The Emergence of Classicality -- Is Decoherence accepted among physicists?

[Question69]

TL;DR

Is Decoherence accepted among physicists?

I have tried to study decoherence theory for a while now, and it does seem to answer why classical distributions arise, instead of quantum ones.Is this fully accepted as an explanation for a classical world out of a quantum one?

 

[throw]

No it is not fully accepted: it is just another attempt to ignore the claim, which the founders of QM spent their lives arguing for, that ('Copenhagen') quantum mechanics does not make sense without the existence of classical mechanics.

 

[PeroK]

Summary:: Is Decoherence accepted among physicists?

I have tried to study decoherence theory for a while now, and it does seem to answer why classical distributions arise, instead of quantum ones.Is this fully accepted as an explanation for a classical world out of a quantum one?

I don't know. I had a quick look online, but I didn't find very much. Wikipedia devotes just one line to "criticism" of decoherence.

https://en.wikipedia.org/wiki/Quantum_decoherence#Criticism

 

[f95toli]

Please be a bit careful with the wording here.
The "technical" definition of decoherence (as in loss of coherence) is not at all controversial and the underlying physics isn't very complicated (it is at the heart of e.g. MRI/NMR). We also mostly understand WHY there is decoherence and mitigating decoherence in quantum devices/systems is in many cases just down to better engineering (in the case of e.g. quantum computing things like better filtering).
Hence, this part of not at all controversial

Now, whether this "explains" the emergence of the classical world (as well as the measurement problem etc) is -right now- largely down to which interpretation you prefer
Hence, the "foundational" aspect is a completely different -and only tangentially related- question.

 

[Nullstein]

Decoherence does explain how classical probability distributions arise from QM. It does not explain why they collapse upon measurement. For example, decoherence explains why you don't see an interference pattern anymore if you don't conduct a double slit experiment with electrons in a vacuum (You just see two overlapping blobs as expected classically).

It is debated how the collapse happens or even whether it requires explanation in the first place. In classical probability, one could just interpret it as Bayesian updating. In QM, it's not that simple.

 

[Question69]

Please be a bit careful with the wording here.
The "technical" definition of decoherence (as in loss of coherence) is not at all controversial and the underlying physics isn't very complicated (it is at the heart of e.g. MRI/NMR). We also mostly understand WHY there is decoherence and mitigating decoherence in quantum devices/systems is in many cases of just down to better engineering (in the case of e.g. quantum computing things like better filtering).
Hence, this part of not at all controversial

Now, whether this "explains" the emergence of the classical world (as well as the measurement problem etc) is -right now- largely down to which interpretation you prefer
Hence, the "foundational" aspect is a completely different -and only tangentially related- question.

According to Dieter Zeh, the founder of decoherence, decoherence was largely started to support Everett's proposal of the splitting of worlds.

 

[Nugatory]

According to Dieter Zeh, the founder of decoherence, decoherence was largely started to support Everett's proposal of the splitting of worlds.

Do you have a citation/link for this claim? I’m not asking because I disagree, I’m asking because it’s a good habit (and often expected here) to provide them.

And with that said, yes, decoherence may resolve one of the issues with MWI, the “preferred basis problem”: the wave function splits, but why does it always split in ways that correspond to classical outcomes?

 

[f95toli]

According to Dieter Zeh, the founder of decoherence, decoherence was largely started to support Everett's proposal of the splitting of worlds.

Again, be careful about the meaning of the words here. I must admit I've never heard of Zeh but according to his wiki page he was born in 1932 and was working on QM Foundations; this is NOT how the word is usually used.

Decoherence in the "technical" sense is a much older concept, e.g. the Bloch equations were introduced in 1946. While these are in a sense phenomenological and -initially- focused on spin systems there are much closer to how the word "decoherence" is used among working physicists (i.e. T1 and T2 times and/or transverse and longitudinal relaxation).

 

[Question69]

Do you have a citation/link for this claim? I’m not asking because I disagree, I’m asking because it’s a good habit (and often expected here) to provide them.

And with that said, yes, decoherence may resolve one of the issues with MWI, the “preferred basis problem”: the wave function splits, but why does it always split in ways that correspond to classical outcomes?

It was in one of his papers: https://arxiv.org/abs/quant-ph/9905004

 

[gentzen]

According to Dieter Zeh, the founder of decoherence, decoherence was largely started to support Everett's proposal of the splitting of worlds.

It was in one of his papers: https://arxiv.org/abs/quant-ph/9905004

Either you linked to the wrong paper, or you read more into Zeh's words than he actually said:

However, it would be severely misleading if this formalism (based on the concept of a density matrix) gave rise to the impression of deriving a real collapse by just taking into account the interaction with the environment. If real physical states are described by wave functions, there are only two possibilities: deviations from the Schrödinger equation or the Everett interpretation.

 

[Question69]

Well it did seem to me that he was alluding to Everett being the right way to go if you were to take decoherence seriously, since it preserves global wavefunctions, and given that a collapse would require some modifications of the Schrodinger equation that would only come about if for some reason superpositions don't hold above some value, for some arbitrary reason.

 

[A. Neumaier]

Well it did seem to me that he was alluding to Everett being the right way to go if you were to take decoherence seriously

But this was his personal view.

Others view decoherence - independent of any interpretation issues - just as a conceptual tool for understanding the rapid decay of off-diagonal entries in a reduced density matrix when written in a practically relevant basis. You might wish to read the review article

• M. Schlosshauer, Quantum decoherence, Physics Reports (2019).

or his earlier book.

 

[Question69]

But this was his personal view.

Others view decoherence - independent of any interpretation issues - just as a conceptual tool for understanding the rapid decay of off-diagonal entries in a reduced density matrix when written in a practically relevant basis. You might wish to read the review article

• M. Schlosshauer, Quantum decoherence, Physics Reports (2019).

or his earlier book.

By conceptual, do you mean that they do not think it is a physical process?

 

[A. Neumaier]

By conceptual, do you mean that they do not think it is a physical process?

No. Conceptual means theoretical, through a well-defined concept of decoherence. The decoherence process is physical and ubiquitous. In fact, it is the dominant process making quantum computing so difficult. Building an efficient quantum computer requires first of all curtailing decoherence as much as possible!

 

[Question69]

No. Conceptual means theoretical, through a well-defined concept of decoherence. The decoherence process is physical and ubiquitous. In fact, it is the dominant process making quantum computing so difficult. Building an efficient quantum computer requires first of all curtailing decoherence as much as possible!

Right, but then if we were to ask about the ontology of the interactions, since the whole process is unitary, wouldn't this imply MW? Because the other eigenstates must have gone somewhere.

 

[PeroK]

Because the other eigenstates must have gone somewhere.

They've gone to the great Hilbert Space in the sky!

 

[A. Neumaier]

Right, but then if we were to ask about the ontology of the interactions, since the whole process is unitary, wouldn't this imply MW? Because the other eigenstates must have gone somewhere.

Decoherence is independent of the ontology employed.

All interpretations equivalent to standard quantum mechanics allow for unitary time evolution, hence no interpretation is implied by the latter. They differ in what they find plausible in the context of measurement. Thus the interpretation of what happens in a measurement to the state and how it is interpreted depends on your personal preferences.

 

[Fra]

One can analyse and seek answers for what happens in "a measurement" from two perspectives. The intrinsic or the extrinsic perspective. From the intrinsic perspective, it's I think obviously a "surprise" or "new information" or "collapse". But from an extrinsic (and BIGGER) perspective, you can of course "see the appearance of a surprise coming". But this is neither a conflict, nor a solution to anything IMO. It's just the perspective from two different observers. It gives not clues about the emergent interactions between these observers. Its like trying to remove the boundary of a box, by making the box bigger. But you are just moving the boundary outwards.

/Fredrik

 

[CoolMint]

What is 'observers'? The reality we perceive in the brains arises from quantum mechanical principles in the brain that cannot be explained by classical physics. Electrical impulses are generated in the cell membrane of brain cells by ionization of single atoms. Single atoms cannot be explained by classical mechanics. They are potentials actualized upon measurement. Your thoughts depend on qm rules and apparently arise from our qft model that stipulates that fields are the ground basis of reality. Your thoughts above are electrical impulses generated by ionization of single atoms that relay electrons in brain cells. Then they go down via the spine and control your muscles and movements when you write down your comments here. Tracing it all back to spontaneous ionization and resultant electricity from atomic potentials leads to the question - are we the fields observing themselves? Is this what observers means?

 

[anuttarasammyak]

AFAIK whether the brain function is originated from quantum mechanics or not is still controversial among the scientists. As a fact most brain scientists are not so well educated in QM.

 

[CoolMint]

AFAIK whether the brain function is originated from quantum mechanics or not is still controversial among the scientists. As a fact most brain scientists are not so well educated in QM.

Your muscles contract upon electrical impulses generated in brain cells membranes. The process involved to generate the electricity is ionization.

 

[CoolMint]

The neural pathways serve to transfer the generated electricity down to your muscles which contract and release when you type a sentence.

 

[Fra]

What is 'observers'?

Definitely not constrained to humans/brains. This is a persistent confusion due to terminology.

A general observer as I mean it is just a part of the universe that interacts with and encodes relations with the rest of the universe (its surrounding).

The problem is though that in QM as it stands, the observer must a classical system that can communicate ane exchange information with the rest of the environment. Without that solid support, its difficult to define QM. This is problematic itself, and is reason for various interpretations if one wants to generalize the notion.

/Fredrik

 

[CoolMint]

Definitely not constrained to humans/brains. This is a persistent confusion due to terminology.

A general observer as I mean it is just a part of the universe that interacts with and encodes relations with the rest of the universe (its surrounding).

The problem is though that in QM as it stands, the observer must a classical system that can communicate ane exchange information with the rest of the environment. Without that solid support, its difficult to define QM. This is problematic itself, and is reason for various interpretations if one wants to generalize the notion.

/Fredrik

I agree with this. As long as you understand the evident take-away from quantum theory(whether you believe in mwi or another interpretation) that what you refer to as 'classical world' is just one tiny aspect of the so called universe. A very special case.
Whenever in doubt, see the effects in experiments that hinder decoherence that show the true invisible nature of reality. Puzzling.

 

[Lord Jestocost]

What is 'observers'?

What really happens is that the quantum-mechanical description of an event ceases to be meaningful as the observer changes the point of reference from before the event to after it.

Freeman Dyson in "Thought Experiments in Honor of John Archibald Wheeler" (In Science and Ultimate Reality, Cambridge University Press, New York, 2004, p.89)

 

[CoolMint]

What really happens is that the quantum-mechanical description of an event ceases to be meaningful as the observer changes the point of reference from before the event to after it.

Freeman Dyson in "Thought Experiments in Honor of John Archibald Wheeler" (In Science and Ultimate Reality, Cambridge University Press, New York, 2004, p.89)

That quote seems too short to make sense of. Is he saying that whenever there are observers/detectors, quantum particles cease to be quantum and become classical(-like)?
I remember Wheeler saying something along those line.

 

[Lord Jestocost]

See: https://www.edge.org/response-detail/25350

 

[CoolMint]

'According to the rules of quantum mechanics, the motions of objects are unpredictable. The wave-function tells us only the probabilities of the possible motions. When an object is observed, the observer sees where it is, and the uncertainty of the motion disappears. Knowledge removes uncertainty. There is no mystery.'The remaining mystery is the consistency of the 'classical' world.

 

[Lord Jestocost]

The hope of classical physics was that the “world of experience” (the experiential reality) would reveal something of an “ontic” world beyond it, a world of objective reality. I think that this hope was a reason for some to settle in the philosophy of materialism. However, since the advent of quantum mechanics, it became clear that this hope might not be met. To my mind, it thus merely troubles and disappoints some people that this hope of classical physics could indeed not be fulfilled. But the “world of experience” itself isn’t neither weird nor mystic.

 

[Fra]

However, since the advent of quantum mechanics, it became clear that this hope might not be met. To my mind, it thus merely troubles and disappoints some people that this hope of classical physics could indeed not be fulfilled. But the “world of experience” itself isn’t neither weird nor mystic.

I think the core of the issue here is not that worldview of classical realism fails, the problem is that the quantum mechanical view (which is different), still relies on a effectively classical context. This is NOT a problem for subatomic systems, as the classical context is there, and solid for all practical purposes.

But as mentioned in post 28, challenges of unification of all forces (including gravity), the unsolved problem is still to build a new understanding without this context. But that simplies to QM, for subatomic systems, and that is consistent the a priori unexplain stability of the common context and classical reality. But we need an "explanation" that does not secretly rest on something that we can not justify. In this perspective, decoherence does not seem to provide the deepest explanations.

/Fredrik