What is the mechanism behind Quantum Entanglement?

[RUTA]

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

See my Post #62. You can add this one as well:

Stuckey, W.; Silberstein, M.; McDevitt, T.; Kohler, I. Why the Tsirelson Bound? Bub’s Question and Fuchs’ Desideratum. Entropy 2019, 21, 692 https://www.mdpi.com/1099-4300/21/7/692

 

[RUTA]

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

In the axiomatic reconstructions of QM, this is called Information Invariance & Continuity. It is indeed equivalent to the mechanism of "average-only" conservation, as detailed in the first paper linked in my Post #62.

 

[vanhees71]

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:

I think we had this discussion a while ago. In fact in the here described EPR-paradox version with angular momentum (spin) a la Bohm, it's easy to see that of course the conservation laws hold event by event. To prove this, of course, you have to measure the angular-momentum component in the same direction on both particles, because you can only determine one momentum component due to the commutator relations of angular-momentum components. The $J=0$ state of the total system is an exception, because for this state all three components take simultaneously the same value, 0.

If you measure different components of the angular momentum on both particles, of course you cannot verifty angular-momentum conservation, because the outcomes of the measurements are random with a probabilities for the possible outcome for the two angular-momentum components given by Born's rule. This also includes, of course, the violations of Bell's inequality, which cannot be described with local deterministic hidden-variable theories. This has, however, nothing to do with the claim that the conservation laws only hold "on average".

I'll try to read the papers, as soon as I find the time.

 

[gentzen]

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

Thanks for answering me. Maybe I was a bit snarky (as DrChinese puts it) towards you and vanhees71. But if I would have tried to explain in a B-level thread how the (dagger) compact closed category structure typical of the QM of small systems leads to a nicer duality between observation and action compared to classical mechanics, I probably would have gotten quite some flak.

What I said is exactly true and very easy to understand.

The more interesting question for me is how easy it will be to discuss details of "interpretation" and "language" of your proposal with you.

Your claim that "the mechanism is average-only conservation" at first seems to contradict my experience that assuming "event-by-event conservation" typically is the right thing to do for theoretical computations or Monte-Carlo simulations of atomic phenomena. This raises the question of what you mean exactly by your claim. In "Answering Mermin’s challenge with conservation per no preferred reference frame" I found:

In short, the explicit conservation that obtains for Alice and Bob’s Stern-Gerlach spin measurement outcomes in the same reference frame holds only on average in different reference frames, not on a trial-by-trial basis.

So you even mention that explicit conservation will be observed if the measurement is done in a way that this outcome is even possible to begin with. Therefore your "trial-by-trial" means something different from my "event-by-event". But then we are back to a sort of measurement problem, how non-conservation in a "trial including preparation and measurement" should be possible, if no single physical process by itself violates conservation.

I have a bit the impression that your "non-conservation in a single trial" arises from a sort of Copenhagen like collapse interpretation of single trials. Therefore, Heisenberg's defense that collapse arises from a sort of boundary condition for open system might help to bring some more clarity: We are looking here at experiments whose boundary conditions are incompatible with the symmetries corresponding to exact conservation. So the boundary conditions have broken a symmetry. This makes sense, because a Stern-Gerlach magnet does its magic in a certain sense by suitably breaking a symmetry.

Of course, you probably won't be happy with this reinterpretation of your nice "reference frames" as "boundary conditions that break symmetries in various ways". But it allows to ask a related question: will "conservation trial-by-trial" always hold, when the "implicit boundary conditions" would in principle allow it? (One problem here is that the idealized implicit boundary conditions are not identical to the boundary conditions realized in an actual experiment.)

That is, “there is no mention in relativity of exactly how clocks slow, or why meter sticks shrink” (no “constructive efforts”), nonetheless the principles of special relativity are so compelling that “physicists always seem so sure about the particular theory of Special Relativity, when so many others have been superseded in the meantime

I thought the mechanism would be "relativity of simultaneity," together with a definition of how to measure lengths based on measuring times (for why meter sticks shrink). However, there is also a second component, why it can be an advantage that the universe does not need a global perfectly synchronized clock. If you do parallel computation, the requirement to always be in perfect sync would slow you down badly.

So even for QM, if the principle explanation is to be perfectly similar to the principle explanation of SR, then I would also expect a similar explanation of the advantages of violating the classical principles.

 

[bhobba]

In the axiomatic reconstructions of QM, this is called Information Invariance & Continuity. It is indeed equivalent to the mechanism of "average-only" conservation, as detailed in the first paper linked in my Post #62.

I know a bit about those reconstructions, and I think you may be correct. But would like to see what others say on the issue. I also like the video you posted. I know the five reasonable axioms paper well but have not kept up with developments beyond that. Since reading that paper years ago, I have always thought of QM as a generalised probability theory - the simplest after ordinary probability theory that allows 'continuity' as you put it and hence the methods of calculus. It reaches its full flowering when generalised to Rigged Hilbert Spaces, which I also have noticed books are now pointing out is the real QM space - not Hilbert Spaces (although Von Neumann did show how it could be formulated using just Hilbert Spaces).

Thanks
Bill

 

[bhobba]

I thought the mechanism would be "relativity of simultaneity," together with a definition of how to measure lengths based on measuring times (for why meter sticks shrink).

I think in modern times, the foundations of Special Relativity are well known to be the symmetries of an inertial reference frame and the POR. C is simply a constant that appears in the theory that needs to be fixed by experiment. From both theoretical and experimental considerations, it is the speed of light but is not an axiom at its foundations. See a paper I post a lot:
http://www2.physics.umd.edu/~yakovenk/teaching/Lorentz.pdf

Thanks
Bill

 

[RUTA]

I think we had this discussion a while ago. In fact in the here described EPR-paradox version with angular momentum (spin) a la Bohm, it's easy to see that of course the conservation laws hold event by event. To prove this, of course, you have to measure the angular-momentum component in the same direction on both particles, because you can only determine one momentum component due to the commutator relations of angular-momentum components. The $J=0$ state of the total system is an exception, because for this state all three components take simultaneously the same value, 0.

If you measure different components of the angular momentum on both particles, of course you cannot verifty angular-momentum conservation, because the outcomes of the measurements are random with a probabilities for the possible outcome for the two angular-momentum components given by Born's rule. This also includes, of course, the violations of Bell's inequality, which cannot be described with local deterministic hidden-variable theories. This has, however, nothing to do with the claim that the conservation laws only hold "on average".

I'll try to read the papers, as soon as I find the time.

Now that would be, as Peter Donis said, an interpretation of the mainstream view of Bell state data.

The mainstream view of the data is that when Alice and Bob measure a Bell state (say, a triplet state in the symmetry plane) at some particular SG magnet orientation, then both will always get the same result, half the time they will both get +1 and half the time they will both get -1. If Bob changes his SG magnet orientation making an angle $\theta$ with Alice's orientation, then they will observe "average-only" conservation. By that I mean that if Alice partitions the data according to her equivalence relation (her +1 and -1 results), she will see that Bob's results average to $\pm \cos{\theta}$, respectively. If Bob partitions that same data according to his equivalence relation, he will see that Alice's results average to $\pm \cos{\theta}$. I'm using the phrase "average-only" conservation per standard physics lingo to characterize this mainstream view of Bell state data as follows.

Alice can say that if Bob had not changed his SG magnet orientation, he would have measured +1 when she measured +1, as required to conserve spin angular momentum. So, when he measured at angle $\theta$ with respect to her, he should have gotten the projection of his +1 result, i.e., $\cos{\theta}$ (analogously with their -1 result). So, his results are only satisfying the conservation of spin angular momentum on average according to her partition of the data. Indeed, his results are not a Gaussian about $\cos{\theta}$, but they give a binary distribution whereby he never measures $\cos{\theta}$ (thus, "average-only" conservation). Of course, Bob can say the same thing about Alice's results per his partition of the data.

This is totally analogous to their partitions of M4 when they occupy different reference frames related by Lorentz boosts, i.e., uniform relative motion. There Bob can partition the events of M4 per his equivalence relation (his surfaces of simultaneity) and say that Alice's meter sticks are short and her clocks run slow. And, of course, Alice can partition the events of M4 per her equivalence relation (her surfaces of simultaneity) and say that Bob's meter sticks are short and his clocks run slow. This is called the relativity of simultaneity and was a key concept in Einstein's development of special relativity (according to John Norton, anyway).

You can make the analogy stronger by noting that the different SG orientations are related by spatial rotations and spatial rotations relate inertial reference frames in both Galilean and Lorentz transformations. So, you could characterize the relativity of simultaneity and "average-only" conservation as consequences of the relativity principle (as Einstein did for the former), i.e., "no preferred reference frame." But, that would be a proposal, as Peter Donis said, because these facts could also hold where there is a preferred reference frame. Indeed, Unnikrishnan is a strong advocate for a preferred reference frame, that's why he does not support our proposal to invoke NPRF to explain his average conservation. He believes average conservation is enough to resolve the mystery of entanglement and a universal preferred reference frame resolves other mysteries for him.

Sorry for the confusion. Despite the fact that I've been teaching college physics for over 40 years, I'm not a very good teacher as it turns out :-(

 

[RUTA]

I think in modern times, the foundations of Special Relativity are well known to be the symmetries of an inertial reference frame and the POR. C is simply a constant that appears in the theory that needs to be fixed by experiment. From both theoretical and experimental considerations, it is the speed of light but is not an axiom at its foundations. See a paper I post a lot:
http://www2.physics.umd.edu/~yakovenk/teaching/Lorentz.pdf

Thanks
Bill

Here is an interesting paper by John Norton (I mentioned this in my Post #69, but did not reference it) "Einsteinʼs Special Theory of Relativity and the Problems in the Electrodynamics of Moving Bodies that Led him to it." On p. 6 at the outset of Section 2.2 "Relativity of simultaneity" he writes:

Einstein pointed out immediately that the two postulates were “apparently irreconcilable.” His point was obvious. If one inertially moving observer measures c for the speed of some light beam, what must be measured by another inertially moving observer who chases after the light beam at high speed—say 50% of c or even 99% of c? That second observer must surely measure the light beam slowed. But if the light postulate respects the principle of relativity, then the light postulate must also hold for this second, inertially moving observer, who must still measure the same speed, c for the light beam.

How could these conflicting considerations be reconciled? Einstein’s solution to this puzzle became the central conceptual innovation of special relativity. Einstein urged that we only think the two postulates are incompatible because of a false assumption we make tacitly about the simultaneity of events separated in space. If one inertially moving observer judges two events, separated in space, to be simultaneous, then we routinely assume that any other observer would agree. That is the false assumption. According to Einstein’s result of the relativity of simultaneity, observers in relative motion do not agree on the simultaneity of events spatially separated in the direction of their relative motion.

The mystery of entanglement (understood via spin measurements, anyway) follows from the structure of an elementary piece of quantum information, i.e., the quantum bit or qubit. Contrary to the classical bit, it is possible to change the SG magnet orientations in continuous fashion and still get your binary $\pm 1$ outcomes for a qubit. A classical bit has discrete choices of measurement, e.g., open box A or box B, to go along with its discrete binary outcomes, e.g., a ball is in the box or not. Your classical intuition applied to spin would be to expect fractions of $\pm 1$ as you rotate your SG magnets, e.g., $\pm \cos{\theta}$. But, you don't, you always get $\pm 1$ for all $\theta$. That's what Hardy discovered in what is considered to be the first axiomatic reconstruction of QM based on information-theoretic principles,

Quantum Theory From Five Reasonable Axioms​

He writes, "If Axiom 5 (or even just the word "continuous" from Axiom 5) is dropped then we obtain classical probability theory instead." Here is what Koberinski and Mueller had to say about it in "Quantum Theory as a Principle Theory: Insights from an Information-Theoretic Reconstruction":

We suggest that (continuous) reversibility may be the postulate which comes closest to being a candidate for a glimpse on the genuinely physical kernel of ``quantum reality''. Even though Fuchs may want to set a higher threshold for a ``glimpse of quantum reality'', this postulate is quite surprising from the point of view of classical physics: when we have a discrete system that can be in a finite number of perfectly distinguishable alternatives, then one would classically expect that reversible evolution must be discrete too. For example, a single bit can only ever be flipped, which is a discrete indivisible operation. Not so in quantum theory: the state |0> of a qubit can be continuously-reversibly ``moved over'' to the state |1>. For people without knowledge of quantum theory (but of classical information theory), this may appear as surprising or ``paradoxical'' as Einstein's light postulate sounds to people without knowledge of relativity.

But, while you always get $\pm 1$ instead of $\pm \cos{\theta}$ per your classical intuition for the spin qubit, the $\pm 1$ do average to $\pm \cos{\theta}$ when, say, you're making measurements at $\theta$ with respect to the z axis on the spin up/down z state. As Weinberg said in "The Trouble with Quantum Mechanics", measuring an electron's spin via SG magnets constitutes the measurement of "a universal constant of Nature, Planck's constant h." So, demanding all SG orientations (inertial reference frames related by spatial rotations) must measure the same value of h, just like the light postulate, we can use "average-only" projection (and normalization) for the qubit to obtain the probabilities $P(+1|\theta) = \cos^2{\left(\frac{\theta}{2}\right)}$ and $P(-1|\theta) = \sin^2{\left(\frac{\theta}{2}\right)}$, which gives "average-only" conservation for qubits entangled in a Bell state.

So, there are a lot of analogies with SR if you consider the relativity principle and light postulate, but this is a proposal, not a statement of mathematical facts alone :-)

 

[PeroK]

Certainly a nice paper, but my goal was for RUTA to acknowledge that "relativity of simultaneity" is indeed a concrete mechanism in case of SR, actually explaining why meter sticks shrink.

Alternatively, "simultaneity" is not a concept that has any physical significance. And metre sticks "shrink" when they are rotated in spacetime.

 

[gentzen]

Alternatively, "simultaneity" is not a concept that has any physical significance. And metre sticks "shrink" when they are rotated in spacetime.

Sorry, I deleted my answer for the moment, to have a bit more time for editing, and better understanding how the papers relate to that answer to vanhees71. For example, table 3 in "Answering Mermin’s challenge with conservation per no preferred reference frame" (the paper which triggered my comment) reads:

Empirical Fact: Alice and Bob both measure c,	Empirical Fact: Alice and Bob both measure $\pm 1(\frac{\hbar}{2})$,
regardless of their motion relative to the source	regardless of their SG orientation relative to the source
Alice(Bob) says of Bob(Alice): Must correct time and length measurements	Alice(Bob) says of Bob(Alice): Must average results
NPRF: Relativity of simultaneity	NPRF: Relativity of data partition

So the totally analogous thing for "relativity of simultaneity" was not "average-only conservation" (which provoked the reactions by vanhees71 and me), but instead "relativity of data partition". And this "relativity of data partition" is indeed quite a quantum thing, where often the quantum mysteries arise via post-selection.

 

[RUTA]

Sorry, I deleted my answer for the moment, to have a bit more time for editing, and better understanding how the papers relate to that answer to vanhees71. For example, table 3 in "Answering Mermin’s challenge with conservation per no preferred reference frame" (the paper which triggered my comment) reads:

Empirical Fact: Alice and Bob both measure c,	Empirical Fact: Alice and Bob both measure $\pm 1(\frac{\hbar}{2})$,
regardless of their motion relative to the source	regardless of their SG orientation relative to the source
Alice(Bob) says of Bob(Alice): Must correct time and length measurements	Alice(Bob) says of Bob(Alice): Must average results
NPRF: Relativity of simultaneity	NPRF: Relativity of data partition

So the totally analogous thing for "relativity of simultaneity" was not "average-only conservation" (which provoked the reactions by vanhees71 and me), but instead "relativity of data partition". And this "relativity of data partition" is indeed quite a quantum thing, where often the quantum mysteries arise via post-selection.

Sorry I haven't responded to you directly, gentzen. I appreciate your questions, so let me do so here.

First, I would be careful with this table. We argue for NPRF ("no preferred reference frame" aka the relativity principle) to give SR and QM a common principle basis, but the addition of NPRF is an opinion/proposal -- the relativity of simultaneity and relativity of data partition are true even if you don't believe in NPRF. The relativity of simultaneity and relativity of data partition are just mainstream physics concerning M4 and the Bell states, whereas NPRF is a contentious add on (surprisingly to me, given the situation with SR today, but ...).

Second, the phrase "relativity of data partition" in our paper refers to the symmetry of the Bell state data referenced in the preceding table row -- Alice(Bob) says Bob(Alice) must average his(her) results -- and that is referring to average-only conservation. The way you're thinking about the relativity of data partition seems to be much broader than our use of the term. Maybe yours is a more robust way to relate SR and QM, we were just trying to garner support for a principle explanation of entanglement via parallels with SR :-)

 

[RUTA]

Alternatively, "simultaneity" is not a concept that has any physical significance. And metre sticks "shrink" when they are rotated in spacetime.

Yes, the standard view is that the length and time between events are relative (can differ from reference frame to reference frame), but the spacetime distance between events is the same for all reference frames.

 

[PeterDonis]

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.

"Mechanism" does not mean the same thing as "mystery". If all you are claming is that "average only conservation" is a way of describing the mystery of quantum entanglement, that's fine, the references you give are open to everyone to read and decide for themselves. But to claim that average only conservation is the "mechanism" of entanglement, which was your original claim in this thread that I objected to, is to claim that average only conservation solves the mystery. When the OP asks what the "mechanism" is behind quantum entanglement, they are not asking for a description of the mystery, they are asking for a solution of the mystery. And the correct answer to that (with which you appear to agree) is that there is no generally accepted solution; various QM interpretations propose solutions, but none are generally accepted and all of them leave issues unresolved.

 

[RUTA]

"Mechanism" does not mean the same thing as "mystery". If all you are claming is that "average only conservation" is a way of describing the mystery of quantum entanglement, that's fine, the references you give are open to everyone to read and decide for themselves. But to claim that average only conservation is the "mechanism" of entanglement, which was your original claim in this thread that I objected to, is to claim that average only conservation solves the mystery. When the OP asks what the "mechanism" is behind quantum entanglement, they are not asking for a description of the mystery, they are asking for a solution of the mystery. And the correct answer to that (with which you appear to agree) is that there is no generally accepted solution; various QM interpretations propose solutions, but none are generally accepted and all of them leave issues unresolved.

I took "mechanism of entanglement" to mean "what is responsible for the mystery of entanglement." Knowing the mechanism in that sense may suffice to resolve the mystery, as average-only conservation does for Unnikrishnan and his followers. I have the same issue with the phrase "explain entanglement." Sometimes that means to simply convey the mystery of entanglement, e.g., the Mermin device. But, sometimes when someone says they are going to "explain entanglement" they mean their explanation will also solve the mystery. Semantics is always open to interpretation.

 

[bhobba]

"Mechanism" does not mean the same thing as "mystery".

IMHO none of this stuff addresses the mystery of QM. All it does is help elucidate why its formalism is the way it is. I generally don't like using 'reality' in these discussions, but here I think it is useful. It helps to understand QM phenomenologically, but not the 'reality' (whatever that is). As I mentioned, we do not have direct experience with the quantum world; we only know about it via interactions with things we do have direct experience with.

Thanks
Bill

 

[Fra]

'reality' (whatever that is). As I mentioned, we do not have direct experience with the quantum world; we only know about it via interactions with things we do have direct experience with.

If one takes this seriously, it applies to anything in the observers/agents environment, not only small things, so the observer/agent has no "direct experience" with anyhing. It all has to "pass" the inference system of the agent. There is no bypass for information.

If one then acknowledges that any agent is just matter, then this gets us into a potential deep insight of the relational nature of interactions and nature of law.

This is how i choose to understand the bell experiment in the first place as the inference shield is what protects the ansatz in bells theorem as it assumes that non-inferrable information (ie hidden variables) rules the way parts of the universe respond to other parts. Instead they respond to the actual information - which is the prepared state(as long as the entanglement isn't broken) and the local detector settingsm. The latter is described by QM.

This is the simple way to understand the a priori problem with bells naive ansatz.

/Fredrik

 

[gentzen]

I thought the mechanism would be "relativity of simultaneity," together with a definition of how to measure lengths based on measuring times (for why meter sticks shrink).

I think in modern times, the foundations of Special Relativity are well known to be the symmetries of an inertial reference frame and the POR. C is simply... From both theoretical and experimental considerations, it is the speed of light but is not an axiom at its foundations. See a paper I post a lot:

Certainly a nice paper, but my goal was for RUTA to acknowledge that "relativity of simultaneity" is indeed a concrete mechanism in case of SR, actually explaining why meter sticks shrink. He has done exactly this in his answer to vanhees71 now, and explained what is the totally analogous mechanism in QM from his perspective:

This is totally analogous to their partitions of M4 when they occupy different reference frames related by Lorentz boosts, i.e., uniform relative motion. There Bob can partition the events of M4 per his equivalence relation (his surfaces of simultaneity) and say that Alice's meter sticks are short and her clocks run slow. And, of course, Alice can partition the events of M4 per her equivalence relation (her surfaces of simultaneity) and say that Bob's meter sticks are short and his clocks run slow. This is called the relativity of simultaneity and was a key concept in Einstein's development of special relativity (according to John Norton, anyway).

Interestingly, this explanation can also be found in the papers. At the moment, it feels like a nice explanation to me, and as basically what I asked for.

You can make the analogy stronger by noting that ...

After some reflection, besides that confusion (now cleared-up) with "average-only conservation," I feel that SR is not given sufficiently credit, and that this makes the analogy weak. For example, "simultaneity" in SR doesn't just tell you which events happen at exactly the same instant, but also which happened earlier or which will happen later.

No idea whether there should be something analogous for QM, or what an appropriate grouping for QM should look like.

Alternatively, "simultaneity" is not a concept that has any physical significance. And metre sticks "shrink" when they are rotated in spacetime.

Sorry, I deleted my answer for the moment, to have a bit more time for editing, ...

Well, this is the edited answer now. Turns out this was a bad idea, I won't do it again.

Instead of simultaneity, Mermin's book also looks at the order in which the observer learns about the events, assuming a signal is sent from each event directly to the observer with the speed of light. One might argue that this order has more physical significance than the derived concept of "simultaneity". I believe Roger Penrose's Twistor theory takes something like this as its starting point, but I could be wrong.

 

[bhobba]

If one takes this seriously, it applies to anything in the observers/agents environment, not only small things, so the observer/agent has no "direct experience" with anyhing. It all has to "pass" the inference system of the agent.

With my Mentors hat on, I would ask we do not take such discussions any further. That is why I don't like to use 'reality' as it opens the door to philosophy. We recognise that some basic philosophical issues inevitably arise when discussing QM foundations, so allow some leeway. Where to draw the line has no hard and fast rules, but posts like the above IMHO cross the line. All I noted is that we have direct interaction with objects from the Newtonian world and intuitively have a good picture of what is going on. Such is not the case with the Quantum World. That's it, that's all. The status of what reality is etc is not part of that - it is a philosophical issue to be taken up not here but on a philosophy forum.

Thanks
Bill

 

[Lord Jestocost]

All I noted is that we have direct interaction with objects from the Newtonian world and intuitively have a good picture of what is going on. Such is not the case with the Quantum World. That's it, that's all.

Maybe, too many false hopes have indeed been awakened by classical physics: That the world of experience would sooner or later reveal something of an ontic world beyond it, a world of 'objective reality'.

 

[Morbert]

Asking for the mechanism behind quantum entanglement doesn't seem right to me.

If we are allowed to accept quantum mechanics as given, then the question is straightforwardly answered: entanglement is established in the preparation.

If we are not allowed to accept quantum mechanics, i.e. if we are really asking for a mechanism behind the existence of correlations that are classically forbidden, then the question seems to presuppose a mechanical rather than nomological grounding of entanglement, which is contrary to the character of fundamental physical laws
"what turns out to be true is that the more we investigate, the more laws we find, and the deeper we penetrate nature, the more this disease persists . Every one of our laws is a purely mathematical statement in rather complex and abstruse mathematics . Newton's statement of the law of gravitation is relatively simple mathematics . It gets more and more abstruse and more and more difficult as we go on. Why? I have not the slightest idea" -- Richard Feynman

 

[Fra]

Asking for the mechanism behind quantum entanglement doesn't seem right to me.

If we are allowed to accept quantum mechanics as given, then the question is straightforwardly answered: entanglement is established in the preparation.

If we are not allowed to accept quantum mechanics, i.e. if we are really asking for a mechanism behind the existence of correlations that are classically forbidden, then the question seems to presuppose a mechanical rather than nomological grounding of entanglement, which is contrary to the character of fundamental physical laws
"what turns out to be true is that the more we investigate, the more laws we find, and the deeper we penetrate nature, the more this disease persists . Every one of our laws is a purely mathematical statement in rather complex and abstruse mathematics . Newton's statement of the law of gravitation is relatively simple mathematics . It gets more and more abstruse and more and more difficult as we go on. Why? I have not the slightest idea" -- Richard Feynman

This is a good point which should encourage ourselves to ask, why are we asking why questions of scientific facts? Is it so we can sleep at night, or do we have a better reason?

I am not even sure that we all can agree upon an answer to that question.

/Fredrik

 

[vanhees71]

"Mechanism" does not mean the same thing as "mystery". If all you are claming is that "average only conservation" is a way of describing the mystery of quantum entanglement, that's fine, the references you give are open to everyone to read and decide for themselves. But to claim that average only conservation is the "mechanism" of entanglement, which was your original claim in this thread that I objected to, is to claim that average only conservation solves the mystery. When the OP asks what the "mechanism" is behind quantum entanglement, they are not asking for a description of the mystery, they are asking for a solution of the mystery. And the correct answer to that (with which you appear to agree) is that there is no generally accepted solution; various QM interpretations propose solutions, but none are generally accepted and all of them leave issues unresolved.

No, the claim the conservation laws were valid only on average is empirically ruled out. There has been a then famous theory by Kramers and Bohr, which has been disproven by Bothe using his coincidence measurements on Compton scattering demonstrating the event-by-event validity of the conservation of energy and momentum, and that's, of course in accordance with standard quantum theory.

Also standard QT is in accordance with the non-relativistic or special-relatistic space-time symmetries. In fact these fundamental concepts are at the heart of their formulation, providing the operator algebras describing observables.

The "mechanism" behind entanglement is also simply standard QT. It may seem unfamiliar to our everyday intuition, but it's no mystery. Standard QT describes to an amazing precision the corresponding correlations.

 

[sakshiverma]

Quantum entanglement is a strange phenomenon that has puzzled scientists for years. It occurs when particles are linked together in such a way that they share information instantaneously regardless of the distance between them. This means that if one particle is in state A, then another particle will be in state B no matter how far apart they may be.

This bizarre property has been used to achieve some amazing things, like sending messages through space without wires and creating secure communications between two quantum systems. In addition, it provides us with an understanding of the nature of reality as we know it and opens up many opportunities for future technology developments.

There are still many unanswered questions about quantum entanglement which continues to intrigue scholars and enthusiasts alike.

 

[vanhees71]

Which scientific questions are unanswered? All known experiments confirm the predictions of QT, and indeed, these very generic quantum properties now become applied on an engineering level, which indeed shows in a very convincing way that it is a very well understood phenomenon!

 

[RUTA]

No, the claim the conservation laws were valid only on average is empirically ruled out. There has been a then famous theory by Kramers and Bohr, which has been disproven by Bothe using his coincidence measurements on Compton scattering demonstrating the event-by-event validity of the conservation of energy and momentum, and that's, of course in accordance with standard quantum theory.

You clearly haven't understood the papers I referenced. Your comment has nothing to do with the average conservation explained in those papers. It is a complete non sequitur.

 

[vanhees71]

I'm not referring to any papers but to the claim that conservation laws were valid only on average. This is an empirically falsified statement!

 

[RUTA]

In post #76, I pointed out that people sometimes say they will "explain entanglement," but they do not mean they will solve the mystery of entanglement. Here is an example of that by John Preskill in a 2020 talk titled, "Entanglement Explained!" Therein he makes no attempt whatsoever to resolve the mystery of entanglement.

 

[RUTA]

I'm not referring to any papers but to the claim that conservation laws were valid only on average. This is an empirically falsified statement!

But, you entered your claim via a response to a post about the average conservation in the referenced papers, which is a empirically verified concept and perfectly consistent with textbook QM. Why would you make your claim in response to a comment about average conservation in those papers when what you're talking about has absolutely nothing to do with those papers?

 

[RUTA]

Which scientific questions are unanswered? All known experiments confirm the predictions of QT, and indeed, these very generic quantum properties now become applied on an engineering level, which indeed shows in a very convincing way that it is a very well understood phenomenon!

One etymology of the word "science" is from the Latin "scientia" which means "knowledge," and science is considered by many to be the search for knowledge. Therefore, one answer to your question is that many of us are seeking more knowledge about entanglement. We understand the QM formalism with its empirical verification and technological applications. We want to know something else, i.e., is there a reason why Nature harbors entanglement as given by QM? We use forums such as this to share our different answers to that question. If you don't have any ideas to contribute, you don't have to participate.

 

[gentzen]

There are still many unanswered questions about quantum entanglement which continues to intrigue scholars and enthusiasts alike.

Which scientific questions are unanswered? All known experiments confirm the predictions of QT, and indeed, these very generic quantum properties now become applied on an engineering level, which indeed shows in a very convincing way that it is a very well understood phenomenon!

How about the following?

• Whether Bit commitment is still possible in a world with quantum computers & co. You might object that there are proofs that unconditionally secure ("provably unbreakable") quantum bit commitment is impossible. But that is not the point, only exponentially hardness is requested, just like in the classical case.
• Whether it is possible in principle to build scalable quantum computers, in a similar way as it is possible to build scalable classical computers. Again you might object that it was already proven that this is possible in principle. But was it really proven?
• Whether quantum randomness allows to draw a random number at a specific point in time, and provide proof (again just exponential hardness is requested, not an "unconditional proof") that the random number indeed was drawn at the claimed point in time, that it was not known before, and that it was not manipulated. And again you might object that it was already proven that this is possible. But was it really proven?

Or maybe you instead object that you never heard of that stuff, and that this is not what you meant by "scientific questions". Perhaps actual discussion about whether some specific experiment qualifies or not, like here for device-independent quantum key distribution might convince you that such questions at least feel scientific to the involved scientists.