Can Quantum Computers Validate the Many-Worlds Interpretation?

[sokrates]

Technically Hurkyl is right. But let me ask him, why do we give equal rights to CI and MWI but not to SR and LET?

No he isn't.

And we do not give equal rights to any of them because they are different.

Currently, there's only heuristic evidence -- in the future, there will be an exact branching to one of these (or even a newer one)

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 ?

Considering the current difficulties in distinguishing them, scientists are being cautious and are not ruling out any of these,

but just because some wiki article treats them on equal footing DOESN'T mean they are equal or not being tested.

 

[Dmitry67]

I am MWI fan :)
AFAIK Hurkyl prefers it too. You see, we are all on the same side!

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.

 

[Fredrik]

Technically Hurkyl is right. But let me ask him, why do we give equal rights to CI and MWI but not to SR and LET?

Because special relativity is easier to work with than Lorentz (a)ether theory, and easier to generalize to a theory of gravity.

(I don't know Lorentz's theory. I'm assuming that their predictions are the same, just because that claim has been made in this forum without getting shot down).

 

[sokrates]

Did Bohr and Heisenberg actually say that the "collapse" in their interpretation must be exact? I mean, why would anyone actually think that it is? They were certainly smart enough to realize that if the component parts of the measuring device obey QM, the whole thing can't be completely classical.

Fredrik, there's historical evidence that they thought exactly the way you described above.

Murray Gell-Mann, in his book ( Quark and the Jaguar ) relates this to the fact that the founders of QM were coming from a classical physics school and they just could not comprehend the fact that the universe is governed by QM and QM only...

In fact, Bohr thought there must be (invariably) a classical object around when making a measurement. These ideas might seem weird, you even go as far as saying "Why would anyone actually think this is the case?"... But they did. The best minds of the worl d at the time thought that.

But we know that QM governs the entire universe now, thanks to the recent developments culminating in the works of Zurek, Hartle, Everett, Gell-Mann and many others.

Founders of QM were very smart people but they didn't "certainly realize" that as you say.

And wavefunction collapse; as ugly and as alien as it seemed, survived all the way up to these days. Because nobody had a better explanation, so these discussions must have been made in thousands of equivalent "physics" forums before. The good news is, the "collapse" is reaching its "collapse". (with MWI or something else!)

A few years from now, these ideas will be flowing in the mainstream layman forums and people will not choose their favorite interpretation based on their taste.

So yes, MWI may not be the correct replacement, but CI is definitely not "exact"

And no interpretation is treated equally, at least in scientific circles. People know that there are problems with CI and they are working on clearing those up.

Assuming that they are all equal is naive. Why so?

 

[Fredrik]

Fredrk, there's historical evidence that they thought exactly the way you described above.

I assume that you meant that they thought a measuring apparatus is completely classical. That's more consistent with what you said next. (If it wasn't for that I would have interpreted the text I just quoted in the exact opposite way).

In fact, Bohr thought there must be (invariably) a classical object around when making a measurement. These ideas might seem weird, you even go as far as saying "Why would anyone actually think this is the case?"

I think it's clear that there must be an object around that for all practical purposes is classical. A measurement is an interaction that entangles the eigenstates of the measured system with macroscopically distinguishable states of the f.a.p.p. classical system.

And no interpretation is treated equally, at least in scientific circles. People know that there are problems with CI and they are working on clearing those up.

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.

 

[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).