Can Quantum Computers Validate the Many-Worlds Interpretation?

[Dmitry67]

Einstein would be probably happy to know that God does not play dice - at least in MWI...

 

[Dmitry67]

sokrates: but there is one small but very important piece of the puzzle missing. I am almost happy with how MWI explains why we see the world around us as classical. Almost, because we need to understand what the qualia is ( http://en.wikipedia.org/wiki/Qualia ). Qualia defines the preferred (and in fact, the only important) basis for the Quantum Decoherence.

 

[Fredrik]

sokrates: but there is one small but very important piece of the puzzle missing. I am almost happy with how MWI explains why we see the world around us as classical. Almost, because we need to understand what the qualia is ( http://en.wikipedia.org/wiki/Qualia ). Qualia defines the preferred (and in fact, the only important) basis for the Quantum Decoherence.

Huh? There's no evidence for the existence of qualia, and the "preferred basis" chosen by decoherence is the one that consists of eigenvectors of the observable being measured and position "eigenstates" of the measuring device. A measurement of $J_z$ of an electron is an interaction that (almost) diagonalizes the density operator of the "electron+apparatus" system in the $\{|m\rangle\otimes|x\rangle\}$ basis. It is in that specific sense, and in that sense only, that this basis is "preferred".

(Yes, I'm simplifying a lot because the whole story is really complicated and I don't know all the details).

 

[Hurkyl]

thus has no bearing on your previous claim that empirical evidence favors MWI over CI.

I never claimed that. You wanted to see it that way. My point is crystal clear.

I refer you to

Sure -- and these experiments support MWI in favor of non-quantum theories. But they have nothing to say regarding whether MWI should be favored over CI. (or Bohm, or RQM, or ...)

...

They do say MWI should be favored over CI.

But things have now been cleared up -- we are in agreement that there is no empirical evidence that favors MWI over CI. (right?)

 

[ThomasT]

You are asking the wrong question, the question is:

Why should we give equal rights to all the interpretations considering that they are predicting entirely different physical mechanisms ?

Because they predict the same instrumental behavior, and because there are reasons to think that MWI and deBB, to name two, are not qualitatively accurate descriptions of the real world apart from their ability to duplicate the instrumental predictions of standard QM?

But the sad truth is that we don't have anything to disprove CI - not for us - we all know that it is insanity: conscious observers, knowledget et cetera - but for those who still think in CI terms.

What exactly is insane about an instrumentalist interpretation of QM?

Quantum wavefunctions do contain what is quantitatively known about the systems they describe, don't they? The uncertainty relations and the Born rule do hold experimentally, don't they? You do have to communicate experimental setups and designs in terms of manipulating macroscopic objects, don't you? If so, then, apart from some speculative assumptions and modeling regarding an underlying reality, you are affirming the most important elements of the same instrumentalist, probabilistic interpretation that the MWI purports to obviate. Or so it seems to me.

QM tells us, within certain statistical limits predicted by the theory, how instruments are going to behave.

Obviously, there's more to be learned about electron, photon, etc. emissions and their interaction with measuring devices. And, from the instrumental behavior, it seems reasonable to infer certain things about the reality of the underlying processes.

But it doesn't seem to me that MWI or any other of the more or less exotic interpretations of quantum theory are good approaches to solving the real measurement problem, which goes far beyond whether quantum wavefunctions are a close qualitative approximation of the reality underlying instrumental behavior or whether some underlying waveform, apparently propagating from emitter to detector, is completely or approximately, or not at all, 'collapsing' during interaction with various obstacles. I don't see MWI contributing in any physically meaningful way to technological advances which might help to clarify the vagueness surrounding what might or might not be inferred about an underlying reality.

In fact, Bohr thought there must be (invariably) a classical object around when making a measurement.

He thought that the only way we could know if an interaction had occurred was if we had some objective (publicly verifiable) record of it. Doing science does require the generation and processing of data, doesn't it?

But we know that QM governs the entire universe now...

In what sense does QM govern the entire universe? How could we possibly know that?

 

[Dmitry67]

1 What exactly is insane about an instrumentalist interpretation of QM?

2 QM tells us, within certain statistical limits predicted by the theory, how instruments are going to behave.

3 Obviously, there's more to be learned about electron, photon, etc. emissions and their interaction with measuring devices. And, from the instrumental behavior, it seems reasonable to infer certain things about the reality of the underlying processes.

2,3 yes, if you want to use QM as a practical tool then it is ok: we apply one rules to QM world and another rules to the macroscopic world, our measurement devices.

So it valid in the same sense as epicycles are valid: they gave correct predictions for the positions of the planets.

1 CI defines the behavior of QM particles based on the high level, macroscopic thing called 'observer's knowledge'. But observer itself is a QM system. So the picture is deadly recursive.

 

[ThomasT]

2,3 yes, if you want to use QM as a practical tool then it is ok: we apply one rules to QM world and another rules to the macroscopic world, our measurement devices.

So it valid in the same sense as epicycles are valid: they gave correct predictions for the positions of the planets.

Yes, and we have reasons to believe that both epicycles and quantum states aren't qualitatively accurate descriptions of the real world, don't we?

Which interpretation makes more sense (seems more sane), one (MWI) that takes a 'superposition' such as, say, a detection both being recorded and not being recorded at a certain location during a certain time interval as a description of 'reality' (and 'explains' the fact that we always only see one or the other by way of the 'uncollapsed' wavefunction producing the alternate result in some parallel 'reality'), or one (CI) that takes this 'superposition' to mean simply that we will observe one or the other instrumental possibility during that time interval?

It seems to me that MWI has, in effect, elevated QM's 'epicycles' to the level of reality.

1 CI defines the behavior of QM particles based on the high level, macroscopic thing called 'observer's knowledge'. But observer itself is a QM system. So the picture is deadly recursive.

You only have the problems that MWI purports to solve if you make unwarranted assumptions about the reality of quantum wavefunctions. CI makes no such assumptions, so it seems to me to be the saner interpretation.

 

[Count Iblis]

What is this "reason" why quantum mechanics would not be an accurate description of the real world? What experiment contradicts quantum mechanics?

 

[sokrates]

Because they predict the same instrumental behavior, and because there are reasons to think that MWI and deBB, to name two, are not qualitatively accurate descriptions of the real world apart from their ability to duplicate the instrumental predictions of standard QM?

Could you elaborate more on the definition of a 'qualitatively accurate' description of the real world?

I think proponents of Copenhagen Interpretation have been vehemently defending the idea that any theory is qualitatively equivalent and accurate ---as long as --- it can replicate all the quantitative predictions of another theory.

The important argument here is 'simplicity'. And the experimental setups that could amplify the nuances between the interpretations.

Which one is simpler and more robust is the question here.

In what sense does QM govern the entire universe? How could we possibly know that?

In physical sense.

As far as we know, QM is "exactly" correct. Unlike any of its predecessors. And surely, We do not need a classical world or any classical instrument to describe QM. That's what's wrong with the instrumentalist approach. It's ridiculously anthropocentric. What is measurement?

Let me stop here and refer you to the following paper:

http://prola.aps.org/abstract/PRD/v28/i12/p2960_1"

 

[sokrates]

I refer you to




But things have now been cleared up -- we are in agreement that there is no empirical evidence that favors MWI over CI. (right?)

As I have stated a number of times, I reject the idea that comparisons between different theories rely only on empirical evidence.

If you remember the debate was not about the predictive powers of different interpretations, rather it was about which one could be "favored" over the other.

"Simplicity" and "Beauty" in physics have been used as a rule of thumb for selecting the "better" option from time immemorial. In this context, MWI can and , believe it or not , is favored over CI because it is SIMPLER.

Ignoring these obvious differences is serious denial, at least, in my view.

 

[Hurkyl]

Epicycles are reality -- any motion whatsoever can be perfectly described by epicycles. Their only drawback is they have essentially no predictive power.

Which interpretation makes more sense (seems more sane),

Science is not constrained by your personal biases.

You only have the problems that MWI purports to solve if you make unwarranted assumptions about the reality of quantum wavefunctions.

What's unwarranted? What tests have unitary evolution failed? The clash with GR aside, TMK the only real argument against the reality of quantum wavefunctions lost pretty much its entire foundation with the discovery of decoherence.

CI makes no such assumptions, so it seems to me to be the saner interpretation.

Yes it does. If it didn't, it wouldn't be able to say anything about reality.

All scientific theories come with the caveat "this is a theory with a certain level of empirical support, and may eventually be replaced with a new theory with more support".

Unless you're in the habit of rejecting the reality of anything that any scientific theory has to say about anything, I don't see how you can consider it "sane" to reject what quantum mechanics has to say about reality.

(And even if you are in such a habit, it is incredibly misleading to argue as if you're criticizing MWI specifically)

 

[sokrates]

Only the version with an exact collapse has problem, so the easiest way to clean it up is to replace the assumption of exact collapse with an assumption of approximate collapse.

What is approximate collapse?

Are you saying that CI could easily get away by making a small modification?

Does this mean that the celebrated Measurement Problem is resolved?

http://en.wikipedia.org/wiki/Measurement_problem"

See the first sentence in the article.

 

[Hurkyl]

As I have stated a number of times...

Why do you have such a problem saying:

Yes, I agree with you: there is no empirical evidence that favors MWI over CI.​

?

I want to establish this as a point of fact. No red herrings, no diversions, no anything -- just say "yes, I agree", or say "no, I disagree: _______ is a piece of empirical evidence that does favor MWI over CI".

Don't worry -- you are allowed favor MWI over CI and still admit to this fact.

 

[Hurkyl]

Huh? There's no evidence for the existence of qualia, and the "preferred basis" chosen by decoherence is the one that consists of eigenvectors of the observable being measured and position "eigenstates" of the measuring device.

Think of the kinetic theory of gasses for a moment.

We know, from physical experiment, that ideal gases satisfy laws like PV=nRT.

We know the kinematic properties of individual gas atoms.

We can then hypothesize that the observed macroscopic properties of gasses are consequences of the known microscopic behaviors of gas atoms.

Under this hypothesis, some things seem clear, such as pressure being caused by atoms bouncing against the container.

But it takes some more work to demonstrate that the hypothesis really is valid.


My vague understanding is that apparently classical macroscopic behavior being a consequence of microscopic quantum behavior is analogous. The discovery of relative states and decoherence killed the argument that this couldn't happen, but there is still a long way to go to demonstrate that this really does happen.

 

[sokrates]

Yes, I agree with you: there is no empirical evidence that favors MWI over CI.

No problem. It's already implied in what I say.

It's just not the important point for me.

Why do you have a problem saying:

Yes, I agree that empirical evidence is NOT the only criterion in selecting the more favorable theory.

 

[Count Iblis]

History matters in physics. If Bohr had proposed the MWI, it would have been the other way around (it probably is in a parallel world).

 

[Hurkyl]

Why do you have a problem saying:

Yes, I agree that empirical evidence is NOT the only criterion in selecting the more favorable theory.

Yes, I do agree.

I don't have a problem saying it, but you never asked...

 

[Ilja]

IF a new theory simplifies and/or removes the assumptions of an older theory, while still making ALL the predictions of the old theory, then scientific method replaces the old theory with the new one.

Tutorial:

new theory = decoherence
old theory = Copenhagen

extra assumptions/specifications in old theory = wavefunction collapse

Could somebody give a reasonable explanation for that under Copenhagen? = No.

Could decoherence do that? = Yes.

Decoherence cannot do anything. Decoherence is a technique applicable if the whole quantum theory is already defined. Moreover, it needs a decomposition into systems to start.

Thus, any interpretation based on decoherence has to define some additional structure.

http://arxiv.org/abs/arXiv:0903.4657"

 

[Count Iblis]

Decoherence cannot do anything. Decoherence is a technique applicable if the whole quantum theory is already defined. Moreover, it needs a decomposition into systems to start.

Thus, any interpretation based on decoherence has to define some additional structure.

http://arxiv.org/abs/arXiv:0903.4657"

Section 7, "What is wrong with the Many Worlds solution",

is not so clear to me.

Is the physics in the different worlds described by the same theory as observed by observers really different in realistic models of the universe?

 

[Dmitry67]

Moreover, it needs a decomposition into systems to start.

Decoherence answers a question: how the quantum reality around system X is observed by that given system X? In other words, X is a parameter.

So yes, QD requires a decomposition, but it is not a problem. It is just a parameter. You don't complain that you can't calculate sin without providing the value of x, right?

If you don't want to make a decomposition, then you can use pure QM and enjoy the untary evolution of the wavefunction of the universe. But at the moment you ask 'but why I don't see both cats?' you make a decomposition of the whole universe into YOU as an observer, a CAT and a BOX.

 

[Dmitry67]

Thesis 2. Decoherence does not allow the derivation of the classical limit without an additional physical structure | a special decomposition into systems | which has to be defined independently by the quantum theory. This additional structure is physically important, different choices define different physics.

So the quoted from the article is absolutely irrelevant.

 

[sokrates]

Decoherence cannot do anything ...

Very convincing and insightful.

 

[ThomasT]

I think proponents of Copenhagen Interpretation have been vehemently defending the idea that any theory is qualitatively equivalent and accurate ---as long as --- it can replicate all the quantitative predictions of another theory.

Because the only qualitative reality that you can unambiguously demonstrate and communicate is at the level of instrumental behavior. The more or less 'realistic' reformulations of standard QM, as well as standard QM itself, all contain mechanisms or objects which have no apparent physical meaning apart from their existence as elements of the mathematical formalism. CI is a different sort of interpretation in that it represents an effort to say all that can be said about the physical meaning and implications of the QM formalism wrt the experimental phenomena.

The important argument here is 'simplicity'. And the experimental setups that could amplify the nuances between the interpretations.

Which one is simpler and more robust is the question here.

Insofar as CI isn't a theory about a reality underlying instrumental behavior, but rather seeks only to clarify the physical meaning and implications of an existing formalism and associated experiments, then it isn't competing with MWI or deBB or any other 'realistic' alternative to bare bones QM.

As far as we know, QM is "exactly" correct.

Which is to say that we don't know how closely it approximates the underlying reality. It does of course produce very accurate statistical averages for large data sets. But then so does regular probability theory wrt a set of 'random' dice. You wouldn't consider that to be a 'description' of what's 'really' happening, would you? So, in what sense is QM a description of what's 'really' happening. Only insofar as it accurately predicts statistical averages.

I believe that the Born rule can be derived, ie., would be evident, in a realistic wave mechanical approach. However, insofar as any so called 'realistic' alternative to standard QM is still using the nuts and bolts of standard QM to calculate predictions (like the various MWIs), then it's just another exotic probability theory and not really so realistic at all.

And surely, We do not need a classical world or any classical instrument to describe QM.

Ok. Then how do you want to go about communicating? For that matter, how would you go about ascertaining whether, or how closely, your theory corresponded to reality?

That's what's wrong with the instrumentalist approach. It's ridiculously anthropocentric. What is measurement?

What about those "experimental setups that could amplify the nuances between the interpretations"?

The measurement process isn't entirely well understood. And yes everybody wants a deterministic accounting of the underlying reality of it. But I think it's very misleading, very obfuscating to talk about what's happening in terms of other worlds or universes. Obviously, quantum disturbances impinging on detectors produce changes in the detectors and the incident quantum disturbance is irreversibly modified. The splitting, branching of MWIs, decoherence, etc. is far to simplistic.

Let me stop here and refer you to the following paper:

Wavefunction of the Universe

Thanks for the link. I like some sort of wave approach. It's one approach among many to modeling our universe. Maybe it accurately describes some aspect(s) of our universe in some simplistic way. (Of course we won't know unless we make some measurements.) It's a huge stretch from this paper to saying that QM 'governs' the entire universe. But I'll agree with you in that I believe that the deep reality does have to do with wave behavior.

 

[ThomasT]

Epicycles are reality -- any motion whatsoever can be perfectly described by epicycles. Their only drawback is they have essentially no predictive power.

We think a bit differently about this then.

Science is not constrained by your personal biases.

Nor yours it seems. But one can hope, eh?

What's unwarranted?

The assumption that a quantum wavefunction describing the probabilities of possible instrumental configurations is in, or close to, a one to one correspondence with the evolution of a quantum disturbance propagating from emitter to detector in an experimental setup that the quantum wavefunction is associated with. And the further assumption that the so endowed quantum wavefunction isn't altered in some physically intuitive way vis interaction with the detection obstacle but rather branches in a way which leads to all of the instrumental possibilities for any trial actually happening in that trial. But we only see one instrumental possibility per trial actualized -- which of course leads to the only logical conclusion that the other possible results must have happened in other universes.

I believe that QM (along with other things) gives us good reasons to assume that Nature is fundamentally waves in a hierarchy of media. But I'm pretty sure that this 'picture' doesn't necessarily lead to an infinitude of virtual universes or virtual worlds in our universe. Can't the branching be thought of, and modeled as, a simplification of the complex wave interaction that's occurring in, and only in, our universe without the need for other universes to 'explain' why we don't see all the results possible for a given trial? If not, then I would suggest that a different approach is called for.

There are reasons to believe that other universes are possible, even highly probable. But, these are cosmological, and of course highly speculative anyway. I don't think that MWI provides the reason for, or any indication of, their existence. I can understand how proponents of MWI might get excited about the idea that they're on to something really heavy. But they aren't, at least not as I understand it.

What tests have unitary evolution failed?

Since the results of the individual trials are random, it seems that the description of the system vis evolution in unitary space is somewhat at odds (pun intended - really ) with reality.

The clash with GR aside, TMK the only real argument against the reality of quantum wavefunctions lost pretty much its entire foundation with the discovery of decoherence.

We can all agree that there's something moving from emitter to detector, and that it has wavelike characteristics. Then again, it also has particlelike characteristics. Depending on the setup. There's the emission and filtration and detection materials and settings. Lots of models. The measurement problem is that there isn't a definitive description of what's going on when the s**t hits the fan, so to speak. Decoherence doesn't solve the problem. So I don't understand why you think it affirms the 'reality' of quantum wavefunctions.

Sure, in some way, they must, it seems, correspond to what's happening in the underlying reality. But exactly how and to what extent is still a mystery. This is what I mean when I say that MWI makes an unwarranted assumption about the wavefunction and CI doesn't.

Yes it does. If it didn't, it wouldn't be able to say anything about reality.

It says what can be said from the experimental evidence

Unless you're in the habit of rejecting the reality of anything that any scientific theory has to say about anything, I don't see how you can consider it "sane" to reject what quantum mechanics has to say about reality.

What MWIers say that QM says about reality isn't what CIers say, or I think, that QM says about reality. The way I read, and insofar as I have read, the extant experimental evidence, MWI isn't supported by it. So, the way I see it, proponents of MWI are rejecting what QM and observations have to say about reality.

Remember, you're the one who equated epicyles with reality.

(And even if you are in such a habit, it is incredibly misleading to argue as if you're criticizing MWI specifically)

I am criticizing MWI specifically. I've learned some things from this thread, but I think that MWI, as an approach to a better theory or better understanding of standard QM or the real world, is pretty much a waste of time.

Of course I might be wrong, so I'll continue to read up on MWI as time permits (I've compiled a list of more than 30 articles from major journals on the various MWIs), and any thoughtful criticisms of anything I've said are always welcomed as I feel sure that you and other posters in this thread know more of MWI than I do.

 

[Dmitry67]

If you don't believe in other branches you need to provide (and prove) some branch-cutting mechanism, like wavefunction collapse in CI. Or particles in BM which go into some waves, leaving other waves empty (as I understand it).

 

[ThomasT]

If you don't believe in other branches you need to provide (and prove) some branch-cutting mechanism, like wavefunction collapse in CI. Or particles in BM which go into some waves, leaving other waves empty (as I understand it).

Only if I assume that these other branches actually exist, or are describing reality -- which I don't.

A realistic description is going to require a somewhat different and more detailed approach than this virtual branching stuff, imo.

 

[Dmitry67]

Other branches exist for the very same reason. Because they are not different from the branch we observe.

Tell me, when is more logical:
1. To expect that space exists beyond what we call our cosmological Horizon (say, 100Billions ly away) because we don't expect that far from us there is something fundamentally different;
2. To claim that the existence of the space beyond our Hubble volume can not be proved, hence, it is logical to assume that there is nothing there.

 

[ThomasT]

Other branches exist for the very same reason. Because they are not different from the branch we observe.

Tell me, when is more logical:
1. To expect that space exists beyond what we call our cosmological Horizon (say, 100Billions ly away) because we don't expect that far from us there is something fundamentally different;
2. To claim that the existence of the space beyond our Hubble volume can not be proved, hence, it is logical to assume that there is nothing there.

We believe that our universe extends beyond the cosmo horizon because our ability to see farther and farther, ie. see more and different stuff, has increased.

In constrast, we have zero ability to see the parallel worlds of MWI. They exist as a fantasy interpretation of standard QM. This branching or splitting into parallel worlds is an unsatisfactory explanation (actually it's no explanation at all) for how the recorded instrumental results are generated vis an underlying reality (ie., vis a realistic description).

The MWI approach to a realistic theory is doomed because it's born out of (pun intended) a mostly non-realistic, statistical theory.

 

[Count Iblis]

Actually, it is precisely the unitary time evolution that restricts the way information can flow. Theories that postulate non-unitary time evolution are potentially vulnerable to making strange predictions like being able to observe what happens in another parallel world.

You cannot rule out that if CI turns out to be correct you could build a quantum computer and then control the non-unitary effects such that the thought experiment in my OP could be modified such that the observer now can have knowledge of what the spin was in the two branches.

 

[Dmitry67]

We believe that our universe extends beyond the cosmo horizon because our ability to see farther and farther, ie. see more and different stuff, has increased.

So, how much did it increase during our lives? :)
No, no, comsologists do not need to wait billion years to be sure how universe looks like in other Hubble spaces. So what you are saying is right, but please admit, it plays NO role.

 

[Fredrik]

If you don't believe in other branches you need to provide (and prove) some branch-cutting mechanism, like wavefunction collapse in CI. Or particles in BM which go into some waves, leaving other waves empty (as I understand it).

Other branches exist for the very same reason. Because they are not different from the branch we observe.

As I pointed out in #30, there are two possible intepretations of a density matrix. (Either it represents an ensemble, or it represents a single system in a specific but unknown state). If your claim is that the fact that "system+environment" is in a mixed state after a measurement implies the existence of other worlds (because there's no difference between the terms that represent reality and the other terms), then you're making a non sequiteur. There's no valid reason to assume that the mixed state can only be interpreted as an ensemble. It can also be interpreted as a specific but unknown state of a single system.

Another possibility is that Hilbert spaces don't actually describe reality, and are nothing more than convenient mathematical tools used in calculations of probabilities of possible results of experiments.

What you said about what's outside of the visible region of the universe is very different, because the theories we have actually predict unambiguously that there is something out there, so if there's nothing there, it would invalidate the theory. (We would never find out, but that's another matter). QM doesn't predict the existence of many worlds, at least not unambiguously, since there are at least two other possibilities.

 

[Hurkyl]

But we only see one instrumental possibility per trial actualized

MWI agrees that we only see one "instrumental possibility per trial actualized".

which of course leads to the only logical conclusion that the other possible results must have happened in other universes.

MWI says, given that we saw result X, that result Y didn't happen.


The point you're missing is that you keep trying to turn these conditional statements into absolute ones. It is physically impossible (for internal observers) to differentiate between a universe of definite outcomes and a universe of indefinite outcomes.

If we've seen result X, it is impossible to empirically test whether or not result Y happened -- the only thing we can now test is whether or not Y happened given that we've already seen result X.

Sure, we can always change "reference frames"^* to switch our physical description of the system from one where the result is indeterminate to one where the result is determinate if we so desire -- but that's a very different thing than insisting there's some physical mechanism that forces the universe to be in that particular reference frame.

*: I am not sure if this is actually a feature of MWI. But it is definitely a philosophical position I support.

I believe that QM (along with other things) gives us good reasons to assume that Nature is fundamentally waves in a hierarchy of media. But I'm pretty sure that this 'picture' doesn't necessarily lead to an infinitude of virtual universes or virtual worlds in our universe.

"Many worlds" is what Schrödinger's equation says happens... Heck, even classical waves have superpositions and what-not.

There are reasons to believe that other universes are possible, even highly probable. But, these are cosmological, and of course highly speculative anyway. I don't think that MWI provides the reason for, or any indication of, their existence.

MWI has absolutely nothing to do with "cosmological universes". (Or, at least what I understand that term to mean)

Since the results of the individual trials are random, it seems that the description of the system vis evolution in unitary space is somewhat at odds (pun intended - really ) with reality.

Don't forget that probabilities naturally deal with indefinite outcomes. It takes a lot of jumping through hoops to reconsile probability theory with having definite outcomes.

We can all agree that there's something moving from emitter to detector, and that it has wavelike characteristics. Then again, it also has particlelike characteristics.

It has wave-function like characteristics, always. Some situations approximate classical waves. Some situations approximate classical particles. But only approximately.

Decoherence doesn't solve the problem. So I don't understand why you think it affirms the 'reality' of quantum wavefunctions.

What relative states solved is how quantum wavefunctions evolving unitarily could be physically indistinguish from a collapsed state. What decoherence proved that wavefunctions (rapidly) tend to such situations.

Thus, quantum states evolving unitarily is known to yield (approximately) classical behavior as an emergent property. The only remaining question is whether or not it yields the right (approximately) classical behavior.

It says what can be said from the experimental evidence

There is no experimental evidence of definite outcomes. There cannot be. Yet, CI insists upon it.

 

[sokrates]

MWI agrees

MWI has absolutely nothing to do with "cosmological universes". (Or, at least what I understand that term to mean)

There is a somewhat vague connection proposed by Tegmark.

http://space.mit.edu/home/tegmark/multiverse.pdf"

If you are interested...

 

[ThomasT]

MWI says, given that we saw result X, that result Y didn't happen.

MWI is no different, in this respect, than the standard way of looking at it.

The point you're missing is that you keep trying to turn these conditional statements into absolute ones. It is physically impossible (for internal observers) to differentiate between a universe of
definite outcomes and a universe of indefinite outcomes.

The universe of our perception, the universe of experiments and statistics, is the universe of definite outcomes. From the organization of the universe as it
reveals itself to us vis physical science we can infer some things about the underlying reality.

If we've seen result X, it is impossible to empirically test whether or not result Y happened ...

If X and Y are mutually exclusive results of the same experimental trial, then yes.

... the only thing we can now test is whether or not Y happened given that we've already seen result X.

No, not if X and Y are mutually exclusive results of the same trial.

It seems like you're thinking of probability distributions as descriptions of reality.

Sure, we can always change "reference frames"^* to switch our physical description of the system from one where the result is indeterminate to one where the result is determinate if we so desire -- but that's a very different thing than insisting there's some physical mechanism that forces the universe to be in that particular reference frame.

I'm not sure what you mean by switching reference
frames.

Are you talking about the underlying physical mechanisms as one frame of reference and the definite outcomes of our experience as another?

Afaik, the assumption is that there are underlying dynamics determining the definite outcomes of our experience. Exactly what those dynamics are is still an open question. They're somewhat different for different experimental preparations.

"Many worlds" is what Schrödinger's equation says happens ...

There's no physical basis for that 'interpretation'. The 'many worlds' are just the mutually exclusive, possible instrumental configurations at the end of each trial. So, the 'many worlds' terminology is somewhat misleading regarding what's known, and what should be inferred about the underlying reality from that.

... Heck, even classical waves have superpositions and what-not.

Of course, we can actually see wave superpositions in various media. And, afaik, and along with you I think,
there's no obvious reason to assume that waves in the reality underlying our perception, in media that we can't see, are governed by essentially different mechanics than waves in media that we can see. But the 'superpositions' of instrumental level configurations vis QM probability distributions aren't themselves descriptions of underlying wave interactions. Part of the confusion is due to the close relationship between probability theory and the mechanics of physical waves. The unitarity of the wave
equation is due to the requirements of probability, isn't it?

Don't forget that probabilities naturally deal with indefinite outcomes. It takes a lot of jumping through hoops to reconsile probability theory with having definite outcomes.

No it doesn't. Just roll some dice.

It has wave-function like characteristics, always. Some situations approximate classical waves. Some situations approximate classical particles. But only approximately.

Both
particles and waves exist. Assuming that waves are fundamental, then the measurement problem is how do particulate structures and media arise in a universe that is fundamentally waves. That is, how do persistent, bound structures, or quantum protectorates, or higher organizing principles which seem to be independent of a fundamental wave dynamic, or basketballs, or toasters, or individual data bits emerge? The Schrodinger equation isn't the solution to this problem.

There are lots of hints from everyday experience about how this happens. Remember those videos on YouTube? Suppose you put some sand on a drum head and set the drum to vibrating at some frequency. The sand assumes specific configurations depending on the vibrational frequencies. We can't see it but we know that there's wave interactions, superpositions happening in the drum head, the drum, and the air in the drum media that produce those static and persistent configurations in the sand medium.

Of course, reality is quite a bit more complex, and the various media aren't just interfacing with each other, they're interspersed.

What relative states solved is how quantum wavefunctions evolving unitarily could be physically indistinguish from a collapsed state. What decoherence proved that wavefunctions (rapidly)
tend to such situations.

Thus, quantum states evolving unitarily is known to yield (approximately) classical behavior as an emergent property. The only remaining question is whether or not it yields the right (approximately) classical
behavior.

The unitarity has to do with the probabilities. The probabilities have to do with the behavior of instruments, ie., an accounting of definite results amenable to our senses without an associated description of the underlying dynamics, and the hardware technology, precise enough to produce anything but random results for individual trials. The question(s) is(are) much deeper than that. And the answers to those questions, the solution to the real measurement problem will have to do with developing a more realistic fundamental conceptual approach. As a famous physicist (Robert Laughlin I think) once said, "Seeing is the beginning of understanding."

There is no experimental evidence of definite outcomes. There cannot be. Yet, CI insists upon it.

Actually, conventional usage insists on it vis the definition of 'definite outcomes' in statistics.

 

[Dmitry67]

There's no physical basis for that 'interpretation'. The 'many worlds' are just the mutually exclusive, possible instrumental configurations at the end of each trial.

1. What is a trial, in terms of QM? What configuration/interaction of particles is called a trial?
2. They are not absolutely mutually exclusive. Quantum decoherence is a gradual process, so you can actually study how 'other branch' is starting to go away... like when you driving on a highway and hit a fork, you see cars taking other root dissapearing from your sight... not immediately...