I Could there be a non local theory without action at a distance?

[syed]

So as far as I know, with regards to the EPR experiments, in bohmian mechanics, one entangled particle's measurement outcome can affect the other in a causal way where there is a privileged frame of reference. However, this is done without any sort of signal propagating between them.

Is there anything within physics that rules out an interpretation involving something being passed between entangled particles that travels through space time?

 

[DrChinese]

Is there anything within physics that rules out an interpretation involving something being passed between entangled particles that travels through space time?

If there were, it needs to travel at a speed well in excess of 10,000 c.

Bounding the speed of `spooky action at a distance'

 

[Nugatory]

Is there anything within physics that rules out an interpretation involving something being passed between entangled particles that travels through space time?

To be consistent with experimental observations that something must (as pointed out by @DrChinese above) travel at superluminal velocities so we would have to discard relativity. That is a very heavy lift; whether you consider this “ruling out” the possibility completely or merely rendering it so extremely unlikely as not to be worth speculating about unless and until someone comes up with a reasonable theory is a matter of personal taste.

It is worth noting that, unlike superluminal speeds, the correlation between measurements of entangled particles does not violate relativistic causality. That seems to me a strong hint that we should try to understand entanglement without picking a fight with relativity.

 

[syed]

If there were, it needs to travel at a speed well in excess of 10,000 c.

Bounding the speed of `spooky action at a distance'

Got it, what does speed mean here though in a relativity context? I'm just curious as to how these measurements were calculated

 

[syed]

To be consistent with experimental observations that something must (as pointed out by @DrChinese above) travel at superluminal velocities so we would have to discard relativity. That is a very heavy lift; whether you consider this “ruling out” the possibility completely or merely rendering it so extremely unlikely as not to be worth speculating about unless and until someone comes up with a reasonable theory is a matter of personal taste.

It is worth noting that, unlike superluminal speeds, the correlation between measurements of entangled particles does not violate relativistic causality. That seems to me a strong hint that we should try to understand entanglement without picking a fight with relativity.

Interesting, so it would have to basically violate relativity if true, which I understand, but so does Bohmian Mechanics as far as I'm aware, correct?

Also, isn't your second paragraph circular? Of course, if we assume relativity to be false, then it wouldn't violate relativistic causality. But as far as I know, there is no explanation for how/why the correlations are maintained once the particles are separated and measured, and any explanation must violate relativity since no local theory could explain it?

 

[Nugatory]

Interesting, so it would have to basically violate relativity if true, which I understand, but so does Bohmian Mechanics as far as I'm aware, correct?

I won't presume to speak for the Bohmians, but if I understand properly, non-locality in Bohmian mechanics avoids conflict with relativity more or less as below.

Also, isn't your second paragraph circular? Of course, if we assume relativity to be false, then it wouldn't violate relativistic causality. But as far as I know, there is no explanation for how/why the correlations are maintained once the particles are separated and measured, and any explanation must violate relativity since no local theory could explain it?

There is a difference between "the theory must be non-local" and "a non-local theory must violate relativistic causality". The point I'm trying to make is that any candidate theory that proposes an influence travelling between the first measurement and the second is making a stronger assumption than non-locality (by assuming that the influence is created at one measurement and felt at the other, not required to explain the experimental results which are symmetrical) and therefore is picking an unnecessary fight with relativity.

 

[syed]

I won't presume to speak for the Bohmians, but if I understand properly, non-locality in Bohmian mechanics avoids conflict with relativity more or less as below.
There is a difference between "the theory must be non-local" and "a non-local theory must violate relativistic causality". The point I'm trying to make is that any candidate theory that proposes an influence travelling between the first measurement and the second is making a stronger assumption than non-locality (by assuming that the influence is created at one measurement and felt at the other, not required to explain the experimental results which are symmetrical) and therefore is picking an unnecessary fight with relativity.

Let us suppose that there is no influence travelling between measurement outcomes, or that there is no spooky action at a distance.

Now, it still remains true that measurement outcomes are correlated with each other, and they cannot be represented independently when in an entangled state. So what would then be the explanation for why they remain correlated? If one says that the explanation is the joint wave function, this is the part that seems circular to me, since the joint wave function just is the fact that they are correlated. In other words, it seems that we have no further explanation for the phenomenon.

Thus, it seems that unless you are in the many worlds camp (which preserves relativity I believe), it seems that any explanation must violate relativity, which presumably is why some people don't find it unnecessary to think of an explanation that violates it.

 

[syed]

If there were, it needs to travel at a speed well in excess of 10,000 c.

Bounding the speed of `spooky action at a distance'

In an earlier post, you said

"[An additional note: I don't think their formal conclusion would be considered as strong today as when it was originally published in 2008. There is evidence from some CMBR studies that Earth might have a velocity relative to some cosmic reference frames that are in the neighborhood of 10^-3 c (or greater), which would then violate one of the assumptions for their specific conclusion.]"

Does this apply to the study you linked as well?

 

[Nugatory]

Let us suppose that there is no influence travelling between measurement outcomes, or that there is no spooky action at a distance.

The difficulty with any "travelling between measurement outcomes" model is that it implies a direction of travel: in any particular instance, does the influence propagate from measurement A to influence the result at B, or does it propagate from measurement B to influence the result at A? We're hypothesizing a physical effect here, so this direction of travel should be a frame-independent invariant - but it cannot be if the two measurements are spacelike-separated, and that creates the conflict with both relativity and the common-sense notion that causes must precede effects.

The correlation between entangled particles is of a different nature. The change of system state upon measurement is the same no matter which measurement we consider to be first (which suggests that maybe that's not something that we need be considering) so there is no way of identifying one measurement as the source of the influence and the other as the recipient. Indeed, the impossibility of communicating by entanglement follows from the way that the math describing entanglement is explicitly acausal.

So the non-local correlation is spooky, more so because it cannot be explained as I would, for example, explain spooky haunted house phenomena ("Nonsense - that's not some supernatural creature, it's mice in the walls or windows rattling in the wind or something and I haven't figured out what yet") by assuming that there's a causal mechanism that I haven't figured out yet. But the conflict with relativity only appears because we've introduced this assumption of a propagating influence, demanded by our classical intuition but not in the math or in the observational results.

 

[syed]

The difficulty with any "travelling between measurement outcomes" model is that it implies a direction of travel: in any particular instance, does the influence propagate from measurement A to influence the result at B, or does it propagate from measurement B to influence the result at A? We're hypothesizing a physical effect here, so this direction of travel should be a frame-independent invariant - but it cannot be if the two measurements are spacelike-separated, and that creates the conflict with both relativity and the common-sense notion that causes must precede effects.

The correlation between entangled particles is of a different nature. The change of system state upon measurement is the same no matter which measurement we consider to be first (which suggests that maybe that's not something that we need be considering) so there is no way of identifying one measurement as the source of the influence and the other as the recipient. Indeed, the impossibility of communicating by entanglement follows from the way that the math describing entanglement is explicitly acausal.

So the non-local correlation is spooky, more so because it cannot be explained as I would, for example, explain spooky haunted house phenomena ("Nonsense - that's not some supernatural creature, it's mice in the walls or windows rattling in the wind or something and I haven't figured out what yet") by assuming that there's a causal mechanism that I haven't figured out yet. But the conflict with relativity only appears because we've introduced this assumption of a propagating influence, demanded by our classical intuition but not in the math or in the observational results.

The events being space like separated is exactly what is creating the confusion for me though. As you say, in one frame, measurement result A occurs before result B. In another frame, result B occurs before result A. But in either frame, before either result is measured, neither result is determined. Yet, as soon as one result is measured, the other result is determined, in both frames. This determination process is what seems to result in paradoxes as far as I know, whether you call this "causal" or not

 

[PeterDonis]

in either frame, before either result is measured, neither result is determined. Yet, as soon as one result is measured, the other result is determined, in both frames.

No, this is not correct. Since the time ordering is frame-dependent, there cannot be any "determination" process involved such as you describe--because such a process would have to violate relativity.

And in fact, no such process is needed, because the two measurements commute--their results do not depend on the order in which they are done. That fact in itself means that whatever is going on, it can't be one result determining the other.

This is one of those cases where you simply have to discard all of your pre-quantum intuitions about how things have to work. Nature simply doesn't work the way you are thinking things have to work.

 

[syed]

No, this is not correct. Since the time ordering is frame-dependent, there cannot be any "determination" process involved such as you describe--because such a process would have to violate relativity.

And in fact, no such process is needed, because the two measurements commute--their results do not depend on the order in which they are done. That fact in itself means that whatever is going on, it can't be one result determining the other.

This is one of those cases where you simply have to discard all of your pre-quantum intuitions about how things have to work. Nature simply doesn't work the way you are thinking things have to work.

Let's look at a particular frame of reference. In a particular frame, measurement A occurs before measurement B. But before either occur, measurement A has a 50% chance of being an up spin or a down spin. The same applies to measurement B.

But as soon as measurement A is measured to be a positive spin, then measurement B can be predicted, with certainty, to be a negative spin. Measurement B occurs after measurement A within this frame as noted already.

How is this not an obvious case of an influence?

Note that this is similar to the EPR argument where Einstein suggests it's either a) a causal influence or b) local predeterminism, except b) was ruled out.

You're suggesting a third option but I'm not seeing any evidence of that. So I don't think this has to do with "pre-quantum intuitions". It's just a simple logical fork.

 

[PeterDonis]

as soon as measurement A is measured to be a positive spin, then measurement B can be predicted, with certainty, to be a negative spin. Measurement B occurs after measurement A within this frame as noted already.

How is this not an obvious case of an influence?

Because the measurements commute; it doesn't matter which one occurs first. The results are the same.

In other words, when you think of A coming before B in this frame, so that it's A's measurement that makes the prediction of B's measurement certain, you are not actually talking about any physics. You're just talking about your own personal thought processes being misled by a frame-dependent thing. The only actual physics involved is that the measurements commute.

You're suggesting a third option

No, I'm not. I'm making no claims whatever about what is "really happening". I'm simply pointing out the reasons why what you think is "really happening", can't be correct.

If you ask what model of "what's really happening" is correct, the answer is that we don't know. Nobody has a model of "what's really happening" that is generally agreed to be correct. But that doesn't change the fact that there are some such models that can't be correct, and the one you have in mind is one of them.

 

[syed]

Because the measurements commute; it doesn't matter which one occurs first. The results are the same.

In other words, when you think of A coming before B in this frame, so that it's A's measurement that makes the prediction of B's measurement certain, you are not actually talking about any physics. You're just talking about your own personal thought processes being misled by a frame-dependent thing. The only actual physics involved is that the measurements commute.


No, I'm not. I'm making no claims whatever about what is "really happening". I'm simply pointing out the reasons why what you think is "really happening", can't be correct.

If you ask what model of "what's really happening" is correct, the answer is that we don't know. Nobody has a model of "what's really happening" that is generally agreed to be correct. But that doesn't change the fact that there are some such models that can't be correct, and the one you have in mind is one of them.

I agree that the measurement operators commute, which means quantum theory doesn't assign a preferred time-ordering to them. But that’s a formal, operator-level statement within the theory. This doesn’t settle the ontological question I’m asking: How do the correlations actually arise? In any given Lorentz frame where A happens before B, the outcome at A lets us predict B with certainty. That sure looks like a directional influence in that frame, unless we posit something else (like predetermination which was ruled out by Bell) to explain it.

You’re saying this is just a "frame-dependent thought process," but that dismisses the fact that relativistic causality itself is frame-dependent: what counts as a cause or an influence can legitimately depend on the ordering in a particular frame, so long as it respects light cones. I'm not asserting that A objectively causes B across all frames, but rather just that in this frame, the correlation appears to reflect an influence unless there's an alternative mechanism. And when you say "some models can't be correct"..okay...but which models can? What third option is there if we reject both non local causality and local predetermination? (apart from perhaps many worlds)

So I don't see or think that this is a matter of clinging to "pre quantum intuitions". Saying "we don’t know what's really happening" is fine, but then you can't go ahead and claim to rule out influence either. What you seem to be doing is denying the aforementioned options while offering no alternative: that’s dodging the explanatory burden.

 

[PeterDonis]

This doesn’t settle the ontological question I’m asking: How do the correlations actually arise?

There is no generally accepted answer to this question. Read the last paragraph of my post #13.

relativistic causality itself is frame-dependent

No, it isn't. Relativistic causality involves timelike and lightlike separations, for which time ordering is invariant. There is no concept of relativistic causality that involves causal connections between spacelike separated events.

Saying "we don’t know what's really happening" is fine, but then you can't go ahead and claim to rule out influence either.

Yes, you can, at least for any concept of "influence" that respects relativistic causality. See above. The fact that we don't know what's really happening doesn't mean we don't know anything.

denying the aforementioned options while offering no alternative: that’s dodging the explanatory burden.

If I had an explanation that would satisfy everyone, I'd have a Nobel Prize. I don't. Nobody does. Again, read the last paragraph of my post #13. If I'm "dodging the explanatory burden", so is everyone else. But we can't just sit on our hands until someone comes up with an explanation that satisfies everyone. We still have to do physics, so we do the best we can with what we have.

 

[syed]

Yes, you can, at least for any concept of "influence" that respects relativistic causality. See above. The fact that we don't know what's really happening doesn't mean we don't know anything.

Then you simply misunderstood, since the proposed influences obviously do not respect relativistic causality. The purpose of talking about each frame and the general EPR argument was to show that there seems to be no way to make sense of the correlations without an influence, but of course, that influence would not cohere with relativity.

 

[PeterDonis]

the proposed influences obviously do not respect relativistic causality

In other words, this is personal speculation on your part--unless you can give a reference for such a model in the literature.

The purpose of talking about each frame was to show that there seems to be no way to make sense of the correlations without an influence,

I understand that it seems that way to you. That doesn't mean you're right. It also doesn't mean it seems that way to everyone.

As the rules for this subforum explain, this kind of dispute is not resolvable. Ultimately it comes down to personal preference: what kinds of counterintuitive models are you willing to accept, and what kinds are you not willing to accept? There's no way to resolve such disputes.

 

[syed]

In other words, this is personal speculation on your part--unless you can give a reference for such a model in the literature.

...?

Bohmian mechanics already exists.

Realistic theories with superluminal, contiguous influences also exist: https://digital.library.unt.edu/ark:/67531/metadc1105618/

Your assertion that this is all personal speculation...is ironically personal speculation on your part.

 

[PeterDonis]

Bohmian mechanics already exists.

Where do the "influences" you talk about appear in Bohmian mechanics?

Realistic theories with superluminal, contiguous influences also exist: https://digital.library.unt.edu/ark:/67531/metadc1105618/

As I commented to you in a PM thread, this paper looks dubious on its face. But now that you've posted it here in a public thread, others can take a look and see what they think.

 

[PeterDonis]

Your assertion that this is all personal speculation...is ironically personal speculation on your part.

No, it's what you should expect when you make claims without giving references. Now that you've given one, others can look at it. That helps.

 

[syed]

Where do the "influences" you talk about appear in Bohmian mechanics?


As I commented to you in a PM thread, this paper looks dubious on its face. But now that you've posted it here in a public thread, others can take a look and see what they think.

Influence as in a causal influence, as in there is a preferred frame of reference in Bohmian mechanics, where there is a privileged time ordering. The first measurement outcome in that ordering causes or influences the second measurement outcome.


From: Dürr, Goldstein, Norsen, Struyve & Zanghì (2014), “Can Bohmian mechanics be made relativistic?”
Link to paper: https://pmc.ncbi.nlm.nih.gov/articles/PMC3896068/

“In Bohmian mechanics, quantum entanglement leads to instantaneous influences between spatially separated particles: the velocity of one particle depends on the positions of others, even if they are space-like separated. In order to formulate this in a relativistic context, one must specify which configuration is 'simultaneous.' This requires a privileged foliation of space–time.