultaneity. Maybe a form of 'local presentism' can be saved but it is surely counter-intuitive (I personally lean towards some form of presentism and I admit that I am troubled by this). — boundless

I have done an advocatus diaboli thread defending the compatibility of relativity and presentism, so I maintain that they're not incompatible. SR says that the preferred frame cannot be determined given the special case after which it is named. But inability to detect such a frame does not mean that there isn't a special one. Presentism doesn't even require it to be a inertial frame, and no presentist that knows their physics seems to assert that it corresponds to such a frame. The foliation is always bent, which has the interesting paradoxical implication that no two stationary observers are simultaneous in each other's inertial frames. I find that hilarious, but not paradoxical. — noAxioms

Ok, I see. Interesting, thanks! I wonder if this can be used to reconcile SR with pilot-wave theory... :smile:

Note, however, that the 'presentism' that I had in mind was somehow different. I am not sure of how to explain it - so, I'll leave it for now.

As he puts it, decoherence gives us quasi-classical worlds (branches) but not actual classical worlds. Which means that decoherence can be treated as irreversible and the worlds as classical for all practical purposes. Nonetheless interference between branches continues to happen in accordance with quantum mechanics. — Andrew M

Ok, I see!

I think splitting might be implied only because Copenhagen and Consistent Histories don't specify any physical mechanism, whereas MWI does. But since some other unknown physical mechanism can't be ruled out at this point, then being silent seems a reasonable option (and treating interference as unactualized potential). — Andrew M

Well, yeah, this would explain the silence :smile:

Another curiosity: what do you think about the problem of interfering branches in MWI (and maybe in RQM if no selection mechanism is accepted)? As 'I aM' (see here) noted it is true that due to the decoherence the interference term becomes very small. Yet, rigorously, it is not exactly 'zero'. Given the fact that decoherence occurs a lot of times, it seems possible that - sooner or later - interference will be observed. In other words, it seems that decoherence gives (multiple but) definite outcomes only 'for all practical purposes' (I remember to have read that decoherence is said to solve the measurement problem 'only for all practical purposes' but I am not sure that this the reason why it is said so...).

Nit-pick: you think that maths, a human invention, is more fundamental than the stuff of which the universe is built?
— Pattern-chaser
I consider it something discovered, not invented. If invented, pi would not be the same value in another world.
— noAxioms

:up: — Wayfarer

+ 1 :wink:

(More precisely, I believe that there is something in maths that is discovered. It cannot be totally invented)

I simply meant that without the selection postulate, it seems that RQM implies the splitting.

Anyway, I agree with you. RQM seems simply silent on this point.
— boundless

Maybe it's embarrassed. :yikes: — Wayfarer

Well, possibly! :razz:

I see what you mean, but 'pre-measurement Alice' can predict that 'she' will be 'remembered' by both 'post-measurement Alici'. — boundless

Therefore the future does exist to Alice, just not a specific state. The cat exists to Bob, even when in superposition of dead and alive. — noAxioms

Ok! Agreed! :smile:

Ok, I agree. But my point was another. If you say that 'your' present exist (the 't=0' 3D hypersurface), then the Andromeda Paradox is unavoidable. — boundless

This hypersurface exists. So does this different hypersurface. That's just two different things, not a paradox. — noAxioms

Well, I think I see where you are getting at but I am not sure you can really avoid the paradox if you say that all events in the hyper surface are in a definite state. I am not saying you are wrong, I just do not know.

The answer to this objection is to not regard what is outside the light cone in the same way of what is inside from an ontological point of view. — boundless

Totally agree. Two observers at the same place but different frames might disagree about what is going on at Andromeda, but they'll agree entirely about what has been measured. The light cone from that location is a frame independent thing. — noAxioms

Yes! In Relativity the ordering of events in every light cone is an invariant (unless one accepts tachyons or any FTL influence).

On the other hand, it seems intuitive to accept the 'existence' of the present (e.g. I will observe the present state of the Sun at t=8 minutes). — boundless

Interesting corollary for a presentist, who by definition cannot observe any existing thing. In 8 minutes, the thing I observe will not be the present state of the sun. It will be an observation of something nonexistent. — noAxioms

Yep! Presentism is somewhat problematic in Relativity. I would say that 'global presentism' is simply incompatible with relativity of simultaneity. Maybe a form of 'local presentism' can be saved but it is surely counter-intuitive (I personally lean towards some form of presentism and I admit that I am troubled by this).

There is nothing physical that connects my current state to that past state as opposed to any other random arrangement of matter. Identity is abstract, not real. There are plenty of philosophical arguments that demonstrate this. — noAxioms

I sort of agree with this (but the reasons are not exactly the same...as I said I have a different view about mind) - it seems that there is some kind continuity without, however, a persisting identity (but we are digressing maybe...). This is not IMO however a complete denial of the existence of 'individuality' (and 'identity' in some sense). — boundless

Agree that what I said depends on my personal choice for philosophy of mind. Some interpretations do give identities to things. Mine just happens not to. — noAxioms

I see!

Not sure how you combine your mind interpretation with your QM one. Does the pre-Alice ontologically become one of the post-measurement Alici to the exclusion of the others because the mind-identity can only follow one of them? That's a very different QM interpretation. — noAxioms

Well, note that I do not currently accept RQM as 'my interpretation'. But it is one of my favorites.

Anyway, I do not believe that the mind is something immutable. So, for me, it is more like a 'stream of consciousnesses'. When I said that, in some sense, 'individuality' is preserved I meant that these 'streams' or 'continuums' are distinguishable. Yet, I do not believe that there is 'something' that 'persists' in the process (a 'substance') - in other words, I do not believe in a substantial identity.

Personally, I do not like the idea of the 'branching'/'splitting' - that's why I am insisting with the 'selection' postulate. On the other hand, I think that this kind of position about the mind is logically consistent with the 'branching' idea. Furthermore, I do not believe that the 'splitting' is a necessary feature of RQM. Before this discussion, I believed that there was a selection postulate in RQM. I now think that the theory is simply silent on it.

I agree. But that interpretation of RQM would only be a semantic difference from MWI, not a substantial one. My understanding is that RQM is a more abstract interpretation that captures what Rovelli considers to be the key elements of QM and nothing more. For example in his RQM paper he says, "From the point of view discussed here, Bohr’s interpretation, consistent histories interpretations, as well as the many worlds interpretation, are all correct." That is, they all share those key elements (albeit they commit to further things as well that differentiates them from each other, such as many worlds versus a single world). — Andrew M

:up: I completely agree!

But unless one adds a selection postulate, I believe that before the measurement 'Alice'/'Wigner's friend' can safely say that all 'Alice-s'/'Wigner's friends' will remember 'her'/'him'. What do you think? — boundless

I'm not sure I see the issue you're raising here. But I would agree that post-measurement, Wigner's friend (or friends on a MWI-style reading) would have a memory of themselves prior to measurement.

Regarding a selection postulate for RQM, I think it's just unknown and RQM doesn't commit to anything specific. — Andrew M

Well, I worded it badly. I simply meant that without the selection postulate, it seems that RQM implies the splitting.

Anyway, I agree with you. RQM seems simply silent on this point.

idk, by my understanding decoherence would render interaction between specific separate branches highly improbable. But because there are SO MANY separate branches, it would happen regularly. Sorta like what happens with quantum tunneling. — i aM

:up:

Well, that's another good argument IMO against the view that decoherence is enough to solve the measurement problem (even in MWI).

Yeah, sorry. I was a bit flippant.

The point is that in your example the interaction would give you some information of the other world(s). In MWI, you would observe superposition due to the interference of the branches (which would be very weird).

The fact we do not observe superposition is explained in MWI using decoherence, which suppresses the interference (well, technically, it renders it negligible...).

I'm not sure how weird it is. As an analogy in the classical world suppose that I forget to blow out a candle before I go to sleep and the house burns down. I'll experience regret because there is some world out there where I did not forget to blow out the candle and the house did not burn down. — i aM

...

If no alternative world in which the house did not burn down existed, it would never occur to me to be more careful in the future. — i aM

But you can explain regret in that way only if you accept the concept of parallel universes, i.e. if you accept the idea that whatever is possible, happens which is precisely what says MWI.

Honestly, I do not find any compelling reason to accept the idea of parallel universes/branches etc.

My reading of RQM (and Rovelli) is that RQM doesn't accept the existence of more than one Alice (or, at least, need not). Per RQM, all that is known to Wigner is that Wigner's friend has made a measurement and that the value is (physically) indefinite for Wigner until it is localized in his reference frame. — Andrew M

Well, more or less I always understood RQM in that way! :smile: ... After my dialogue with noAxioms, I am not sure about it. In fact, the 'relativization' of existence makes perfect sense in RQM. For each 'Alice' (each 'Wigner's friend') the other(s) cannot be said to 'exist'. But unless one adds a selection postulate, I believe that before the measurement 'Alice'/'Wigner's friend' can safely say that all 'Alice-s'/'Wigner's friends' will remember 'her'/'him'. What do you think?

I should reword. Yes, the odds are almost a certainty from the beginning that the unicorn will occur in some world, but I meant given a single measurement giving one random collapse. You only get one try. From the beginning of the universe, there's not even a planet on which a single measurement might hope to collapse a unicorn. I would presume an existing Earth with life already on it would raise the odds of a unicorn considerably from the odds from a blank slate. — noAxioms

Ok, I agree!

To summarize, in RQM, according to the pre-measurement 'Alice' both 'Alice-s' (or 'Alici' :wink: ) will exist. — boundless

Such statements are why I balk at A-series wordings like that. Under RQM, both post-measurement Alici (the plural is so stupid I am compelled to use it) consider the pre-measurement Alice to be part of their history. To pre-measurement Alice, the other two do not exist. The future is unmeasurable and thus doesn't exist to that instance of Alice. So there's no 'will-exist' except to indicate that certain future events (post-measurement Alici) consider certain past events to exist and others (like the one where Alice didn't measure it at all) to not exist. — noAxioms

I see what you mean, but 'pre-measurement Alice' can predict that 'she' will be 'remembered' by both 'post-measurement Alici'. This is not too very different from what MWI says. According to this view, MWI and RQM would be similar (not the same, but similar...).

I agree about the lack of contradiction. I know what you're saying and agree with it, but I don't like the A-series wording of it. 'Will exist' makes it sound like existence is something objective that occurs, and not the relation to something. The future Alici cannot exist ever to the pre-measurement one because there is no 'ever' to that version. She's an event, and events don't move into the future. — noAxioms

OK! That's fine. Note however that if one accepts presentism the past and the future do not 'exist'. Only the present does. It is true that the past can be said to 'exist' in the sense that there are data, in the present, about it. At the same time, it is also true that it is possible to make predictions about the future.

So, it seems to me that the relation between the future Alici and the past Alice is more or less the same of the reverse. So, before the experiment, Alice can predict the 'appearance' of the future Alici. So, for her, it seems legitimate to say that a 'split' happens.

SR is also quite consistent for the same reason: different orderings of events are not contradictory if they're from different perspectives. — noAxioms

Ok, I agree. But my point was another. If you say that 'your' present exist (the 't=0' 3D hypersurface), then the Andromeda Paradox is unavoidable. The answer to this objection is to not regard what is outside the light cone in the same way of what is inside from an ontological point of view. On the other hand, it seems intuitive to accept the 'existence' of the present (e.g. I will observe the present state of the Sun at t=8 minutes). If you follow your intuition, you end up with the Andromeda Paradox. I am not absolutely certain that the intuition is wrong, though (if not, dBB supporters would be very happy).

Well, 'I', from an RQM standpoint, am an event, despite my whole me being an abstract worldline. So in that event sense, I don't exist to myself, I only have memory of some past consistent state. From a pure event perspective, any two events (the table lamp and I at two specific moments) cannot exist in relation to each other. Neither exists to the other if the two events are space-like separated, and only one might exist to the other if not. It isn't paradoxical since no such mutual existence relation is ever posited. — noAxioms

Well, this seems also the implication of presentism plus SR/GR.

All different events, so not comparing the same thing. There is no 'the lamp' any more than there is a 'me' making that decision. We're both a series of events, any of which can relate to other events. The fact that a certain event in the past is considered 'also me, yesterday' is an abstract designation I make. There is nothing physical that connects my current state to that past state as opposed to any other random arrangement of matter. Identity is abstract, not real. There are plenty of philosophical arguments that demonstrate this. — noAxioms

I sort of agree with this (but the reasons are not exactly the same...as I said I have a different view about mind) - it seems that there is some kind continuity without, however, a persisting identity (but we are digressing maybe...). This is not IMO however a complete denial of the existence of 'individuality' (and 'identity' in some sense).

(But I am not sure about this :smile: we are probably digressing here)

I think I see what you are getting at*. But I do not believe that this really solves the problem that I have in mind. Unless you specify a duration for the events. — boundless

I'm sorry, but what was the problem? I thought the lack of duration was exactly what solved the problem. — noAxioms

Yeah, it seems so.

Not even familiar with the term Process Philosophy, but perhaps I am discussing it anyway. I'm a poet and don't even know it. — noAxioms

Well, I do not know very much about it. It is a kind of presentist ontology. 'Objects' are not regarded as substantial entities but rather as patterns in succession of events. On the other hand, existence is not defined in a relational way.

I think that works as well, yes. I seem to have a pretty weak grasp on the panpsychism idea. It doesn't seem to have a consistent interpretation from one person to the next. — noAxioms

Yeah, there are a lot of different versions of it. The parallelist variety, for instance, holds that everything has both a physical and a 'mental' aspect. The more complex an entity is, the more complex are both its physical and mental sides. In other words, mind and matter are like two sides of the same coin. But this is off-topic :sad:

Hi,

How does MWI handle probabilities in its branching of worlds? For instance if there are two possibilities (+ or -) and each has a probability of 50%, it makes sense to say that two separate branches result. — i aM

In MWI, you need an additional axiom AFAIK to include the Born Rule. I know that there have been some attempts to derive the Born Rule but I do not know if any of these attempts are regarded to be satisfactory. Note that some deny that such a derivation is necessary (I disagree, though - FWIW).

I'm not sure why, in MWI, the separate branches are said to not be able to interact with one another. Especially if, as you say, it is all one thing in Hilbert space. — i aM

In MWI, the branches can interact. But the likelihood of this interaction is negligible. (By the way, this is another really weird feature of MWI...)

Thanks for the very informative answer, again. I hope I'll can answer tomorrow (if not, I will on Saturday).

I am still confused about regarding 'observers' (as defined in RQM) as 'events' without duration but considering systems as abstractions denoting a stream of events is very interesting (note that I do not regard my confusion as an objection :smile: ...).

(sorry again for the edit... I misread a part of your answer)

For all those Alices (Alici? :confused:) to exist, you need to change the definition of 'exist' from the RQM one to the MWI one. The change of definition is what distinguishes the two, not that there's all these Alices. — noAxioms

Agreed! Until yesterday I did not fully understand RQM, I believe. My confusion was about the treatment of the ontological status of Alice: for me the 'Alice' that does the observation was the same as the 'Alice' observed by 'Bob'. Which is not true in RQM.

Or maybe it becomes a binary event if the measurement tells only if the decay happened during a certain time interval (I am not sure if you were saying this in the second paragraph). — boundless

Yes, I was forcing a (still imbalanced) binary event from a non-binary situation. — noAxioms

Ok!

I partly disagree. In Tegmark's view, there is no metaphysical split at the level of the universal wave-function. So, in that case there are indeed different 'Alice-s' there. It is not a 'real split' because what is truly real is the universal wave-function. — boundless

Sounds pretty similar to me. There is one universal wave function, some solutions including an Alice in one state or another. — noAxioms

I am not sure that I understand you here but I think we agree :wink:

In MWI, the 'Alice that observed decay' would know that, indeed, 'Alice that observed no-decay' exists. In RQM, however, the answer is, you say, negative. Why? The meaning of 'existence' is different in RQM: it is relational. — boundless

Yes, the other Alices don't exist, per definition. We can still, just like the MWI person does, say that the decay measurement doesn't exist relative to the no-decay Alice. She was a real (and even more probable) outcome of the quite real pre-measurement Alice. The unicorn is more difficult due to the vast improbability of one from a 100 million year ago wave function of Earth. Of course humans are near equally as (if not more) unlikely per that same wave function. — noAxioms

Ok!

Pop quiz: At what distance in the past does the wave function of Earth have the highest probability of there ever being a unicorn today? — noAxioms

I would say at the beginning of the history of the Universe (unless one believes to ancient mythologies that actual unicorns wandered on the Earth).

So, we cannot treat the 'pre-measurement Alice' as the 'Alice that observed decay'. So, a negative answer is perfectly fine. — boundless

Note that above I did treat the pre-measurement Alice as being real to either post-measurement Alice. Both have memory of that state and thus have taken a measurement. — noAxioms

Ok.

To summarize, in RQM, according to the pre-measurement 'Alice' both 'Alice-s' (or 'Alici' :wink: ) will exist. But both post-measurement 'Alice-s' regard the other one as 'non-existent' and the 'pre-measurement' as having existed in the past. There is no contradiction here because the states are perspective dependent.

I know that it is problematic within a relational framework, but it appears that there is no reason to believe that the other event did not occur. — boundless

It is not valid to state "the other event did occur" in a relational framework. The statement is an objective one, and has no meaning in a relational framework. It seems to constitute a counterfactual statement just like "The decay occurred". It didn't. It occurred relative to me, but it didn't just 'occur'. Thus just say that the other event occurred to the measurer of the other outcome. I tried to do what with the no-decay Alice above. I carefully avoided a wording like 'she exists' or 'the decay was not measured'. I tried to be careful to always include the relations. — noAxioms

Yeah, sorry! You are correct :smile:

It seems a bit 'solipsistic' for 'Alice that observed decay' to declare that 'her' counterpart that observed no-decay is simply real, in this case. — boundless

Non-mind idealism of a sort, but not solipsism. If existence hinges on an interactive relation with a subject, then that subject defines its own existence, which is interaction-idealism. But there is symmetry. Everything does it, so it isn't solipsism. The table lamp does it, so it isn't mind-idealism. — noAxioms

Ok, I see what you mean. But since yesterday, I am doubting that RQM is really consistent. But maybe the situation is the same as in the case of SR if one does not accept the 'block universe' (well, to be honest, I am not completely sure that even SR without the 'block universe' is really consistent...).

[But if one accepts some sort of panpsychism it is a mind-idealism :lol: Well, I believe that RQM and Process Philosophy can fit nicely together.]

A table lamp might measure a different version of me and thus I cease to exist in relation to it, but I always exist relative to myself, so I'm here. Under solipsism, I would not exist because only the table lamp (or whatever the one privileged thing is) counts. — noAxioms

Yeah in RQM, 'you' according to yourself and 'you' according to the table lamp are different. The table lamp according to itself is different from the table lamp according to you.

Furthermore, as I said before, I find RQM somewhat vague in the definition of 'perspectives'. According to RQM, every physical system is an 'observer'. Fine, but if we consider, for instance a pen, it can be argued that its parts can be considered a 'physical system'. — boundless

If you get right down to the details, an observer is an event, and a system is not. No pen is in a consistent state with itself since at any given moment, parts of it are separated by about a nanosecond of speed-of-light space, and thus the ball of the pen is in superposition relative to the clicker at the other end. So in that sense, I do not exist as a system with a state. Right now I am completely undefined since no point of me has had time to measure any other part. In hindsight (say a microsecond later), that state is mostly defined and immutable. I mean, suppose that my retina has just measured the decay result from the first photon from the device giving answer to my query. OK, several other parts of the front of me also measured that, but not yet the innards (brain in particular), which are still in superposition of decay happened or not. Relative to different parts of me, the measurement was taken or not. It isn't entirely correct to say that the measurement has been taken relative to me since 'me' is not an event. — noAxioms

I think I see what you are getting at*. But I do not believe that this really solves the problem that I have in mind. Unless you specify a duration for the events.

So, it seems that there is actually a very, very huge number of 'physical systems' (and, consequently, 'perspectives'). I believe that this is a legitimate criticism to RQM (as legitimate as the criticism to MWI to have too many 'branches'). — boundless

As legitimate, yes, but I find neither argument to have any teeth. Yes, it is an obscenely large number. Physics is full of those. One measurement such as a decay has seemingly infinite possibilities (not a discreet list), so even a trivial system already has infinite worlds. If one is to worry about how such a list can be instantiated (where do we find room to put them all?), then you're applying classic wording to a Hilbert-space problem. It is a good argument against the universe as computer simulation hypothesis since the implementation really would need to find room to put it all. — noAxioms

This is not true, however, for CI. In CI, only a specific class of entities can be considered an observer (which kind of 'entity' is subject to interpretation). At the same time, however, Relativity seems to imply something analogous.

I noticed that. It seems not to matter. In relation to me (or to anything else), what has happened is what has happened (fixed, in the past), and what will happen is meaningless since none of it can happen to me. Multiple future possibilities will be able to claim me as prior state, but that fact doesn't change if the list of those future possibilities is determined or not. Hence agnostic: it works either way.
— noAxioms

Ok, it seems so from a RQM point. — boundless

Still, the wave function is a pure function just like it is in MWI. Without interference from outside tweaking the terms, how can it not be deterministic? Sure, RQM doesn't care either way, but the same reasoning that MWI uses also works with RQM.
I'm now arguing against what I said above. I also do not know the meaning of the 'agnostic' designation on the wiki list for RQM. — noAxioms

From the perspective of the 'pre-measurement observer' if no 'selection' is made, then I'd agree it is deterministic. But if you consider the perspective of each 'post-measurement observer', the situation changes. For the 'Alice that observed decay' attributing the status of either 'existence' or 'non-existence' to the 'Alice that observed no decay' is meaningless (and vice versa). So, in this sense maybe we should understand the term 'agnostic'.

(sorry for the late edit!)

To summarize, I believe that RQM has two serious problems.

1) I believe - as I said previously - that there are indeed too much 'perspectives'. If every physical system defines a 'perspective'/'reference frame' (i.e. is an 'observer' according to Rovelli), then there is an incredibly huge number of perspectives.

2) If after a 'measurement' the measuring physical system becomes something else, we are, indeed, implying that after every physical interaction (for Rovelli, measurements are physical interactions) 'creates' new perspectives.

IMO, these two are very problematic features of RQM. YMMV!

Well, I was presuming their possibility. It's just a horse-like thing with an evolutionary feature currently found on a narwhal and arguably a rhino (both mammals). It can happen, no? The being a sucker for human female virgins is implausible if there are no humans around, but I think its still a unicorn without that feature (or the blowing of rainbows out it's arse). Pick a different example if you find them impossible. — noAxioms

Well, yeah in that case it might be a possibility. Who knows :smile:

BTW, regardless unicorns, I believe that the ontological status of possible yet unactualized 'things' it is a very interesting topic. Suppose that X is possible but it is never actualized in the past, the present or the future. Is that possible? Well, yeah, I believe. As I said before I am a free will believer. So, there are choices that I could have made in the past but I chose not to make them. Indeed, they were 'possible'.

Maybe those choices are simply 'unreal'. Yet, they are not 'unreal' in the same sense that dragons are (assuming that they cannot exist). Or in the same sense that the academic career of a dragon is. What makes something possible? What it means for X to be possible?
Using a MWI-like reasoning, one might say that whatever is possible is, in fact, actualized 'somewhere'. But for people like me who do not accept that kind of reasoning, it is an interesting conundrum.

Radioactive decay is a wonderful example of a lot more than two possibilities since it could decay any time, thus infinite possibilities. Alice detecting it after one second is different than the Alice that measures it after two, but there is one Alice that doesn't measure it at all yet. This isn't to say that the one Alice is less probable. That depends on the half-life of the sample.
For the sake of your example, the device measures the decay destroys the 'when' part of it, so all Alice gets is a yes/no from the device when she makes her single query as to if it's happen already or not. Cat dead or alive so to speak, which as I recall was done in this manner. — noAxioms

Ok, well as you say they are all indeed different cases. But suppose that as per above, not everything that is possible actualizes. Hence also in this case, only one 'event' happens. Of course, I am assuming that not everything happens. But note that if you, instead, accept the 'existence' of all those Alice-s, how RQM is really different from MWI (except for the universal wave-function)? I believe that Tegmark pointed this out to Rovelli.

Or maybe it becomes a binary event if the measurement tells only if the decay happened during a certain time interval (I am not sure if you were saying this in the second paragraph).

I knew where you were going with this. It seems solved by MWI by exactly what you quoted from Tegmark: There isn't actually any metaphysical split. There is but the one wave function with different solutions, and thus one Alice in two unequally weighted states. There are not two separate worlds, one metaphysically weighted more than the other. But the weight never changes from the '1' that it always was.
This argument is a great one against the whole metaphysical split interpretation of MWI. — noAxioms

I partly disagree. In Tegmark's view, there is no metaphysical split at the level of the universal wave-function. So, in that case there are indeed different 'Alice-s' there. It is not a 'real split' because what is truly real is the universal wave-function.

I thought we were discussing MWI there. Looking back at the exchange, maybe you mean RQM here. — noAxioms

Yeah, I was referring to RQM. Pardon the lack of clarity.

Under RQM, the Alice that measured no-decay does not exist in relation to the one that measured decay, so there is no weight problem. There is no selection since there is but the one world in relation to any particular state. The Alice that had measured the decay is not the same event as the pre-measurement Alice, so no selecting took place for either of them. That's at least how I've been wording it. — noAxioms

Ok, I think I see what you mean. Nice work!

In MWI, the 'Alice that observed decay' would know that, indeed, 'Alice that observed no-decay' exists. In RQM, however, the answer is, you say, negative. Why? The meaning of 'existence' is different in RQM: it is relational. So, we cannot treat the 'pre-measurement Alice' as the 'Alice that observed decay'. So, a negative answer is perfectly fine.
I know that it is problematic within a relational framework, but it appears that there is no reason to believe that the other event did not occur. It seems a bit 'solipsistic' for 'Alice that observed decay' to declare that 'her' counterpart that observed no-decay is simply real, in this case.

Furthermore, as I said before, I find RQM somewhat vague in the definition of 'perspectives'. According to RQM, every physical system is an 'observer'. Fine, but if we consider, for instance a pen, it can be argued that its parts can be considered a 'physical system'.
So, it seems that there is actually a very, very huge number of 'physical systems' (and, consequently, 'perspectives'). I believe that this is a legitimate criticism to RQM (as legitimate as the criticism to MWI to have too many 'branches').

And if we unite these two aspects we end up with an even more huge number of perspectives!

I noticed that. It seems not to matter. In relation to me (or to anything else), what has happened is what has happened (fixed, in the past), and what will happen is meaningless since none of it can happen to me. Multiple future possibilities will be able to claim me as prior state, but that fact doesn't change if the list of those future possibilities is determined or not. Hence agnostic: it works either way. — noAxioms

Ok, it seems so from a RQM point.

We are starting to agree more and more, making these posts a bit shorter. — noAxioms

Well, I believe that we are certainly understanding each other more and more. Not sure about the agreement :wink: Anyway, thank you very much for the very clarifying explanations. Now I feel that I understand RQM much better!

For those, like me, that are not averse to a Kantian-like sub-interpretation of CI, I suggest also this article by Cuffaro: http://philsci-archive.pitt.edu/14357/1/kant_bohr_hermann.pdf.
This paper also discusses the 'neo-Kantian' views of Grete Hermann (link to Wikipedia) who also much time earlier than Bell discovered that not all hidden variables theories are in conflict with the predictions of QM.

Another Kantian-like perspective as I said before is advocated by for instance Bitbol. For convenience I give again the link to Bitbol's paper: http://www.bourbaphy.fr/bitbol.pdf.

↪boundless Thanks for linking that. The comments from Demystifier and other members whose opinion I respect, align with my impression that this paper doesn't reveal anything new of significance. It effectively just affirms that if you insist on locality then you have to give up on CFD, which is in many ways the same thing as 'objective reality'. Bell told us that in the early sixties. — andrewk

Yes, I agree. This experiment does not 'reveal' anything new. This does not mean that it can be interpreted as an evidence that CFD is problematic, but we already knew that.

There is also a parallel discussion in Physics Forums about this experiment: https://www.physicsforums.com/threads/a-realization-of-a-basic-wigners-friend-type-experiment.968181/

I would not call what a table lamp does 'epistemic', so again, I do not personal hold view described there.
— noAxioms

Ok, I agree. Bad choice of terms on my part. For the sake of generality, let's just say 'not ontic' instead of 'epistemic' and drop using the word 'knowledge' in favor of 'addition of new 'information'' (for the lack of a better term). — boundless

The table lamp does acquire information (physics definition), so I can go with that. — noAxioms

Ok let's keep 'information', then! :smile:

Again, you are right! I should have said, instead: 'non-representational' means that the wave-function simply does not have any ontological meaning. — boundless

It may or it may not. Wasn't this the thing we said we're not sure about? It seems quite interpretation dependent, even to the point of interpreting the meaning of 'ontological'. — noAxioms

Yep!

Tegmark's mathematical universe says that the wave function is what the universe is. It doesn't just describe it, but it actually is it. That's one kind of ontological statement: mathematics is fundamental, not just descriptive — noAxioms

Correct! There is a spectrum of views here.
Tegmark's position is that the Hilbert space is the only true reality, whereas our 3 dimensional space is merely an appearance. That's why in his view it is perfectly safe to say that there is no 'real' splitting.
On the other hand, one can even IMO in MWI take a less ontological view about the 'wave-function' (and the Hilbert space). I believe that a purely descriptive position is consistent with MWI.

Another kind would be two different interpretations of this mathematical universe where the mathematical structure has the property of existing (MWI variant) as opposed to RQM, where 'exists' is a relation, not a property. That's a different sort of ontological statement. RQM doesn't say that the universe doesn't exist (isn't nihilistic). It just gives no meaning to the phrase. — noAxioms

Good point! Existence is relational. The 'universe as a whole' is not in relation with anything, so 'existence' here does not apply. This does not imply that the universe 'does not exist', as you say :smile:

In other words, if we take this reasoning seriously we can speak of 'existence' in the presence of relations. If there are no relations, we cannot speak in terms of 'existence' (maybe of a different kind of 'reality', if 'reality' is taken as a more general term than 'existence'...).

And no, neither MWI nor RQM needs to accept this mathematical foundation. I just used the mathematical universe example to illustrate two different kinds of ontology. — noAxioms

Agreed!

I am not sure if what you are describing here can be applied to the 'version' of RQM that I have in mind where after the observation, for the observer, the other branches do not exist. — boundless

Sounds like what I have in mind as well. For me, unicorns don't exist. For the unicorn, I don't exist. You seem to indicate that what I've described is something else. — noAxioms

Thanks for the clarification but I still do not understand how you say that we can avoid some sort of 'selection' here.

To make it simpler, consider a radioactive decay experiment. There are two possibilities: Alice either observes the occurrence of the decay or not. Let's call these two possibilities, 'decay' and 'no decay' respectively. Let's also say that the probability of 'decay' is much less than the one of 'no decay'. Alice performs the measurement. And, say, she observes 'decay'.
If we do not accept the selection, we should accept that there is 'another Alice' that observes, instead, 'no decay'. And - besides the existence of 'another Alice' that observers 'no decay' - we have the different weights problem that occurs in MWI. In fact, this scenario is not very different from MWI. The only difference is that here we do not have a 'universal wave-function'.

On the other hand, if a selection is accepted, there is only one outcome.

BTW, the table on the Wiki article on the interpretations of QM, says that RQM is 'agnostic' about determinism. So, maybe RQM is simply silent about the selection.

[Also, IMHO unicorns here are not a good example. For me, they are simply impossible (but as you say, better not to be too dogmatic about this :wink: ).]

BTW, I am sympathetic to some kind of realism for mathematics. I do not believe that mathematics is entirely a product of our minds but, at the same time, the usual version of mathematical Platonism does not convince me. So, maybe mathematical relations are 'real' (but not 'physical'). Interesting view — boundless

Or maybe they're physical but not real. — noAxioms

I believe that 'real' is more general than 'physical'. In fact, I just cannot understand how mathematical relations can be 'physical'. :wink:

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). — boundless

I'll now explain why I believe that replacing 'conscious observers' with 'sufficiently complex physical object/system' does not solve anything IMO.

In CI, 'classicality', i.e. having definite values of physical quantities, arises due to collapse of the wave-function. But collapse itself needs in CI a classical physical system, i.e., in this view, a 'sufficiently complex physical system'.

This is a deep issue in CI. Classicality is both a pre-condition to explain 'measurements' and a consequence of measurement. So, the problem of conscious observers is now replaced by another problem: CI cannot explain the arising of classicality - after all, to work CI requires that something must be treated as classical in the first place. Hence, the first occurrence of a 'classical system' is left completely unexplained in CI. Hence as it is said in in an already quoted article by Bitbol (http://www.bourbaphy.fr/bitbol.pdf), according to CI:

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].

This also shows IMO that the problem of the validity of 'ancestral statements' is not solved by replacing 'conscious observers' with 'sufficiently complex physical systems'. CI needs some kind of classical object as a starting point.

If one wants to avoid completely the problems due to giving a special role to conscious observers of some physical systems, one IMO should simply choose another interpretation. After all, there are a lot of interpretations of QM. RQM for instance does not have this problem because it treats all physical systems as 'observers', so there is no problem to explain how 'classicality' arises.

Apologies for slow reply time. — noAxioms

No worries!

A tool (a map of Paris say) may be just a tool and not be the thing it describes, but it very much still describes Paris. Thus I object to the statement that the tool doesn't describe anything. — noAxioms

I see what you mean and IMO this is a good argument against a 'non-representional' reading of the wave-function: it is difficult to accept that the wave-function can have absolutely no ontological meaning and, at the same time, be so useful.

I actually am not sure. On one hand, I am inclined to say that when the 'observation' occurs the other 'branches' do not exist (for the 'observer'). On the other hand, I recognize that the 'non-representational' reading is problematic. Maybe thinking in terms of potentiality/actuality helps.

This is very compatible with the view that collapse is due to an increase of knowledge (i.e. an 'epistemic', not 'ontic' view). — boundless

I would not call what a table lamp does 'epistemic', so again, I do not personal hold view described there. — noAxioms

Ok, I agree. Bad choice of terms on my part. For the sake of generality, let's just say 'not ontic' instead of 'epistemic' and drop using the word 'knowledge' in favor of 'addition of new 'information'' (for the lack of a better term).

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. — boundless

Disturbingly close, yes, to the point where no arbitrarily close inspection will yield a difference. This is not true of the paper map of Paris. — noAxioms

Again, you are right! I should have said, instead: 'non-representational' means that the wave-function simply does not have any ontological meaning.

I don't think there is just the two choices. It is certainly not representational in my opinion, but the wave function has meaning only in relations, not objectively, so it isn't ontological as in 'is real' but more like 'does relate to'. 5 really is less than 7 and that is not just a representational property of the tool that is the integers, and yet the integers don't need to have Platonic realism (ontological meaning) for that relation to be true. — noAxioms

Ok, I see. I am open to the view that there is a 'middle way' between the two positions. At the same time, however, I am not sure if what you are describing here can be applied to the 'version' of RQM that I have in mind where after the observation, for the observer, the other branches do not exist.

BTW, I am sympathetic to some kind of realism for mathematics. I do not believe that mathematics is entirely a product of our minds but, at the same time, the usual version of mathematical Platonism does not convince me. So, maybe mathematical relations are 'real' (but not 'physical'). Interesting view :smile:

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...