A question about quantum entanglement

[Ben vdP]

The most entangled might (in general) be the problem description.

Maybe in first order:

Step 1, suppose there is a process in which two quantum particles are created that move away in opposite directions with opposite spins due to conservation laws.
Here we are talking about two specific instances of quantum particles that are correlated.

In the second step there is time evolution with schrodinger equation or other method, wave function, interaction with other quantum particles or fields and all put together as a "state". This leads to probabilities that can be calculated.
And probabilities lead to statistics and ensembles, although there can be an interpretation dependency.
But at this step it is no longer the same system being described as in step 1, it is now a broader system than "two specific instances of quantum particles that are correlated".

Only at the third step there are measurements made with macro world outcomes and how this affects the quantum system or it is yet another qsystem. Now there is interaction between the quantum particles under investigation with measuring devices. That leads to additional disturbances of the "states". At this step the opinions might be diverging the most on for example what that "state" actually is that you are measuring or what the wave function stands for.



Maybe a minor note, but if a measurement is made and that results into a "state" outcome then there is still limited knowledge of the "state" just before the measurement.

 

[Peter Morgan]

The correlations associated with "thermal nonlocality" can't violate the Bell inequalities, so no, I would not say they're "similar enough", since Bell inequality violations are the key issue that undermines intuitive "classical" explanations of the correlations.

Classical measurement theory in the presence of thermal noise can and should include contextuality as noncommutativity, because it's a useful tool for describing multiple complex experiments. Without noncommutativity, Classical measurement theory is straw-manned relative to quantum measurement theory; using the Poisson bracket to include noncommutativity steel-mans classical measurement theory, so that in my AnnPhys 2020 (arXiv, DOI there) I call the expanded classical measurement theory 'CM+'.
With noncommutativity, Bell-type inequalities can be violated. There is an article by Arthur Fine in PhysRevLett 1982, "Hidden Variables, Joint Probability, and the Bell Inequalities", but the mathematics is much more succinctly stated on just the first page of an article by Lawrence J. Landau in PhysLettA 1987,

I rehearse that same mathematics in Section 7.2 of my AnnPhys 2020. In QFT, microcausality requires an exact relationship between locality and noncommutativity, which must play out in a detailed discussion of how the introduction of noncommutativity into classical measurement theory can model a violation of Bell-type inequalities.

[Unacceptable reference and discussion deleted by the Mentors]

 

[Fra]

How do a pair of particles via entanglement “know” what the other particle is doing? Any help is appreciated.

"I think I can safely say that nobody understands quantum mechanics."
"in mathematics you don't understand things. You just get used to them.”
-- Richard Feynman

Mathematically, they are one entity.
...
That is not one causing the other, and it is not a case of a "hidden variable". Simply, knowing one tells you what the other is.

The OP apparently seeks at plausible conceptual understanding of how nature can possibly logically pull this off, rather than just understanding or learning how to use the mathematics of quantum theory.

As someone subscribing to an agentic qbist like interpretation I can interpret OP question quite cleanly and "conceptually" I would personally answer it like this:

1. The particles/part definitely does NOT "know" what the other particle/part "is doing". And there is no need for it.

2. However, following from how the became entangled(there are different ways), their behaviour to certain environmental perturbations are complementing each other in an exact way provided that theiy are both not further manipulated after beeing entangled. But the environment can not infer their behavioural states without destrying it. Neither can a bell type HV over the states explain it, as that isnt how the interaction works.

This is what can explain the correlations. The hard mathematics that "they have as one unity" is conditional on that they are isolated from interactions which would destroy the entanglement. But in terms of reality, this condition stands and falls with wether the experimental conditions can maintain the isolation and entanglement, which is a nontrivial task experimentally. This is also why no other system in the environment can "find out" or "know" or even parameterize any conventional hidden variables that determines their states, becuase that same inference is exactly what would break entanglement as well. Also one particle really does not "know" if the entanglement to the other particle is instantly broken either. That would requirement some other system to collect information from interactions of them both, directly or indirectly. And this is why bell inequality is always violated.

/Fredrik

 

[Demiurge]

The OP apparently seeks at plausible conceptual understanding of how nature can possibly logically pull this off, rather than just understanding or learning how to use the mathematics of quantum theory.

As someone subscribing to an agentic qbist like interpretation I can interpret OP question quite cleanly and "conceptually" I would personally answer it like this:

1. The particles/part definitely does NOT "know" what the other particle/part "is doing". And there is no need for it.

2. However, following from how the became entangled(there are different ways), their behaviour to certain environmental perturbations are complementing each other in an exact way provided that theiy are both not further manipulated after beeing entangled. But the environment can not infer their behavioural states without destrying it. Neither can a bell type HV over the states explain it, as that isnt how the interaction works.

This is what can explain the correlations. The hard mathematics that "they have as one unity" is conditional on that they are isolated from interactions which would destroy the entanglement. But in terms of reality, this condition stands and falls with wether the experimental conditions can maintain the isolation and entanglement, which is a nontrivial task experimentally. This is also why no other system in the environment can "find out" or "know" or even parameterize any conventional hidden variables that determines their states, becuase that same inference is exactly what would break entanglement as well. Also one particle really does not "know" if the entanglement to the other particle is instantly broken either. That would requirement some other system to collect information from interactions of them both, directly or indirectly. And this is why bell inequality is always violated.

/Fredrik

This is not an explanation but a description.

 

[Fra]

This is not an explanation but a description.

Fair point! I should have written that it is a conceptual framework that "allows" for an explanation - witout one particle in som omnipotent way "knows" what the other remote part is doing. The implicit possibility was that the particle just acts out of its expectations; and these are aligned from the entanglement created.

But yes a detailed explanation is not known by anyone so far, so its not what i talk about. I tried to provie a conceptual answer as detailed as possible without going down in some inappropriate whole.

/Fredrik

 

[FactChecker]

This is also why no other system in the environment can "find out" or "know" or even parameterize any conventional hidden variables that determines their states,

I thought that Bell's theorem/test ruled out the existence of local hidden variables.

 

[Fra]

I thought that Bell's theorem/test ruled out the existence of local hidden variables.

Correc,t there is nothing wrong with Bells theorem. There is no local hidden variables in the way of Bell definitions.

/Fredrik

 

[edmund cavendish]

Spooky action at a distance appears to have been proved evidentially. And seems to be a basis of cryptography...

 

[PeterDonis]

Moderator's note: Several off topic posts arguing about "explanation" vs. "description" have been deleted.

 

[Morbert]

Spooky action at a distance appears to have been proved evidentially. And seems to be a basis of cryptography...

There are well established interpretations with no spooky action at a distance.

 

[PeterDonis]

There are well established interpretations with no spooky action at a distance.

Unless by "spooky action at a distance" one means "violations of the Bell inequalities".

 

[FactChecker]

There are well established interpretations with no spooky action at a distance.

Can you be more specific?
Are any interpretations like that "established", let alone "well established"?
Do you mean popular? (Even that may be too strong.)

 

[Morbert]

Can you be more specific?
Are any interpretations like that "established", let alone "well established"?
Do you mean popular? (Even that may be too strong.)

QBism, Copenhagen (Asher Peres's variety), Consistent Histories, the Oxford branch (no pun intended) of Everettianism, Relational interpretation (Rovelli), Ensemble is at least agnostic.

None of these involve superluminal influence of one system on another.

 

[gentzen]

There are well established interpretations with no spooky action at a distance.

Can you be more specific?

Einstein used „spooky action at a distance“ in his letter to Max Born, where he complained about the wavefunction in 3n-dim configuration space, and the collapse of the wavefunction. That was long before the EPR paper. I am not aware that Einstein used that expression again later, let alone with a different meaning.

Are any interpretations like that "established", let alone "well established"?
Do you mean popular? (Even that may be too strong.)

Any interpretation without wavefunction collapse qualifies, including Bohmian mechanics, consistent histories, and many worlds.

 

[PeterDonis]

Bohmian mechanics

Of which, IIRC, Bell said that he could not imagine an interpretation that Einstein would have liked less. Bohmian mechanics doesn't have "spooky action at a distance" only in the sense that its action at a distance is not at all "spooky", it's explicit, right there in the dynamics, instead of being implicit in wavefunction collapse or something similar.

 

[FactChecker]

Any interpretation without wavefunction collapse qualifies, including Bohmian mechanics, consistent histories, and many worlds.

My objection was to the phrase "well established". How do you "establish" an interpretation?
Maybe "respected" would be a better word.

 

[Sambuco]

My objection was to the phrase "well established". How do you "establish" an interpretation?
Maybe "respected" would be a better word.

I believe that "established" is a reasonable word to refer to those interpretations whose predictions are considered to coincide with those of orthodox/textbook quantum mechanics.

Lucas.

 

[FactChecker]

I believe that "established" is a reasonable word to refer to those interpretations whose predictions are considered to coincide with those of orthodox/textbook quantum mechanics.

Lucas.

I'll accept that. I may read more into the word than I should, and I don't want to further hijack this discussion.

 

[Matterwave]

Am I reading "spooky action at a distance" totally different from everyone else? I read it as "non-local".

Bohmian mechanics is about as non-local as it gets. The wave function guides particles at *any distance* instantaneously. I think Einstein would have considered that throughly "spooky action at a distance".

 

[PeterDonis]

those interpretations whose predictions are considered to coincide with those of orthodox/textbook quantum mechanics.

All interpretations make the same predictions as "textbook quantum mechanics". Things that don't make the same predictions aren't interpretations of QM; they're different theories.

 

[Fra]

In most similar discussions there is always a mess of different use of terms like local, causal, action etc. This looping part of the discussion is not very interesting and quite tiresome, but what remains the enigma, regardless of terminology is to that

There is still after all time no "established" logically coherent mechanism that EXPLAINS the correlations, without introducing either new pathologies, or new abstractions that seem even more impalatable.

We do have a theory that describes what happens: In the paradigm of system dynamics in hilbert space. But this, along with the hamiltonian operators working on it, and why they are what they are, is an abstract mathematical description that no matter how successful, does not provide the "inside picture" that I think we conceptaully need to feel we understand how nature implements this - in particular as the simplest resolution indeed was the classical hidden variable idea - provenly is invalid. The problem isnt just that there is something "wrong" or mistake in with Bells theorem. It's much bigger.

From here on people analyse this from difference conceptual perspectives, often using new "abstractions" that replace the obscure hilbert spaces or system dynamics paradigms.

Edit: I justed wanted to clarify that seeking a "mechanism" does not imply seeking a naive classical mechanics "mechaical" seqeunce. A causal mechanism in the bigger picture is not associated with naive classical physics in any way. I added this, because i know this is another confusion that also often happens with the term "mechanism". causal mechanism <> "classical mechanics".

/Fredrik

 

[gentzen]

Am I reading "spooky action at a distance" totally different from everyone else? I read it as "non-local".

It should be irrelevant how I or everyone else reads it. The phrase goes back to Einstein, so only the context(s) in which he said or wrote it should be relevant for the meaning of that phrase.
I tried to find those contexts now, and only one turned up, in a letter to Max Born from 3 March, 1947:

I cannot make a case for my attitude in physics which you would consider at all reasonable. I admit, of course, that there is a considerable amount of validity in the statistical approach which you were the first to recognise clearly as necessary given the framework of the existing formalism. I cannot seriously believe in it because the theory cannot be reconciled with the idea that physics should represent a reality in time and space, free from spooky actions at a distance. I am, however, not yet firmly convinced that it can really be achieved with a continuous field theory, although I have discovered a possible way of doing this which so far seems quite reasonable. The calculation difficulties are so great that I will be biting the dust long before I myself can be fully convinced of it. But I am quite convinced that someone will eventually come up with a theory whose objects, connected by laws, are not probabilities but considered facts, as used to be taken for granted until quite recently. I cannot, however, base this conviction on logical reasons, but can only produce my little finger as witness, that is, I offer no authority which would be able to command any kind of respect outside of my own hand.

The crucial parts in the original German letter were:

Ich sehe natürlich ein, daß die prinzipiell statistische Behandlungsweise, deren Notwendigkeit im Rahmen des bestehenden Formalismus ja zuerst von Dir klar erkannt wurde, einen bedeutenden Wahrheitsgehalt hat. Ich kann aber deshalb nicht ernsthaft daran glauben, weil die Theorie mit dem Grundsatz unvereinbar ist, daß die Physik eine Wirklichkeit in Zeit und Raum darstellen soll, ohne spukhafte Fernwirkungen.

Einstein used „spooky action at a distance“ in his letter to Max Born, where he complained about the wavefunction in 3n-dim configuration space, and the collapse of the wavefunction. That was long before the EPR paper. I am not aware that Einstein used that expression again later, let alone with a different meaning.

Turns out I was wrong. Here are the crucial parts in the letter to Max Born from 4 December, 1926:

Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the 'old one'. I, at any rate, am convinced that He is not playing at dice. Waves in 3n-dimensional space, whose velocity is regulated by potential energy (for example, rubber bands) ...

Die Quantenmechanik ist sehr achtung-gebietend. Aber eine innere Stimme sagt mir, daß das doch nicht der wahre Jakob ist. Die Theorie liefert viel, aber dem Geheimnis des Alten bringt sie uns kaum näher. Jedenfalls bin ich überzeugt, daß der nicht würfelt. Wellen im 3n-dimensionalen Raum, deren Geschwindigkeit durch potentielle Energie (z. B. Gummibänder) reguliert wird ...

 

[Fra]

I tried to find those contexts now, and only one turned up, in a letter to Max Born from 3 March, 1947:

Einstein said:
"I cannot seriously believe in it because the theory cannot be reconciled with the idea that physics should represent a reality in time and space, free from spooky actions at a distance. I am, however, not yet firmly convinced that it can really be achieved with a continuous field theory"

This quotation comes out insightful even in the light of today. I read this so that his intuition was and still is valid - there must be some "mehanicsm" - but i think the resolution required can not be achieved with a the tentative traditional continuous field theory.

So does it make sense that Einsteins objection and concern was and still is valid, but just that the initial idea to solve it was wrong? But everything tend to start with lots of wrong turns.

/Fredrik