What is the mechanism behind Quantum Entanglement?

[Fra]

The point of Bell's work is that there is a difference in the predictions about probabilities for the outcome of measurements between local realistic hidden-variable theories and QT, no more no less.

Maybe I misunderstand what you mean by "mechanism", but when I say that there is no mechanism behind entanglement I mean there is QT and no other thing (like hidden variables) that may in some sense "explain" the correlations described by entanglement beyond what QT describes.

For me it's not the correlations that need explanation. A hidden variable explains this.

The difficulty is to find a causal mechanism for the physics that happens at bob and alice that is consistent with observations and correlations given that the naive ansatz of bell does not work. This is clearly not explaines by hidden variables in the way bell thought.

QM describes it but shatters the old naive mechanisms but without replacing it. Seeking improvement does not mean i am looking for bell loopholes. My point is that the class of explanations in terms of hidden variables as per bell does not seem exhaustive.

/Fredrik

 

[ohwilleke]

The most comprehensive QT is relativistic QFT, the Standard Model of elementary particle physics is based on, and that's the most successful theory of matter that has been hitherto discovered. It's so successful that it is hard to find "physics beyond the Standard Model", for which HEP physicists strive vigorously, because we know on the other hand that it is incomplete since there's very convincing evidence from astronomy and cosmology that there should be more particles than described by the Standard model (the socalled dark matter) and because there is no satisfactory quantum theory of the gravitational interaction.

However, this so far most successful theory about the known types of matter and the interaction between its constituents is clearly based on locality and causality, and this is so by construction, i.e., it is built in in its very foundations.

Causality means that the state of a (quantum) system can be influenced only by the past and not the future. In relativistic models of spacetime this implies that there cannot be causal influences from space-like separated events.

So it can only be "realism" that's violated by QT. In my opinion it's a very unfortunate choice of naming, because QT in fact is the most "realistic" theory we have, i.e., it describes the phenomena best in accordance with the observations.

I appreciate your careful definitions of these terms which I didn't attempt for fear that I'd miss an important nuance. I also agree with you the "realism" is an unfortunate choice of names for this property as it fails, unlikely causality and locality, to clearly and intuitively describe what it means (although perhaps the problem is that the concept itself isn't in the inventory of common sense ideas).

 

[Fra]

The point of Bell's work is that there is a difference in the predictions about probabilities for the outcome of measurements between local realistic hidden-variable theories and QT, no more no less.

Maybe I misunderstand what you mean by "mechanism", but when I say that there is no mechanism behind entanglement I mean there is QT and no other thing (like hidden variables) that may in some sense "explain" the correlations described by entanglement beyond what QT describes.

I read this again, and if by "there is no other thing" mean there is no accepted theory that does this, I agree fully, if that was your point.

I mainly argued that there is still reasons why the case is not closed. The issues not to probe Bell wrong. There is nothing wrong with bells theorem. The question is - wether it applies to the explanation we are looking for? In retrospect - obviously not, right?

/Fredrik

 

[CoolMint]

If realism fails, both locality and causality lose their meaning and it would be impossible to explain the consistency of the 'classical' world.

 

[PeterDonis]

Moderator's note: Thread moved to QM interpretations forum since the answers to the OP questions are interpretation dependent.

 

[RUTA]

Summary: Quantum Entanglement

What is the mechanism behind Quantum Entanglement? Why do only subatomic particles exhibit Quantum Entanglement?

The mechanism is “average-only” conservation and that happens because everyone must measure the same value for Planck’s constant h, regardless of their orientation relative to the source, i.e., rotational invariance of h. It’s totally analogous to why we have time dilation and length contraction. Those happen because everyone must measure the same value for the speed of light c, regardless of their motion relative to the source, i.e., boost invariance of c. See https://www.physicsforums.com/insig...ciple-at-the-foundation-of-quantum-mechanics/

 

[vanhees71]

As always you forget gravity or leave it for last. Given that the spacetime background is required for formulating QFT it is a major conceptual issue. Although admittedly not a point with the most engineering applications.

/Fredrik

Sure, gravity is the big issue, but that has nothing to do with these apparent philosophical issues of QT. In the entire history of science, philosophical ideas helped only to understand the wider implications of the results of the natural sciences in a larger cultural context. Kuhn's paradigm shifts always happened due to discrepancies between scientific observations and the then valid theories. Purely philosophical speculations never helped to find new theories, and paradigm shifts are really rare (on the timescale of centuries!).

 

[vanhees71]

The mechanism is “average-only” conservation and that happens because everyone must measure the same value for Planck’s constant h, regardless of their orientation relative to the source, i.e., rotational invariance of h. It’s totally analogous to why we have time dilation and length contraction. Those happen because everyone must measure the same value for the speed of light c, regardless of their motion relative to the source, i.e., boost invariance of c. See https://www.physicsforums.com/insig...ciple-at-the-foundation-of-quantum-mechanics/

The conservation laws are not only valid "average-only" but event by event. That's a result known since the 1920ies with Bothe's coincidence measurement of the Compton effect. The Bohr-Kramers theory claiming this "average-only-validity of the conservations laws" was very short-lived ;-)).

 

[RUTA]

The conservation laws are not only valid "average-only" but event by event. That's a result known since the 1920ies with Bothe's coincidence measurement of the Compton effect. The Bohr-Kramers theory claiming this "average-only-validity of the conservations laws" was very short-lived ;-)).

What I said is exactly true and very easy to understand. I even present this to my gen ed students. Read the Insight linked, you can reference any of the published papers therein if necessary.

 

[vanhees71]

Where in this long Insight can I find the claim that the conservation laws hold on average only? As I said, this contradicts very early empirical evidence from the early history of modern quantum theory. Prominent other ideas, like the famous Bohr-Kramers theory, have been refuted by these observations and finally modern QT in its usual form has been found.

 

[Fra]

Sure, gravity is the big issue, but that has nothing to do with these apparent philosophical issues of QT.

I figured by now that you think so, but I disagree even though the the link is indeed far fetched seen in the light of the current models.

But as for the general link, there are others that is associating entanglement with potential connections to quantum gravity.
https://arxiv.org/abs/1306.0533

I don't see how the unification of gravity and QM is going to happen in a reasonable way unless one considers and reconstructs some of the foundations of QM.

Purely philosophical speculations never helped to find new theories, and paradigm shifts are really rare (on the timescale of centuries!).

I suspect many creative people keep these speculations private or inside their own heads, and only present the polished results, as it makes the process look cleaner than it really is. Noone wants to read the ugly process of creating a theory that may be wrong. Only once proven right, maybe you can read a little bit about it in biographies or so, but even there I think the ugly turns are omitted, to make it look more sexy.

/Fredrik

 

[gentzen]

The conservation laws are not only valid "average-only" but event by event. That's a result known since the 1920ies ...

What I said is exactly true and very easy to understand. I even present this to my gen ed students. Read the Insight linked, you can reference any of the published papers therein if necessary.

Well, just because you present it to your gen ed students doesn't mean that they understand it any more than the OP of this B-level thread will understand QFT and the "microcausality principle" brought-up by vanhees71:

I have not done any search in any literature. I am not a university student. I am just curious about how it works. I have no understanding of this.

Even worse, just because you believe that it is "very easy to understand" doesn't even mean that it is strictly true in all contexts.

So let us look at your Insights article how it introduces this "average-only" claim, and how it proves and explains it:

And, just as the light postulate of SR leads to time dilation and length contraction in a perfectly symmetrical fashion between different reference frames (aka the relativity of simultaneity), the “Planck postulate” of QM leads to “average-only” projection and conservation of spin angular momentum in a perfectly symmetrical fashion between different reference frames (explained below).

This introduction not only postpones the proof (which is fine), but also doesn't specify what exactly is meant by "average-only". From the POV of the minimal statistical interpretation (vanhees71's preferred interpretation), the natural interpretation of "average-only" would be that there would exists experiments where actual violations of conservation of momentum, or angular momentum, or energy, or ... would actually be observable. Or to put it differently, his "event by event" conservation claim mean that no statistical significant violation of conservation should ever be observable in any properly performed series of experiments. Even so this is quite a strong claim, I am not aware of any experimental evidence against it.

So, how can this be reconsiled with your proof(s)?

However, given that the radiation is actually composed of indivisible photons, there is a non-zero lower limit to the energy passed by a polarizing filter, i.e., each quantum of energy either passes or it doesn’t. Thus, we understand that the classical “expectation” of fractional amounts of quanta can only obtain on average per the quantum reality, so we expect the corresponding quantum theory will be probabilistic.

The assumption that "the radiation is actually composed of indivisible photons" doesn't hold from the perspective of QFT (vanhees71's preferred perspective), but that is less important than that you cannot nail down individual indivisible photons in experiments. So this argument is (most probably) unable to make verifiable preditions about observable violation of conservation in experiments.

Thus, as argued by Brukner & Zeilinger, a theory of qubits must be probabilistic. Of course, the relationship between classical and quantum mechanics per its expectation values (averages) is another textbook result, e.g., the Ehrenfest theorem.

The provable (weak) relationship between classical and quantum mechanics given by the Ehrenfest theorem doesn't mean that there cannot be stronger relationships between classical and quantum mechanics when it comes to conservation laws.

 

[PeterDonis]

What I said is exactly true

No, what you said is a proposal you have made. It is not something that has been experimentally tested and verified. It's not even clear how it could be experimentally tested and verified.

 

[vanhees71]

So let us look at your Insights article how it introduces this "average-only" claim, and how it proves and explains it:

Thanks for doing the hard work, digging out the claim from a long text!

This introduction not only postpones the proof (which is fine), but also doesn't specify what exactly is meant by "average-only". From the POV of the minimal statistical interpretation (vanhees71's preferred interpretation), the natural interpretation of "average-only" would be that there would exists experiments where actual violations of conservation of momentum, or angular momentum, or energy, or ... would actually be observable. Or to put it differently, his "event by event" conservation claim mean that no statistical significant violation of conservation should ever be observable in any properly performed series of experiments. Even so this is quite a strong claim, I am not aware of any experimental evidence against it.

This has nothing to do with interpretation whatsoever, it's simply an experimentally verified fact without any counterexamples observed yet, and it's known even before or just at the time when modern quantum theory has been discovered.

So, how can this be reconsiled with your proof(s)?

The assumption that "the radiation is actually composed of indivisible photons" doesn't hold from the perspective of QFT (vanhees71's preferred perspective), but that is less important than that you cannot nail down individual indivisible photons in experiments. So this argument is (most probably) unable to make verifiable preditions about observable violation of conservation in experiments.

That's not true either. The "indivisibility of photons" is one of the stringent proves for the existence of photons, i.e., the validity of relativistic QFT. The claim by many textbooks that this were the case for leading-order treatments of the photoelectric effect or Compton scattering is not conclusive, because both follows from the quantization for charged particles (electrons in this case) alone keeping the em. field classical.

Of course one must be a bit more precise in this statement, because what's "indivisible" are the energy quanta of radiation of a certain frequency. There are, of course, processes in non-linear optics, where a photon of some frequency is absorbed and two photons with different frequencies (however with energy and momentum conservation valid) are emitted like in the celebrated parametric-down conversion process to prepare entangled photon pairs, enabling proper one-photon sources.

The provable (weak) relationship between classical and quantum mechanics given by the Ehrenfest theorem doesn't mean that there cannot be stronger relationships between classical and quantum mechanics when it comes to conservation laws.

 

[RUTA]

Where in this long Insight can I find the claim that the conservation laws hold on average only? As I said, this contradicts very early empirical evidence from the early history of modern quantum theory. Prominent other ideas, like the famous Bohr-Kramers theory, have been refuted by these observations and finally modern QT in its usual form has been found.

Right after Eq. 4. A longer explanation with pictures is at the ScienceX Dialogue link. That’s the level I show my gen ed students. As long as they can understand projection, they can understand that explanation.

 

[RUTA]

No, what you said is a proposal you have made. It is not something that has been experimentally tested and verified. It's not even clear how it could be experimentally tested and verified.

What I shared are mathematical facts about the formalism of QM. It is not a mere proposal.

 

[vanhees71]

Right after Eq. 4. A longer explanation with pictures is at the ScienceX Dialogue link. That’s the level I show my gen ed students. As long as they can understand projection, they can understand that explanation.

It's really hard to discuss, if you don't give precise quotations. In your Insight Eq. 4 is something about spin. In the paragraph following it there's nothing about energy and momentum conservation.

So precisely where is this bold claim explained? I'm really puzzled that you teach students (whatever "gen ed" means) such speculative ideas which clearly contradict established empirical facts and the currently established physical theories.

 

[RUTA]

Well, just because you present it to your gen ed students doesn't mean that they understand it any more than the OP of this B-level thread will understand QFT and the "microcausality principle" brought-up by vanhees71:

I can only say that many of my gen ed students’ reactions are consistent with them understanding the explanation.

Even worse, just because you believe that it is "very easy to understand" doesn't even mean that it is strictly true in all contexts.

As I pointed out to Peter Donis, what I have presented are mathematical facts about the formalism of QM. So, they are true in the context of QM.

Here is the 2:45-sec video abstract for the paper that also explains the calculation with figures .

If you read the paper or ScienceX Dialogue, you should be able to understand what is meant by “average-only” conservation. It is relational, exactly like time dilation and length contraction, between reference frames. When Alice and Bob make measurements in different reference frames, Alice(Bob) says Bob(Alice) must average his(her) data according to her(his) partition of the data in order to conserve spin angular momentum. All of this follows from the exact conservation of spin angular momentum responsible for the Bell state with its rotational symmetry to begin with. As long as Alice and Bob are making measurements in the same reference frame (same orientation relative to source) their outcomes will be exactly in accord with conservation of spin angular momentum. And, not surprisingly, that can be easily accounted for via local realism. The “weirdness” of entanglement occurs for measurements in different reference frames. That’s where the relative “average-only” conservation holds and that’s what evades explanation via local realism.

If you want to get into the technical nature of the Bell spin states, read this Insight https://www.physicsforums.com/insights/bell-states-and-conservation-of-spin-angular-momentum/ which is an appendix in this paper https://www.nature.com/articles/s41598-020-72817-7

 

[RUTA]

It's really hard to discuss, if you don't give precise quotations. In your Insight Eq. 4 is something about spin. In the paragraph following it there's nothing about energy and momentum conservation.

So precisely where is this bold claim explained? I'm really puzzled that you teach students (whatever "gen ed" means) such speculative ideas which clearly contradict established empirical facts and the currently established physical theories.

Nothing I presented violates established physics. Quite the opposite, it follows exactly according to established textbook QM. Read the papers and it should be readily obvious to someone with your background. “Gen ed students” means “general education students”, i.e., the business, comm, ed, etc. students taking physics. Sorry, that’s a typical term here in U.S. academe, but maybe not where you’re located.

 

[vanhees71]

Ok, so which papers should I read? Is it so difficult to just give the references?

 

[PeterDonis]

What I shared are mathematical facts about the formalism of QM. It is not a mere proposal.

What you said in post #38 is not "mathematical facts about the formalism of QM". It's a proposal you have made for interpreting QM, or, if you like, for a new axiomatic foundation for QM different from all the other extant ones. Please do not misrepresent your own research.

 

[RUTA]

Ok, so which papers should I read? Is it so difficult to just give the references?

It’s linked at the outset of the Insight I linked.

 

[RUTA]

What you said in post #38 is not "mathematical facts about the formalism of QM". It's a proposal you have made for interpreting QM, or, if you like, for a new axiomatic foundation for QM different from all the other extant ones. Please do not misrepresent your own research.

Sorry but you’re mistaken. The words I used map exactly to mathematical facts. Look carefully and you’ll see that I’m not sharing an interpretation. I’m merely pointing out mathematical facts and using standard physics language to label those facts.

 

[DrChinese]

It's really hard to discuss, if you don't give precise quotations.

Without trying to be snarky, good friend: This is something I've said to you about 100 times.

 

[vanhees71]

It’s linked at the outset of the Insight I linked.

Ok, obviously you don't want to give the precise reference.

 

[PeterDonis]

Sorry but you’re mistaken.

You're referencing an Insights article of yours, which in turn references a published paper of yours. That paper is not just "stating mathematical facts". It's a proposal of yours which is still under review by the physics community in general. Referencing that work here is fine, since it's based on published peer-reviewed work. Claiming that it is mainstream QM is not fine, because, no matter how much you might fervently believe that is true, the physics community in general has not come to agreement with you. And that is the standard we use here at PF for whether something is mainstream. The personal opinion of the person who published the research, by itself, is not enough.

 

[vanhees71]

Sigh. Which of the papers quoted in the Insight are we talking about?

 

[physika]

,
.

What is the mechanism behind Quantum Entanglement?

That is wicked!

Not a true entanglement.
The relevant question is, what is the source or origin (mechanism) of quantum correlations.

quantum entanglement is just a kind of Quantum Correlation.

The Advantages of Not Entangling Macroscopic Diamonds at Room Temperature​

https://www.hindawi.com/journals/jamp/2012/469043/

"The recent paper entitled by K. C. Lee et al. (2011) establishes nonlocal macroscopic quantum correlations, which they term “entanglement”, under ambient conditions. Photon(s)-phonon entanglements are established within each interferometer arm. However, our analysis demonstrates, the phonon fields between arms become correlated as a result of single-photon wavepacket path indistinguishability, not true nonlocal entanglement"

"Nonlocal interactions can be from either entanglement or path indistinguishability (the path integral for larger systems), with the latter being further subdivided as discussed. These two distinct phenomenon have recently been treated often in the literature as essentially identical, which is problematic when utilizing them for practical applications"

"the phonon fields between arms become correlated as a result of single-photon wavepacket path indistinguishability, not a true nonlocal entanglement"

Nonlocal Quantum Correlations: Beyond Entanglement​

https://arxiv.org/abs/1209.1081

​

Quantum Correlations beyond Entanglement and Discord​

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.170404

"Dissimilar notions of quantum correlations have been established, each being motivated through particular applications in quantum information science and each competing for being recognized as the most relevant measure of quantumness. In this contribution, we experimentally realize a form of quantum correlation that exists even in the absence of entanglement"

Quantumness beyond entanglement: The case of symmetric states​

https://journals.aps.org/pra/abstract/10.1103/PhysRevA.105.022433

"Nowadays, it is accepted that truly quantum correlations can exist even in the absence of entanglement. For the case of symmetric states, a physically trivial unitary transformation can alter a state from entangled to separable, and vice versa."

,


.

 

[bhobba]

Regarding ideas as to the 'mechanism' of QM I feel it is important to emphasise that QM is different to other physical theories (and of course, those theories are limiting cases of QM). We do not have direct experience with the subatomic world (ie the QM world) as we do with Newtonian Mechanics. To know about the QM world, we interact with it using what we have direct experience with here in the macroscopic world. We have a mathematical theory about these interactions (ie observations), but as to what is 'really' going on, this makes QM difficult, perhaps nearly impossible, to grasp at that level. I was once really interested in such things, but slowly, over time, I realized it might be something at our current level of knowledge these may not be good questions to ask as they mostly lead down a rabbit hole. Occasionally we get progress, like with Bell, PBR etc, but by and large, it is tough going. I am happy with the idea QM is a phenomenological probabilistic theory about observations. At that level, there is no issue. We know the 'mechanism' of entanglement. It is simply a consequence of the principle of superposition. Suppose we have two systems that can be in state |a> or state |b>. States are simply bookkeeping devices that help us predict the probabilities of observational outcomes. If system 1 is in state |a> and system 2 is in state |b>, that is written as |a>|b>. Conversely if system 1 is in state |b> and system 2 in state |a> it is written as |b>|a>. But the principle of superposition says a possible state is 1/root(2) |a>|b> + 1/root(2) |b>|a>. This is a peculiar state of affairs with no classical analogue. My view is that is the mechanism. But I know opinions on this differ.

Thanks
Bill

 

[RUTA]

You're referencing an Insights article of yours, which in turn references a published paper of yours. That paper is not just "stating mathematical facts". It's a proposal of yours which is still under review by the physics community in general. Referencing that work here is fine, since it's based on published peer-reviewed work. Claiming that it is mainstream QM is not fine, because, no matter how much you might fervently believe that is true, the physics community in general has not come to agreement with you. And that is the standard we use here at PF for whether something is mainstream. The personal opinion of the person who published the research, by itself, is not enough.

The paper:

No Preferred Reference Frame at the Foundation of Quantum Mechanics​

and this earlier one:

Answering Mermin’s challenge with conservation per no preferred reference frame​

do indeed merely state mathematical facts about the Bell spin states that are mainstream QM. "Average-only" conservation is a characterization of relational partitions by different reference frames (related by spatial rotations) of Bell state data per mainstream physics. It's no more a matter of opinion than the relativity of simultaneity, which is a characterization of relational partitions by different reference frames (related by Lorentz boosts) of the events in M4. Here is the Science X Dialogue related to the "Answering Mermin's Challenge" paper:

Einstein's missed opportunity to rid us of 'spooky actions at a distance'​

What makes entanglement mysterious isn't Alice and Bob's measurements of the same spin, those correlations are easy to explain using conservation of spin angular momentum, e.g., Fact 1 about case (a) for the Mermin device. The mystery arises because of the correlations between Alice and Bob's measurements of different spins, e.g., Fact 2 about case (b) for the Mermin device, given their correlations when measuring the same spins. Those correlations satisfy "average-only" conservation in relative fashion. This was shown in 2005 for the singlet state by Unnikrishnan in this paper:

Unnikrishnan, C. Correlation functions, Bell’s inequalities and the fundamental conservation laws. Europhysics Letters 69, 489–495 (2005) (arxiv:quant-ph/0407041).

Unnikrishnan's mistake was to claim this average conservation principle resolves the mystery of entanglement when in fact it is simply another way to characterize the mystery, since it is just a mathematical fact about the Bell spin states. Indeed, you can say that average-only conservation is simply the result of an empirical fact we call spin when spin angular momentum is conserved but measured in different reference frames.

So, there is no opinion or proposal or interpretation going on here when I say the mystery ("mechanism" in the OP) of entanglement is characterized by "average-only" conservation.