The Many-Worlds Interpretation of QM

[kith]

It seems that since you cannot explain the results of this experiment in terms of the MWI(you would have if you could) [...]

Sure I can. Whenever collapse interpretations say collapse happens, the MWI says that decoherence occurs. For every which-path information you get, there is a complementary world where you register that the other path has been taken. These points of view can't be distinguished by experiments. If you don't understand this, you don't understand the mathematics of the MWI.

[...] you hope you can just skip the technical details of the experiement and appeal to authorities.

Yes, I appeal to authorities. Kim's experiment has been done almost 15 years ago. Since then, numerous articles on the MWI have appeared in peer-reviewed physics journals. You claim that all these people are wrong. This is an extraordinary claim which has to be backed up by a reference. And no, Kim's original paper is not supporting your claim.

 

[bhobba]

That's not sufficient to falsify an assumption; it shows that the assumption is unnecessary but does not show that the asumption is untrue. Falsification requires a stronger criterion: the assumption must lead to a prediction that can be shown to be false.

True. But in this case LESS assumptions are made - no collapse assumption is assumed - instead each of the outcomes of the mixture after decoherence continues to evolve with each observer in each element of the mixture experiencing that outcome - it literally takes the formalism at face value.

Thanks
Bill

 

[Quantumental]

True. But in this case LESS assumptions are made - no collapse assumption is assumed - instead each of the outcomes of the mixture after decoherence continues to evolve with each observer in each element of the mixture experiencing that outcome - it literally takes the formalism at face value.

Thanks
Bill

So how are you going to get worlds out of the formalism and how are you going to get probabilities out of the worlds?
I posted 2 papers, one that highlights the factorization problem, no one has rebutted it.
The second paper shows the inherent circularity of relying on decoherence for a basis, when decoherence itself is probabilistic. So you have to magically introduce (just like collapse) the born rule into decoherence and then hope that FAPP will give you worlds.

I'm so tired of people claiming that MWI hass less assumptions and more parsimony when it isn't even a working interpretation yet!

I hate collapse interpretations, I literally dispise them to the end of time, but at least they can have their interpretation work (its obviously wrong, but still). MWI cannot even do that.


So please do me the favor of reading both of those papers and then come back with a rebutle before boasting about how MWI is elegant again.

 

[bhobba]

So how are you going to get worlds out of the formalism and how are you going to get probabilities out of the worlds?
I posted 2 papers, one that highlights the factorization problem, no one has rebutted it.

The first paper was a philosophical analysis and philosophy is notorious for not being able to get a consensus on anything and to be blunt their knowledge of the physics often leaves a lot to be desired. Sorry mate but despite your cajoling you are not going to get too many physics types to take it seriously. Care to give us a run down on its argument? If it sounds promising you may get more people interested in reading it.

MFB rebutted the second.

So you have to magically introduce (just like collapse) the born rule into decoherence and then hope that FAPP will give you worlds.

David Deutch and others beg to differ. You may not agree with it but that is a matter of opinion - saying its magic is not a rebuttal.

I'm so tired of people claiming that MWI hass less assumptions and more parsimony when it isn't even a working interpretation yet!

You know what I am tired of? Statements like the above not backed up by anything. Care to actually tell us why?

I personally think MW is BS - but its a fully valid theory that takes the formalism literally.

Lets be clear exactly what it does. Decoherence, completely deterministically, transforms a superposition into a mixed state. Each element of the mixed state then evolves deterministically - no collapse - no assumption - simply the normal deterministic evolution of a quantum state. How you get the worlds is dead simple - by definition each element of the mixed state is a world. Exactly how is that not a working interpretation?

Thanks
Bill

 

[Fredrik]

So how are you going to get worlds out of the formalism

By an assumption that we can consider a part of the definition of the MWI. It seems to me that this assumption should say that every 1-dimensional subspace of the Hilbert space is a world. It annoys me that the stuff I've read (admittedly not that much) about the MWI doesn't explicitly define what they mean by a "world".

and how are you going to get probabilities out of the worlds?

I think that once a decomposition into subsystems has been chosen, Gleason's theorem gives us the probabilities, but not their significance. It's not clear to me to what extent that problem has, or can, be solved. It certainly hasn't been completely solved.

I posted 2 papers, one that highlights the factorization problem, no one has rebutted it.

I have however written several comments about it. My "rebuttal" is that I don't consider "the factorization problem" a problem at all. (See e.g. my posts #90 and #116). It's only a problem for those who think that at a given moment, there's exactly one set of worlds that make up the universe. The factorization "problem" is however a very interesting feature, that should be highlighted by a proper definition of the interpretation. So I find it pretty weird that this paper is the first mention of this feature that I've seen outside of my own forum posts.

The second paper shows the inherent circularity of relying on decoherence for a basis, when decoherence itself is probabilistic. So you have to magically introduce (just like collapse) the born rule into decoherence and then hope that FAPP will give you worlds.

As I said before, the low quality of the published papers I've read makes me reluctant to read an unpublished preprint.

I'm so tired of people claiming that MWI hass less assumptions and more parsimony when it isn't even a working interpretation yet!

I agree that it still needs work. I don't think that anyone has ever written down a definition of "the MWI" that I would find satisfactory. So maybe it shouldn't be called an interpretation yet. It's an idea that probably can be developed into an interpretation.

I hate collapse interpretations, I literally dispise them to the end of time, but at least they can have their interpretation work (its obviously wrong, but still).

I find those pretty ridiculous too.

 

[martinbn]

Fredrik, as you very well know, 1-dim subspaces are the states of the system. If you call them worlds, it seems to me, that you are just renaming them, that cannot make any difference.

 

[Fredrik]

Fredrik, as you very well know, 1-dim subspaces are the states of the system. If you call them worlds, it seems to me, that you are just renaming them, that cannot make any difference.

The difference is the idea that they all represent something that actually exists, rather than something that could exist. (States should usually be thought of as equivalence classes of preparation procedures, but I guess that wouldn't make sense when we apply QM to the whole universe).

If you (or anyone) know a better definition of "world", I would be interested in hearing it. All I know is that terms like that need to be defined before we can claim to have defined a many-worlds interpretation.

 

[bhobba]

The difference is the idea that they all represent something that actually exists, rather than something that could exist. (States should usually be thought of as equivalence classes of preparation procedures, but I guess that wouldn't make sense when we apply QM to the whole universe).

If you (or anyone) know a better definition of "world", I would be interested in hearing it. All I know is that terms like that need to be defined before we can claim to have defined a many-worlds interpretation.

That's the thing with MW - they think of a state like an electric field - something real - not like for example Ballentine does in his excellent book. Of course you then have the arguments he brings to bear about such a view but I don't think they are deal breakers within the context of MW.

We also have the issues you raised about decoherence in selecting a preferred basis (ie a different decomposition may single out a different basis or even none at all) but the consensus view is that decoherence solves that issue. It certainly does if we make the assumption of decomposing the system in the obvious way - ie observational apparatus and system being observed. The decoherence guys also admit that this is still an area of active research and I have zero doubt it will be clarified in time.

Now taking that into account I think what I wrote, namely the |bi>|ri> where the |bi> are the basis vectors after decoherence and the |ri> the state of the rest of the world can be taken as the worlds the MW guys talk about. They come about deterministically via the decoherence process (ie tracing over the environment) in transforming a superposition into the form of a mixed state sum pi |bi>|ri> and evolve deterministically after decoherence.

From the discussion here I think we can say that MW is a valid interpretation and a very elegant one at that. There are some issues that are controversial such as how probabilities come into it and the preferred basis problem but exactly what's going on there will become clearer with further research.

Because of that I see nothing that justifies some of the hysteria in this thread that it has been disproved or that it is not even a 'working' interpretation.

Thanks
Bill

 

[Fredrik]

We also have the issues you raised about decoherence in selecting a preferred basis (ie a different decomposition may single out a different basis or even none at all) but the consensus view is that decoherence solves that issue.

I would say that it doesn't even address the issue.

It certainly does if we make the assumption of decomposing the system in the obvious way - ie observational apparatus and system being observed.

I think this is like saying that the ether must be comoving with me, because the obvious way for me to choose a coordinate system is to choose the inertial coordinate system that's comoving with me.

What if two different experiments are performed at the same time? Then there are two "obvious" decompositions.

Now taking that into account I think what I wrote, namely the |bi>|ri> where the |bi> are the basis vectors after decoherence and the |ri> the state of the rest of the world can be taken as the worlds the MW guys talk about. They come about deterministically via the decoherence process (ie tracing over the environment) in transforming a superposition into the form of a mixed state sum pi |bi>|ri> and evolve deterministically after decoherence.

Since any complex number times |bi>|ri> represents the same state as |bi>|ri>, I think it's slightly more accurate to say that each 1-dimensional subspace spanned by a member of this basis is a world. This basis has the property that the state operator of the universe is approximately diagonal in it. If it had been exactly diagonal, I think it might have made sense to say that the worlds corresponding to the basis vectors are the only ones. But it's only approximately diagonal, so I don't think it makes sense to say that $|b_i\rangle|r_i\rangle$ represents something that actually exists, while $|b_i'\rangle|r_i'\rangle$ such that $\langle b_i'|b_i\rangle \langle r_i'|r_i\rangle$ is extremely close to 1, doesn't.

I think people who write about the MWI are aware of this. They sometimes refer to the worlds identified by a preferred basis as "the classical worlds" rather than "the worlds". I think it would be appropriate to say that all 1-dimensional subspaces are worlds, and that the ones identified by the basis are the classical worlds, or perhaps more accurately, the most classical worlds. They are the ones in which the classical description of what's going on (we measured the spin and found it to be "up") is as close as possible to being accurate. I think it would be even more accurate to say that the 1-dimensional subspaces corresponding to the members of the preferred basis associated with the chosen decomposition are the classical worlds associated with that decomposition.

I don't think this is a problem that can be "solved". It's just a matter of accepting that at any instant, there are many inequivalent ways to view the universe as consisting of classical worlds. To me, this makes the MWI less crazy, not more.

 

[martinbn]

The difference is the idea that they all represent something that actually exists, rather than something that could exist. (States should usually be thought of as equivalence classes of preparation procedures, but I guess that wouldn't make sense when we apply QM to the whole universe).

If you (or anyone) know a better definition of "world", I would be interested in hearing it. All I know is that terms like that need to be defined before we can claim to have defined a many-worlds interpretation.

I still don't see the difference. It seems simply as more words. If you try to explain what exists means here, I think you'll end up with nothing new just different names.

 

[mfb]

If you (or anyone) know a better definition of "world", I would be interested in hearing it. All I know is that terms like that need to be defined before we can claim to have defined a many-worlds interpretation.

"Real-wavefunction-interpretation"?

From the discussion here I think we can say that MW is a valid interpretation and a very elegant one at that.

:)

What if two different experiments are performed at the same time? Then there are two "obvious" decompositions.

And there are "obvious" decompositions describing both experiments at the same time.

I think people who write about the MWI are aware of this. They sometimes refer to the worlds identified by a preferred basis as "the classical worlds" rather than "the worlds". I think it would be appropriate to say that all 1-dimensional subspaces are worlds, and that the ones identified by the basis are the classical worlds, or perhaps more accurately, the most classical worlds. They are the ones in which the classical description of what's going on (we measured the spin and found it to be "up") is as close as possible to being accurate. I think it would be even more accurate to say that the 1-dimensional subspaces corresponding to the members of the preferred basis associated with the chosen decomposition are the classical worlds associated with that decomposition.

I don't think this is a problem that can be "solved". It's just a matter of accepting that at any instant, there are many inequivalent ways to view the universe as consisting of classical worlds. To me, this makes the MWI less crazy, not more.

That sounds good.

 

[Fredrik]

I still don't see the difference. It seems simply as more words. If you try to explain what exists means here, I think you'll end up with nothing new just different names.

I think that perhaps you expect too much from an interpretation. The starting point of this interpretation is that QM is more than just an assignment of probabilities, it also describes what's actually happening. Since you have been trained to think like a scientist, you're probably immediately thinking that such statements are nonsense unless we define them. You may also have thought about how to define some of them and only come up with useless definitions like "a theory describes what's happening if it makes good predictions about the results of experiments". This definition is useless because if this is how the statement is defined, it doesn't tell us anything.

We are so used to requiring everything to be defined that we sometimes forget that some concepts are simply more fundamental than the things we use to define them. For example, we like to define "integers" in ZFC set theory. But a set theory is defined by a set of axioms that are statements in a formal language, and the formal language is defined by specifying an alphabet of symbols and some other stuff. How can you even write down an alphabet with say five symbols, if the concepts 1,2,3,4,5 are undefined? It seems that we are using those concepts before we have defined them.

This is, unfortunately, how we need to think of the statements that define an interpretation. You don't define what it means for a theory to describe what's happening. You already know what it means. Similarly, if the interpretation says that every 1-dimensional subspace represents something that actually exists, you need to just accept that you already know what that statement means. If you allow such statements to have the meanings your intuition is already assigning to them, then they do tell us something that the theory doesn't.

Is this science? Hell no. But it's not supposed to be. Is it nonsense? Probably not.

 

[martinbn]

Fredrik, I think now I see better what you mean, but it still seems that your proposal doesn't give anything knew as an interpretation, except for new names. Of course, that's my problem, I am not very familiar with any of them.

 

[Quotidian]

There's an interesting Scientific American review of Everett's Many Worlds here. Not that it is very relevant, but he was not a likable, warm human being. He died an emotionally distant, chain-smoking alchoholic aged 51; the article remarks that his son found his body and felt for his pulse, then realized it was possibly the only time he had ever actually touched him.

A lot of his career was spent callibrating the maximum effectiveness for where to drop nuclear weapons. His ethics were also questioned by some of his employers.

Most of his peers thought the actual theory was nonsense and refused to even consider it. However the article notes that attitudes have changed in the last 20 years. I do wonder if this is simply because we have generally developed a much higher tolerance for nonsense. That would not at all surprise me.

I thought a very telling quotation from Everett was this one:

The Copenhagen Interpretation is hopelessly incomplete because of its a priori reliance on classical physics ... as well as a philosophic monstrosity with a “reality” concept for the macroscopic world and denial of the same for the microcosm.

(page 3)

Why 'a denial of the same'? I think it is because the then-prevailing Copenhagen interpretation posited the centrality of the act of observation in the 'collapse of the wave-function'. This, of course, undercuts the notion of objectivity, and is the source of a lot of the debate around the whole issue. (Einstein was said to have remarked in relation to this notion 'does the moon continue to exist when nobody is looking at it?')

So the solution was to posit an uncountable number of worlds. This was preferable to allowing the heresy that consciousness might have a fundamental role in the scheme of things even if it seems such an affront to the natural principles of science.

Strange old world.

This one, anyway.

 

[bhobba]

This was preferable to allowing the heresy that consciousness might have a fundamental role in the scheme of things even if it seems such an affront to the natural principles of science.

I think consciousness causes collapse is basically rubbish - but heresy - no.

It attracted the attention of some pretty high powered and reputable people such as Von-Neumann and Wigner.

The interesting thing however is that MWI was the first interpretation to incorporate decoherence - it was in its infancy then but it certainly signalled the start of its rise. In fact from some early work on decoherence by Zurek Wigner abandoned the consciousness causes collapse idea as no longer being necessary - and he was correct - it isn't - some quite simple interpretive assumptions avoid it eg decoherent histories. Indeed in a poll only about 7% ascribe to it nowadays so one could argue it really is in decline - and MWI either with or without decohenrece polls 20%. My ensemble interpretation polls about the same as consciousness cause collapse at a lousy 7% - looks like shut up and calculate aren't that popular either - which may be good or bad depending how you look at it - I personally think its a good thing because it shows people actually are thinking about it.

Thanks
Bill

 

[Quotidian]

The 'consciousness causes the wave-function collapse' idea is rubbish because...

As regards 'decoherence', I have never read an intelligible account of this idea in English, and actually doubt that there is such a thing. Although I am open to persuasion.

 

[bhobba]

The 'consciousness causes the wave-function collapse' idea is rubbish because...

Well imagine a quantum experiment recording the positions of an interference experiment but the results are stored in a computer. The apparatus is disassembled and centuries later the results viewed. Are you seriously going to tell me that's when the wave-function collapsed and the positions manifested? Indeed exactly what quality distinguishes a computer memory recording results and human memory doing the same thing? You could probably come up with some contrived scheme placing the human brain on a pedestal with some privileged status but exactly for what gain? Do you want the world to be this weird consciousnesses created reality? Is solipsism appealing to you? If it is I can't prove you wrong but there is a reason virtually everyone rejects it - but hey if that's what floats your boat be my guest - I - and I suspect most people won't be there with you.

As regards 'decoherence', I have never read an intelligible account of this idea in English, and actually doubt that there is such a thing. Although I am open to persuasion.

Well obviously guys like Wigner did - which should be a clue there is something to it. Its standard textbook stuff - see for example Schlosshauer - Decoherence: and the Quantum-To-Classical Transition:
https://www.amazon.com/dp/3642071422/?tag=pfamazon01-20

Thanks
Bill

 

[Quotidian]

Indeed exactly what quality distinguishes a computer memory recording results and human memory doing the same thing?

A computer doesn't read or interpret anything whatever. Quite apart from the fact that the computer itself is only an instrument and will only do what it is programmed to do by a human intelligence, whatever data it contains does not amount to information unless it is interpreted by a human. in his lectures on Mind and Matter, Schrödinger argued that there is a difference between measuring instruments and human observation: a thermometer’s registration cannot be considered an act of observation, as it contains no meaning in itself.

We, of course, can argue about that. A computer, however, cannot.

Is solipsism appealing to you?

Solipsism only enters into the equation if you think that yours is the sole intelligence in the universe. I would not be so vain. You will note that we all share the same basic perceptual apparatus, metabolism, brain functioning, and so on. On a slightly higher level of the 'stack' we also share a common culture, language, background assumptions about what is or is not real, and so on. And these things do indeed constitute the fabric of reality.

As regards the book linked to on 'decoherence'. The topic might be 'standard textbook stuff', however my observation was that I have never encounted a comprehensible account of it in English. The reader reviews and editorial information on those titles do nothing to address that.

 

[bhobba]

A computer doesn't read or interpret anything whatever. Quite apart from the fact that the computer itself is only an instrument and will only do what it is programmed to do by a human intelligence, whatever data it contains does not amount to information

So you think until it's printed out of memory or displayed on a screen and interpreted by a human its not information? Like I said you can probably come up with some weird view giving some kind of privileged status to human consciousness - and it looks like you did. All I can say is your view is not held by any computer scientist I am aware of. I would suggest if you promulgated such a view in a computer science class your audience will disperse rather quickly amid quite a bit of chuckling. Still I can't prove you wrong - its simply a very weird view that is at odds with how the majority view such things.

As regards the book linked to on 'decoherence'. The topic might be 'standard textbook stuff', however my observation was that I have never encounted a comprehensible account of it

Well since that is not the reaction of the vast majority of people exposed to it have you considered that the issue may reside in you rather than the material?

You might like to detail exactly what you don't understand. For example what's the issue with tracing over the environment transforming a pure state into an improper mixture?

Thanks
Bill

 

[kith]

[...] in his lectures on Mind and Matter, Schrödinger argued that there is a difference between measuring instruments and human observation: a thermometer’s registration cannot be considered an act of observation, as it contains no meaning in itself.

The question is, why should the difference between "observation by a human" and "registration by an apparatus" in QM be more important than in classical physics? In consciousness-based interpretations and (arguably) the Copenhagen interpretation, this is the case. In the MWI, it is not.

As regards the book linked to on 'decoherence'. The topic might be 'standard textbook stuff', however my observation was that I have never encounted a comprehensible account of it in English.

It's quite difficult to explain decoherence without using a bit of math. I haven't come up with an easy but accurate explanation of it yet. Maybe other people here have?

However, in order to understand how the measurement problem is solved within the MWI and how the worlds are introduced, I think decoherence is a detour. See my post #3 for Everett's initial approach.

 

[Quotidian]

you can probably come up with some weird view giving some kind of privileged status to human consciousness - and it looks like you did.

Right. So believing that 'observation requires an observer' is weird, but entertaining the idea of 'infinite proliferating worlds' is not.

In consciousness-based interpretations and (arguably) the Copenhagen interpretation, this is the case. In the MWI, it is not.

Again - at what cost? Your post #3 says 'There's nothing what singles out one outcome, so we can interpret each of them as belonging to a different world'. But are there actually 'different worlds'? Does it matter? And if it doesn't matter, then does this construction have anything to do with reality? Or is it just like a 'thought-experiment', a kind of conceptual model to do away with something that is 'conceptionally unsatisfying for many people'?

 

[HomogenousCow]

At the end of the day, without any real distinguishing evidence to go by, I think the subject of interpretations is a non-subject, for example if I choose to believe that there are parallel universes which cannot affect us in any way, you cannot prove me wrong but I cannot prove myself right either.
Or you know..if you believe there are omni-powerful beings present in the universe..Oh wait I have a warning point already.

 

[bhobba]

Right. So believing that 'observation requires an observer' is weird, but entertaining the idea of 'infinite proliferating worlds' is not.

MW is equally as weird IMHO as consciousness causes collapse. But weirdness is in the eye of the beholder. My Ensemble interpretation also has a weird aspect - namely exactly how is the improper mixture of decoherence converted into an actual mixture. Observationally they are identical - but their physical preparation is entirely different. Every single interpretation is weird in some way - you simply pick the one that is the least weird to you. Most people, because of the difficulties I alluded to with computers, and other reasons, would say consciousness causes collapse has too much baggage associated with it to be taken seriously - but that doesn't mean its not valid.

BTW in QM observation means any device capable of registering in the macro world - it's a misunderstanding thinking a priori observation requires an observer despite the semantic closeness of the terms. If that is what you believe you are not the only one to be confused by it.

Thanks
Bill

 

[bhobba]

Again - at what cost?

You asked me to justify:

I think consciousness causes collapse is basically rubbish - but heresy - no.

Notice the words - I think.

In any interpretation you are making a judgement - every single interpretation has baggage - it's purely a matter of which you think has the least baggage. But that is your choice - its not subject to experimental verification. Opinions are like bums - everyone has one - it doesn't make it correct.

Thanks
Bill

 

[bhobba]

I think the subject of interpretations is a non-subject

Not really. In order to apply QM you need a way to map the mathematical elements of its formalism to stuff out there. That's the job of an interpretation. Even if you are in the shut-up and calculate camp you still have an interpretation - its called the Minimal Statistical Interpretation.

Think of good old Euclidean Geometry. How to map it is usually very obvious physically and no issues arise. The interesting thing about QM is that mapping is not obvious so you have all these different interpretations and threads like this. Its in the nature of the material.

Interestingly in the past, say about the time Feynman was in his heyday, the shut-up and calculate view was predominate but these days its all over the place. As mentioned previously this may be a good or bad thing depending on how you look at it.

Thanks
Bill

 

[Quotidian]

Very true. Wise words indeed. It is just that us philosophically curious lay-persons are often informed by the likes of Lawrence Krauss and Stephen Hawking that 'physics has superseded philosophy'. Yet the question of 'interpretations' is very much a philosophical question - and one which, no disrespect intended, few physicists seem to have much of a handle on.

I think I will go back to Kant, Schopenhauer and Buddhist studies. Thanks for your responses.

 

[bhobba]

Very true. Wise words indeed. It is just that us philosophically curious lay-persons are often informed by the likes of Lawrence Krauss and Stephen Hawking that 'physics has superseded philosophy'. Yet the question of 'interpretations' is very much a philosophical question - and one which, no disrespect intended, few physicists seem to have much of a handle on.

I think I will go back to Kant, Schopenhauer and Buddhist studies. Thanks for your responses.

Most physicists are like myself and are a bit anti-philosophy along the lines of Feynman.

But note - that itself is a philosophy.

I don't think its fair to say, like Lawrence Krauss and Stephen Hawking says (and I have read what they say and they do say stuff like that), that science now supersedes philosophy. What is fairer to say is science makes extensive use of a tiebreaker in an argument - philosophical or otherwise - actual observation - that's the real issue. Truth is only held provisional while observation supports it. That's the key difference.

Thanks
Bill

 

[Fredrik]

In my opinion, philosophy has a very limited relevance to science and mathematics. It can handle questions like "What is science?", "What is mathematics?" and "What is the best way to define the term 'theory of physics'?". To note that terms like "state" can be defined in a theory-independent way is to do philosophy. To think about what whether QM can or can't be interpreted as a description of what's actually happening, is also to do philosophy.

Unfortunately I don't think philosophers are doing any of those things well. So I can't help wondering if philosophers have any relevance to science.

The only reason I'm not saying that the philosophy of science and mathematics should definitely be left to physicists and mathematicians is that they're not doing these things well either. Philosophers probably just don't understand the subject well enough. But physicists and mathematicians have an attitude problem. They think that this sort of stuff is beneath them.

 

[dx]

To note that terms like "state" can be defined in a theory-independent way is to do philosophy. To think about what whether QM can or can't be interpreted as a description of what's actually happening, is also to do philosophy.

I would like to mention this again, since it is a key point in the copenhagen interpretation (a term I reserve for Bohr's views alone)

"What is actually happening to the system" is clearly a meaningful concept as long as the behavior of the system can be observed without the act of observing the system influencing the system.

But in quantum mechanics, the existence of the quantum of action h implies that there is a lower limit to the interaction between the measuring bodies and the objects under investigation. One can try to control this interaction, by observing the measuring bodies themselves, but in that case those measuring bodies themselves become part of the system being observed, and the additional measuring bodies introduced will again have an uncontrollable interaction with the system.

So the point is, "what is happening to the system" cannot be separated from the question of the observation of "what is happening to the system"

 

[Fredrik]

I would like to mention this again, since it is a key point in the copenhagen interpretation (a term I reserve for Bohr's views alone)

"What is actually happening to the system" is clearly a meaningful concept as long as the behavior of the system can be observed without the act of observing the system influencing the system.

But in quantum mechanics, the existence of the quantum of action h implies that there is a lower limit to the interaction between the measuring bodies and the objects under investigation. One can try to control this interaction, by observing the measuring bodies themselves, but in that case those measuring bodies themselves become part of the system being observed, and the additional measuring bodies introduced will again have an uncontrollable interaction with the system.

So the point is, "what is happening to the system" cannot be separated from the question of the observation of "what is happening to the system"

QM says that any isolated system (or at least any isolated system that can be made to interact with a measuring device) has a state that changes with time as described by the Schrödinger equation. I would say that the main purpose of an interpretation of QM is to provide a possible answer to the question of what is actually happening to an isolated system, while it's still isolated.

Isn't what you're suggesting, and what those Bohr quotes in post #15 are suggesting, only that a measurement breaks the isolation? What is happening to to the system during the measurement can't be separated from the observation of what's happening to the system. I don't think this is relevant to what I had in mind, because when I was talking about the question of whether QM describes "what's actually happening" to a system, what I had in mind was an isolated system.