At every point is 'observation' linked to, and articulated in terms of, the physical set-up of the scientific apparatus in place. — StreetlightX

And at no point, could that apparatus have been set up without a guiding theory. If there were no theory to test, then no apparatus would have been set up. Open and shut. And theory is the product of reason - that is the way that consciousness manifests.

Wheeler says: 'The dependence of what is observed upon the experimental arrangement made Einstein unhappy'.

Why was that? Why would Einstein be unhappy about that?

No shit, but that's nothing but a contingent fact that has nothing to do with the process of 'registration' itself; you may as well say, if there were no microscopes guided by our theories of optics, there'd be no bacteria - but then, you're no longer taking about quantum phenomenon, but broader questions of epistemology. No doubt your whole sthick is to confuse and muddle the two under the cover of the general confusion about QM, but that's entirely your own fuck up.

Oh look an Einstein quote, right on fucking cue. Another rhetorical (non)-argument-by-allusion. Next stop, bookshop stocklists.

QM is about the atomic world and objective reality is about the macroscopic world. I don't think we can carry over the results in one to the other.

If all this is true then there's as much difference between classical physics and QM as there is between religion and science.

Also, to declare there's no objective reality from such an experiment seems to let logic off the hook. As if to say logic, as it is, can't fail. We probably need to work on our logic too before we reject objective reality so soon.

Actually the next sentence is that the reason 'this made Einstein unhappy' is that 'it conflicts with the view that the universe exists "out there" independent of all acts of observation'. Which is the only point that I'm pressing. That is the point. Honestly we should be able to at least debate this point without you lapsing into profanities and histrionics.

Yes, and an observation in a quantum process is a record by an instrument. Your 'point' is to equivocate on this and worm some horseshit in about 'theory' which Wheeler deliberately avows to be a 'separate story'.

you may as well say, if there were no microscopes guided by our theories of optics, there'd be no bacteria — StreetlightX

That is an exact analogy. It is why the question is vexed. It was expected that 'bacteria' (fundamental particles) would be found. It was precisely because of the 'observer effect' that the whole issue is so very controversial. The quarry - atomic particles in this case - were found not to have an independent reality, and they were supposed to be the fundamental building blocks of nature.

I will put something to you. Bohr said, as I said, that 'Quantum physics is shocking'. This is why it's shocking. It's shocking because it seems to imply that the these supposed 'fundamental building blocks' depend for their existence on observation, that they don't exist independently until they're measured. That is the problem in a nutshell. The disgust you're expressing towards me, actually is that 'shock'. I'm actually causing you to seriously consider the outrageous implication of quantum physics, which I sincerely, hand-on-heart believe you haven't actually seen up until now. The revulsion your expressing towards me, is exactly what drove a lot of the heat and passion in the debates over quantum. Heisenberg said he was driven to tears over it, because it genuinely is shocking.

I don't think fdrake has seen this point, either. That's not me 'being a shithead'. or a troll, or an internet crank. I have read a fair amount, I consider myself a responsible correspondent, and this is what I think the issue is.

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). — Andrew M

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.

Calm down. Picture yourself on that balcony on Caïro. In what way do you think QM puts an independent reality radically to question? And independent from what?

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...)

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

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

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

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

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

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

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

Have a question for people who know a lot more about this kind of thing than me.

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?

You outright lie, misrepresent, and quite literally make things up — StreetlightX

I completely reject that. Your criticisms are ad hominem, they're not supported by any references or sources, and they're solely informed by prejudice. You can continue to rant, but that is the last thing I will say to you.

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

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

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

Are you sure about this?

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

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

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

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

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

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

Same thing, different spin.

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

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

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

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

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

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

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

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

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

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

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

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

At no point is the wider body of 'theory' as set out by a community of scientists invoked necessary to bring about a quantum phenomenon: the phenomenon is 'brought to a close' by the interaction with the instruments: it goes no further, and certainly requires no 'consciousness' to swoop in from out of nowhere to make it an observation. — StreetlightX

Isn't the "quantum phenomenon" itself completely theoretical? There are some things, like wave interference which can be observed with the human eyes, and some things like the photoelectric effect which are observed by the interaction with instruments, all the rest, what we call the quantum phenomenon is just theory. Isn't it?

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.

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.

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. — Andrew M

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.

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.

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.

,

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.

My issue with these statements is that it takes too restrictive a view of what 'observation' entails. — Wayfarer

It seems strange that you should say this, when it seems that you are proposing a narrower conception of "observation".

Because it seems to me that the very experiment that is the subject of this thread calls into question the whole notion that nature exists in a given state, whether or not observed. The whole point is that there is not a single, objective 'state of affairs' that is apprehended differently by different observers. — Wayfarer

What exactly do you mean by "given state"? Would saying that nature does not exist in a given state mean that nature is "really" not anything, that there is no 'way things are' at all? If there were no "objective states of affairs that are apprehended differently by different observers", then how could there be any commonality of observation at all? Where would the commonality that is obviously attained originate? If there are no "ways things are", then what is science studying, and how would science then be possible at all?

Again the act of observation is implicated in determining the outcome, which is the point at issue. — Wayfarer

So, is there "a way the act of observation is" which all by itself enables the outcomes to be consistently determined and consistently agreed upon?

"An observation", in this context, is a data-point, one piece of information that is interpreted in the light of theory. That is how 'consciousness' manifests in this context, and the sense in which 'observation' is meaningful. — Wayfarer

If all you want to say here is that human experience is always already concept-laden, then I would agree.

The salient question is as to what conclusions you want to draw from that obvious fact of human experience and what further assumptions and lines of reasoning you would use to attempt to sustain any conclusions you want to maintain. I am yet to see you do that, to be honest.

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. — boundless

I think we observe it every day, but you take it as an ordinary observation. A rainbow is quite impossible to see without superposition. The lens of my eye would not work without it. Most directly, Bob in the OP is able to directly measure superposition even after Alice has taken a measurement.

BUT you need an additional assumption in order to explain why only one outcome actually occurs.

If your interpretation says that a single outcome occurs.

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.

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

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.

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.

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).

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

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).

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?
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.

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

So the inhabitants of those 'other branches' don't continue to post in their forums philosophical discussions about how their world being more real than ours? The Schrodinger equation says they very much do, and none of these additional postulates make any modifications to that.

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.

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'.

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.

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. A worldline doesn't work because it isn't defined after a certain point. So that point (one event) is 'me', and that event cannot take a measurement. It may or may not have an abstract worldline leading up to it, but none has a worldline leading from it. Hence an event (me) can have already taken a measurement, but it cannot take a new one. Such new measurement are taken by future events, and the future does not exist to 'me', thus 'I' cannot take a new measurement, only something else which will happen to include 'me' as part of its past worldline. So 'I' do not select anything and thus what is real never changes for me. One of my future events (say one that measures a vertical polarity) can have already measured that polarity and that event considers that state of affairs to be real to it, but it never becomes real to (is selected by) any particular 'me' event, which, being an event, cannot flow through the selection process to its future, however much it might intuitively anticipate doing so.
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.

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.

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

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.

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.

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.

How does one go about 'finding' an electron? What sort of measurement are we talking about here? With the moon, sure, you look up on a clear day and see if there's one there, but for an electron, it seems more difficult to not find one since they're everywhere. OK, 50 BLY away I might not find one in a sample radius since odds are I encounter fairly empty space, but I'm not sure if that's what you mean.

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)? — boundless

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:

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.

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.

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:

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. — boundless

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.

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).

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.
Tegmark also argues effectively that the 'split' is never a full separation into separate disjoint worlds, and others (e.g. Wayfarer) often do, which I feel misrepresents the view. To quote Tegmark again (same link):
"According to the MWI, there is, was and always will be only one wavefunction, and only decoherence calculations, not postulates, can tell us when it is a good approximation to treat two terms as non-interacting."
So worlds don't actually split off, but different terms simply become sufficiently decoherent for their interaction to become negligible.

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.

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.

Sounds good.

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).

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.

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

Everett was forced to reign in his views in order to gain acceptance. It was dumbed-down to a reasonably finite number of worlds, not this full blown Hilbert space thing. That has since been put back, but I wonder what else never was. It's not like Everett really ever contributed much after being driven from the physics community like he was. He needed more of the PR engine that was behind Einstein. His thinking was too out-of-box for the time.

some persistence over time seems required.

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.

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

Agree. Nothing to collapse a wave function.

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)

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

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

I say it isn't selected at all.

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'.

What exactly do you mean by "given state"? Would saying that nature does not exist in a given state mean that nature is "really" not anything, that there is no 'way things are' at all? If there were no "objective states of affairs that are apprehended differently by different observers", then how could there be any commonality of observation at all? Where would the commonality that is obviously attained originate? If there are no "ways things are", then what is science studying, and how would science then be possible at all? — Janus

Don't you see that this is the whole point of the article in the OP? Read the title of the thread again - it is literally what the experiment is about.

I asked before, why did Niels Bohr say that 'if you're not shocked by quantum physics, then you haven't understood it'? I've tracked down the source of the quote:

In Physics and Beyond Chapter 17, entitled “Positivism, Metaphysics, and Religion (1952)", Heisenberg recounts a conversation he had in Copenhagen with Wolfgang Pauli and Niels Bohr in June 1952. They were reflecting on the interpretation of quantum theory, which had emerged 25 years earlier. Heisenberg recounts Bohr as saying the following:

Some time ago there was a meeting of philosophers, most of them positivists, here in Copenhagen, during which members of the Vienna Circle played a prominent part. I was asked to address them on the interpretation of quantum theory. After my lecture, no one raised any objections or asked any embarrassing questions, but I must say this very fact proved a terrible disappointment to me. For those who are not shocked when they first come across quantum theory cannot possibly have understood it. Probably I spoke so badly that no one knew what I was talking about.

So - why the shock? What is shocking about it? The books I've read about it include David Lindley's book Uncertainty: Einstein, Heisenberg, Bohr, and the Struggle for the Soul of Science. Why is it called 'the struggle for the soul of science?' It's because of the very question that you're asking. Adam Becker's recent book is called 'What is real?' Why is it called 'what is real?' Isn't it just obvious what is real? Don't we know that? Why is that a question? I don't have any kind of solution but at least I think I understand the problem.

Anyway, I'm logging out for the time being to concentrate on work. Bye.