I Can entanglement swapping be explained without nonlocal influences?

[syed]

TL;DR Summary

Nonlocal influences in entanglement swapping

I just finished reading a paper on entanglement swapping and some material on it. So the experiments create entanglement between particles that have never directly interacted. Suppose I have two pairs of entangled particles, (A, B) and (C, D), prepared independently. When particles B and C are subjected to a joint Bell-state measurement, the distant particles A and D, that were previously uncorrelated, become entangled. This seems to beg the question: how can a measurement on B and C instantaneously affect the state of A and D? If we adhere to classical notions of locality, where influences can only propagate at or below the speed of light, such instantaneous correlations appear impossible.

Moreover, subsequent violations of Bell inequalities between A and D after entanglement swapping rule out explanations based on local hidden variables. One natural interpretation is that a nonlocal influence, some form of instantaneous connection or “steering” mediated by the quantum state, links these distant particles. I understand that as of now, we don't have any definite proof for faster-than-light signalling, but I am more interested in the explanation for how this might occur, and whether any explanation could be made without some sort of influence, even if we have no knowledge of how to actually signal this.

I suppose what I find interesting about entanglement swapping specifically is that it creates entanglement between two particles that never interacted and come from independent sources, only after a joint measurement on other particles. This retroactive creation of nonlocal correlations seems hard to explain by simply dropping realism (denying definite properties before measurement) while keeping locality, which seems to be one of the standard ways to "avoid" nonlocal influences. Unlike usual Bell tests, entanglement swapping seems to highlight that the measurement instantaneously affects distant particles with no shared past.

How do physicists who don't believe in any sort of nonlocal influences explain this experiment?

Here is the paper that I am referring to:

“Experimental Entanglement Swapping: Entangling Photons That Never Interacted” by J.-W. Pan, D. Bouwmeester, H. Weinfurter, and A. Zeilinger, Phys. Rev. Lett. 80, 3891 (1998).
https://doi.org/10.1103/PhysRevLett.80.3891

 

[PeroK]

I suppose what I find interesting about entanglement swapping specifically is that it creates entanglement between two particles that never interacted and come from independent sources, only after a joint measurement on other particles. This retroactive creation of nonlocal correlations seems hard to explain by simply dropping realism (denying definite properties before measurement) while keeping locality, which seems to be one of the standard ways to "avoid" nonlocal influences. Unlike usual Bell tests, entanglement swapping seems to highlight that the measurement instantaneously affects distant particles with no shared past.

How do physicists who don't believe in any sort of nonlocal influences explain this experiment?

By no longer thinking classically. And accepting that nature is described by states and not by particles at well-defined locations that communicate with each other. Note that Bohr and Heisenberg(and many other leading physicists of the 1920's and 30's) accepted this idea long before the Bell experiments, let alone entanglement swapping. Why did they do that? Why didn't they reject QM at that point, given that all hope of a classical resurgence had not been lost? One answer is that long before you get to entangment swapping, classical notions can no longer adequately explain atomic and sub-atomic phenomena.

You can look at QM two ways. The first is that it is so unintuitive that it can't possibly be correct and something is badly wrong. The second is that nature is showing us how it works and it's your intuition that is wrong.

I doubt there are many physicists who believe in nonlocal influences. Most believe that nature is described by nonlocal states.

 

[syed]

By no longer thinking classically. And accepting that nature is described by states and not by particles at well-defined locations that communicate with each other. Note that Bohr and Heisenberg(and many other leading physicists of the 1920's and 30's) accepted this idea long before the Bell experiments, let alone entanglement swapping. Why did they do that? Why didn't they reject QM at that point, given that all hope of a classical resurgence had not been lost? One answer is that long before you get to entangment swapping, classical notions can no longer adequately explain atomic and sub-atomic phenomena.

You can look at QM two ways. The first is that it is so unintuitive that it can't possibly be correct and something is badly wrong. The second is that nature is showing us how it works and it's your intuition that is wrong.

I doubt there are many physicists who believe in nonlocal influences. Most believe that nature is described by nonlocal states.

But is there a non classical way of explaining what's happening in entanglement that doesn't just amount to restating the phenomenon or saying "I don't know?". Words must mean something and be clear. Otherwise, they become akin to word salad that are just placeholders for no explanation.

If the answer to the above question is no, and there isn't a clear explanation, then it is not the case that nature is telling us how it works and that it is unintuitive. Rather, it is that you don't know what nature is telling you in the first place. There is a strong difference between the two.

But if the answer to the above question is yes, I'd surely like to know what it is.

 

[PeroK]

But is there a non classical way of explaining what's happening in entanglement that doesn't just amount to restating the phenomenon or saying "I don't know?". Words must mean something and be clear.

QM is essentially mathematics. Utimately, the words don't matter.

Otherwise, they become akin to word salad that are just placeholders for no explanation.

The "explanation" is the mathematics. Heisenberg first realised this: that at the fundamental level nature can only really be understood mathematically.

If the answer to the above question is no, and there isn't a clear explanation, then it is not the case that nature is telling us how it works and that it is unintuitive. Rather, it is that you don't know what nature is telling you in the first place. There is a strong difference between the two.

QM and QFT are the best that humanity can do. If it doesn't satisfy you, then too bad. QM is not my invention; nor that of any individual physicist. It's the product of thousands of specialists - many of them quite brilliant - who dedicated their lives to developing a theory that explains atomic and subatomic phenomena as best they could. And, along the way, intuitive realism was lost.

It's your prerogative not to accept modern physics - and to demand more by way of intuitive explanation. And, you may say that humanity has failed in this respect - and that the best physicists who ever lived didn't understand what nature was telling them. But, you're not actively researching how to move physics forward. You can propose dead ends and hopeless endeavours as what physicists ought to be doing. It's their prerogative to reject your ideas and accept QM and do the best they can with the best theories that we have available.

 

[syed]

QM is essentially mathematics. Utimately, the words don't matter.

The "explanation" is the mathematics. Heisenberg first realised this: that at the fundamental level nature can only really be understood mathematically.

It's your prerogative not to accept modern physics - and to demand more by way of intuitive explanation. And, you may say that humanity has failed in this respect - and that the best physicists who ever lived didn't understand what nature was telling them. But, you're not actively researching how to move physics forward. You can propose dead ends and hopeless endeavours as what physicists ought to be doing. It's their prerogative to reject your ideas and accept QM and do the best they can with the best theories that we have available.

The claim that quantum mechanics is “just mathematics” and that “the explanation is the math” is not a neutral scientific statement, but a philosophical position. This is otherwise known as instrumentalism.

Ironically, in dismissing the need for intuitive or conceptual explanations, you're engaging in philosophy while denying it. Science doesn't operate in a philosophical vacuum; it always rests on assumptions about what counts as an explanation, what reality is, and how we come to know it. Saying the math is all that matters assumes a view about the nature of theories and their relation to the world, which is again a metaphysical and epistemological stance. An alternative stance is that math by itself is just description and not the full explanation, and you can't argue against that view through science itself. So when you insist that we should stop asking interpretive questions and just accept the formalism, you're not avoiding philosophy; you're actually doing it, just uncritically.

 

[PeroK]

This is not a philosophy forum. It's a physics forum. We explicitly should not get into a debate about the philosophy of science.

QM is generally accepted by modern physicists as fundamentally a mathematical theory. Some insist on a preferred interpretation (I think MWI is the most popular) and some are agnostic. The interpretations are generally of secondary importance. Empirical success is first and foremost.

The key point is this. If you produce a theory that ticks all the philosophical boxes and it doesn't match experiment, then it's wrong. There is no philosphical argument to be had on a physical forum. This is what Feynman said in one of his lectures:

It doesn't matter what your name is; and, it doesn't matter how clever you are; if it doesn't match experiment, it's wrong. That's modern physics.

PS I don't revere Feynman, any more than any other great physicist, but I think I'm very close to his way of thinking about the role of mathematics and experiment in modern physics. And, if Feynman would fail a first year philosophy of science exam, then I don't care!

 

[syed]

This is not a philosophy forum. It's a physics forum. We explicitly should not get into a debate about the philosophy of science.

QM is generally accepted by modern physicists as fundamentally a mathematical theory. Some insist on a preferred interpretation (I think MWI is the most popular) and some are agnostic. The interpretations are generally of secondary importance. Empirical success is first and foremost.

The key point is this. If you produce a theory that ticks all the philosophical boxes and it doesn't match experiment, then it's wrong. There is no philosphical argument to be had on a physical forum. This is what Feynman said in one of his lectures:

It doesn't matter what your name is; and, it doesn't matter how clever you are; if it doesn't match experiment, it's wrong. That's modern physics.

PS I don't revere Feynman, any more than any other great physicist, but I think I'm very close to his way of thinking about the role of mathematics and experiment in modern physics. And, if Feynman would fail a first year philosophy of science exam, then I don't care!

“Physics is like sex: sure, it may give some practical results, but that’s not why we do it." - Richard Feynman

 

[Demystifier]

TL;DR Summary: Nonlocal influences in entanglement swapping

How do physicists who don't believe in any sort of nonlocal influences explain this experiment?

The same way they explain experiments that show violation of Bell inequalities (or any other experiment related to QM for that matter). They compute the relevant probabilities from standard QM formalism, and since they match the observations, they count these formal computations as an explanation. Since this formalism does not contain nonlocal influences in any explicit form, for them it's an explanation without any sort of nonlocal influences. I know that you are not satisfied with such an explanation (neither am I), but you asked how they explain it. That's how they do it, whether one likes it or not.

 

[PeroK]

The same way they explain experiments that show violation of Bell inequalities (or any other experiment related to QM for that matter). They compute the relevant probabilities from standard QM formalism, and since they match the observations, they count these formal computations as an explanation. Since this formalism does not contain nonlocal influences in any explicit form, for them it's an explanation without any sort of nonlocal influences. I know that you are not satisfied with such an explanation (neither am I), but you asked how they explain it. That's how they do it, whether one likes it or not.

When I studied pure mathematics, I had to retrain my intuition and change my intellectual expectations of what mathematics was about. For example, eventually a continuous function is where "the preimage of every open set is open". And, some sort of abstract intuition follows. The notion that a continuous function is where "you can draw the graph without lifting your pencil off the paper" is gone.

The question is what level of intellectual development is required to understand modern physics? One answer is none! Our native intellect is already sufficient and we must seek theories that appeal to that and nothing more.

Another answer, the one I would give, is that we have to be willing to extend our intellect and consider ideas, like nonlocality, that seem counterintuitive to us.

 

[martinbn]

I think the problem is with the word explain. When you use it, you mean to explain with a calssical mechanism.

 

[PeroK]

I think the problem is with the word explain. When you use it, you mean to explain with a calssical mechanism.

And this is illustrated long before we reach the intricacies of entanglement swapping. The first step is to understand the humble hydrogen atom not as a system of two well-defined particles with classical trajectories, but as a bound energy state.

That answers my earlier unanswered question about why Bohr and Heisenberg etc followed QM principles. Even the simple hydrogen atom required a departure from classical thinking, Bohmian mechanics notwithstanding.

 

[Demystifier]

When I studied pure mathematics, I had to retrain my intuition and change my intellectual expectations of what mathematics was about. For example, eventually a continuous function is where "the preimage of every open set is open". And, some sort of abstract intuition follows. The notion that a continuous function is where "you can draw the graph without lifting your pencil off the paper" is gone.

The question is what level of intellectual development is required to understand modern physics? One answer is none! Our native intellect is already sufficient and we must seek theories that appeal to that and nothing more.

Another answer, the one I would give, is that we have to be willing to extend our intellect and consider ideas, like nonlocality, that seem counterintuitive to us.

My main problem with standard Copenhagenish QM is not that it is too abstract, but that it is not abstract enough. By that I mean the fact that it gives a special fundamental status to the role of measurement, without giving a general abstract explanation of what measurement is, but leaving it to our naive anthropomorphic intuition to decide what is measurement and what is not.

 

[gentzen]

Science doesn't operate in a philosophical vacuum; it always rests on assumptions about what counts as an explanation, what reality is, and how we come to know it. ... An alternative stance is that math by itself is just description and not the full explanation, and you can't argue against that view through science itself.

But how should I know what counts as an explanation for you? How should philosophy help, if everybody decides for himself anyway, which explanations are acceptable?

I think the problem is with the word explain. When you use it, you mean to explain with a classical mechanism.

I wish I would know what people mean when they use the word explain. In my experience, they mostly mean "short, high level, and not overly complex".

 

[Demystifier]

I wish I would know what people mean when they use the word explain. In my experience, they mostly mean "short, high level, and not overly complex".

Or maybe "in terms they are familiar with and have a good intuitive understanding of".

 

[martinbn]

Or maybe "in terms they are familiar with and have a good intuitive understanding of".

And often this is impossible. They need to familiarise themselves with new concepts and develop new intuition.

 

[weirdoguy]

So when you insist that we should stop asking interpretive questions

Who are "we"? Because there is a huge difference between when layman/non-physicist ask those questions, and when scientists do. Usually when layman ask those, they expect things that cannot be achieved. We've seen that multiple times here on PF. Maybe even in this thread.

 

[syed]

I think the problem is with the word explain. When you use it, you mean to explain with a calssical mechanism.

Wanting an explanation doesn’t automatically mean demanding a classical mechanism (it seems that you're assuming this). The real issue is that Copenhagen doesn’t seem to give any physical account of what’s happening. It just tells you what measurements will show, not why. So when people ask “what’s going on?”, they’re not necessarily stuck in classical thinking. They’re asking for a coherent picture of the world that makes sense of the formalism. Physics didn’t stop at Newton because the math worked: we kept asking what the math meant. Why should we stop now?

Who are "we"? Because there is a huge difference between when layman/non-physicist ask those questions, and when scientists do. Usually when layman ask those, they expect things that cannot be achieved. We've seen that multiple times here on PF. Maybe even in this thread.

Sure, laypeople sometimes ask for impossible things. But I don't think that's what’s happening here. Interpretive questions are being asked by serious physicists (Bell, Bohm, Einstein, etc) so it's not like this is a fringe concern. They’re simply attempts to make sense of a theory that works incredibly well but says very little about the underlying reality. If one says those questions shouldn’t be asked, that’s not protecting science, but is rather narrowing it.

Keep in mind that saying “there is no further explanation” has no more objective basis than saying “there is one.” So even declaring that the search for explanation is over is itself a kind of interpretation, not a scientific conclusion.

 

[DaveC426913]

Surely the reason non-locality is so hard to accept is because it seems to violate the cause-effect relationship that all science and logic rests on, no?

If we abandon cause-and-effect, do we have anything left but ... magic?

Or am I barking up the wrong tree?

 

[syed]

Surely the reason non-locality is so hard to accept is because it seems to violate the cause-effect relationship that all science and logic rests on, no?

If we abandon cause-and-effect, do we have anything left but ... magic?

Or am I barking up the wrong tree?

Not necessarily, since there are nonlocal theories that posit a preferred frame of reference. This contradicts relativity but this allows for there to be an objective time ordering to events, such that one measurement outcome occurs before the other in this preferred frame. When seen this way, there are no causal paradoxes.

There are also proposed experiments that could potentially test whether this frame exists: https://arxiv.org/pdf/0910.5324 (not sure how accurate this is though)

 

[PeroK]

Wanting an explanation doesn’t automatically mean demanding a classical mechanism (it seems that you're assuming this).

We risk arguing about words. MWI is an explanation. It's just not the sort of nice, intuitive, classical explanation that you want.

The real issue is that Copenhagen doesn’t seem to give any physical account of what’s happening. It just tells you what measurements will show, not why. So when people ask “what’s going on?”, they’re not necessarily stuck in classical thinking. They’re asking for a coherent picture of the world that makes sense of the formalism.

If you don't like Copenhagen, then take MWI. You can't get more coherent than MWI. But, that's not classical either. This is the point, all potential explanations are non-classical, so they are never going to be intuitive in that sense.

Physics didn’t stop at Newton because the math worked: we kept asking what the math meant. Why should we stop now?

Because QM challenges our intuition more than Newton. It's a hollow argument that what is true for Newton must be true for modern physics theories.

Sure, laypeople sometimes ask for impossible things. But I don't think that's what’s happening here. Interpretive questions are being asked by serious physicists (Bell, Bohm, Einstein, etc) so it's not like this is a fringe concern.

Einstein is long dead. He never liked QM and did his best to undermine it. Ironically, you want a theory that rejects relativity..

The critical point, IMO, was the tests of Bell's theorem. That should have silenced the critics. That's why Aaronson says nature is screaming "genuine indeterminism" as loud as possible.

They’re simply attempts to make sense of a theory that works incredibly well but says very little about the underlying reality. If one says those questions shouldn’t be asked, that’s not protecting science, but is rather narrowing it.

Keep in mind that saying “there is no further explanation” has no more objective basis than saying “there is one.” So even declaring that the search for explanation is over is itself a kind of interpretation, not a scientific conclusion.

I'm saying something less conclusive than that. I'm saying that it's probably a waste of time looking for an intuitive explanation of QM. And, rejecting relativity, doesn't fill me with confidence. And, superdeterminism is giving up on science altogether.

These do seem to me to be on the fringes. If I were a young, theoretical physicist I wouldn't wouldn't waste my time on these things (and I certainly wouldn't be guided by what lay-people or people who have never opened a physics book think I should do).

It's true that in the development of QM, challenges to the theory helped move it forward. The critical point was the tests of Bell's theorem. That should have settled the matter. We should accept what nature is telling us and move on.

PS the way forward in physics (quantum gravity; further back towards the Big Bang etc.; discovering another elementary force?) is even more mathematical. We shouldn't expect the theory of the weak nuclear force, say, to be replaced with an intuitive model. Instead, we should expect things to get even more mathematical.

 

[weirdoguy]

If one says those questions shouldn’t be asked, that’s not protecting science, but is rather narrowing it.

They should be asked, but by people that know physics on a level of working scientist, because, as threads on PF show throughout the years, people who do not know physics on that level, don't know what they are asking about and tend to get argumentative with people who know physics, because they don't like the answers they get. And of course bringing up Einstein is a must

 

[PeroK]

We can take this a step further. Does spacetime emerge from a deeper theory? We start with some deeper laws of physics and show how spacetime emerges. How could that theory possibly be intuitive? How is it even possible to think of physics intuitively without space and time to begin with? The demand for Newtonian-like intuitive simplicity is a dead-end in modern physics.

 

[syed]

They should be asked, but by people that know physics on a level of working scientist, because, as threads on PF show throughout the years, people who do not know physics on that level, don't know what they are asking about and tend to get argumentative with people who know physics, because they don't like the answers they get. And of course bringing up Einstein is a must

The reason why he and other modern physicists are brought up is because people also very commonly and falsely imply that these kinds of questions come from some "misunderstanding" and "bad classical intuitions" rather than for purely rational reasons

 

[martinbn]

Wanting an explanation doesn’t automatically mean demanding a classical mechanism (it seems that you're assuming this). The real issue is that Copenhagen doesn’t seem to give any physical account of what’s happening. It just tells you what measurements will show, not why. So when people ask “what’s going on?”, they’re not necessarily stuck in classical thinking. They’re asking for a coherent picture of the world that makes sense of the formalism. Physics didn’t stop at Newton because the math worked: we kept asking what the math meant. Why should we now?

Let me give an example from classical physics, any will do. You throw something horizontally from a given height. It falls following a parabola. Why? The usual explanation goes like this: given the initial position and velocity we can calculate, using Newtons equation, what the path will be. Everyone is happy with Newtonian mechanics. Quantum mechanics, no interpretations just textbook QM, does the same thing. But you are unhappy with it. In fact you didn't even mention the derivation in your posts. Why? My guess is you not familier enough with quantum mechanics. Have you seen the analysis of the entanglment swapping? Or any problm solved using QM?

 

[Fra]

We start with some deeper laws of physics and show how spacetime emerges. How could that theory possibly be intuitive? How is it even possible to think of physics intuitively without space and time to begin with?

That seems quite possible if we consider that spacetime is an internal property used to index the internal map of the environment/terrotory. Wether the spacetime is "real" property of the actual environment we will never know, as that is doomed to be a black box. One could even argue that it does not matter which, as long as the population of maps are synchronised, no joint inference is able to reject the external spacetime that can be seem as real if we can form an equivalence class out of all the maps.

This is very intuitive I would say, especially to humans, as that is much like we think the human brain works like. It encodes a "map" of the uknonwn environment, and this map is used for navigation. In this sense, the set of MAPS are the real thing, and the territory is like hologram.

/Fredrik

 

[Morbert]

My main problem with standard Copenhagenish QM is not that it is too abstract, but that it is not abstract enough. By that I mean the fact that it gives a special fundamental status to the role of measurement, without giving a general abstract explanation of what measurement is, but leaving it to our naive anthropomorphic intuition to decide what is measurement and what is not.

Chapter 12 of Asher Peres's "Quantum Theory: Concepts and Methods" gives a concrete account of measurement from a modern instrumentalist perspective.

 

[gentzen]

Chapter 12 of Asher Peres's "Quantum Theory: Concepts and Methods" gives a concrete account of measurement from a modern instrumentalist perspective.

Maybe "one of the few elaborated accounts of Bohr's variant of the Copenhagen interpretion" would be a better description. Which is actually quite an achievement and very useful, because Bohr himself was famously unable to express himself clearly.
What I find strange is that Chris Fuchs started out working and publishing together with Asher Peres, but later switched to an even more radial subjective interpretation than Heisenberg's variant of the Copenhagen interpretation.

 

[Fra]

Bohr himself was famously unable to express himself clearly.

I always felt that Bohr tried to as clearly as possible express what we know/knew, without speculation. This is also a kind of clarity itself. After all at the time, the macroscopic context is where from where measurements are prepared and launched.

Bohr supposedly said "Never express yourself more clearly than you think", and I think there was a point with that.

/Fredrik

 

[Demystifier]

Chapter 12 of Asher Peres's "Quantum Theory: Concepts and Methods" gives a concrete account of measurement from a modern instrumentalist perspective.

Yes, the instrumentalist version of Copenhagen interpretation is OK, I have no objections against it. Essentially, the instrumentalist version says: Look, this is what we know so far and it works in practice. But it does not say that it is all what we can know, in principle. The instrumentalist version does not claim that QM in its current form is complete. It does not try to be deep, it's satisfied with the fact that it works.

The problem with other versions of Copenhagen interpretation is that they try to say something deep, which turns out to be unconvincing to say the least, and insist that QM in its general principles is complete.

 

[Morbert]

In discussions of instrumentalism, questions like "What is a measurement?", "What counts as a measurement apparatus?", "Where is the classical/quantum divide?" etc normally arise quickly. Peres gives these questions sharp answers, clearing away the ambiguities people normally associate with instrumentalism.

 

[Fra]

Quick reflection on the book looking at chapter 12.

Peres writings is very good, I like them, but the "sharpness" still rests on a few concepts that conceptually take the "role" of the heisenberg cut, some of the keys are

"Consistency thus requires the measuring process to be irreversible. There are no superobservers
in our physical world."
Peres, p366

This is course makes perfect sense, but I have a critical reflections

The notion of "reversibility" is plauged by the similar thing such as "randomness". While all someone can say is that: I have not means to predict, or i have not means to "control" and reverse, so the process I can observer seems "random" and "irreversible". Thus these concepts are dependent on the "capacity" of the "observing part". And its why it requires, there are no superobservers. But there are still observers of vastly different "capacity" to distinguish, encode and process their empirically accessible information. So the question of a scale independent notion of reversibility and timeless law is still fuzzy.

So the question of how the apparenty dynamical law, evolves with complexity here, is not sharpy addressed at all. But I think Peres is doing a decent job to be as sharp as possible.

/Fredrik

 

[Morbert]

Quick reflection on the book looking at chapter 12.

Peres writings is very good, I like them, but the "sharpness" still rests on a few concepts that conceptually take the "role" of the heisenberg cut, some of the keys are

"Consistency thus requires the measuring process to be irreversible. There are no superobservers
in our physical world."
Peres, p366

This is course makes perfect sense, but I have a critical reflections

The notion of "reversibility" is plauged by the similar thing such as "randomness". While all someone can say is that: I have not means to predict, or i have not means to "control" and reverse, so the process I can observer seems "random" and "irreversible". Thus these concepts are dependent on the "capacity" of the "observing part". And its why it requires, there are no superobservers. But there are still observers of vastly different "capacity" to distinguish, encode and process their empirically accessible information. So the question of a scale independent notion of reversibility and timeless law is still fuzzy.

So the question of how the apparenty dynamical law, evolves with complexity here, is not sharpy addressed at all. But I think Peres is doing a decent job to be as sharp as possible.

/Fredrik

I don't understand this. Sorry.

 

[PeroK]

I don't understand this. Sorry.

I think I've only ever understood about two or three of Fra's posts!