I Entanglement and FTL signaling in professional scientific literature

[PeterDonis]

they decided to analyse the idea of a quantum information signaling and make the requisite assumptions and see what conclusions they could draw

"The requisite assumptions" can't in themselves be inconsistent with relativity because there is a relativistic theory, QFT, that correctly predicts the correlations. The assumption of a preferred frame does not seem to me to be "requisite" to explain the correlations; it seems to me to be an additional assumption driven only by the particular method of analysis chosen. The fact that that additional assumption is inconsistent with relativity seems to me to be far more likely to be an issue with the assumption and the method of analysis that drove it than with relativity itself, particularly in view of the fact that, as above, we can correctly predict the correlations with a relativistic theory.

 

[DrChinese]

"The requisite assumptions" can't in themselves be inconsistent with relativity because there is a relativistic theory, QFT, that correctly predicts the correlations.

You assume that all explicitly nonlocal/nonrelativistic quantum theories/interpretations are wrong (of which Bohmian Mechanics is just one). I'm sure you are aware of some others (such as GRW). Those generally make identical predictions for Bell tests as does QFT (which has many other advantages),

That was their point, without them saying so directly. IF there exists a universal preferred reference frame, AND the Earth's motion relative to it is within a threshold, THEN: a Bell test like the one they ran will in fact show differences in some directions (which are not predicted by garden variety QM). It doesn't, so one should rule those out by experiment. Of course, defenders of those alternate candidate theories will attempt to explain away the experiment or further modify the theory...

 

[PeterDonis]

You assume that all explicitly nonlocal/nonrelativistic quantum theories/interpretations are wrong (of which Bohmian Mechanics is just one).

No, I'm not assuming that. It's perfectly possible to have an "explicitly nonlocal/nonlrelativistic interpretation" that still acknowledges and accounts for the fact that, in domains where relativistic effects are significant, our best current experimental data says that those relativistic effects are present. For example, the version of Bohmian Mechanics that @Demystifier has published is explicitly nonlocal and nonrelativistic, but it still has Lorentz invariance as an emergent symmetry that is expected to be observed in all of our experimental tests to date.

The particular paper in question, though, is "explicitly nonlocal and nonrelativistic" in a different way: the method of analysis it is using appears to me to explicitly deny Lorentz invariance even as an "emergent" symmetry valid in the domain we have experimentally tested. And I would like to see some explanation of why they adopted that method of analysis, when it seems open to the obvious objection that we have observed Lorentz invariance as a valid symmetry in all of our experimental tests to date, which certainly include relative speeds larger than the limits the paper is using, so we should not expect to see preferred frame effects in this domain. (This question becomes even more acute, btw, if the paper's conclusion is, as it seems to be, that the version of "quantum signaling" they are analyzing is questionable or debunked.)

 

[PeterDonis]

Those generally make identical predictions for Bell tests as does QFT (which has many other advantages),

But if that's the case, how can any such interpretation possibly be ruled out by an experiment that confirms those same predictions for Bell tests?

IF there exists a universal preferred reference frame, AND the Earth's motion relative to it is within a threshold, THEN: a Bell test like the one they ran will in fact show differences in some directions (which are not predicted by garden variety QM).

But if there is any such interpretation, it would make predictions for Bell tests different from the predictions of standard QM/QFT. But you said above (in what I quoted) that these interpretations make identical predictions for those tests to standard QM/QFT. So what, exactly, is being ruled out by this experiment?

 

[DrChinese]

But if that's the case, how can any such interpretation possibly be ruled out by an experiment that confirms those same predictions for Bell tests?But if there is any such interpretation, it would make predictions for Bell tests different from the predictions of standard QM/QFT. But you said above (in what I quoted) that these interpretations make identical predictions for those tests to standard QM/QFT. So what, exactly, is being ruled out by this experiment?

You missed the modifying word "generally". The purpose of the cited paper was to demonstrate a little known consequence of some of the outlier candidate/competing theories. Thus showing that there are some predictions which are not identical.

Clearly, the race is on (and has been on) to discover any possible theoretical consequence of the various interpretations/candidate quantum theories, no matter how small, and see if an experiment can be formulated to distinguish. No different than in the study of dark matter vs MOND vs everything else. Each accounts for many of the basics alike, but some have discrepancies which are coming to light.

 

[PeterDonis]

The purpose of the cited paper was to demonstrate a little known consequence of some of the outlier candidate/competing theories. Thus showing that there are some predictions which are not identical.

How have proponents of those theories/interpretations responded?

 

[vanhees71]

You assume that all explicitly nonlocal/nonrelativistic quantum theories/interpretations are wrong (of which Bohmian Mechanics is just one). I'm sure you are aware of some others (such as GRW). Those generally make identical predictions for Bell tests as does QFT (which has many other advantages),

Of course, within non-relativistic QM there is no such apparent tension between causality and the assumption that there's instantaneous influence between far-distant objects, and thus there's nothing to be debated in this direction within non-relativistic QM and even interpretational extensions of it like Bohmian mechanics. GRW is another case, because that's a new theory beyond QM. There's not the slightest hint for its necessity though.

I don't know, how often I have to repeat it: By construction microcausal relativistic QFT exclude causal connections between space-like separated events. In our context "events" are the registration of two photons at far distant places, and if these registration events are space-like separated, the observed correlations due to entanglement cannot be caused by the measurement on one photon influencing the measurement of the other. The only conclusion is that the correlations are due to the state preparation, and that's indeed the standard interpretation of the entire formalism: The entanglement is due to the preparation of the two photons (in this case by parametric down conversion). It describes long-ranged correlations but not faster-than-light signalling, and I think that's also the interpretation the authors of the said paper have about their results, and indeed, for space-like separated events it doesn't even make sense to talk about some "signal propagation" or "causal order".

This is very similar to the fact that phase velocities as well as group velocities in classical field theory (e.g., electrodynamics) can be and in fact are larger than $c$ without indicating any violation of relativistic causality. This is known since 1907!

That was their point, without them saying so directly. IF there exists a universal preferred reference frame, AND the Earth's motion relative to it is within a threshold, THEN: a Bell test like the one they ran will in fact show differences in some directions (which are not predicted by garden variety QM). It doesn't, so one should rule those out by experiment. Of course, defenders of those alternate candidate theories will attempt to explain away the experiment or further modify the theory...

So, why have you claimed the opposite before?

 

[Morbert]

Obviously: Alice's selection of a measurement basis causes (by assumption only) distant Bob's later measurement to be synchronized with Alice's results. This occurs FTL, possibly instantaneously, and further the causal direction is ambiguous. Put a different way: the relative timing of Alice's and Bob's measurements in a Bell test are irrelevant (this is consistent with any interpretation). Whatever influence occurs to make this happen, is labeled "quantum nonlocality" in the more modern usage of the term, and this is generally accepted as well.

Surely this is interpretation dependent and therefore not obvious. Many interpretations would not frame quantum correlations as evidence of nonlocal influences.
https://www.webofstories.com/play/murray.gell-mann/165

 

[vanhees71]

If by nonlocal influences you mean faster-than-light causal influences then any interpretation which assumes them is in plain contradiction with the mathematical facts of microcausal relativistic QFTs!

 

[Demystifier]

If by nonlocal influences you mean faster-than-light causal influences then any interpretation which assumes them is in plain contradiction with the mathematical facts of microcausal relativistic QFTs!

This statement cannot be found in professional scientific literature, which this thread is supposed to be about.

 

[martinbn]

This statement cannot be found in professional scientific literature, which this thread is supposed to be about.

I cannot cite one now, but I am sure I have seen published papers that state or suggest that the causal structure as described by relativity is incorrect. My impression is that is most Bohmians even if they don't say it out loud.

 

[Demystifier]

I cannot cite one now, but I am sure I have seen published papers that state or suggest that the causal structure as described by relativity is incorrect. My impression is that is most Bohmians even if they don't say it out loud.

Sure, I have written it myself, but such papers emphasize that it is not in contradiction with "the mathematical facts of microcausal relativistic QFTs". The idea of such papers is that relativistic QFT is right but incomplete. Completeness is often an additional tacit assumption in standard QFT literature, but completeness is not a mathematical fact.

 

[vanhees71]

This statement cannot be found in professional scientific literature, which this thread is supposed to be about.

Of course it can. The microcausality condition is precisely imposed to avoid faster-than-light signal propagation.

 

[Demystifier]

The microcausality condition is precisely imposed to avoid faster-than-light signal propagation.

That's true, but signal is the key word. Your original statement was wrong because it didn't contain the word signal. Standard QFT forbids FTL propagation of signals, but it doesn't forbid FTL propagation of influences. I know that you don't distinguish signal from influence, but professional scientific literature does.

 

[vanhees71]

For me signal and influence is a synonym. The point is to distinguish causal signals/influences and long-ranged correlations. A correlation does not necessarily imply a causal connection/signal/influence between two events, and space-like separated events can never be causally connected by construction in microcausal relativistic QFT. This is at least standard in all treatments of relativistic QFT in the HEP community. Obviously it's not so often explicitly stated in the quantum-optics literature although of course both communities use the same QED.

 

[Demystifier]

For me signal and influence is a synonym.

For you, not for the professional scientific literature.

This is at least standard in all treatments of relativistic QFT in the HEP community.

That's a wrong community for such questions. The relevant community is the community professionally working on quantum foundations (Bell theorem etc). I know that the HEP community thinks that they have nothing to learn from quantum foundations community, but they are wrong.

 

[vanhees71]

Then, please, define what you precisely understand under "influence" vs. "signal" or point to the "professional scientific literature". I always thought, mathematical facts do not depend on the community using this math ;-)).

 

[martinbn]

Sure, I have written it myself, but such papers emphasize that it is not in contradiction with "the mathematical facts of microcausal relativistic QFTs".

They can emphasize whatever they want to, but it is inconsistent. You cannot say that two space-like events are in cause effect relation and that your interpretation doesn't contradict relativity.

The idea of such papers is that relativistic QFT is right but incomplete. Completeness is often an additional tacit assumption in standard QFT literature, but completeness is not a mathematical fact.

Incompleteness will not save you, because the inconsistency is with what is already there. Adding more to the theory to complete it will not help.

 

[martinbn]

That's true, but signal is the key word. Your original statement was wrong because it didn't contain the word signal. Standard QFT forbids FTL propagation of signals, but it doesn't forbid FTL propagation of influences. I know that you don't distinguish signal from influence, but professional scientific literature does.

Different terminology makes no difference. If your theory says that A is the cause of B, and A and B are space-like separated, then you are saying that relativity is way off. Which is by itself ok, the problem is that it is just wishful thinking with no shred of evidence to support it.

 

[PeroK]

If by nonlocal influences you mean faster-than-light causal influences then any interpretation which assumes them is in plain contradiction with the mathematical facts of microcausal relativistic QFTs!

I don't see how you can appeal to mathematics as "facts". All mathematics can be is consistent. There should be nothing mathematically inconsistent about any theory, whether it's right or wrong.

That's a wrong community for such questions. The relevant community is the community professionally working on quantum foundations (Bell theorem etc). I know that the HEP community thinks that they have nothing to learn from quantum foundations community, but they are wrong.

They only have something to learn, IMO, if the relativistic QFT championed by @vanhees71 turns out to be incomplete! I don't share his evangelical confidence in QFT, but neither do I share the philosophical doubts that seem to trouble many.

 

[vanhees71]

My argument simply is that microcausal relativistic QFT excludes by construction faster-than-light influences, i.e., within this theory it's a "mathematical fact" that space-like separated events cannot be causally connected. So far relativistic QFT is utmost successful in describing all known matter and its interactions. This is a good reason to "believe" in it as a good physical theory. This doesn't mean that it's complete, which for sure it is not, but indeed not for philosophical but hard reasons, i.e., it's inability to satisfactorily describe also the gravitational interaction.

 

[Vanadium 50]

A correlation does not necessarily imply a causal connection/signal/influence between two events

This is important. (A fun example is below)

Somehow the combination of the ideas that "QM is fundamentally probabilistic" and "you can have a state consisting of multiple particles" confuses and/or bothers people. They true and shoehorn ithis into the idea that each particle is independent, except for a spooky causal connection that causes a measurement of one to influence the other.

This is the same sort of thinking that is confusing about virtual particles: the quantum world is classical except for these little unseen virtual particles bouncing around.

 

[PeterDonis]

By construction microcausal relativistic QFT exclude causal connections between space-like separated events.

This statement is too strong. The condition you call "microcausality" says that spacelike separated measurements must commute. That is not the same as saying they cannot be causally connected. A causal connection that is consistent with them commuting is still possible.

 

[PeterDonis]

You cannot say that two space-like events are in cause effect relation and that your interpretation doesn't contradict relativity.

Yes, you can, as long as whatever cause effect relation you are postulating is consistent with the spacelike separated measurements commuting. See my post #113 in response to @vanhees71 just now.

 

[vanhees71]

This statement is too strong. The condition you call "microcausality" says that spacelike separated measurements must commute. That is not the same as saying they cannot be causally connected. A causal connection that is consistent with them commuting is still possible.

Can you give an example for such a scenario? The microcausality principle says that all local operators that represent observables commute at space like separation of their arguments, particularly the Hamilton density commutes with such an operator at space-like-separated arguments!

 

[martinbn]

Yes, you can, as long as whatever cause effect relation you are postulating is consistent with the spacelike separated measurements commuting. See my post #113 in response to @vanhees71 just now.

And what kind of a cause effect relation is consistent with space-like separated measurements commuting?

 

[PeterDonis]

Can you give an example for such a scenario?

The very scenario we've been discussing: spacelike separated Bell-type measurements on a pair of entangled particles. The measurements commute, as QFT says they must, but the correlations violate the Bell inequalities, so there is still some kind of connection between them that cannot be explained classically.

The microcausality principle says that all local operators that represent observables commute at space like separation of their arguments, particularly the Hamilton density commutes with such an operator at space-like-separated arguments!

Yes. So what?

what kind of a cause effect relation is consistent with space-like separated measurements commuting?

The very one we've been discussing. See above.

 

[vanhees71]

The very scenario we've been discussing: spacelike separated Bell-type measurements on a pair of entangled particles. The measurements commute, as QFT says they must, but the correlations violate the Bell inequalities, so there is still some kind of connection between them that cannot be explained classically.

Of course, they cannot be explained classically. We talk about relativistic QUANTUM field theory. The correlations are due to the preparation of the photon pair as an entangled quantum state, i.e., for the explanation of the correlations you don't need to invoke causality between the measurement on photon at place A and that on the photon at place B but just to the causal connection due to their common creation, but that's my argument all the time!

Yes. So what?

It rules out that there are causal connections between space-like separated observations.

The very one we've been discussing. See above.

 

[martinbn]

The very scenario we've been discussing: spacelike separated Bell-type measurements on a pair of entangled particles. The measurements commute, as QFT says they must, but the correlations violate the Bell inequalities, so there is still some kind of connection between them that cannot be explained classically.

Don't get it! Why does this imply that there is a cause effect relation?

 

[DrChinese]

Surely this is interpretation dependent and therefore not obvious. Many interpretations would not frame quantum correlations as evidence of nonlocal influences.
https://www.webofstories.com/play/murray.gell-mann/165

I watched the clip (4 minutes) and loved hearing Gell-Mann speak on this. Thanks for that.

However, I certainly don't think his attack (in the clip) on the word "nonlocal" represents the majority view of the community given its near constant use (at least 3188 papers have this in their titles in the past 10 years, for example). Apparently many others find the word "nonlocal" meaningful and representative. Gell-Mann places some of the blame for usage of that word at the feet of John Bell, whom did tend to believe something ("influence", "synchronization", whatever, but not a signal) nonlocal was occurring.

Gell-Mann dismisses Bell's essential point by claiming that it is explained by the "decoherent [sometimes consistent] histories" interpretation. So I guess in that respect, your point about my description being "interpretation dependent and therefore not obvious" has some merit. I just don't see how Alice's selection of a measurement basis *here* - which casts distant Bob's outcome into a precisely synchronized result *there* - should not be described as a (quantum) nonlocal influence. It doesn't happen by coincidence, as Bell showed us...

Bell test results ALWAYS appear to be sharply defined by either Alice's choice of measurement basis, [and/]or of Bob's, along with a random element for an outcome consistent with both measurements per the quantum mechanical prediction (either a perfect correlation, or a statistical one). Since Alice and Bob are spacelike and causally separated, the context is nonlocal - and there is no escape for that (even in decoherent histories). I guess we could agree that the context is nonlocal, but argue that there is no "influence" which is FTL. And that description seems OK to me as well.