What is the mechanism behind Quantum Entanglement?

[curiosity1]

TL;DR Summary

Quantum Entanglement

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

 

[sumfor]

I don’t think anyone knows.

 

[malawi_glenn]

What is the mechanism behind Quantum Entanglement?

It kinda depends on what level you want the answer, and from which viewpoint.

Why do only subatomic particles exhibit Quantum Entanglement?

Same as why a huge rock does not exhibit any quantum phenomena

 

[curiosity1]

It kinda depends on what level you want the answer, and from which viewpoint.

Same as why a huge rock does not exhibit any quantum phenomena

Please tell me more. Thank you.

 

[malawi_glenn]

Please tell me more.

What search in the literature have you done to try to answer these questions yourself?
What is your current understanding?

 

[vanhees71]

Summary: Quantum Entanglement

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

Well, it's a direct conclusion from the formalism of QT. Why Nature is well-described by QT, is a question, one cannot answer within the realm of the natural sciences.

It's also not true that only subatomic particles exhibit quantum entanglement. E.g., the entanglement between two diamonds (even at room temperature) has been demonstrated some time ago:

https://physicsworld.com/a/diamonds-entangled-at-room-temperature/

 

[malawi_glenn]

https://physicsworld.com/a/diamonds-entangled-at-room-temperature/

That is wicked!

 

[CoolMint]

Summary: Quantum Entanglement

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

Because they are quanta. Because of the HUP. The HUP operates on objects small enough to be classified as quantum.

 

[curiosity1]

What search in the literature have you done to try to answer these questions yourself?
What is your current understanding?

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

 

[sumfor]

Spooky action at a distance was Einstein’s comment about entanglement. Even for particles near one another it’s spooky. But I’m thinking that if one particle can be in two different places, take two different paths when passing through a half-silvered mirror, then perhaps two different particles could become one particle when entangled and so behave as such.

 

[ohwilleke]

Summary: Quantum Entanglement

What is the mechanism behind Quantum Entanglement?

As noted in #6, because the equations of quantum physics says that it should happen.

While it is true that we don't know, it is also true that experiments have essentially ruled out "classical" physics type mechanisms. We don't know because all of the "first guess" answers scientists had for a mechanism have been ruled out.

One shorthand way of describing the issue is that quantum entanglement is not possible unless one or more of the following is not true in quantum entanglement cases:

1. Locality.
2. Causality.
3. Reality.

The exact definitions of those terms are somewhat technical, but all seem like very reasonable axioms to have about how physics works. And, all three of these assumptions hold true for all other aspects of physics. Yet, at least one of them must not be universally true or quantum entanglement would not give rise to the phenomena that we observe.

There are a variety of speculative hypotheses for the mechanism of quantum entanglement, but none of them can be singled out as correct with existing experiments.

 

[StevieTNZ]

You might find this book useful - https://www.springer.com/gp/about-springer/media/press-releases/corporate/quantum-chance/32820

https://link.springer.com/book/10.1007/978-3-319-05473-5

 

[PeroK]

Summary: Quantum Entanglement

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

Quantum mechanics is a mathematical model that predicts the behaviour of microscopic systems. This includes systems where the particles are entangled. There is no underlying "mechanism", although there are "interpretations" of the mathematics.

Note that there is also no "mechanism" in classical mechanics to describe how the gravitational force is propagated. Newton, himself, was at pains to emphasise this.

 

[vanhees71]

As noted in #6, because the equations of quantum physics says that it should happen.

While it is true that we don't know, it is also true that experiments have essentially ruled out "classical" physics type mechanisms. We don't know because all of the "first guess" answers scientists had for a mechanism have been ruled out.

One shorthand way of describing the issue is that quantum entanglement is not possible unless one or more of the following is not true in quantum entanglement cases:

1. Locality.
2. Causality.
3. Reality.

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

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

Causality means that the state of a (quantum) system can be influenced only by the past and not the future. In relativistic models of spacetime this implies that there cannot be causal influences from space-like separated events. In both classical and quantum relativistic theories this has been realized by a strict use of the paradigm of local field theories. In quantum field theory it is realized by a formal mathematical demand called the "microcausality principle", i.e., the quantum fields are the building blocks for all the operators that describe observables at a point in spacetime (usually densities like charge density, energy-momentum densities, etc.) must commute with the Hamilton density for space-like separated space-time arguments. This rules out any "spooky actions at a distance", i.e., causal effects can only be due to signals that propagate with a speed less than or equal to the speed of light in vacuum.

So it can only be "realism" that's violated by QT. In my opinion it's a very unfortunate choice of naming, because QT in fact is the most "realistic" theory we have, i.e., it describes the phenomena best in accordance with the observations. In fact there is not a single reproducible empirical fact contradicting quantum theory. To the contrary all the predictions of Q(F)T are in accordance with the observations at very high precision, including the incompatibility of these observations with what's called "realism". A better naming would be "determinism", where determinism means that all possible observables always take determined valued, and a statistical (probabilistic description) is only necessary, because we cannot know all these predetermined values due to complexity. That's the paradigm of classical statistical physics: We are not able to follow the trajectories of the about $10^{24}$ particles contained in 1 mol of a substance, and thus we describe the "relevant macroscopic observables" in a "coarse-grained" description by the means of probability theory.

The probabilities in QT are of a different nature though: They are not simply due to our inability to take account of a very complex situation but they are inherent in the nature of observabiles themselves, i.e., according to QT there is no state, where all observables of a system take predetermined values. This is the content of Heisenberg's uncertainty principle, valid for all kinds of observables. Most famous is the uncertainty principle for position and momentum of a single particle. If you prepare a particle to have a very precisely determined position, i.e., if you somehow confine it to be in a small volume around a certain point in space, then its momentum must be very indetermined and vice versa. So QT says that there is "irreducible randomness" in Nature.

In a famous paper by Einstein, Podolsky, and Rosen the authors claimed to have found a contradiction in the foundations of quantum theory. Einstein also proposed the idea that there might be socalled "hidden variables", i.e., observables we don't know but whose values would determine the values of the observables which cannot be determined simultaneously in any state within QT, and then the statistical nature of QT would be of the same kind as in classical statistical physics: It's introduced to describe our ignorance about the hidden variables.

It took about 30 years until this purely philosophical prejudice was made a clear scientific statement that could be objectively tested experimentally by John Bell. The idea was that there might be a "realistic local hidden variable theory". The great achievement was that Bell didn't need to assume more, let alone formulate any such theory concretely to find clear contradictions for the proabilities for the outcome of measurements between what QT and any such kind of local realistic hidden-variable theory predicts, the socalled Bell inequalities.

The contradictions arise precisely for entangled states, and it took another 20 years to become possible to realize it in experiments. In the early 1980ies Aspect et al started to experiment with entangled photon pairs from a atom cascade, and already these early tests clearly demonstrated that realistic local hidden-variable theories are ruled out. Today this is known to an amazing precision and significance. So indeed the most "realistic" theory we have is Q(F)T, based on the causality and locality but violating "realism" in the sense of EPR.

The exact definitions of those terms are somewhat technical, but all seem like very reasonable axioms to have about how physics works. And, all three of these assumptions hold true for all other aspects of physics. Yet, at least one of them must not be universally true or quantum entanglement would not give rise to the phenomena that we observe.

There are a variety of speculative hypotheses for the mechanism of quantum entanglement, but none of them can be singled out as correct with existing experiments.

The only "mechanism" that explains entanglement is simply Q(F)T itself. There is no other mechanism behind it, at least not one that we know about today.

Finally one has to stress that QT is not only describing "microscopic sytems" of a few particles but all aspects of matter, including the macroscopic stuff around us. In fact the very basic experience that matter is stable, given its atomistic structure, is already not possible to describe within classical physics, but you need QT for that.

 

[gentzen]

There is no underlying "mechanism", although there are "interpretations" of the mathematics.

Note that there is also no "mechanism" in classical mechanics to describe how the gravitational force is propagated. Newton, himself, was at pains to emphasise this.

But Newton emphasised this, because the gravitational force was nonlocal, before Einstein discovered general relativity. So gravity seems rather unsuitable (from my POV) as an argument to dismiss the search for a deeper theory or better understanding of how nonlocal effects arise.

 

[WernerQH]

Causality means that the state of a (quantum) system can be influenced only by the past and not the future.

I think this is too narrow a view of causality that is at odds with quantum theory. We are habituated to think of the past as influencing the future, just like we are accustomed to read from left to right (at least in English). But quantum theory offers a more time-symmetric picture. It is well known that the decay rate of a particle, for example, depends on the density of available final states. And surely you must be aware that QFT achieves its magic with propagators that extend also into the backward light cone. I think this is the "mechanism" that @curiosity1 is wondering about. Having forward and backward traveling waves (linking what we somewhat arbitrarily distinguish as "cause" and "effect") ensures that the events that we observe follow a consistent pattern. In my view, the consistent histories interpretation, the transactional interpretation, and the closed time-path formalism all favour a time-symmetric picture.

 

[Fra]

I agree with most, i just wanted to add that we should not forget the causal mechanism. Reality itself is I think coupled to the presume causal mechanisms it takes part with, otherwise it would be an empty concept.

Causality means that the state of a (quantum) system can be influenced only by the past and not the future.
...
causal effects can only be due to signals that propagate with a speed less than or equal to the speed of light
...
So it can only be "realism" that's violated by QT.

These terms are always the focus of the discussions. In previous discussions I have tend to separate "causality" as in the the causal ordering you refer to, and the paradigm for causal mechanisms. This may have caused miscommunication.

The standard causal mechanism is somehow that in the view of of system dynamics, the future state depends only on the past states, as per a fixed eternal law(that we do not ask why questions about, we just "discover" them). This is the natural way we think of things in classical mechanics (ie. where obserers play no central role).

But in QM, it becomes important to think about what ontology we assign to the state spaces themselves, to understand in which space to apply the causal mechanisms? In which space does the real causal mechanism happen in QM?

What seems rational is that a single observers "expecation" of the infinitesimal future states, at least in the depends only on it's present knowledge, seems rational as otherwise one would have to add new information and the observer would have to make a measurement and the state revised. Statements about finite future times (as per fixed laws) does not follow from pure rationality arguments - it is empirical only.

The issue in Bells theorem is, just because you assume there are "hidden variables"(existing and beeing encoded by at least one observer), does this necessarily mean that expected future state as per onother observer, are causally depending on these variables? (like ignorance) I think, this does not follow from any logic I know of!

I think Bell assues it from the habit from the tradtional causal mechanisms, which IMO is intermixed with a notion of "realism" that assumes that "mechanisms" must have an objective description. This is a naive "realist type" of causal mechanism. I think it's instructional to see that insight shouldbe possible to get even before you find that QM violates the inequality I think. I think this mixes the notions of "realism" and "causal mechanisms" as we talk about this! This is what makes QM hard to grasp think.

But suppose hidden variables of a system are real, in the senes that they are de facto existing in the system(inside observer) itself, but the nature of this is so that it is can not be communicated due to constraints to other external observers, then this hidden variable will still influence the expexcation of this one inside observer, and potentially explain causal mechanisms, but NOT the expectation of other observers. So a theory built in those principles, could as I see still employ hidden variables that can be thought of as real, and still violate bells theorem. But such theoy would not be deterministic or allow any observer to get rid of the randomness. This is why I would like to say that causal mechanism is just as suspsect as reality.

But I agree, I see no reason to destroty the causal order anywhere.

/Fredrik

 

[vanhees71]

I think this is too narrow a view of causality that is at odds with quantum theory. We are habituated to think of the past as influencing the future, just like we are accustomed to read from left to right (at least in English). But quantum theory offers a more time-symmetric picture. It is well known that the decay rate of a particle, for example, depends on the density of available final states. And surely you must be aware that QFT achieves its magic with propagators that extend also into the backward light cone. I think this is the "mechanism" that @curiosity1 is wondering about. Having forward and backward traveling waves (linking what we somewhat arbitrarily distinguish as "cause" and "effect") ensures that the events that we observe follow a consistent pattern. In my view, the consistent histories interpretation, the transactional interpretation, and the closed time-path formalism all favour a time-symmetric picture.

There is not the slightest hint at any causality violation at all. Where do you get the idea from that were the case?

 

[vanhees71]

I agree with most, i just wanted to add that we should not forget the causal mechanism. Reality itself is I think coupled to the presume causal mechanisms it takes part with, otherwise it would be an empty concept.

These terms are always the focus of the discussions. In previous discussions I have tend to separate "causality" as in the the causal ordering you refer to, and the paradigm for causal mechanisms. This may have caused miscommunication.

The standard causal mechanism is somehow that in the view of of system dynamics, the future state depends only on the past states, as per a fixed eternal law(that we do not ask why questions about, we just "discover" them). This is the natural way we think of things in classical mechanics (ie. where obserers play no central role).

But in QM, it becomes important to think about what ontology we assign to the state spaces themselves, to understand in which space to apply the causal mechanisms? In which space does the real causal mechanism happen in QM?

What seems rational is that a single observers "expecation" of the infinitesimal future states, at least in the depends only on it's present knowledge, seems rational as otherwise one would have to add new information and the observer would have to make a measurement and the state revised. Statements about finite future times (as per fixed laws) does not follow from pure rationality arguments - it is empirical only.

The issue in Bells theorem is, just because you assume there are "hidden variables"(existing and beeing encoded by at least one observer), does this necessarily mean that expected future state as per onother observer, are causally depending on these variables? (like ignorance) I think, this does not follow from any logic I know of!

I think Bell assues it from the habit from the tradtional causal mechanisms, which IMO is intermixed with a notion of "realism" that assumes that "mechanisms" must have an objective description. This is a naive "realist type" of causal mechanism. I think it's instructional to see that insight shouldbe possible to get even before you find that QM violates the inequality I think. I think this mixes the notions of "realism" and "causal mechanisms" as we talk about this! This is what makes QM hard to grasp think.

But suppose hidden variables of a system are real, in the senes that they are de facto existing in the system(inside observer) itself, but the nature of this is so that it is can not be communicated due to constraints to other external observers, then this hidden variable will still influence the expexcation of this one inside observer, and potentially explain causal mechanisms, but NOT the expectation of other observers. So a theory built in those principles, could as I see still employ hidden variables that can be thought of as real, and still violate bells theorem. But such theoy would not be deterministic or allow any observer to get rid of the randomness. This is why I would like to say that causal mechanism is just as suspsect as reality.

But I agree, I see no reason to destroty the causal order anywhere.

/Fredrik

There is no mechanism. Causality is just an assumption you make in all physical theories, and you build the physical theories in such a way that it is fulfilled. This is not different in QT in any way.

The realization of the causality principle is, however, different in Newtonian and relativistic physics. In Newtonian physics it's just realized by time ordering, and time is "absolute". Thus there is no need of a locality principle, and the standard paradigm to describe interactions is to use a "action-at-a-distance model", as in Newton's theory of the gravitational interaction. Thus in Newtonian mechanics there is no need for locality in both classical and quantum descriptions.

In special relativity there can be causal order only between time and light-like separated events, while space-like separated events have no fixed time-ordering when Lorentz boosting from one to another inertial reference frame. For any two given space-like separated events you always find an inertial frame, where the events are simultaneous (wrt. to the coordinate time of this frame), and you can always find inertial frames where the one or the other event is "earlier" (wrt. the coordinate times of these frames). That's why causal effects can only be due to influences that propagate at most with the speed of light, and thus there cannot be actions at a distance a la Newton in any relativistic theory.

Since Faraday and Maxwell the way out of this dilemma is the locality principle, i.e., the interaction between two distant particles is due to the presence of fields, and due to the field at the position of the particle. In the case of quantum theory the only yet known way to achieve a causal theory is to formulate it as a local QFT obeying the microcausality principle for local observables.

 

[WernerQH]

There is not the slightest hint at any causality violation at all. Where do you get the idea from that were the case?

I wasn't talking of causality "violation" at all, just the standard QFT formalism. (What would that even mean?)
Of course, for a pair of time-like separated events you can always label the earlier one the "cause" and the later one "effect" to maintain a convention. You may prefer to think of an electron moving backwards in time as a positron moving forwards in time, but you do not have to.

 

[vanhees71]

That's a pop-sci distortion of what's really done: To have a local microcausal theory the quantized free electron field must consist of annihilation operators in front of the positive-frequency modes and a creation operator in front of the negative-frequency modes, leading to the prediction of anti-particles (positron). Everything is moving forwards in time and each mode refers to positive-energy eigenstates (provided you quantize the Dirac field correctly as fermion field).

 

[weirdoguy]

And btw in my opinion the moderators occasionally should show some tolerance in speculative posts because that's how new science is born, by speculations.

By speculations of people who has huge background knowledge. Do you have enough knowledge on quantum physics to work as a scientist in this field? If not, then your speculations are not meaningfull. Einstein was not speculating out of nothing, he was a working physicist and knew exactly what was known and unknown.

 

[Fra]

There is no mechanism.

If Bell thought so, why did he make the ansatz that the total probability is a written as a sum over a hidden variable, where for each value there was a presumed outcome? This is not a explicit mechanism itself, but imposes a "structure/constraint" on the causal mechanisms.

That the mechanism is unknonwn or not observer independent, and thata there exists no mechanism is not the same. If we resign for a mechanism, then what we are doing is just describing things, and not explaining it in any way deeper than that things seem to follow the empirial rules. Nothing wrong with empirics per see, but theoretical progress often seeks to back the empirics up by more understanding.

If you mean we do not know the mechanism, then I agree, but isn't the search for it is what keeps the fire here?

/Fredrik

 

[vanhees71]

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

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

 

[PeroK]

If you mean we do not know the mechanism, then I agree,

There's no mechanism specified within mainstream QM.

but isn't the search for it is what keeps the fire here?

I'm not sure about. I suspect most physicists believe that only an abstract mathematical description of nature is possible at the fundamental level.

 

[vanhees71]

As far as we know today, QT describes everything right, including the precise statistics about the correlations between entangled parts of a quantum system, and that's all the fire there is. There's not even a little smoke indicating anything that's not described by QT.

 

[Nugatory]

If Bell thought so, why did he make the ansatz that the total probability is a written as a sum over a hidden variable, where for each value there was a presumed outcome?

Bell was demonstrating that that particular ansatz implied his inequality and suggesting that the contrapositive (a violation of the inequality implies that the ansatz is wrong) could be used to invalidate any theory consistent with the ansatz.

That doesn’t tell us much about Bell’s thinking, except that he thought that the implications of that ansatz were interesting.

 

[Fra]

I suspect most physicists believe that only an abstract mathematical description of nature is possible at the fundamental level.

Yes, so do I but i think it could be done with very different levels of understanding and i am not happy with the models we have.

For example aa A mathematical that is the result of fitting model parameters with experimental data will be good but have less explanatory value than a mathematical model that is constructed by more guiding principles and that requires fitting of fewer parameters and be more natural.

/Fredrik

 

[gentzen]

I suspect most physicists believe that only an abstract mathematical description of nature is possible at the fundamental level.

But even abstract principles like Einstein's relativity principle or Stuckey's NoPreferredReferenceFrame principle which he explained in various insights articles might provide hints how to answer questions like "What is the mechanism behind Quantum Entanglement?"

Or maybe semi-mathematical non-quantum ideas like Spekkens toy model provide some insights into what is so special about quantum entanglement.

 

[Fra]

There's not even a little smoke indicating anything that's not described by QT.

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

/Fredrik

 

[Fra]

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

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

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

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

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

/Fredrik

 

[ohwilleke]

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

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

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

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

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

 

[Fra]

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

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

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

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

/Fredrik

 

[CoolMint]

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

 

[PeterDonis]

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