Confused by nonlocal models and relativity

[SteveF]

Hello everyone!

Recently I saw this paper: https://arxiv.org/pdf/1304.4801.pdf ("Any nonlocal model assuming “local parts” conflicts with relativity " by Antoine Suarez).

He mentions standard experimental configuration with beam-splitters and detectors. Then he distinguishes possible models by assuming "decision" about the outcome at beam-splitters (like Bohmian mechanics); or by assuming "decision" at detection.

After that he shows that the first group of models conflict with relativity. Moreover, he uses the notion of "nonlocality at detection" as opposed to "Bell's nonlocality" etc. And this logic can be find in almost every paper by this author (especially, about the so called "before-before" experiments). For example,
-https://arxiv.org/abs/1204.1732
or
-https://www.unige.ch/gap/quantum/_media/publications:bib:suarez.pdf
or
-https://arxiv.org/pdf/0708.1997.pdf

One time the author claimed that "after the before-before experiment Bohm’s interpretation can hardly be considered a valid alternative to Copenhagen".

I have several questions and would be grateful if anybody made some clarification:
1. Somehow Suarez opposes predictions of standard QM and such models as Bohmian mechanics. I thought that at some foundamental level their predictions are the same. Did I make a mistake?

2. Suarez considers Bohmian mechanics to contain some "locality". Again, I was sure that nonlocality is an important feature of BM. What do I miss?

3. Finally, does the conclusions in those papers prevent any attempts to make BM relativistic?

Sorry if these questions are too amateurish.

 

[bobob]

Suarez with Gisin and others have done a number of experiments along those lines. One reason their papers might be confusing is that they have been doing those experiments to test various schemes to make Bohmian mechanics consistent with relativity and have consistently failed to do so and instead just provided more evidence that stadard quantum theory (which is LOCAL) is correct. Just remember that despite any misguided notions to the contrary, stndard quantum mechanics is a LOCAL theory. Find the paper that Suarez references with regard to the moving beam splitters.

I'm not really sure why Suarez and his collaborators haven't given up on finding a way to skirt standard quantum mechanics and instead, try to understand what quantum mechanics is telling them after every experiment they've done excludes their theory attempting to skirt quantum mechanics. If you are confused, it's because they seem confused and more than a few of their papers are more about being being puzzled over the results even though just taking standard quantum mechanics to be correct would clear that up. They've done some great experiments, but seem to have difficulty accepting the implications.

Yes, every experiment they have done has shown that their attempts to make Bohmian mechanics relativistic have failed spectacularly to do that.

 

[SteveF]

Thank you for your reply, but I still have some objections/confusions.

They have been doing those experiments to test various schemes to make Bohmian mechanics consistent with relativity and have consistently failed to do so and instead just provided more evidence that stadard quantum theory (which is LOCAL) is correct.

I will repeat that in my view standard quantum theory and Bohmian mechanics made the same predictions. So, if their experiments confirmed standard QT, then they confirmed BM too. You may say that they made some predictions about relativistic BM, but I haven't find any formal description of the model they test. And even if they somehow worked out this model in details, why do they compare it to nonrelativistic version of QT then?

Finally, I thought that in terms of relativity BM always uses a preferred frame. And in experiments mentioned above this aspect is disregarded.

 

[bobob]

Well, Bohmian mechanics fans would like to say they make the same predictions, but obviously the experiment in question blows that idea out of the water. The requirement for a preferred frame is a killer here. A rather good critique of Bohmian mechanics and the machinations required to try to assert that they mke the same predictions may be found in: "Lost Causes In and Beyond Physics," by Ray Streater (physicist and co author with Wightman of PCT, Spin, Statistics and All That."

Oh, btw, one of their failed models is called "multisimultaneity," and if the way they explain it is confusing, that is because it isn't very clear that it means anything other than "we are going to jack around with the idea of simultaneity until it no longer has any real connection to the concept of simultaneity in relativity." The strangest thing about this research group is they've done a couple of outstanding experiments that validate standard quantum theory, but seem totally perplexed by that and think there must be some other explanation. Sort of like people who can't let the ether die.

 

[Demystifier]

1. Somehow Suarez opposes predictions of standard QM and such models as Bohmian mechanics. I thought that at some foundamental level their predictions are the same. Did I make a mistake?

You did not. Perhaps Suarez did.

2. Suarez considers Bohmian mechanics to contain some "locality". Again, I was sure that nonlocality is an important feature of BM. What do I miss?

Perhaps the crucial word is "some". BM is nonlocal in some aspects and local in other aspects.

3. Finally, does the conclusions in those papers prevent any attempts to make BM relativistic?

It depends on what exactly one means by "relativistic".

 

[SteveF]

It depends on what exactly one means by "relativistic".

Demystifier, I waited for your comments:)

Could you please mention some properties which could differ in various relativistic versions of BM?

For example, in the model of "multisimultaneity" "a causal temporal order is defined for each measurement. And the relevant reference frame for each measurement is the inertial frame of the massive apparatus. As in the pilot-wave model, each particle emerging from a beam splitter follows one (and only one) outgoing mode, hence particles are always localized, although the guiding wave follows all paths, in accordance with the usual Schrödinger equation. When all beam splitters are at relative rest, this model reduces to the pilot-wave model and has thus precisely the same predictions as quantum mechanics. However, when two beam splitters move apart, then there are several (i.e., two) relevant reference frames, each defining a time ordering, hence the name of multisimultaneity."

And what could be the alternatives?

Sorry again for my vague questions.

 

[Demystifier]

And the relevant reference frame for each measurement is the inertial frame of the massive apparatus.

This seems too vague, because one apparatus can be a non-rigid body with parts in motion relatively to each other.

If you want to see how I view the issue of relativity in BM, see the paper linked in my signature below (which can be thought of as an alternative to the usual BM) .

 

[bobob]

And what could be the alternatives?

Let me add a little food for thought. Unlike classical theory in which the electrons would spiral into the nucleus in very short order, quantum mechanics explains this just like it is without any additional frufru added. Similarly, we have the fact that there exist things like electrons which have spin 1/2, which not only doesn't describe any sort of spinning like a top, but which is something totally unfamiliar to anything encountered in classical theory or experienced by us in any way. Now, given that quantum theory taken at face values solves all sorts of things that classical theory fails miserably to explain, what in the world would provide any incentive to add all sorts of convoluted additions and imagine particles that violate relativity with faster than light travel or separate a nicely intergated whole into real physical properties associated with marble like particles that behave in a way determined by a wavefunction that is in reality no more accessible than in standard quantum mechanics all for the sake of wishing to believe that things like electrons are really pretty much like classical things, even though classical mechanics fails miserably to describe the behaviour of things like electrons?

You are still stuck with a wave function (excuse me, guiding equation), which if anything begs the question of how the hell does this work such that I can apply my classical intuition to things that don't behave classically? Really? Is it really important to believe that things like electrons are actual particles in the classical sense to feel comfortable with quantum theory, even though quantum theory solves the problems and merely asks one to believe that the microscopic world just might be a little different or classical observations of the world can bias one to believe everything is really like that? (Actually, from reading some comments made by Hiley, neither Bohm nor Hiley could be considered supporters of bohminan mechanics in the sense it is presented by sheldon goldstein and company. They seem (seemed in bohm's case) to have a bit more sophisticated idea about this stuff, but I'm not sure what that is since the cheerleaders for bohmian mechanics (e,g., goldstein, durr, etc.) make up the bulk of what they call bohmian mechanics.

Once you throw relativity into the picture, there really is no option but to either cave in and discard one's classical preconceptions or else discard relativity and I think a lot of people are really having more difficulty with the relativistic part. In a relativistic setting, two events which are spacelike cannot have any causal connection. Therefore the photon or electron or whatever in an epr experiment cannot be a cause for the result measured on the other photon. Youy cannot be rescued with some additional assumption that there really is a preferred frame in which one particle really is detected first, to wriggle out of the problem.

The reason is, relativity also tells you that spacelike events CANNOT be time ordered, so that observers who each make a measurement on one of the particles can easily disagree on who made a measurement first. This is not a shortcoming of relativity. The inability to absolutely time order spaceline events is essential for relativity to be a consistent theory in which physical events are described by consistent mathematics. At this point one can discard relativity as a description of spacetime and head down a dead end, but if you accept relativity, you are stuck with the conundrum of having to explain measurements that are correlated, but the requirements of relativity force you accept that there really is NO time ordering you can impose to allow you to believe one measurement really did cause the other. (Also recall that the lightlike trajectories of the photons allows you to find a frame in which both photons are measured at exactly the same time even if in the lab frame you might be tempted to think a photon measured 1 inch from the sour had to "really" be measured before a photon that was measred in another galaxy. A lorentz transformations can mame any two spcelike separated events happen at the same time in some frome). Keep in mind that because photons propagate on null rays, you are no more correct in asserting that you measure your photon first than some wookie on the star wars death star claiming the same thing, despite being so didtsntly separated that you find this weird. Maybe it's not quantum theory people are convinced is really weird, but the actual implications of taking relativity seriously when faced with null rays and trying to explain physics that goes along with that.

In this respect, (standard) quantum theory really does provide precisely what is needed to make sense out of this. Results which are correlated do not necessarily have any cause and effect relationship. Just taking quantum theory at face value and realizing the measurements have no relationship more stringent than being correlated means quantum theory as it is provides exactly the freedom required to be consistent relativity. Personally, I regard that as almost mirulous in how nature allows the pieces to fall together with exactly the right properties to make this coesxistence existence exactly what it must be, no more, no less. Seriously, how could hope for such a compatibility to be any better than that?

As a final piece of the puzzle here, ponder the fact that when two entangled photons are produced, the proper time and proper distance between the source and measuring device is ZERO. In light cone coordinates, the photons do nothing but carry the phase of the source, which coincidentally, the unmeasurability of which is where electromagnetism originates. Try to pin down too many properties to satisfy some preconception and you begin to come in conflict with well known theories which by virtue of experiment are widely held to be correct, leaving you to reinvent the wheel.

Just because we see the world with a particular classical bias that makes sense to us, doesn't mean that nature has to agree with us and mangle a simple non classical world into a psuedo classical nightmare to rescue some preconceptions of how things should be. In any argument, nature has never come out the loser.

 

[kurt101]

In a relativistic setting, two events which are spacelike cannot have any causal connection.

If measuring 1 entangled photon instantly affects the other entangled photon; there is no way to determine this happened except by statistical analysis. What does special relativity have to say about statistical analysis of entangled photons? Nothing. So the idea that special relativity somehow prevents the possibility that there is non-local action between entangled photons seems wrong. There is just no way for us to really know what happened between those entangled photons, because they are no longer entangled as soon as we measure one.

 

[bobob]

If measuring 1 entangled photon instantly affects the other entangled photon; there is no way to determine this happened except by statistical analysis. What does special relativity have to say about statistical analysis of entangled photons?

I think you missed my entire point about relativity being wht is confusing people, not the quantum mechanics because you seem to be missing the entire point about time ordering of spacelike events. Spacelike events CANNOT be time ordered, whether you are talking about entangled photons or any other events with a spacelike seperation.

In other words, when you say something about an instantaneous influence, you can only be referring to an influence that is instantaneous in one frame and that frame cannot be considered in any way more special than any other. By making a lorentz transform, in some other frame, the measurement of one photon will happen before the measurement of the other and by making yet another lorentz transform to another frame, the time order of the measurements will be reversed.

There is no possible way to make some claim that for two spacelike separated events, that one happened before or after the other or at the same time because any two spacelike events will happen at the same time in some frame and in different time orders in others. It is meaningless to try attach any absolute significance to some time relationship between those events. THAT is the entire point and the fact that you are dealing with entangled photons has no relevance to this fact.

The relevance to entangled photons arises because relativity prevents you from making any statement about which photon is measured before the other or even at the same time. If it's physically meaningless to say one photon was detected before the other, it's just as physically meaningless to try to assign some cause and effect relationship to the measurements. There can't be a cause and effect relationship when relativity tells you that the two events cannot even be given a time ordering.

Statistics has nothing to do with this. Statistics only enter the picture when doing a real experiment to rule out experimental errors by repeating the same experiment many times. The statistics has nothing to do with the impossibility of one event not being able to influence another event which is spacelike seperated.

If this is not clear, then what you are not understanding is relativity. I personally think this where people actually falther when trying to invoke some cause and effect. To do that, you have to throw relativity out the window, not quantum mechanics.

And yeah, relativity specifically excludes non-local interactions or else it would be galilean relativity. That is the entire point of special relativity which is why standard quantum theory is a LOCAL theory and axctually doesn't violate any known physics. The only thing that gets violated is the preconception of reality that a lot people seem to want to cling to without having a personal experience looking at the world like an elementary particle would see it. Is it really better to try to circumvent well known and experimentally verified physics to save a preconception of the world based on the observations of only things far removed from and bearing no resemblance to the things you are trying to describe with a theory? If you want to hang on to the idea that electrons and other particles are just likr everyday objects, please describe and everyday object with a spin 1/2.

 

[vanhees71]

If measuring 1 entangled photon instantly affects the other entangled photon; there is no way to determine this happened except by statistical analysis. What does special relativity have to say about statistical analysis of entangled photons? Nothing. So the idea that special relativity somehow prevents the possibility that there is non-local action between entangled photons seems wrong. There is just no way for us to really know what happened between those entangled photons, because they are no longer entangled as soon as we measure one.

It does not instantly the other entangled photon. Entanglement describes correlations which have been prepared at the moment where the photons were created, it does not describe an action at a distance when a measurement on one of these photons is made. The interactions are, according to the very fundamental construction of relativistic QFTs of which QED is the paradigmatic example, local and also only those QFTs are successful which obey the microcausality principle and thus ensure that there are no faster-than-light information transmitting signals possible.

 

[DrChinese]

It does not instantly [affect] the other entangled photon. Entanglement describes correlations which have been prepared at the moment where the photons were created, it does not describe an action at a distance when a measurement on one of these photons is made.

From the reference below (diametrically opposite to your comment): In the standard entanglement case, the measurement of anyone of the particles instantaneously changes the physical description of the other. This result was described by Einstein as ”spooky action at a distance”.

The truth is: no one knows the details of when entangled particles become entangled*, nor exactly when they cease their relationship. I believe this was more or less Kurt101's point. I don't think it is helpful to speak of your preferred interpretation (minimalist) as if it provides such a description, nor that your description of that is what's generally accepted. What is generally accepted** is that the common elements of QT can be applied to provide useful predictions.*For example: The entanglement swap can occur such that the entangled photons never co-exist. So obviously the entanglement did not occur at their creation - they were not created together (as you proposed). Variations on entanglement swapping wreck attempts to describe in terms of locality and/or (micro)causality.

Entanglement Between Photons That Have Never Co-existed

 

[Adel Makram]

There is no possible way to make some claim that for two spacelike separated events, that one happened before or after the other or at the same time because any two spacelike events will happen at the same time in some frame and in different time orders in others. It is meaningless to try attach any absolute significance to some time relationship between those events. THAT is the entire point and the fact that you are dealing with entangled photons has no relevance to this fact.

The relevance to entangled photons arises because relativity prevents you from making any statement about which photon is measured before the other or even at the same time. If it's physically meaningless to say one photon was detected before the other, it's just as physically meaningless to try to assign some cause and effect relationship to the measurements. There can't be a cause and effect relationship when relativity tells you that the two events cannot even be given a time ordering.

But still, no matter what frame you choose to ride with any time order of the events you get, the physical theory must explain the quantum correlation between the measurement outcomes.
In other words, the problem of understanding the quantum entanglement is not because the time order of the space-like events is meaningless in SR but because any observer will find the experiment outcomes always violates BI (Bell`s Inequality). And this can even happen in a non-moving frame where suppose that there are two observers each is attached to the measurement device and each claim that his device made the first measurement. In this setup, BI is still violated not because that there is no significance to talk about the order of the measurements but because something else is missing, whether to call it non-locality or retro-causality,,,etc.
I think there is a difference between what we comprehend and what we understand. The comprehension comes from rigorous mathematical approach to solve problem but the understanding comes from how this all relates to our common sense. QM is a comprehensive theory but not understandable one.

 

[kurt101]

I think you missed my entire point about relativity being wht is confusing people, not the quantum mechanics because you seem to be missing the entire point about time ordering of spacelike events. Spacelike events CANNOT be time ordered, whether you are talking about entangled photons or any other events with a spacelike seperation.

I understood you the first time as well as I understood you the second time. You made some very strong statements about causality which if false, should be corrected.

if you accept relativity, you are stuck with the conundrum of having to explain measurements that are correlated, but the requirements of relativity force you accept that there really is NO time ordering you can impose to allow you to believe one measurement really did cause the other.

Special relativity allows us to compare events we can measure. With entangled photons, when you measure one of the photons you don't know what happened to the other photon. So we can't actually know or not know whether measuring the first photon caused some instant effect on the second photon. Since we can't know either way, your statement that "relativity force you accept" is not true.

The truth is: no one knows the details of when entangled particles become entangled*, nor exactly when they cease their relationship. I believe this was more or less Kurt101's point. I don't think it is helpful to speak of your preferred interpretation (minimalist) as if it provides such a description, nor that your description of that is what's generally accepted. What is generally accepted** is that the common elements of QT can be applied to provide useful predictions.

I agree with this statement and thank you for helping me with my point.

The entanglement swap can occur such that the entangled photons never co-exist. So obviously the entanglement did not occur at their creation - they were not created together (as you proposed). Variations on entanglement swapping wreck attempts to describe in terms of locality and/or (micro)causality.

Now I am going to disagree with you for sort of the same reason as the other issue. I think that the entanglement in the experiment you referenced only happens when the photons interact; and the result of the interaction is non-locally propagated to the previous entanglement (if not already severed). So only through non-local causal action do these never co-existed photons achieve a correlation. I have reasons to think my explanation is the better one, but I have no definitive proof any more than I think you do. Though if you do have proof or even strong reasons, I would like to be convinced to the better or correct view.

 

[vanhees71]

From the reference below (diametrically opposite to your comment): In the standard entanglement case, the measurement of anyone of the particles instantaneously changes the physical description of the other. This result was described by Einstein as ”spooky action at a distance”.

The truth is: no one knows the details of when entangled particles become entangled*, nor exactly when they cease their relationship. I believe this was more or less Kurt101's point. I don't think it is helpful to speak of your preferred interpretation (minimalist) as if it provides such a description, nor that your description of that is what's generally accepted. What is generally accepted** is that the common elements of QT can be applied to provide useful predictions.*For example: The entanglement swap can occur such that the entangled photons never co-exist. So obviously the entanglement did not occur at their creation - they were not created together (as you proposed). Variations on entanglement swapping wreck attempts to describe in terms of locality and/or (micro)causality.

Entanglement Between Photons That Have Never Co-existed

Well, in my opnion, this is a fundamentale misunderstanding. There are no instantaneous interactions in relativistic local QFTs (that's why they are called local, and this they are by construction with the simple reason because this type of theories is very successful).

Also entanglement swapping as described in the above paper is not contradicting this fundamental mathematical facts. To achieve entanglement swapping you also use the entanglement of photon pairs which are entangled due to their preparation by a local process (like parametric down conversion in a BBO) and to make the other pair entangled you need to perform local measurements on one photon of each pair but no spooky action at a distance.

I don't see, what else is needed to understand that this successful class of relativistic QTs, namely local relativistic QFTs, are consistent with Einstein causality and of course allow to describe entanglement, which is a well-established observational fact too. I don't see, where there's any "explanation" lacking or where something is not understood.

 

[DrChinese]

Well, in my opinion, this is a fundamental misunderstanding.
...
I don't see, where ... something is not understood.

Let's skip to the chase: 2 photons are entangled that never interacted. They never even need to exist in each others' past light cone. Obviously, it's not a local process being described. You can call QFT local and [Einstein] causal all you like, but that would be ignoring what is right in front of your face in the cited experiment*. Bell precludes that anyway, so not sure why anyone would try to sneak that by.

No one is disputing that QFT is good, it's just your characterization of it which is incorrect. This experiment demonstrates unambiguously that there is no local source for the entanglement seen, and there is also no conflict with standard quantum theory either.

To tie this into the OP: because quantum theory does not require a causal ordering of [entanglement] events for its predictions, relativistic ordering is not an issue. That would only be a problem if QT postulated that there were causal effects ('a first measurement by Alice causes a change to Bob' versus 'a second measurement by Bob causes a change to Alice') that needed consideration (as the 2 Alice/Bob cases are experimentally indistinguishable). So adding relativistic considerations does not change anything.

In OP's cited paper (by Suarez), assumptions regarding when things happen in entanglement events are shown to lead to conflict with signal locality (the "relativistic consideration" of the previous paragraph). The logical conclusion is to reject those assumptions. There are a variety of interpretations that reject them anyway, one example being Relational Blockworld**. I am not entirely sure if the Suarez result actually eliminates any currently viable interpretations, but it might.*In previous discussions, IIRC you have expressed some reservations about whether entanglement swapping is a valid effect. Not sure if that is still an issue with you or not, but there are no end of papers by top teams documenting this important source of entanglement (and I'd be happy to cite as many as you like). So hopefully we can agree on this point without further wrangling.

** Which calls itself "acausal"; since no individual event within a block is considered to be the fundamental or preferred cause of the outcome. In an entanglement swapping case, the 2 entangled photon sources and 2 photon detections form a block of [at least] 4 vertices, and time ordering is not a consideration.

 

[kurt101]

This experiment demonstrates unambiguously that there is no local source for the entanglement seen

I don't see it as unambiguous. In the experiment you referenced where it says photons 1 and 4 exhibit a correlation despite the fact they never coexisted; try taking the position that 1 and 4 don't have any entanglement at the time of their respective creations. Also assume Vanhees71 is correct that entanglement happens due to their preparation by a local process (parametric down conversion). If you use these assumptions where does that lead?

Here is how I think of it:
1 and 2 start off entangled.
1 is measured and non-locally gives the result of this measurement to 2.
3 and 4 start off entangled.
2 and 3 become entangled at the beam splitter and this interaction results in 3 non-locally giving the result of this interaction to 4
Now 4 effectively contains some of the particle 1 state and the correlation is achieved

So I just gave an alternative interpretation of this experiment. Do you still see this experiment as unambiguously demonstrating there is no local source for entanglement?

Also for what it is worth, my interpretation is completely consistent with the basic EPR experiment that we see in the original Alain Aspect 1983 experiment where the polarizier angles are rotated right before the entangled photons are measured preventing any causal effect at the speed of light, but the non-local correlation remains. Assuming you hold causality as a fundamental truth, the most apparent conclusion (IMHO) is that the measurement of one of the entangled photons changed the state of the other photon instantly. And even if my causal interpretation of this EPR experiment is wrong, you would still expect the same non-local behavior in the swapping experiment.

 

[PeterDonis]

I just gave an alternative interpretation of this experiment.

Which explicitly says it's nonlocal:

1 is measured and non-locally gives the result of this measurement to 2.

So, just as @DrChinese says, there is no local source for the entanglement seen.

 

[kurt101]

It does not instantly the other entangled photon.

What is your level of certainty (0 to 100) that there is not a more fundamental model of the universe than QFT that acts in this way, where measuring one of the entangled photons imparts the result of this interaction to the other photon? I am trying to gauge if you are even willing to see things from a different perspective. And if you are not, I am still interested in understanding why your conviction is so strong.

Entanglement describes correlations which have been prepared at the moment where the photons were created

I agree with this.

The interactions are, according to the very fundamental construction of relativistic QFTs of which QED is the paradigmatic example, local and also only those QFTs are successful which obey the microcausality principle and thus ensure that there are no faster-than-light information transmitting signals possible.

I think of QFT as a mathematical framework that allows us to calculate the probability of events. I don't think of it as saying very much about single events. Is this a reasonable interpretation?

 

[kurt101]

So, just as @DrChinese says, there is no local source for the entanglement seen.

Yes, I think you are right. I have been reading what @DrChinese has been saying incorrectly. In my mind, I was thinking of photons 1 and 4 as correlated, but not really entangled and thinking of photons 1 and 2 or 3 and 4 as truly entangled. However, I guess that is not how @DrChinese is seeing the definition of entanglement. Sorry for the misunderstanding!

 

[vanhees71]

View attachment 245801
I don't see it as unambiguous. In the experiment you referenced where it says photons 1 and 4 exhibit a correlation despite the fact they never coexisted; try taking the position that 1 and 4 don't have any entanglement at the time of their respective creations. Also assume Vanhees71 is correct that entanglement happens due to their preparation by a local process (parametric down conversion). If you use these assumptions where does that lead?

Here is how I think of it:
1 and 2 start off entangled.
1 is measured and non-locally gives the result of this measurement to 2.
3 and 4 start off entangled.
2 and 3 become entangled at the beam splitter and this interaction results in 3 non-locally giving the result of this interaction to 4
Now 4 effectively contains some of the particle 1 state and the correlation is achieved

So I just gave an alternative interpretation of this experiment. Do you still see this experiment as unambiguously demonstrating there is no local source for entanglement?

Also for what it is worth, my interpretation is completely consistent with the basic EPR experiment that we see in the original Alain Aspect 1983 experiment where the polarizier angles are rotated right before the entangled photons are measured preventing any causal effect at the speed of light, but the non-local correlation remains. Assuming you hold causality as a fundamental truth, the most apparent conclusion (IMHO) is that the measurement of one of the entangled photons changed the state of the other photon instantly. And even if my causal interpretation of this EPR experiment is wrong, you would still expect the same non-local behavior in the swapping experiment.

This hits the nail on its head! The interactions are "local", the correlations are, in a delicate sense "non-local". I'd rather call it "long-range correlation" than "non-local correlation" to make the meaning of entanglement clear. That's precisely what I said: The entanglement of the final state is due to the quantum correlations described by the entanglement of the photon pairs in the beginning and the local (filtering) processes done on the single photons.

 

[vanhees71]

Which explicitly says it's nonlocal:
So, just as @DrChinese says, there is no local source for the entanglement seen.

Again, you have to be more precise: The "non-locality" refers to the correlations. In my opinion it's bad language. I'd call it rather long-range correlations than "non-locality". What's local here is precisely what's meant when you call a relativistic QFT "local" (the Standard Modell including QED is in this sense "local"): The interactions of the single photons with the detectors, beam-splitters etc. The entanglement swapping is made possible by these local interactions together with the long-range correlations described by the entanglement of each of the two photon pairs prepared (by local processes each!) in the very beginning.

There is no contradiction between the "locality of the interactions described by standard relativistic QFTs and the Standard Model and the long-range correlations described by entanglement.

 

[vanhees71]

What is your level of certainty (0 to 100) that there is not a more fundamental model of the universe than QFT that acts in this way, where measuring one of the entangled photons imparts the result of this interaction to the other photon? I am trying to gauge if you are even willing to see things from a different perspective. And if you are not, I am still interested in understanding why your conviction is so strong.I agree with this.I think of QFT as a mathematical framework that allows us to calculate the probability of events. I don't think of it as saying very much about single events. Is this a reasonable interpretation?

There may be a more fundamental model of the universe than QFT. However, the experiment is fully designed and understood by standard QED with local interactions and long-ranged correlations described by entanglement. There's no contradiction between standard QFT and this experiment.

My conviction is so strong, because I think that locality of interaction/micro-causality is the very fundamental assumptions put into the construction of relativistic QFTs and particularly the Standard Model/QED.

 

[DrChinese]

Yes, I think you are right. I have been reading what @DrChinese has been saying incorrectly. In my mind, I was thinking of photons 1 and 4 as correlated, but not really entangled and thinking of photons 1 and 2 or 3 and 4 as truly entangled. However, I guess that is not how @DrChinese is seeing the definition of entanglement. Sorry for the misunderstanding!

Yes, 1 & 4 are fully entangled on the polarization basis (also a couple of other bases). This is demonstrated by violation of a Bell Inequality. Think of it like this: if you were to check the polarization of 1 & 4 at ANY specific angle, the outcome can be predicted. That can't happen UNLESS they are entangled.

The citation I gave is one of many for entanglement swapping. In other variations, 1 & 4 are measured AFTER they are cast into the entangled state. So here are the key variations to consider:

a. 1 & 4 measured after they are cast into an entangled state.
b. 1 measured before 4 created, and before they are cast into an entangled state.
c. 1 & 4 both measured before they are cast into an entangled state.

An important thing to take away from these variations: the statistics for 1 & 4 do NOT change as you switch from a to b to c. Time ordering is not relevant, which is of course confusing if you attempt to assign causality. Which is what the OP's question relates to.

 

[DrChinese]

... The entanglement swapping is made possible by these local interactions together with the long-range correlations described by the entanglement of each of the two photon pairs prepared (by local processes each!) in the very beginning.

There are no sources that back up your position on entanglement swapping, and I can cite as many more as you like. I realize you feel strongly about local micro-causality, but Bell prohibits that anyway so I don't see what your sticking point is. QFT is not local deterministic, and of course I agree that there is no disagreement between QFT and the cited experiment. So:

Photons 1 & 4 are entangled, period. That's what entanglement is, and it is certified by correlations that exceed the Bell boundary. Those photons never interacted. If they are NOT cast into an entangled state by a suitable manipulation of photons 2 & 3, then they will NOT be correlated (i.e. entangled). If the manipulation of 2 & 3 is modified only so that 2 & 3 are distinguishable (say by adding a sufficient time delay to 2), then they will NOT be correlated*. That's because the cast of photons 1 & 4 into an entangled state is dependent on the action at 2 & 3 being successful. Photons 1, 2&3, and 4 can be measured in places non-local to each other and the entanglement still occurs. This completely flies in the face of your attempted description.*If you were correct, then inserting a time delay for photon 2 would not make any difference to the outcome for the 1 & 4 entanglement correlations. You would still select the same 1 & 4 pairs either way (from the detector results at 2 & 3). But the indistinguishable nature of 2 & 3 is an absolute requirement for entanglement of 1 & 4, which are otherwise far away.

 

[kurt101]

Yes, 1 & 4 are fully entangled on the polarization basis (also a couple of other bases). This is demonstrated by violation of a Bell Inequality. Think of it like this: if you were to check the polarization of 1 & 4 at ANY specific angle, the outcome can be predicted. That can't happen UNLESS they are entangled.

A full correlation is exactly what I would expect in the causal scenario I outlined. If you don’t believe me, I can go through the actual calculations or even provide a simulation if I have to.

In other variations, 1 & 4 are measured AFTER they are cast into the entangled state. So here are the key variations to consider:

a. 1 & 4 measured after they are cast into an entangled state.
b. 1 measured before 4 created, and before they are cast into an entangled state.
c. 1 & 4 both measured before they are cast into an entangled state.

An important thing to take away from these variations: the statistics for 1 & 4 do NOT change as you switch from a to b to c. Time ordering is not relevant, which is of course confusing if you attempt to assign causality. Which is what the OP's question relates to.

It is not confusing at all if you use causality. The causal algorithm is very simple and will work for all the scenarios you mentioned and give the same probability.

 

[PeterDonis]

there is disagreement between QFT and the cited experiment

There is? I was not aware that there was any experiment that disagreed with QFT.

 

[DrChinese]

There is? I was not aware that there was any experiment that disagreed with QFT.

Oops! NO disagreement. I went ahead and edited to fix that.

 

[DrChinese]

A full correlation is exactly what I would expect in the causal scenario I outlined. If you don’t believe me, I can go through the actual calculations or even provide a simulation if I have to.

It is not confusing at all if you use causality. The causal algorithm is very simple and will work for all the scenarios you mentioned and give the same probability.

I don't follow your thinking. Are you a) saying 1 & 4 are entangled, and b) it is due to an effect that both flows from past to future AND is local? Because your example features non-locality, I quote: "1 is measured and non-locally gives the result of this measurement to 2. " I have no problem with a Bohmian type interpretation of entanglement swapping, which is deterministic but nonlocal. Bell rules!

 

[kurt101]

I don't follow your thinking. Are you a) saying 1 & 4 are entangled, and b) it is due to an effect that both flows from past to future AND is local? Because your example features non-locality, I quote: "1 is measured and non-locally gives the result of this measurement to 2. " I have no problem with a Bohmian type interpretation of entanglement swapping, which is deterministic but nonlocal. Bell rules!

Great that you have no problem with the "Bohmian type interpretation", I was still trying to make a defense of it.

I will assume your definition of entanglement is the accepted one, but I find the statement "photons are entangled that never interacted" confusing/misleading and without careful explanation it makes the universe seem strange and mysterious when it is not necessarily so. Also it is not an accurate description in the causal model I outlined where it would make more sense to define entanglement as a shared state that is created and destroyed by local interaction.

 

[DrChinese]

1. Great that you have no problem with the "Bohmian type interpretation", I was still trying to make a defense of it.

2. I will assume your definition of entanglement is the accepted one, but I find the statement "photons are entangled that never interacted" confusing/misleading and without careful explanation it makes the universe seem strange and mysterious when it is not necessarily so. Also it is not an accurate description in the causal model I outlined where it would make more sense to define entanglement as a shared state that is created and destroyed by local interaction.

1. No need for me. Bell specifically mentions that as viable in his groundbreaking paper.

2. ...I find the statement "photons are entangled that never interacted" confusing/misleading and without careful explanation it makes the universe seem strange and mysterious when it is not necessarily so...

Well, the title of the cited paper is: "Entanglement Between Photons That Have Never Co-existed". So they didn't interact. Below is another citation (although it is behind a paywall), and if that doesn't say it, I don't know what will. To be fair, I was not intending to make a big deal of how strange and mysterious it is.

Experimental Entanglement Swapping: Entangling Photons That Never Interacted

On the other hand, it is a bit strange and mysterious to me.

 

[Demystifier]

Entanglement describes correlations which have been prepared at the moment where the photons were created

What does it mean "to prepare correlations"? Correlations of what? One cannot prepare correlations themselves. One can only prepare the objective properties. But you seem to deny the existence of objective properties themselves. This is what Mermin calls "correlations without correlata" https://arxiv.org/pdf/quant-ph/9801057.pdf .

 

[vanhees71]

There are no sources that back up your position on entanglement swapping, and I can cite as many more as you like. I realize you feel strongly about local micro-causality, but Bell prohibits that anyway so I don't see what your sticking point is. QFT is not local deterministic, and of course I agree that there is no disagreement between QFT and the cited experiment. So:

Photons 1 & 4 are entangled, period. That's what entanglement is, and it is certified by correlations that exceed the Bell boundary. Those photons never interacted. If they are NOT cast into an entangled state by a suitable manipulation of photons 2 & 3, then they will NOT be correlated (i.e. entangled). If the manipulation of 2 & 3 is modified only so that 2 & 3 are distinguishable (say by adding a sufficient time delay to 2), then they will NOT be correlated*. That's because the cast of photons 1 & 4 into an entangled state is dependent on the action at 2 & 3 being successful. Photons 1, 2&3, and 4 can be measured in places non-local to each other and the entanglement still occurs. This completely flies in the face of your attempted description.*If you were correct, then inserting a time delay for photon 2 would not make any difference to the outcome for the 1 & 4 entanglement correlations. You would still select the same 1 & 4 pairs either way (from the detector results at 2 & 3). But the indistinguishable nature of 2 & 3 is an absolute requirement for entanglement of 1 & 4, which are otherwise far away.

I still not understand what you consider wrong with microcausality and what you think is proven wrong with it by entanglement swapping. This would mean nothing less than claiming that QED, one of the best tested theories ever, is wrong, and this I think one would have heard about very clearly, if this were true.

As I always emphasized you have to be concrete in which experiment you want to do. Which time delay do you want to introduce and how?

No matter how, what's very sure from the very foundations of relativistic QFT (microcausality) no space-like separated events can be causally connected in any way, and thus the time order in the sense of any coordinate time in a fixed inertial reference frame is irrelevant for whether your partial ensemble of photon pairs consisting of photons 1&4 is entangled or not. In the experiment described in the quoted paper they are entangled. It's also irrelevant that they have never directly interacted (that's of course true here by the setup of the experiment) but they are entangled through their selection by local measurements on one of the photons of the pair 1&2 and on one of the photons of pair 3&4. Then entanglement of the selected set of pairs 1&4 is due to the entanglement of the pairs 1&2 and 3&4 respectively, and this entanglement is due to the preparation of each of these pairs before (in the invariant sense, i.e., the preparation process of these pairs is at timelike distance to the measurement events leading to entanglement of the partial ensemble of 1&4 pairs after the described selection process).

Again, I do not understand what's wrong with this simple and logical interpretation based on solely on the mathematical formalism of relativistic QFT.

 

[vanhees71]

1. No need for me. Bell specifically mentions that as viable in his groundbreaking paper.

2. ...I find the statement "photons are entangled that never interacted" confusing/misleading and without careful explanation it makes the universe seem strange and mysterious when it is not necessarily so...

Well, the title of the cited paper is: "Entanglement Between Photons That Have Never Co-existed". So they didn't interact. Below is another citation (although it is behind a paywall), and if that doesn't say it, I don't know what will. To be fair, I was not intending to make a big deal of how strange and mysterious it is.

Experimental Entanglement Swapping: Entangling Photons That Never Interacted

On the other hand, it is a bit strange and mysterious to me.

This paper is very clear in what's done, and there's indeed nothing strange and mysterious but it's all in full accordance with QED again. My main point is even explicitly stated and emphasized by the experimenters

Note that the Bell-state analysis relies on the interfer-
ence of two independently created photons. One, there-
fore, has to guarantee good spatial and temporal overlap at
the beam splitter and, above all, one has to erase all kinds
of path information for photon 2 and for photon 3.

The key issue indeed is, as the authors also very clearly demonstrate in the entire introductory passage above: The key of this experiment is to project the pair 2&3 (which was NOT entangled before) by a Bell measurement to one of the Bell states given in Eq. (2) to select an ensemble of entangled pairs 1&4. This selection process consists of local manipulations on the photons 2&3 (see the quoted passage).

 

[DrChinese]

1. I still not understand what you consider wrong with microcausality ...

2. As I always emphasized you have to be concrete in which experiment you want to do. Which time delay do you want to introduce and how?

1. There cannot be both causality [deterministic effects flowing from past to future] and locality (per Bell). I am guessing that you are pushing some special meaning of "microcausality" that purports to avoid Bell, but I don't know what that would be. If that is the case, perhaps you can explain relative to the example below.

2. Sure, and the reference diagram is from here.

a) In the various swapping experiments cited, entanglement of photons 1 & 4 occurs when photons 2 & 3 are cast via a Bell State Analyzer (the BSA is to be set at 0 degrees). The BSA is in the upper middle balloon of the diagram. The 1 & 4 pairs are spacetime separated, post selected, and the setup calls for the 1 & 4 to arrive at their beamsplitters near simultaneously (within a suitable coincidence window - so note that this is NOT the variation where the photons never coexisted). For simplicity, let's say we post select only those pairs that are psi+ entangled (same polarization when both measured at 45 degrees or any angle). By selecting 0 degrees for the BSA, and 45 degrees for the 1 & 4 pair, you can't learn anything about polarization of 1 & 4 when you perform the BSA on 2 & 3. The BSA on 2 & 3 rings either HH or VV for psi+, and again, the coincidence window is suitably small so that 2 & 3 are indistinguishable.

The 1 & 4 pairs will be matched in polarization about 100% assuming an ideal setup for the group we post select.

b) Now, we insert a 100 picosecond delay into the path of photon 2 (but nothing done to any of the other paths). The 1 & 4 pairs we want will still arrive within the requisite time window (relative to each other). But the 2 & 3 pairs will be easily distinguished as photon 2 will arrive about 100 ps later than photon 3. Importantly, we can still identify the 2 & 3 pairs as being either HH or VV as before, so that we can still post-select the proper 1 & 4 pairs. If there is local microcausality, as you claim, this modification should NOT affect the results at 1 & 4. (There is no causal connection between events at the BSA and detections at 1 & 4.)

But that is not the case. Instead, the 1 & 4 polarization matching will be about 50%, no correlation*. How is it that the same pairs arriving at 1 & 4 - which are spacelike separated - are suddenly giving Product state statistics rather than the Entangled state statistics? The only change was to the path length of photon 2, which is spacelike separated from photon 1 (and 4)? Because of the separation, there cannot be any causal connection between the decision to add the 100 picosecond delay and the matching between photons 1 and 4.

And yet this is all standard QFT. Which of course follows Bell, which rejects local determinism (a/k/a local realism). In case anyone is wondering if my version of this setup has ever been tested: EVERY entanglement swapping experiment demonstrates this! They always start with the 2 & 3 photons being distinguishable and then attempt to tune it to the indistinguishable version. When the 1 & 4 stats move from being Product state stats to Entangled state stats, they know they have succeeded. QED.*The stats would be as follows: The matches between entangled photons 1 & 2 will be 50% (i.e. random) due to the 45 degree separation in polarization angles being measured (cos^2 theta). Ditto for entangled photons 3 & 4. 1 & 4 are not entangled, and so their match rate will likewise be random. Of course, we are still only post selecting the portion that are denoted by HH or VV at the BSA.