# Proper mix randomness

1. Dec 23, 2013

### kye

In decoherence.. it is said that it is in an improper mixed FAPP (for all purposes) which is the same as collapse. Are you saying that decoherence is deterministic and not random (probabilities)? Random or probabilities only occured in proper mix? Why. Is it not possible to calculate exactly the wavelets and waveforms of the wave functions to tell how exactly the decohering interference occur and from there determine why it is in certain state (deterministic)? Why not?

2. Dec 23, 2013

### Staff: Mentor

Its saying the mathematical form of the state of an improper mixture and a proper mixture is exactly the same, namely ∑ pi |bi><bi|, where the |bi> are eigenvectors of the observable corresponding to the observation being made. A proper mixture is the states |bi><bi| randomly selected and presented for observation. The state is there prior to observation, and since it is an eigenvector of the observable will not be changed by the observation. The probability of getting |bi> is the corresponding pi. Collapse never occurs - everything is honkey-dorey - QM - no issues. The state of an improper mixture is created by decoherence and is exactly the same. There is no way to tell the difference. If it was a proper mixture collapse would have actually occurred. That is what is meant by apparent collapse - it looks exactly the same - but really isn't.

Its all in the paper I have posted about it innumerable times:
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

See section 1.2.3

BTW the jig is up here - I cant explain the math in layman's terms - if the math is beyond you - sorry you will need to learn it to understand it.

Thanks
Bill

Last edited: Dec 23, 2013
3. Dec 23, 2013

### atyy

I prefer the terminology that decoherence solves the "pointer basis" problem.

What I find confusing about the "apparent collapse" language, especially the claim that it is interpretation free is that in the naive textbook interpretation, collapse is how definite outcomes are produced. Decoherence cannot produce definite outcomes, so without an interpretation to produce definite outcomes, decoherence is not equivalent to collapse for all practical purposes, since there are no definite outcomes. In contrast, in the many-worlds interpretation, there are definite outcomes without collapse, and there is only apparent collapse, so in the many-worlds interpretation it does seem that decoherence is as good as collapse for all practical purposes.

4. Dec 23, 2013

### Staff: Mentor

The standard formalism, which decoherence uses, produces definite outcomes so decoherence doesn't have to explain it.

Why it does that is the problem of definite outcomes - only some interpretations answer that eg MWI.

If you don't want to believe decoherence explains collapse FAPP, go ahead, you are not the only one. Its simply my and many others view.

Just to elaborate further, a proper mixed state has definite outcomes because its assumption is a particular outcome is randomly selected.

Thanks
Bill

Last edited: Dec 24, 2013
5. Dec 24, 2013

### kye

Hi Bill, is the paper shared above peer reviewed? It doesn't appear in arxiv (is it rejected.. why didn't it appear there)... how many percentage of physicists believe that paper is valid or how could one exclude possibility of author's bias or some subtle misconception?

6. Dec 24, 2013

### Staff: Mentor

It's, as it states, an essay reviewing standard stuff - its not really anything new - simply a write-up in one place.

It's contains the same stuff as Schlosshauer's standard textbook:
https://www.amazon.com/Decoherence-Classical-Transition-Frontiers-Collection/dp/3540357734

But if you are worried get Schlosshauer's textbook - its where I learnt it from.

Thanks
Bill

Last edited by a moderator: May 6, 2017
7. Dec 24, 2013

### kye

If Decoherence were true, why hadn't Bohr, Schroedinger, Born, Heisenberg, Pauli, von Neumann, Einstein thought of it? Why only Zurek? And why is there no Nobel Prize for Zurek (seriously) if he beat all of them?

And about the C60 or C70 Buckyball. Is there no possibility it could still be explained by collapse? Maybe regional/sectional collapse that we mistook for decoherence? Maybe Bohr could still explain it by collapse if he were alive today?

8. Dec 24, 2013

### atyy

Bas Hensen's essay http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf is basically sound. It is based largely on Schlosshauer's book https://www.amazon.com/Decoherence-...rs-Collection/dp/3540357734?tag=vglnk-c905-20 that bhobba linked to above. I haven't read Schlosshauer's book, but Hensen's essay seems consonant with Schlosshauer's article in Reviews of modern Physics http://arxiv.org/abs/quant-ph/0312059.

There is much superb work that is not recognized by a Nobel Prize. Also, other physicists have worked on decoherence. For example, it is used to explain why some molecules are chiral, for which Schlosshauer references Harris and Stodolsky's 1981 paper, and Zeh's 2000 paper.

Decoherence is undoubtedly correct, but it does not favour one interpretation over another. It is most powerful in the many-worlds interpretation, where it seems quite convincing in explaining the appearance of collapse. In the Copenhagen interpretation (by which I mean the naive textbook interpretation), decoherence does not remove the need for collapse, but it goes some way to explaining why a classical apparatus may be outside one Heisenberg cut (or von Neumann cut), but inside another.

Last edited by a moderator: May 6, 2017
9. Dec 24, 2013

### audioloop

of course, there are, objective reduction models.

http://plato.stanford.edu/entries/qm-collapse/

----
http://www.perimeterinstitute.ca/videos/recent-developments-collapse-models
"they explain why measurements always have definite outcomes, distributed according to the Born probability rule"

.

10. Dec 24, 2013

### kye

Remember Bohr was able to refute Einstein great attempt to bypass HUP by a complicated setup. Maybe objection reduction models can be converted into pure Bohr reasoning? How.. section by section collapse?.. or maybe Bohr explaining since collapse is only on paper and not actually there existing, the calculations can show collapse even for C60? How would Bohr explain the Buckyball experiments if he were alive today? Or there is not another human as incredibly genius as Bohr?

11. Dec 24, 2013

### atyy

The GRW theory is not equivalent to quantum mechanics. It is beyond the standard model, and predicts violations of quantum mechanics.

12. Dec 24, 2013

### Staff: Mentor

It wasn't only Zureck - there were many others.

It basically actually started with MW and developed from there.

It takes time for ideas to ferment and percolate.

Thanks
Bill

13. Dec 24, 2013

### Staff: Mentor

Well put.

And indeed decoherence really is very elegantly applied to MW. It is also extremely valuable in Consistent Histories leading to its alternate name - Decoherent Histories.

But there are people like Ballentine, while in no way denying decoherence, you cant really because the formalism implies it, believe its of zero value as far as interpretations go.

Thanks
Bill

14. Dec 24, 2013

### kye

Does decoherence disprove Bohr's division into quantum and classical world or does Bohr still have surprises up his sleeve? His objections are powerful. For example, he countered Einstein EPR argument by saying distances didn't matter because it was one big experiment setup. Could he explain the buckyball experiment by saying it didn't show otherwise and there is still divisions between the classical and quantum world simply because the wave function doesn't exist in the objects but just an operation we perform on paper, isn't this what also some still believed? Or do 100% of physicists agree that the buckyball experiment disprove Bohr's division of quantum and classical world?

15. Dec 24, 2013

### Staff: Mentor

There is no division - everything is quantum and this was Bohr's mistake:
http://scitation.aip.org/content/aip/magazine/physicstoday/article/58/11/10.1063/1.2155755 [Broken]

'Bohr’s version of quantum mechanics was deeply flawed, but not for the reason Einstein thought. The Copenhagen interpretation describes what happens when an observer makes a measurement, but the observer and the act of measurement are themselves treated classically. This is surely wrong: Physicists and their apparatus must be governed by the same quantum mechanical rules that govern everything else in the universe. But these rules are expressed in terms of a wavefunction (or, more precisely, a state vector) that evolves in a perfectly deterministic way. So where do the probabilistic rules of the Copenhagen interpretation come from?

Considerable progress has been made in recent years toward the resolution of the problem, which I cannot go into here. It is enough to say that neither Bohr nor Einstein had focused on the real problem with quantum mechanics. The Copenhagen rules clearly work, so they have to be accepted. But this leaves the task of explaining them by applying the deterministic equation for the evolution of the wavefunction, the Schrödinger equation, to observers and their apparatus. The difficulty is not that quantum mechanics is probabilistic—that is something we apparently just have to live with. The real difficulty is that it is also deterministic, or more precisely, that it combines a probabilistic interpretation with deterministic dynamics.'

Decoherence is a major part of that considerable progress.

Thanks
Bill

Last edited by a moderator: May 6, 2017
16. Dec 24, 2013

### kye

This assumes the states are real and there in the objects. But in Bohr's view. Everything is really classical, probability only came about in calculations and only occurred in ensembles. I talked with a physics professor. He laugh off the idea of interpretations. He said it's all about eigenvalues and eigenfunction and state vectors and this is what we can know about objects.

Can others who still support Bohr here share why Bohr may still be right? Note decoherence is inspired from Bohmian and Many Worlds and these two are trying to make the quantum understandable or conformed to the thoughts and bias of the human mind. Maybe everything is really still classical and probability and wave functions are just to compute probabilities of occurences. If this is true, does this refute decoherence?

Last edited by a moderator: May 6, 2017
17. Dec 24, 2013

### Staff: Mentor

That's got nothing to do with anything.

I think you need to understand Copenhagen and exactly what later interpretations like Consistent Histories did to fix up it's issues, which in the great scheme of things were in fact relatively minor.

Lubos, although for me he can come across as bit closed minded on occasion, although usually correct, and often interesting, explains this fairly well:
http://motls.blogspot.com.au/2011/05/copenhagen-interpretation-of-quantum.html

Thanks
Bill

18. Dec 24, 2013

### kye

I read the lubus paper and understood it fully. Bottomline is. Docoherence is only interesting for Bohmian and Many Worlds. It is not important for Copenhagen because since collapse is not real in the objects. May as well just accept Collapse. Decoherence just makes it more complicated. In the Buckyball experiment, we can say that collapse occur regionally for each interaction with the environment. Unless you are saying that this is not possible and the language of decoherence is required 100%?

19. Dec 24, 2013

### Staff: Mentor

That really has me perplexed:

Measurement devices are classical

The fifth rule says that the measurement devices follow the rules of classical physics. This is another source of misunderstandings.

The Copenhagen school surely didn't want to say that quantum mechanics couldn't be applied to large systems. Indeed, many people from the Copenhagen school were key researchers who helped to show that quantum mechanics works for large systems including molecules, bubble chambers, solids, and anything else you can think of.

Instead, this rule was meant as a phenomenological rule. If you measure something, you may assume that the apparatus behaves as a classical object. So in particular, you may assume that these classical objects - especially your brain, but you don't have to go up to your brain - won't ever evolve into unnatural superpositions of macroscopically distinct states.

Is that true? Is that a sign of a problem of the Copenhagen interpretation?

It is surely true. It's how the world works. However, one may also say that this was a point in which the Copenhagen interpretation was incomplete. They didn't quite understand decoherence - or at least, Bohr who probably "morally" understood what was going on failed in his attempts to comprehensibly and quantitatively describe what he "knew".

However, once we understand decoherence, we should view it as an explicit proof of this fifth principle of the Copenhagen interpretation. Decoherence shows that the states of macroscopic (or otherwise classical-like) objects whose probabilities are well-defined are exactly those that we could identify with the "classical states" - they're eigenstates of the density matrix. The corresponding eigenvalues - diagonal entries of the density matrix in the right basis - are the predicted probabilities.

Because the calculus of decoherence wasn't fully developed in the 1920s, the Copenhagen school couldn't have exactly identified the point at which the classical logic becomes applicable for large enough objects. However, they were saying that there is such a boundary at which the quantum subtleties may be forgotten for certain purposes and they were damn right. There is such a (fuzzy) boundary and we may calculate it with the decoherence calculus today. The loss of the information about the relative phase of the probability amplitudes between several basis vectors is the only new "thing" that occurs near the boundary.

Again, this point was the only principle of the Copenhagen interpretation that was arguably "incomplete" but their proposition was definitely right! To make this point complete, one didn't have to add anything new to quantum mechanics or distort it in any way. One only needs to make the right calculation of the evolution of the density matrix in a complicated setup. In that way, one proves that they always treated the measuring devices in the right way even though they couldn't fully formulate a fully quantum proof why it was the right way.

End of quote

Decoherence is needed in Copenhagen to fix up the issue it had with dividing the world into classical and quantum.

Copenhagen was basically correct, and in the great scheme of things it was minor, but it was a problem, and it needed fixing.

IMHO modern interpretations like Decoherent Histories and my Ignorance Ensemble Interpretation that incorporate it from the outset are better. But that's just a matter of elegance and personal taste - its got nothing to do with its validity.

Thanks
Bill

Last edited: Dec 24, 2013
20. Dec 24, 2013

### kye

The question is. Do 100% of physicists agree with Lubus? Or is it Lubus own thoughts or ideas?

If 100% of physicists agree with Lubus and Decoherence is totally true. Why not give Nobel Prize to Zurek, Zeh, Joos? 3 people can get Nobel for the same prize. In the Higgs. Only 2 or 3 get them inspite of having 8 people originally working on it. Likewise in decoherence, 2 or 3 can be nominated. Why hasn't this occurred? When this occurred, then I'd be convince Decoherence is fully accepted and not just some esoteric viewpoints of some physics elite. Maybe it's possible some just don't care about it that is why the Nobel committee is not persuaded with it?