What is the mechanism behind Quantum Entanglement?

[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.

 

[bhobba]

It occurs when particles are linked together in such a way that they share information instantaneously regardless of the distance between them.

The principle of superposition clearly explains what is going on. As Bell explained, it leads to statistical correlations different to standard probability theory unless they can share information instantaneously. You either have a probability scheme of a more general sort or standard probability with spooky instantaneous action at a distance. Take your pick. Until an experiment can be devised to separate the two, it is simply what you prefer - like interpretations. My view is we do not have direct experience with the Quantum world. Hence, I have no problem with the idea it may involve a probability structure different to everyday intuition.

While 'strange', 'unusual' etc can all apply, I certainly don't find it a mystery, but I suppose everyone is different.

Thanks
Bill

 

[bhobba]

Therein he makes no attempt whatsoever to resolve the mystery of entanglement.

I don't find any mystery in QM being a different generalised probability theory than standard probability theory. Maybe it's just me.

Thanks
Bill

 

[Fra]

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.

I find it amusing that we adore the current scientific knowledge of corroborated theories, but the scientific method which is what keeps evolving science falls into the realm of "philosophy" and is something that is considered irrelevant for science.

For an experimental physicists or engineer, I totally get that discussing methodology is somehow off topic, as the feedback is direct experimental feedback. If you have such prime feedback, indeed it seems like a waste of time to worry about alternative ways to model the same thing.

But for a theoretical physicist that is using existing data(sometimes very old data) and existing theories and tries to expand the theory or merge theories one needs abstract constructing principles or consistency requirements that themselves as methods are by definition now "scientific facts" but which somehow seem necessary. Indeed excellent examples of theoretical constructing principles are the "no preferred frame" or "observer equivalence" principles, or various other symmetry arguments, they can be extremely powerful. Yet I don't think we can call such design tools are hardly "scientific facts" in the ordinary sense. It's like the unreasonable effectiveness of mathematics that you may or may not find interesting to discuss.

So where to draw the line here in the discussion must be different depending on where in the experimental-theoretical range you are??

/Fredrik

 

[Fra]

I don't find any mystery in QM being a different generalised probability theory than standard probability theory. Maybe it's just me.

I symphatize with this. But for me, when this "generalized probability" is digested, a new conceptual understanding emerges for me at least, and when trying to understand physics in these terms, things just does not add up right. How do understand gravity in QM and also entanglement are it seems two key areas. So while I am with you in the "generalized probability" line of reasoning, many details are unclear to make the puzzle complete.

/Fredrik

 

[RUTA]

I don't find any mystery in QM being a different generalised probability theory than standard probability theory. Maybe it's just me.

Thanks
Bill

And why this particular generalized probability theory? There are others besides QM. No preferred reference frame is one answer to that question. It selects QM over classical probability theory and the PR box, thereby providing a reason for the Tsirelson bound. Everyone has their own terminus for their curiosity, so someone might then ask, "Why NPRF?" And so on.

 

[vanhees71]

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.

Well, I'd like to understand, what these problems might be. Theoretical physics is about the mathematical description of the observable objective phenomena of Nature. QT is the hitherto most comprehensive model describing all hitherto known matter. The question, why the natural laws are as we observe them cannot be answered by the scientific method.

Entanglement and the implied long-range strong correlations between far-distantly observed parts of an entangled system is indeed a good example: The "philosophical problems" EPR had at their time with this consequence of QT has been answered with the following development of the issue thanks to Bell's work, who found a way to transform the vague philosophical question posed in the EPR paper to a clear scientifically testable hypothesis, and all the experiments testing the corresponding Bell inequalities following from local deterministic hidden-variable theories are contradicting these inequalities in precisely the way as it is predicted by QT.

What else then is it, what's still "not understood"? To participate in a scientific discussion, one has to understand, what's discussed! If I get an answer clearly contradicting empirical facts, I think it's legitimate to clarify where I might have misunderstood something and it's also legitimate to critizise such obviously empirically refuted claims.

 

[vanhees71]

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.

Indeed, I think from the theoretical point of view, these questions are answered, and the problem is indeed to realize, e.g., scalable quantum computers, is now in the realm of an engineering problem and as such it's of course a scientific one.

It's, however, not a problem about the foundations of quantum theory. Of course, if in the work towards this goal observations are made, which reproducibly contradict QT, then there's a fundamental problem with QT, but it's not a philosophical quibble about some "ontological implication of QT" but a scientific problem to modify QT to describe the new empirical discoveries. That's how the natural sciences work!

 

[vanhees71]

I find it amusing that we adore the current scientific knowledge of corroborated theories, but the scientific method which is what keeps evolving science falls into the realm of "philosophy" and is something that is considered irrelevant for science.

But it's not "philosophy" which evolves the natural sciences but ever more precise observation and theory building based on that observations.

For an experimental physicists or engineer, I totally get that discussing methodology is somehow off topic, as the feedback is direct experimental feedback. If you have such prime feedback, indeed it seems like a waste of time to worry about alternative ways to model the same thing.

That's also not true. To find new methods to solve problems within a given theoretical framework is the bread-and-butter work of theoretical physicists. E.g., it's of great value to have formulated one and the same QT in various different ways and to understand that these different ways are indeed descriptions of the same theory. This has been true even historically, where QT has been discovered almost simultaneously in two different formulations (matrix and wave mechanics). In this case it was rather quickly understood that both are indeed formulations of the same theory, which could be formulated in a more general framework (by Dirac and mathematically rigorously by von Neumann). Somewhat later Feynman found his path-integral formulation. All of these formulations of the same theory have their merits in providing calculational tools to apply the theory to all kinds of different problems.

But for a theoretical physicist that is using existing data(sometimes very old data) and existing theories and tries to expand the theory or merge theories one needs abstract constructing principles or consistency requirements that themselves as methods are by definition now "scientific facts" but which somehow seem necessary. Indeed excellent examples of theoretical constructing principles are the "no preferred frame" or "observer equivalence" principles, or various other symmetry arguments, they can be extremely powerful. Yet I don't think we can call such design tools are hardly "scientific facts" in the ordinary sense. It's like the unreasonable effectiveness of mathematics that you may or may not find interesting to discuss.

This is also right, but if you impose assumptions, like the invalidity of the conservation laws on an event-by-event basis, which are empirically refuted for almost a century, this cannot be a valid tool to extent theories. I've still to read the papers, where this claim is made. So far I only followed the discussion here, and from this I get that this statement is contradicting both the theory and empirical well-known facts. Of course, QT obeys the "no preferred frame" principle by construction.

So where to draw the line here in the discussion must be different depending on where in the experimental-theoretical range you are??

/Fredrik

No, experiment and theory are in close relationship, but theorists must be careful not to loose contact to the established empirical facts, among them the validity of conservation laws on an event-by-event basis.

 

[CoolMint]

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.

Some in the community have thrown the idea of realism out the window a decade ago, esp. after Bell. It does resolve a lot of conceptual issues and some why questions. Would it resolve some/most of your issues with entanglement if suddenly it appeared that realism at the quantum scale is no longer tenable?

 

[Fra]

The question, why the natural laws are as we observe them cannot be answered by the scientific method.

While I think may be true, just like science can never claim verification of anything, only corroboration...

"To suppose universal laws of nature capable of being apprehended by the mind and yet having no reason for their special forms, but standing inexplicable and irrational, is hardly a justifiable position. Uniformities are precisely the sort of facts that need to be accounted for. Law is par excellence the thing that wants a reason. Now the only possible way of accounting for the laws of nature, and for uniformity in general, is to suppose them results of evolution."
-- Charles Sanders Peirce, 1891, https://www.jstor.org/stable/27896847?searchText=The+architecture+of+theories&searchUri=%2Faction%2FdoBasicSearch%3FQuery%3DThe%2Barchitecture%2Bof%2Btheories%26so%3Drel&ab_segments=0%2FSYC-6451%2Fcontrol&refreqid=fastly-default%3Af66b3685f9ddbaaddc446656bc4c9b92#metadata_info_tab_contents

Pursuing this further by definition takes us into the philosophy of science so I will pass details. But I still think that as we ponder about the deepest theories of the world, and the presumably deepest laws, it is very hard to ignore questioning the very inference process that leads us there, although Popper tried to play down the inductive reasoning and highlight the selection.

I think we should not interpret this in the way that we should "question empirical facts". It also does not mean we should "think our way to enlightment". We need real feedback of course. After all, we do not "directly observe" laws of nature. It's usually described as and abduction of best explanation from more "raw observations". So even if we do not question the "raw feedback", it seems sound to reflect the way the abduction is done, as the resulting theory is obviously not unique. As we know there are many possible "theories" to explain the same "raw observations", and we tend to use the simplest one, but what is the qualified measure of simplicity?

So let me rephrase the question of "why these laws" into "why do we abduce these laws and not others". From the point of view or falsification, it is indeed irrelevant HOW anyone came up with a hypothesis. But from the perspective of intelligent learning it seems critical? If we can not answer that, it would effectively treat theory makers as monkeys, which has actually not been a bad suggestion in other fields.
https://www.forbes.com/sites/rickferri/2012/12/20/any-monkey-can-beat-the-market/

/Fredrik

 

[Fra]

But it's not "philosophy" which evolves the natural sciences but ever more precise observation and theory building based on that observations.

Over time, it indirectly does as philosophy has evolved the scientifid method. Long time ago in the dark ages a probable opinion, was simply that of an educated person or authority, and the careful person would not oppose. Philosophical reasoning lead to search for a more "rational beliefs" that was was quantitative and more objective, which lead to probability theory for example. In this sense, the foundation of scientic method follows from philosophical arguments.

No, experiment and theory are in close relationship, but theorists must be careful not to loose contact to the established empirical facts

100% agreed. But we must not confused actualy raw empirical facts with abduced beliefs, which I think is often done.

/Fredrik

 

[RUTA]

Well, I'd like to understand, what these problems might be. Theoretical physics is about the mathematical description of the observable objective phenomena of Nature. QT is the hitherto most comprehensive model describing all hitherto known matter. The question, why the natural laws are as we observe them cannot be answered by the scientific method.

We're not trying to answer that question by the scientific method. Did you happen to check the name of the forum you're posting in? Quantum Interpretations and Foundations is not restricted to what can be known by the scientific method. These topics have a strong philosophical bent. If you are not interested in discussing such issues, you do not have to be here telling everyone that you're not interested. Do you also visit forums on how to play chess and tell them they're not discussing the scientific method?

Entanglement and the implied long-range strong correlations between far-distantly observed parts of an entangled system is indeed a good example: The "philosophical problems" EPR had at their time with this consequence of QT has been answered with the following development of the issue thanks to Bell's work, who found a way to transform the vague philosophical question posed in the EPR paper to a clear scientifically testable hypothesis, and all the experiments testing the corresponding Bell inequalities following from local deterministic hidden-variable theories are contradicting these inequalities in precisely the way as it is predicted by QT.

Would Bell have produced his work without the philosophical discussions by Einstein, Bohr, etc.?

What else then is it, what's still "not understood"? To participate in a scientific discussion, one has to understand, what's discussed!

What else is there to understand? I pointed it out and you repeated it in your post. Five sentences later and you've forgotten what you wrote?

If I get an answer clearly contradicting empirical facts, I think it's legitimate to clarify where I might have misunderstood something and it's also legitimate to critizise such obviously empirically refuted claims.

There is nothing contradicting scientific facts in average-only conservation. Read the papers and ask for help if you need it before making fallacious assertions.

 

[vanhees71]

Ok, let's start. I'm now reading

https://doi.org/10.1038/s41598-020-72817-7

It's clear that both non-relativistic QM and special relativistic QFT are constructed in such a way to be compatible with the underlying spacetime symmetries (Galilei and Poincare symmetries, respectively). So it's no surprise that it predicts that any spin component of a particle has the same spectrum, i.e., $(-s \hbar, (-s+1)\hbar,\ldots (s-1) \hbar, s \hbar)$.

If I understood the rest of the paper right (restricting myself to the much simpler standard representation in the Methods section rather than the overcomplicated "Mermin-device narrative"), the issue simply is that if you have an entangled spin state and measure spin components in different directions on the two particles, you cannot verify angular-momentum conservation. That's no surprise, and I don't see, where there should be any problem with that. It's also a strange wording to say you change the reference frame just by measuring different spin components.

Concerning angular-momentum conservation this state is due to the decay of a spin-0 particle (in its rest frame) to two spin-1/2 particles, and indeed the total angular momentum stays 0 in this case, and you can verify this conservation law by measuring both particles in the same direction, which can be arbitrarily chosen. That's a special case, because the $S=0$ states are rotational invariant and here indeed all total-spin components take simultaneously the determined value 0.

It's also clear that it makes a difference whether you have prepared the two-particle system in the (unique) spin singulett state, $1/\sqrt{2} (|ud \rangle-|du \rangle)$, which is of course rotational invariant, because it's the $S=0$ state, or one of the three triplet states $|S=1,M=1 \rangle=|uu \rangle$, $|S=1,M=0 \rangle =1/\sqrt{2} (|ud \rangle + |du \rangle)$, and $|S=1,M=1 \rangle=|dd \rangle$, because this basis is not rotationally invariant but transforms under the $S=1$ representation.

Concerning conservation laws, one of these states is realized by the decay of a spin-1 particle (in its rest frame) to two spin-1/2 particles. To verify the angular-momentum conservation law you have to prepare this mother particle in a specific spin state $M=1$, $M=0$, or $M=-1$ for an arbitrary component of the spin (which usually is called the $z$ direction). To verify angular-momentum conservation in the decay process you now have to measure the spin components of both decay products in the same $z$ direction. In any other direction already the spin component of the mother particle has been indetermined and correspondingly the components of the total angular momentum of the decay particles in this direction is indetermined either, but indeed there are the strong correlations in spin measurements in this other direction described by the entanglement of each of the three triplet states. This doesn't imply that the conservation law were valid only on average.

I still miss which problem has been solved by this and similar papers. I still do not understand what Mermin and Weinberg are after claiming there were some physics not understood. The predictions of quantum theory are in accordance with all observations, including the strong correlations due to entanglement. Of course, it "feels" unfamiliar given our experience "trained" by living in a "classical world", but it's to be expected that we find out surprising deviations from our everyday experience, when we consider situations, we are not used to. That's true for the (in)famous relativistic kinematical effects like length contraction, time dilation, "relativity of simultaneity", etc. because we simply don't move with relative velocities close to the speed of light wrt. the Earth as well as for "quantum phenomena", because we never have contact with systems, for which decoherence destroys the correlations described by entanglement between far-distant parts of a quantum system.

 

[RUTA]

If I understood the rest of the paper right (restricting myself to the much simpler standard representation in the Methods section rather than the overcomplicated "Mermin-device narrative"),

Well, the part you skipped is what motivated the title of the paper. Mermin's challenge is the reason the rest of the paper was written.

the issue simply is that if you have an entangled spin state and measure spin components in different directions on the two particles, you cannot verify angular-momentum conservation. That's no surprise, and I don't see, where there should be any problem with that. It's also a strange wording to say you change the reference frame just by measuring different spin components.

Brukner and Zeilinger also use this language in "Information and fundamental elements of the structure of quantum theory" where they associate a complete set of complementary spin measurements with a particular reference frame. Establishing what constitutes a reference frame is necessary to using the relativity principle aka "no preferred reference frame" (NPRF), which is the foundation of our answer to Mermin's challenge.

Concerning angular-momentum conservation this state is due to the decay of a spin-0 particle (in its rest frame) to two spin-1/2 particles, and indeed the total angular momentum stays 0 in this case, and you can verify this conservation law by measuring both particles in the same direction, which can be arbitrarily chosen. That's a special case, because the $S=0$ states are rotational invariant and here indeed all total-spin components take simultaneously the determined value 0.

It's also clear that it makes a difference whether you have prepared the two-particle system in the (unique) spin singulett state, $1/\sqrt{2} (|ud \rangle-|du \rangle)$, which is of course rotational invariant, because it's the $S=0$ state, or one of the three triplet states $|S=1,M=1 \rangle=|uu \rangle$, $|S=1,M=0 \rangle =1/\sqrt{2} (|ud \rangle + |du \rangle)$, and $|S=1,M=1 \rangle=|dd \rangle$, because this basis is not rotationally invariant but transforms under the $S=1$ representation.

Each of the three triplet states is rotationally invariant in a particular plane, as we explain in the paper and I explained in the Insight, "Exploring Bell States and Conservation of Spin Angular Momentum." Putting that together with NPRF per the reference frames of complementary spin measurements tells us that the SU(2) invariance of eigenvalues between different spin measurement operators per Information Invariance & Continuity entails the SO(3) invariance of spin measurement outcomes between those different inertial reference frames. Then add the fact that such measurements are actually measurements of Planck's constant h (Weinberg) and we have an exact analogy with the light postulate, NPRF + c, i.e., we have NPRF + h. So, the "mysteries" of time dilation and length contraction are due to NPRF + c while the "mysterious" Bell state correlations are due to NPRF + h. That's our answer to Mermin's challenge. Very simple, right?

Concerning conservation laws, one of these states is realized by the decay of a spin-1 particle (in its rest frame) to two spin-1/2 particles. To verify the angular-momentum conservation law you have to prepare this mother particle in a specific spin state $M=1$, $M=0$, or $M=-1$ for an arbitrary component of the spin (which usually is called the $z$ direction). To verify angular-momentum conservation in the decay process you now have to measure the spin components of both decay products in the same $z$ direction. In any other direction already the spin component of the mother particle has been indetermined and correspondingly the components of the total angular momentum of the decay particles in this direction is indetermined either, but indeed there are the strong correlations in spin measurements in this other direction described by the entanglement of each of the three triplet states. This doesn't imply that the conservation law were valid only on average.

For those who are interested in how one might actually prepare a Bell triplet state, see this paper: Dehlinger, D. & Mitchell, M. Entangled photons, nonlocality, and Bell inequalities in the undergraduate laboratory. American Journal of Physics 70, 903–910 (2002).

I still miss which problem has been solved by this and similar papers. I still do not understand what Mermin and Weinberg are after claiming there were some physics not understood.

Keep in mind that you're simply making a statement of your ignorance here. These and many other highly accomplished physicists did and do discuss issues concerning the understanding of QM. Once you understand what it is that bothers them, then you can address their concerns (if you so choose) rather than simply expressing the fact that you are ignorant of them.

The predictions of quantum theory are in accordance with all observations, including the strong correlations due to entanglement. Of course, it "feels" unfamiliar given our experience "trained" by living in a "classical world", but it's to be expected that we find out surprising deviations from our everyday experience, when we consider situations, we are not used to. That's true for the (in)famous relativistic kinematical effects like length contraction, time dilation, "relativity of simultaneity", etc. because we simply don't move with relative velocities close to the speed of light wrt. the Earth as well as for "quantum phenomena", because we never have contact with systems, for which decoherence destroys the correlations described by entanglement between far-distant parts of a quantum system.

"I think I can safely say that nobody understands quantum mechanics." Feynman, Probability and Uncertainty; The Quantum Mechanical View of Nature.

"All of modern physics is governed by that magnificent and thoroughly confusing discipline called quantum mechanics. It has survived all tests and there is no reason to believe that there is any flaw in it. We all know how to use it and and how to apply it to problems; and so we have learned to live with the fact that nobody can understand it." Gell-Mann in The Unnatural Nature of Science, p. 144.

"Everybody who has learned quantum mechanics agrees how to use it. 'Shut up and calculate!' There is no ambiguity, no confusion, and spectacular success. What we lack is any consensus about what one is actually talking about as one uses quantum mechanics. There is an unprecedented gap between the abstract terms in which the theory is couched and the phenomena the theory enables us so well to account for. We do not understand the meaning of this strange conceptual apparatus that each of us uses so effectively to deal with our world. ... What the hell are we talking about when we use quantum mechanics? For practical purposes ordinary everyday quantum mechanics is just fine, and what I have to say is of little or no interest. It is my hope to interest those who, like me, are impractical enough always to have been bothered, at least a bit, by not knowing what they are talking about." Mermin, Making Better Sense of Quantum Mechanics. 2019 Rep. Prog. Phys. 82 012002

You may just have to accept the fact that you will never understand what bothered Einstein, Weinberg, Mermin, Gell-Mann, Feynman, and many others about QM. You simply cannot relate, so you have nothing to contribute to such discussions. I wish I could help you!

 

[vanhees71]

Well, the part you skipped is what motivated the title of the paper. Mermin's challenge is the reason the rest of the paper was written.Brukner and Zeilinger also use this language in "Information and fundamental elements of the structure of quantum theory" where they associate a complete set of complementary spin measurements with a particular reference frame. Establishing what constitutes a reference frame is necessary to using the relativity principle aka "no preferred reference frame" (NPRF), which is the foundation of our answer to Mermin's challenge.

Well, they use way more conventional language. At least Zeilinger seems to be a Bohrian Copenhagian and thus much more inclined to the orthodox interpretation than using some unusual language.

Each of the three triplet states is rotationally invariant in a particular plane, as we explain in the paper and I explained in the Insight, "Exploring Bell States and Conservation of Spin Angular Momentum." Putting that together with NPRF per the reference frames of complementary spin measurements tells us that the SU(2) invariance of eigenvalues between different spin measurement operators per Information Invariance & Continuity entails the SO(3) invariance of spin measurement outcomes between those different inertial reference frames. Then add the fact that such measurements are actually measurements of Planck's constant h (Weinberg) and we have an exact analogy with the light postulate, NPRF + c, i.e., we have NPRF + h. So, the "mysteries" of time dilation and length contraction are due to NPRF + c while the "mysterious" Bell state correlations are due to NPRF + h. That's our answer to Mermin's challenge. Very simple, right?

Of course the states are invariant under rotation around the "quantization axis". This is also no mystery but follows from the representation theory of the rotation group or rather its covering group SU(2).

For those who are interested in how one might actually prepare a Bell triplet state, see this paper: Dehlinger, D. & Mitchell, M. Entangled photons, nonlocality, and Bell inequalities in the undergraduate laboratory. American Journal of Physics 70, 903–910 (2002).Keep in mind that you're simply making a statement of your ignorance here. These and many other highly accomplished physicists did and do discuss issues concerning the understanding of QM. Once you understand what it is that bothers them, then you can address their concerns (if you so choose) rather than simply expressing the fact that you are ignorant of them.

Indeed, I don't understand, where the problem is. That's why I'm asking!

"I think I can safely say that nobody understands quantum mechanics." Feynman, Probability and Uncertainty; The Quantum Mechanical View of Nature.

"All of modern physics is governed by that magnificent and thoroughly confusing discipline called quantum mechanics. It has survived all tests and there is no reason to believe that there is any flaw in it. We all know how to use it and and how to apply it to problems; and so we have learned to live with the fact that nobody can understand it." Gell-Mann in The Unnatural Nature of Science, p. 144.

Again, I don't understand what Gell-Mann thinks is not understood. Of course all these celebreties of science you quote have very well understood quantum mechanics (including Einstein, who was an opponent against it). So what are Feynman and Gell-Mann find incomplete in our understanding given that QT is the most comprehensive and so far never empirically falsified theory about nature we have? The only thing not understood is a satisfactory quantum description of gravitation, but that's obviously not what they are after in these statements.

"Everybody who has learned quantum mechanics agrees how to use it. 'Shut up and calculate!' There is no ambiguity, no confusion, and spectacular success. What we lack is any consensus about what one is actually talking about as one uses quantum mechanics. There is an unprecedented gap between the abstract terms in which the theory is couched and the phenomena the theory enables us so well to account for. We do not understand the meaning of this strange conceptual apparatus that each of us uses so effectively to deal with our world. ... What the hell are we talking about when we use quantum mechanics? For practical purposes ordinary everyday quantum mechanics is just fine, and what I have to say is of little or no interest. It is my hope to interest those who, like me, are impractical enough always to have been bothered, at least a bit, by not knowing what they are talking about." Mermin, Making Better Sense of Quantum Mechanics. 2019 Rep. Prog. Phys. 82 012002

What meaning? It's well known what we are talking about, i.e., devices to set up and observe "quantum systems" of various kinds. QT precisely predicts the outcomes of these measurements, and there are no cases, where the predictions of QT turned out to be wrong.

You may just have to accept the fact that you will never understand what bothered Einstein, Weinberg, Mermin, Gell-Mann, Feynman, and many others about QM. You simply cannot relate, so you have nothing to contribute to such discussions. I wish I could help you!

:-(