So what Carroll means is that fields are fundamental and particles (and everyday things generally) emerge from the interaction of those underlying fields.

Or, as Wikipedia puts it, "QFT treats particles as excited states (also called quanta) of their underlying fields, which are—in a sense—more fundamental than the basic particles."
— Andrew M

‘in a sense’ ;-)

When it comes to this matter, that phrase carries a lot of weight. — Wayfarer

I would have thought that one sense was enough. How many senses are you requiring? ;-)

Right, we are talking about the temporal ordering of two events, when the light reaches the front of the traincar, and when the light reaches the back. Special relativity allows for contradiction in the ordering of these events. — Metaphysician Undercover

No, the relative ordering of events necessarily follows from the invariant speed of light in different reference frames. Since the traincar is moving away from the train platform then the light emitted from the middle of the traincar, travelling at velocity c in the train-platform observer's reference frame, will take longer to reach the front of the traincar that is moving away from it than the back of the traincar that is moving toward it.

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

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

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

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'. — boundless

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).

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.

"In the same sense" means using the words in the same way. It has nothing to do with reference frames unless "temporal order" has a different meaning from one reference frame to the next. — Metaphysician Undercover

The truth of a statement depends on its reference in the world. For example, when it is noon in London, the statement "It is noon" will be true for London observers while false for Sydney observers.

Now consider the train-and-platform scenario (including the two traincar pictures). Per special relativity, the statement "The light reached the front and back of the traincar simultaneously" is true for an observer on the moving traincar while false for an observer on the train platform.

As with the noon example, there is no contradiction when the statement is indexed to an observer (or, equivalentally here, a reference frame).

So a field would be the property of something because there needs to be something actual which has that potential For example an electromagnetic field is a property of an object. — Metaphysician Undercover

And that's not the way it's modeled in QFT. In QFT, objects (including particles) emerge from the interactions of more fundamental fields. That is, the existence of the object is dependent on the existence of the fields.

Sure, absolutely. I'm not querying whether fields are real or the effectiveness of field theory. The point I was taking issue with was 'Particles are what we see. Fields are what reality is made of.' — Wayfarer

So what Carroll means is that fields are fundamental and particles (and everyday things generally) emerge from the interaction of those underlying fields.

Or, as Wikipedia puts it, "QFT treats particles as excited states (also called quanta) of their underlying fields, which are—in a sense—more fundamental than the basic particles."

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. — boundless

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.

I think the assertion that fields are what ‘reailty Is made of’ indicates deep confusion. We don’t even know what fields are - all we see is effects in respect of those particular phenomena in which field effects are visible. But what if there are non-physical fields, like Rupert Sheldrake’s morphic fields, or other forms of fields, like mental fields? There’s nothing to say there can’t be. Oh, I know - ‘scientists don’t think so.’ But that’s because their entire approach is based on studying matter, particles, radiation, and the other phenomena that can be studied using physical instruments. What’s that great analogy? 1. Metal detectors have had far greater success in finding coins and other metallic objects in more places than any other method. 2. Therefore what metal detectors reveal to us (coins and other metallic objects) is all that is real. — Wayfarer

QFT provides a physical mechanism (mathematically specified) for what is observed that is predictive and testable. It's been extremely successful and many physicists, including Feynman, have won nobel prizes for their work on it.

Regarding non-physical hypotheses, it's not that scientists don't think so. It's that without a physical mechanism, there's nothing to test. One person's speculation is as valid as anyone else's. So a physical mechanism provides a constraint on speculation.

I think that the relativity of simultaneity allows for the same type of contradiction. It allows that it is true that two events are simultaneous, and also true that two events are not simultaneous. That is contradiction, plain and simple. The relativity of simultaneity undermines the objectivity of the law of non-contradiction in a very fundamental way. This law states that the same predication cannot be both true and false at the same time. The relativity of simultaneity allows discretion, choice, in the judgement of "at the same time". — Metaphysician Undercover

Per the LNC, there is also "and in the same sense". In this case, the reference frames differ. Do you reject special relativity?

Whether or not I agree with Carroll that reality is made of fields is irrelevant to the issue here. — Metaphysician Undercover

The issue was whether fields are real in the ontology of QFT which Carroll's comments confirm.

This emphasizes the fact that what's commonly thought of as objective is a psychic construction which happens to be mostly wrong.

Taking QM into account means we have to change which 3rd person statements we consider to be true. So the OP is really just a matter of semantics. — frank

:up:

Redefining "objective reality" so that contradiction is acceptable in an objective reality is not what I would consider as an acceptable solution. — Metaphysician Undercover

There isn't a contradiction. Do you accept the relativity of simultaneity in special relativity? If so, then you already accept that a correct account of events can be reference-frame dependent and not absolute.

As far as I understand "fields", they are always modeled as potentials, and this includes "the more fundamental fields" of QFT. If you understand them as a model of something actual, then I think you misunderstand the ontology of QFT. But perhaps I'm wrong, and you can show me how a field is modeled as something actual. — Metaphysician Undercover

Sean Carroll gave a lecture a few years ago entitled, Particles, Fields and The Future of Particle Physics. I recommend listening to his discussion of one of the slides (between 28:00 - 30:40) that includes the line, "Particles are what we see. Fields are what reality is made of." Do you disagree with Carroll's characterization of QFT?

This is the point of the op then. The classical sense of "object" can be rejected altogether, and we no longer have any objective reality, everything is a "relative state". — Metaphysician Undercover

Objects and objective reality remain, but counterfactual definiteness, an assumption from classical mechanics, is rejected. In other words, there is no view of an object from nowhere but only in relation to an observer (that is similarly internal to the universe).

As Banno analyzed earlier here, this already has a precedent in relativistic physics which is consistent with an objective reality.

In the case of the Wigner's friend experiment, Wigner sees interference while Wigner's friend sees a definite result. And QM is used to translate between the observations such that Wigner and his friend both agree that Wigner sees interference and Wigner's friend sees a definite result.

The field itself is constructed as potential, then you layer another potential on top. — Metaphysician Undercover

The field is not constructed as potential. QFT says that the physical things that we observe emerge from the interactions of more fundamental physical fields. That is, those physical fields (one per particle type) are part of the ontology of QFT.

Take an electron for example. It must be a particle in the classical sense, because the structure of molecules and atoms is dependent on those particles. — Metaphysician Undercover

No, the classical sense (with absolute state) can be rejected altogether. On a relational model such as Rovelli's RQM, particles, atoms and molecules (and apples, desk lamps and human beings) are all quantum systems with relative state.

OK, so as I explained above to andrewk, I'll accept that a field is assumed to be more than imaginary. The problem is that it is modeled as the potential for activity, rather than a real active thing. So the issue is with the modeling technique, not the assumption that an imaginary thing is real. Therefore there is an inconsistency between the assumption, that the field is a real active thing, and the modeling of the field, as the potential for activity. — Metaphysician Undercover

On a quantum fields model, the fields for each particle type are real whereas it is particles that are potentials between interactions. For example, in the double-slit experiment the photon emitter acts by producing a disturbance in the electromagnetic field that propagates as a wave through the slits and toward the back screen. The wave represents a potential photon and a subsequent measurement actualizes the photon (say, at a slit detector or the back screen).

don't think physics provides any reason to doubt that the elementary particles (as described in the Standard Model) exist and have measurable physical properties just as everyday macroscopic objects do.
— Andrew M

I don't agree with this. I've spoken to physicists who say that there is no reason to believe that what they call "particles" in the Standard Model, are actually particles at all. That's just the word that is used. — Metaphysician Undercover

That's like saying that what we call "apples" aren't actually apples, that's just the word we use. So it's really a semantic issue. If one understands particles in a classical sense (i.e., as having an absolute state) then, I agree, physics gives us no reason to think such things exist. However if one understands particles (and apples) in a quantum/relativistic sense (as having a relative or relational state) then there is no problem - it's a natural fit.

The issue here is that a "field" is an imaginary thing, created by mathematics. — Metaphysician Undercover

Or, conversely, it's not imaginary since it has physical consequences. Perhaps consider it a manifestation of the measurement problem that can be understood in terms of potentiality.

Could you explain what a Higgs field is? — frank

I'm not familiar enough with QFT to do so. But here is someone else's explanation that I found helpful.

That's the interpretive problem in a nutshell. — Wayfarer

:up:

Isn’t the ontological status of fundamental particles also precisely what is at issue in all this? The ‘Copenhagen interpretation’ does, after alll, say that ‘the particle doesn’t exist until it’s measured.’ — Wayfarer

That's one framing of the issue, sure. But, more impartially, what is at issue is the solution to the measurement problem. That is, what does it mean for a physical system to be in a superposition of states given that it will always be measured in a definite state?

Yes, I think that is the case, electrons are measured as effects, and most forms of measurement are like this. — Metaphysician Undercover

I don't think physics provides any reason to doubt that the elementary particles (as described in the Standard Model) exist and have measurable physical properties just as everyday macroscopic objects do. Whatever issues QM raises apply to particles and macroscopic objects alike.

So you see it like - everything is a quantum system, just sometimes the corrections from quantum mechanics to macroscopic systems are sometimes negligible? — fdrake

Yes. The Schrödinger equation doesn't make a micro/macro distinction or specify a collapse of the wave function. Which is what gives rise to the measurement problem.

The physical properties of a photon are able to be measured in the same way as for any other particle. If you want to know a photon's position or speed, you set up an experiment and measure it.
— Andrew M

What is measured is the effect of the photon. — Metaphysician Undercover

If so, then it would seem that the same principle should apply to an electron. One would be measuring the effect of the electron (on a measurement device), not a property of the electron itself.

Are you singling out the measurement of photons as unique here or claiming a general principle for the measurement of all particles and, by extension, all physical objects?

Sure, QM is consistent with special relativity, but I don't think it's correct to call the properties of a photon "physical". A photon has an effect on physical things, and it might have a physical cause, and it is described by mathematics, but according to special relativity, light does not have spatial-temporal properties. The speed of light is the limit to spatial-temporal properties. So how exactly would you describe these "physical properties" which light quanta have? — Metaphysician Undercover

The physical properties of a photon are able to be measured in the same way as for any other particle. If you want to know a photon's position or speed, you set up an experiment and measure it.

See https://en.wikipedia.org/wiki/Photon#Physical_properties

Quantum particles are even smaller, aka subatomic particles, and not properly described as particles but that's language for you. — Benkei

It's worth noting that superpositions have been created for objects with up to trillions of atoms (as in the case of the piezoelectric "tuning fork"). Probably most physicists would consider QM to be a universal physical theory (i.e., applicable to everything). Which is part of the point of the Schrodinger's Cat and Wigner's Friend thought experiments.

Or as Dustin Lazarovici reacted to the paper: "A group of physicists claims to have found experimental evidence that there are no objective facts observed in quantum experiments. For some reason, they have still chosen to share the observations from their quantum experiment with the outside world.

...

In particular, it doesn’t mean that measurement outcomes, once obtained, are not objective. It rather reminds us that a measurement is not a purely passive perception but an active interaction that “brings about” a particular outcome and can affect the state of the measured system in the process."

Basically, the only way we would have to let go the assumption of an objective reality is if we were to insist on locality (against this and other experimental results) in which case nothing really exists but thank God it's local! (pace Tim Maudlin) — Benkei

Here's the DailyNous link for those quotes. The assumption of reality, as defined in Bell test experiments, is simply that there is a definite value for a measurement that has not been performed (counterfactual definiteness).

Almost all QM interpretations reject counterfactual definiteness, with Bohmian Mechanics being the main exception (as it happens, both the individuals you quoted above hold the Bohmian interpretation).

So what experimental results actually show is that counterfactual definiteness, locality and free choice can't all be true. Those interpretations that assume locality are compatible with special relativity.

What is necessary is to either release the confines of special relativity, allowing light to have physical properties, and describe those properties, or devise a way of observing non-physical properties. — Metaphysician Undercover

There is no implication of non-physical properties. In QM, light quanta (photons) have physical properties. And QM is consistent with special relativity.

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. — noAxioms

Yes, that's a good way to put it.

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

Agreed. A human observer and an artifact will interact differently with their environment based on their physical characteristics. And no observer will pick up all the information available during an interaction. However since whether or not there is a hole in the dish is a physical characteristic then a subsequent observation could detect it (either because the human observer directs their attention to it or because the machine is modified to detect it).

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. — boundless

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

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... — boundless

:up:

The photon definitely takes both paths relative to the interferometer because it takes no measurements until the paths join up again. — noAxioms

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.

Now the issue is that any information collected by the "reference frame", as "inanimate object", needs to be interpreted by human beings before it is useful as observational evidence. The interpretation is theory laden. So the idea that a reference frame can give observational information which is independent of intentionality is false.

The problem is that the theory laden interpretation cannot account for all the possibilities. For example, I put a dish of water in the sun, as my observer. I measure that water every fifteen minutes and derive a rate of evaporation as the day progresses. But I am assuming that evaporation is the only thing happening, I don't know if something else happened to the water, like a creature went and drank some when I wasn't looking. So the inanimate reference frame, as an observer, is only as good, and reliable, as the principles used to interpret the information. — Metaphysician Undercover

I'm not sure I understand your claim. Your example seems to merely raise ordinary epistemic issues around observations and experiments. There's always the possibility of some factor undermining your conclusion regardless of how careful you are or how you define your terms. For example, perhaps you observed the dish all day, but there was leak in the dish resulting in you recording an incorrect evaporation rate.

I've seen that people use the word 'know' a lot when talking about observation/information transmission/interactions involving energy exchange, but 'know' looks to be used in different contexts to where just 'correlation' would be appropriate - like, if 2 particles are entangled, it doesn't seem typical to say that one particle 'knows' the state (distribution) of the other. Examples in this thread are the use of 'Alice knows that Bob knows...' towards the start in the discussion between boundless, @noAxioms and @Andrew M.

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

"Know" would generally indicate that a measurement had been made and the result stored. As you suggest, the spin values of an entangled particle pair would be correlated but not known until a measurement occurs.

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. — boundless

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.

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? — boundless

I don't think there is another branch for RQM. A superposition merely indicates that there is no actual value for O' prior to an interaction. The reason that O' will agree with O is simply that an interaction allows the value that O has obtained to also become actual for O'. This does not constitute a hidden variable because the rule is that the value only exists as the result of a local interaction between the two systems.

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).

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

There is still interference. See Brukner's discussion on this below:

The key element of the experiment is that the message contains no information about which outcome has occurred and thus should not lead to a collapse of the quantum state assigned by the superobserver. Imagine that the observer encodes her message in state |message>₅ of system 5. This state is factorized out from the total state, |psi(t')> = 1/sqrt(2)(|z+>₁|z+>₂|z->₃|knows "up">₄ + |z->₁|z->₂|z+>₃|knows "down">₄) |message>₅, and thus the communication of the message does not destroy the superposition. — On the quantum measurement problem, p18 - Caslav Brukner

What human beings and table lamps have in common is that they are substantial and have form.
— Andrew M

And what they DON'T share is 'the ability to form a perspective'. — Wayfarer

Right! I think we're in agreement.

Right, but we support abstract theories with empirical evidence gathered from observations. If, what is called an "observer" is not really an observer by rigorous standards, then the biases inherent within that definition of "observer" must be accounted for or else "empirical evidence" will not really be empirical evidence. — Metaphysician Undercover

Yes, the term observer has two uses, so we should always pay attention to the context to avoid equivocation. If an inanimate object is called an observer, then no intentionality is implied, it's just a reference frame. Whereas human observers have an intentional view (and can additionally serve as a reference frame).

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

For RQM, the path travelled is only counterfactually indefinite for the observer outside the interferometer. It says nothing about what the full-silvered mirrors within the interferometer might measure (the result of which gets subsequently erased by the final beam splitter). Bell's Theorem is inapplicable for RQM because there are no hidden variables in the observer's reference frame.

Of course, if the photon did travel only one path for the full-silvered mirrors, that still leaves open the question of what produces interference for the observer. But, logically, a one path explanation has not been closed off.

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

Yes. Physically it's a front/back reflection. And it can also potentially be perceived as a 3D object that is half rotated around the up/down axis and reflected left/right. Perhaps a 2D/3D gestalt effect.

I don't see that as superimposing or interference. — noAxioms

Thanks, problems noted!

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

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

One thought here is the idea of reflection. When Alice looks in the mirror and raises her left arm, it appears in the mirror that she is raising her right arm. Now Alice's action and her reflection are both the results of local physical processes. Bob, observing from afar, might receive light signals directly from Alice and indirectly via the mirror that end up superimposing for him, so it seems that Alice is raising both her left and right arms at the same time. That would be a physical interference effect. However when Bob and Alice interact, he discovers and they agree that she was raising her left arm.

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

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!

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

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.

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: — boundless

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.

Interestingly, it seems that RQM agrees with the Consistent Histories interpretation about the lack of a 'single history'.
...
Well, I really find this interpretation (at least as it is presented there) somewhat difficult to understand. So, I still have not formed an idea about it. In fact, I somehow have a problem to really distinguish it from MWI.

What do you think of Consistent Histories? — boundless

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.

Then, as Bitbol points out, we overlook or forget the "role of the observer" in all of this, assuming that we're viewing the world as if from no perspective or no point of view. — Wayfarer

What human beings and table lamps have in common is that they are substantial and have form. What RQM says is that that form is relative to the observer, not intrinsic, so a point-of-view is explicitly recognized.

But RQM also says that an observer is internal to the universe. As a consequence, RQM allows an observer to be part of a system that is itself being observed by another observer. That is the scenario that the Wigner's friend thought experiment presents.

But we don't need RQM in order to recognize that. We observe others in everyday life and they can observe us. The principle is the same.

The problem is that anything within that "system" needs to be interpreted according to standards before the system has any observational value. The "system" has no observational value without those human standards. — Metaphysician Undercover

Which is fine. The point is only that QM is an abstract theory about the mechanics of physical systems generally, regardless of the specific systems one is interested in modelling (which will include context-specific information).

An observer is one who takes notice of (and this means pays attention to) the thing which is observed. — Metaphysician Undercover

Right. And by my reckoning table lamps can’t do that. — Wayfarer

Simply substitute system for observer if that helps. That is how Rovelli is using the term.

They're almost the same thing, with different definitions of 'is real'. RQM says this world is real to me and a different world is real to anything else (the cat say), so they're both talking about different worlds. MWI says they're all equally real, and RQM says none are real, only that there are relations between worlds and observers.
...
Still, there are very 'real' differences between MWI and RQM, and I don't see how either Rovelli and Tegmark are going to convince the other that they hold the opposite view. — noAxioms

Thanks! My own reading of RQM for a while was as a relational semantics abstracting over a Many Worlds dynamics. But as boundless also suggested, I think Rovelli would reject an underlying Many Worlds dynamics.

My answer to the above question is that perhaps there isn't anything, and thus there is no need to have to explain its being. — noAxioms

I recall that you accept Tegmark's mathematical universe hypothesis, so that would seem to follow. :-)

What is interference? It seems to be 'maybe' as an answer to a question not yet asked, a measurement not yet taken. So X = square root of 2 is 1.414 but also -1.414 and both those values can work through my equation until a choice must be make before the mathematics can continue. That's a measurement, and now there are two equations that proceed in different directions using a now real value for X instead of one in superposition. — noAxioms

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

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

Regarding Kaster's approach I am not sure to call it 'non-local'. — boundless

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.

All that is physically going on are systems interacting locally with other systems. The time of closure of the cafe is system-dependent and is determined by physical interactions.

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

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

I am simply undecided. — boundless

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.

Thanks !

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

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

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

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

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

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

Thanks, that was interesting. So my suggestion differs in at least two ways. First, as with Rovelli, I think that quantum mechanics is local. Second, as with Aristotle, potentialities don't "do" anything, only actual systems do.

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

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

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

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

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

As Brukner says in that link (my italics):

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

I'd rather see them sink RQM. Always best to have ones own cage rattled once in a while. — noAxioms

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

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

Yes, another great discussion! I also liked Wallace's explanation of the wave function at 11:25.

So here's the setup and exchange between Valentini and Rovelli:

[4:55] Valentini sets up the scenario as Rovelli deciding to speak at the conference (or not) based on measuring a particle spin as spin up (or spin down). Rovelli measured spin up and so here they are talking at the conference. However a super-intelligent being in the future measures interference.

[53:52] Rovelli: Antony asks, "Carlo, some super-intelligent is believing that you are not here because in his wave function you're superimposing, there's no fact of the matter. Does this bother you?"

I think it doesn't because it's exactly the kind of thing that happens in theoretical physics all the time. I think it's very similar to what happens in special relativity. If I take Einstein's simultaneity convention, right now in Andromeda there is something which has already happened with respect to the - not the past cone but the simultaneity convention - with respect to which I haven't happened yet with respect to this.

This makes no sense whatsoever but that's the structure of the world. The relation between when things happen for who are complicated. I think with this guy in the future, I could talk if I could survive until then, I could talk to him and we would agree and the fact that now, for him, in the future before I interact with him there is a discrepancy in what we see doesn't really bother me.

Valentini: For him, there wouldn't be a fact of the matter about the past?

Rovelli: That's right.

Here's also the relevant passage from the Physics Forum Insights article that you linked to earlier: The Block Universe – Refuting a Common Argument.

(3) All events in the past light cone of a given event are real (i.e., fixed and certain) for an observer at that event.

The reason this accounts for all of our observations is that information can’t travel faster than light, so anything we observe at a given event can only give information about the past light cone of that event.

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

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

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

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

For a brief discussion of that prediction in one of the experiment's referenced papers, see https://arxiv.org/abs/1507.05255 (p18, point 1).

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

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

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

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

One of the two propositions in such instances must be true and the other false, but we cannot say determinately that this or that is false, but must leave the alternative undecided. One may indeed be more likely to be true than the other, but it cannot be either actually true or actually false. It is therefore plain that it is not necessary that of an affirmation and a denial, one should be true and the other false. For in the case of that which exists potentially, but not actually, the rule which applies to that which exists actually does not hold good. — Aristotle, On Interpretation, §9

--

It seems not contradictory at all for Bob to find the state of the photon still in superposition, despite the conflict wording in the article. — noAxioms

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

I believe that Rovelli himself treats the wave-function as not descriptive. So, he would not say that there are 'many physical branches'. — boundless

Thanks - that would be my reading as well.

The reason is that in MWI you regard the entire universe as the single 'real system' and you need to add an 'additional structure' in order to decompose the universe into subsystems. In RQM, the subsystems are the 'primary' because they are given by experience (in MWI, instead you try to derive experience from the universal wavefunction). — boundless

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

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

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

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

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

Well, yeah I honestly do not know how you can explain that if you assume that the wave-function is not 'real'. So, I unfortunately cannot give you a response. — boundless

No worries! This is the difficulty for me regarding RQM. I can understand how MWI works, but not RQM in this regard.

If the wave-function is taken as 'real', then the situation is still different from MWI IMO (as I explained above, hoping that it made some sense LOL...). Mauro Dorato apparently tried to explain RQM in terms of dispositions, check: https://arxiv.org/pdf/1309.0132.pdf — boundless

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

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

Don't know what you mean by 'reverse polarity' — noAxioms

By reverse, I just mean that the measurement process can be undone by applying appropriate (inverse) unitary transformations.

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

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

Maybe also Andrew M and @noAxioms might find the above linked videos interesting. — boundless

Thanks boundless - they're excellent videos and well worth watching for anyone with an interest in the philosophical aspects of QM. Fun quote from Rovelli at 36 mins: "When I told Max (Tegmark) that he was a relationist, he told me that he is going to convince me that I'm, without knowing, a Many World believer." Anyway, Rovelli has a slide at 40:15 that says:

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

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

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

"I have analyzed a fixed physical sequence of events E, from two different points of observations, the one of the observer and the one of a third system, external to the measurement. I have concluded that two observers give different accounts of the same physical set of events (main observation)." [Rovelli] — boundless

As I'm guessing you're well aware, Rovelli's main observation exactly describes the Wigner's friend scenario that this thread is about. But it's worth shining a light on. Also of interest, Rovelli commented on the OP experiment in a recent New Scientist article:

I do take it as a great piece of evidence directly supporting the relational interpretation. I agree in full with the way they interpret it,” he says. “It is fantastic that ‘ideal experiments’ of the past become real experiments of today. — Rovelli in New Scientist (Quantum experiment suggests there really are ‘alternative facts’)

The funny stuff aside; how can you "measure" a superposition or how can a measurement be in a superposition? I thought any observation causes the wave function to collapse in a single eigenstate and a measurement, I would think, involves an observation. — Benkei

It's illustrated in the article's image of the experiment (Figure 2 in the paper). Bob's friend's measurement occurs in the gray box (with a record that a definite result occurred). There are detectors on the far right after the beam splitter that Bob observes (and similarly for Alice on the far left). The statistics collected from the detectors over multiple runs indicate quantum interference which means that Bob's friend's measurement in the isolated box was in superposition. It's analogous to the interference pattern in the double-slit experiment which indicates that the emitted particle was in superposition when it passed through the slits.

What this experiment challenges is the idea of objective collapse. Instead collapse is observer-dependent - there is collapse for Bob's friend but not collapse for Bob.