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...

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

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

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

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

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

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

Ok.

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

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

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

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

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

Ok, I agree!

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

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

The first section says:

Dalla Chiara shows that the duality in the description of state evolution, encoded in the ordinary (i.e. von Neumann's) approach to the measurement problem, can be given a purely logical interpretation: “If the apparatus observer O is an object of the theory, then O cannot realize the reduction of the wave function. This is possible only to another O′, which is ‘external’ with respect to the universe of the theory. In other words, any apparatus, as a particular physical system, can be an object of the theory. Nevertheless, any apparatus which realizes the reduction of the wave function is necessarily only a metatheoretical object ” (Dalla Chiara 1977, p. 340).
...
O cannot have a full description of the interaction of S with himself (O), because his information is correlation information and there is no meaning in being correlated with oneself. If we include the observer into the system, then the evolution is still unitary, but we are now dealing with the description of a different observer.

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

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

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

There is, however a problem here. If Wigner asks his friend before entering in the lab if he sees a definite result, it appears that at this point Wigner already knows that everything in the lab is in a definite state. So, superposition is now destroyed. At this point, it seems it is not very different from the classical case of an unseen coin. The problem is that Wigner still does not know in which state the lab is (including is friend). But it seems that when he enters Wigner and his friend must be in agreement! So for them, S is in the 'up' state.

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

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


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

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

I do not think that it is reversible, hence I'd say that time symmetry is broken!

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

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

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

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

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

I see. But I would say that the comparison is apt for Vulcan and the definite state of the Moon, not the Moon.

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

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

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

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

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

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

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

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

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

Well, yes. So, it must 'exist' even if unobserved!

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

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

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

I see. Then maybe we are in agreement! The unobserved electron is not really non-existent. More precisely, it does not exist in a definite state. So, before measurement it simply does not make sense to talk about electrons in definite states.

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

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

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

IMO, because results of observations are random.

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

Agreed! That's definitely a problem for MWI.

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

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

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

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

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

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

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

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

Agreed!

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

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

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

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

In which case, how does this address the 'observer problem'? The observer might occupy a particular reference frame, but without an observer, then what is being measured/observed? I guess I will have to find time to go back and look at those resources that boundless provided in more detail. — Wayfarer

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

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

None of this means I cannot collapse my own state in sufficient hindsight, which is sort of what I meant with my first reply to this issue. — noAxioms

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

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

No worries!

This isn't the QM I know. There is no 'big difference' between the two cases. The cat is both dead and alive (superposition), not either dead or alive (result of an unobserved coin toss). The latter probability function is said expression of ignorace about how the coin toss turned out, but the probability function of the cat is not ignorance. It really is dead and alive until measured, else it could not interfere with itself. — noAxioms

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

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

I see. Maybe I misunderstood.

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

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

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

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

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

As you say, however, at the level of practice this does not make any difference. After all, in RQM it is still true that for 'our' perspective, 'we' are 'special'. And our knowledge is perspectival. In a 'Kantian-like' philosophy it is impossible to know 'things as they are' because phenomena (in our immediate experience) are conditioned by a-priori categories of our mind. So, for Bitbol, we can only be (almost) sure about our perspective.

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

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

I believe that the registration act involves 'something' that is able to store information. E.g., a computer that stores '0' ('1') for spin 'up' ('down') when the spin of an electron is measured. But anyway, if one follows Rovelli in saying that everything can store information in that way, I believe that Wheeler's CI becomes RQM.

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

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

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

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

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

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

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

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

Yes, sorry.

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

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

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

Ok!

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

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

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

LOL. You are right! :smile:

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

Ok, I see. Good point!

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

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

I think I agree!

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

Right, I agree.

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

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

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

:up:

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

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

Ditto :wink:

,

Just a curiosity...

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

In a probabilistic theory the limiting cases of a probability equal to 1 or 0 are equivalent to asserting that the corresponding proposition (e.g., “the system has property P”) is, respectively, true or false. In the histories approach probabilities are linked to frameworks, and for this reason the notions of “true” and “false” are also framework dependent. This cannot lead to inconsistencies, a proposition being both true and false, because of the single framework rule. But it is contrary to a deeply rooted faith or intuition, shared by philosophers, physicists, and the proverbial man in the street, that at any point in time there is one and only one state of the universe which is “true”, and with which every true statement about the world must be consistent. In Sec. 27.3 of Griffiths (2002a) this belief is referred to as unicity, and it is what must be abandoned if the histories interpretation of quantum theory is on the right track.

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

What do you think of Consistent Histories?

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

Mmm, I see!

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

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

Agreed!

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

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

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

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

Now, what happens if WE open the door of the lab. to see what happened in it? As previously, we see a well-defined chain of characters on the screen of the computer, we see that the cat is either dead or alive, and we hear a colleague saying that he had already seen (well before our arrival) what we are now discovering. And as previously, we must modify suddenly our tool of probabilistic prediction in order to account for our certainty of finding everything in the same configuration, if we come back in the lab. some time later. In the same way as we had to reduce the probability function of the whole measurement chain, we now have to reduce its wave function. There is a big difference between the two cases, however: whereas the ordinary probability functions could be considered as the expression of our ignorance about what effectively occurred in the lab., this is not true for the wave functions of quantum mechanics. The wave function of the measurement chain just provides a probabilistic prediction of the result of a possible (uncontrollable) interaction between it and something else. And this manifests itself through the persistent presence of a term W (for “wavering” or “wavelike effects”), in the probabilities calculated from the wave function.
...
Let me show how this indexical “solution” (or “dissolution”) of the measurement problem of quantum mechanics works. It is based on the far-reaching, and already documented, difference between a property and an observable. A property of an object is defined in the absolute, whereas an observable refers to a possible relation between the object and something else. Each time one wishes to make the relation explicit, one has to introduce the “something else” into the field of description. But then, the characterization of this something else is itself relative to a possible interaction with a third element, and so on and so forth. Isn't there a natural end to this chain of relations? Yes, there is. WE are this end. Of course, I am not trying to say that WE are unique or privileged beings in nature (this would be collective solipsism of an absurd sort), but only that WE are privileged beings for US! As soon as we establish a relation with an element of the measurement chain, this element acquires a determination relative to US. Nothing has thus to be changed in the physical description, since determinations of the measurement chain are still relative to something. But everything is different for US, since the determinations of the measurement chain are now relative to US. And a relation of which WE are one term is something quite peculiar, even if it is only peculiar ... from OUR point of view.

Oddly, I think that Bitbol's view seems a mixture of CI and RQM, but in fact I think it is still a form of CI. In usual formulations of CI, there is only a 'classical world'. Yet, Bitbol's point is that Wigner still has to consider his friend in superposition. In other words, just like in the case of Rovelli, in Bitbol's take of CI, there is no single 'history', so to speak. As Bitbol points out in the other article I linked before (i.e. this: http://www.bourbaphy.fr/bitbol.pdf):

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

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

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

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

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

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

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

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

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

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

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

According to Bitbol, collapse is due to an increase of knowledge. It is not a physical process. The wave-function is regarded as a tool.
Of course, it becomes more problematic if a minimalist take on the wave-function is untenable.

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

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

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

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

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

Ok...Yes, I agree that inter-subjectivity and objectivity in the usual sense are different. What moral relativism denies, however, is that there are no universal ethical truths. I find it somewhat irrational, actually. In a sense, it is somewhat dogmatic: 'perspectivism' does not entail that it is impossible to find something that is considered 'good' for everyone. Also, it has absurd consequences: I believe that, indeed, there are actions that can be considered good/bad by everyone. And I do not believe that rejecting 'objectivity' in the usual sense denies that. (I referred to a discussion on ethics when I referred to 'off-topic')

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

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

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

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

Ok!

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

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

Take for instance even an electron. Its state might be different for the various 'perspectives'. But for all physical systems it is indeed an 'electron'. So, you need something invariant, after all. Can we still have invariants if we do not have something perspective-independent? I am struggling with this point and I am not arrived to a solution.

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

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

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

Thanks!

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

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

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

I’ll spell out my bottom line - that all measurement or observation has a subjective aspect, i.e. it is undertaken by a subject, and the subject is (obviously) never disclosed in the act of observation, because observation is always of objects or at least of the objective domain. As Bitbol says, the inability to recognise the ubiquitous presence of the subject is precisely the blind spot of modern science. Philosophers (including Kant and Schopenhauer) understand this but many scientists do not, because it’s not, by definition, a scientific issue. But the fact that it becomes a subject of discussion here is because scientists are perhaps operating at ‘the limit of objectivity’. — Wayfarer

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The Wigner interpretation I referenced. — noAxioms

Yep!

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

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

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

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

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

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

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

It is meaningful to say that there are individual systems like S? That sounds an awful lot like counterfactual definiteness. A system with states still in complete superposition is still an existing system. There was no description of S existing in relation to R. Just 'there is a system S', which sounds more like MWI and not RQM. — noAxioms

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

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

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

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

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

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

Thank you :blush:

You're welcome.

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

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

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

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

I do not believe that RQM per se goes as far as denying that we can speak of individual objects, i.e. a 'no-thing' view, so to speak. After all, perspectives here are defined in relation to these objects. But it certainly comes close to this idea: ultimately there are no 'objects' (which should not be taken as saying that 'there is no reality'). Rather objects are useful abstractions or approximations :smile: