I Entanglement and FTL signaling in professional scientific literature

[Demystifier]

Where do you find this in Ballentine's book,

I quoted it several times before for you, so I'll not do it again.

I don't understand the claim that I propose something non-standard here. It's all consistent with modern QFT textbooks, including particularly Weinberg's!

The standard QFT textbooks, including the Weinberg's, don't talk about the meaning of the Bell theorem in QFT. So whatever you say about their relation, right or wrong, is nonstandard.

 

[vanhees71]

Where do you find this in Ballentine's book, which by the way is about non-relavistic quantum theory, where there is no microcausality principle and also no restriction in the velocity of signal propagation. I don't understand the claim that I propose something non-standard here. It's all consistent with modern QFT textbooks, including particularly Weinberg's!

I looked it up. There's a statement about locality and contextuality on page 606, and as expected this is not nonlocality in the sense of microcausality of relativistic QFT but "nonlocality" in the sense of long-ranged correlations of measurement outcomes due to entanglement. That's of course non contradiction, because as with any quantum theory also in relativistic local (or better microcausal) QFT you can have such "long-range correlations" between entangled parts of a quantum system. These correlations are inherent in the state since it was prepared and is not caused in any way by the local measurements on the far-distant parts.

 

[vanhees71]

I quoted it several times before for you, so I'll not do it again.The standard QFT textbooks, including the Weinberg's, don't talk about the meaning of the Bell theorem in QFT. So whatever you say about their relation, right or wrong, is nonstandard.

Well, I don't think that Weinberg discusses Bell etc. in his QFT textbooks, but what he discusses at length is the microcausality principle and its meaning and its implications, and this implies that there is no faster-than-light influence between local measurements of local observables. I don't understand, why you think that were "nonstandard".

Bell's theorem and all that usually is discussed in Quantum Optics textbooks, which use "standard QED" too. So there's still nothing beyond nonstandard QFT in my arguments either!

 

[PeroK]

There is a flaw in the argument, but on your side. Relativistic local QFT does not admit nonlocal signaling, but it admits nonlocal influences. Even Weinberg, in Lectures on Quantum Mechanics, says: "Copenhagen interpretation relies on something happening during a measurement that is outside the scope of quantum mechanics" (2nd edition, page 97).

If we take entanglement at face value, then the measurement outcomes at A and B are spacelike separated and yet correlated. That only implies "things happening outside QM" and "nonlocal influences" if we do not accept non-locality as a building block of nature. I do not have the confidence to say either "well, of course non-locality (in this sense at least) can be a building block of nature - why not?"; nor to say "this sort of non-locality is sufficiently counterintuitive that there must be something wrong".

Whether threre is a problem with QM in this regard depends on what we are prepared to accept from nature. And I can't find within myself any clear argument that determines whether I can take quantum entanglement at face value without feeling a fool; nor, to reject it without feeling innately prejudiced!

 

[PeterDonis]

For example, Alice can use quantum measurement to create a random number and use entanglement to instantaneously share this random number with Bob.

Not quite. Bob can't know which random number Alice shared until he receives information from Alice by normal signals and combines that information with the information from the measurement he made on a particle entangled with Alice's particle.

 

[vanhees71]

There's nothing necessary "outside QM" or rather microcausal QFT to understand the long-ranged correlations, which are in full accordance with the microcausality principle. Here's a very nice answer about the issue on stackexchange:

https://physics.stackexchange.com/q...ith-or-even-predicted-by-quantum-field-theory

 

[Demystifier]

These correlations are inherent in the state since it was prepared and is not caused in any way by the local measurements on the far-distant parts.

That's your interpretation, not Ballentine's. At pages 609-610 (2nd edition) he says:
"Einstein’s locality postulate, which is the key to Bell’s theorem, is strongly
motivated by special relativity. Thus the conflict between quantum mechanics
and locality suggests a deep incompatibility between quantum mechanics and
relativity. ... It is not valid to object that we have based our analysis on nonrelativis-
tic quantum mechanics. In fact, only the properties of spin and polarization
have been used, and these are essentially identical in both the relativistic and
nonrelativistic theories."

 

[Demystifier]

Bell's theorem and all that usually is discussed in Quantum Optics textbooks, which use "standard QED" too. So there's still nothing beyond nonstandard QFT in my arguments either!

Can you quote some quantum optics book that confirms your interpretation of the Bell theorem?

 

[Demystifier]

Not quite. Bob can't know which random number Alice shared until he receives information from Alice by normal signals and combines that information with the information from the measurement he made on a particle entangled with Alice's particle.

You are simply wrong (in a way that, fortunately, does not depend on interpretation). For example, let the entangled state be $|0\rangle |0\rangle + |1\rangle |1\rangle$. Suppose that Alice performs a measurement at time $t$ and obtains the result $0$. (The chance for this result was 50%, so it's a random number.) Bob can know that this random number is $0$ by performing the measurement at the same time $t$, without any additional classical information.

Note that this is not quantum teleportation (which requires a classical information), because here Bob knows in advance the entangled state.

 

[PeterDonis]

this is not quantum cloning (which requires a classical information), because here Bob knows in advance the entangled state.

Ah, I see; yes, I was confusing this scenario with quantum teleportation (not "cloning" since cloning implies making a copy of a quantum state without changing the original, which is impossible by the no cloning theorem).

 

[PeterDonis]

Bob can know that this random number is $0$ by performing the measurement at the same time $t$, without any additional classical information.

The time when Bob makes his measurement doesn't matter, does it? The key thing is the direction of his spin measurement (it has to be the same as Alice's).

 

[Demystifier]

Ah, I see; yes, I was confusing this scenario with quantum teleportation (not "cloning" since cloning implies making a copy of a quantum state without changing the original, which is impossible by the no cloning theorem).

Yes, I meant teleportation, I've corrected it now.

 

[Demystifier]

The time when Bob makes his measurement doesn't matter, does it? The key thing is the direction of his spin measurement (it has to be the same as Alice's).

Yes.

 

[Demystifier]

There's nothing necessary "outside QM" or rather microcausal QFT to understand the long-ranged correlations, which are in full accordance with the microcausality principle.

Agree on that. But the point is that microcausality prevents FTL signaling, not FTL influences. Standard QFT does not prevent FTL influences simply because it does not say anything about influences. Influence is not a concept that you can define within standard QFT. That's because standard QFT is an instrumental theory, while influence (unlike signal) is not an instrumental concept. An instrumental theory is a theory talking only about how humans can use it, while influence refers to how nature itself behaves, irrespective of humans. You can say that "FTL influence" is philosophy and hence irrelevant, I'm fine with that, but you can't say that FTL influence is disproved by standard QFT. A theory cannot disprove something which is not even a well-defined concept in this theory.

 

[vanhees71]

Can you quote some quantum optics book that confirms your interpretation of the Bell theorem?

Good question. I don't know, whether there's a quantum-optics book which states the microcausality principle explicitly at all, but they use the standard QFT formalism to describe the violation of Bell's inequality etc. and this formalism fulfills the microcausality principle, be it mentioned explicitly or not, and this shows that non-local correlations are indeed compatible with the microcausality principle, which excludes faster-than-light signal propagation, i.e., it's not the measurement at A which causes the outcome of a measurement at B, when the measurement outcomes are registered at spacelike separated events. The observed correlations are due to the preparation of the system in the entangled state, and these correlations can only be observed by comparing the registered measurement results afterwards.

 

[vanhees71]

The time when Bob makes his measurement doesn't matter, does it? The key thing is the direction of his spin measurement (it has to be the same as Alice's).

Indeed, if you want to have 100% correlations A and B must measure the spin component in the same direction, and B knows what A will find as soon as he gets his measurement result given (!) that he knows that the particles are prepared before (!) in an entangled state. It doesn't matter whether A measures her spin before or after Bob or simultaneously. The microcausality principle implies that if the measurement outcomes are registered as space-like separated events, it cannot be B's measurement that causes A's outcome and vice versa. To verify the 100% correlation, of course A and B must share their measurement protocols, which cannot done with any faster-than-light communication either.

 

[vanhees71]

Agree on that. But the point is that microcausality prevents FTL signaling, not FTL influences. Standard QFT does not prevent FTL influences simply because it does not say anything about influences. Influence is not a concept that you can define within standard QFT. That's because standard QFT is an instrumental theory, while influence (unlike signal) is not an instrumental concept. An instrumental theory is a theory talking only about how humans can use it, while influence refers to how nature itself behaves, irrespective of humans. You can say that "FTL influence" is philosophy and hence irrelevant, I'm fine with that, but you can't say that FTL influence is disproved by standard QFT. A theory cannot disprove something which is not even a well-defined concept in this theory.

What then in fact IS "an influence"? For me signal and influence in this context are simply the same thing. We are talking about physics, not philosophy here. We are (still ;-)) in the scientific part of the quantum forum!

 

[PeterDonis]

What then in fact IS "an influence"?

It's whatever it is that allows correlations that violate the Bell inequalties, without FTL signaling taking place.

 

[vanhees71]

But the standard QFT doesn't need an influence, because the correlation is due to the state preparation and not due to some faster-than-light "influence" of one measurement on the other. This is, of course, argued within microcausal QFT. If you admit some "influences" outside the dynamics of this QFT you can claim anything you like, including FTL "influences", but the point is that microcausal QFT is a theory that is compatible with the impossibility of FTL "influences" on the one hand and the observed correlations described by entangled states.

 

[Demystifier]

But the standard QFT doesn't need an influence, because the correlation is due to the state preparation

No, standard QFT does not say that it's due to the state preparation. Standard QFT is agnostic on that, because it's a philosophical question and standard QFT is not philosophy.

 

[PeterDonis]

the standard QFT doesn't need an influence, because the correlation is due to the state preparation

As a plain statement of "fact", this is obviously false, since we can get different correlations by changing what measurements we choose to make. So the measurements also play a role in what we observe.

One could interpret your statement here as saying that standard QFT doesn't include any "influences" in its model--it just has state preparations and measurements, and the quantum fields that let us predict probabilities for measurement results based on state preparations--but that is just agreeing with @Demystifier.

Or one could interpret your statement as a claim along the lines of "no hidden variables"--nothing else is required to explain the correlations besides state preparations, quantum fields, and measurements--in which case we would move any further discussion of this subtopic to the intepretations subforum, since this kind of claim is a claim about what kinds of interpretations of the math are valid or reasonable, and many people do not agree with your position about this, and there is no way of testing it by experiment since all interpretations make the same predictions for all experimental results.

 

[Demystifier]

What then in fact IS "an influence"?

I've explained it in my first post here.

For me signal and influence in this context are simply the same thing.

Maybe for you, but not so in standard QFT, and certainly not in standard texts on quantum foundations.

We are talking about physics, not philosophy here. We are (still ;-)) in the scientific part of the quantum forum!

But you are talking about philosophy a lot. I can't remember that I ever disagreed with you when you were talking about science (maybe once or twice, which is really negligible). When I criticize you, I always criticize your philosophy, not your science. Your science is almost flawless. Your philosophy is very far from that, but I think that you should take it as a compliment; if your philosophy was any good it would mean that you take philosophy seriously, which would offend you.

 

[DrChinese]

Summary: Is it absolutely certain according to our current best understanding, that entanglement doesn't imply faster than light signaling?

According to professional scientific literature and to our best understanding, are there any suggestions that entanglement might imply some sort of faster than light signaling between the entangled particles?

There are substantial experimental suggestions that there exists a type of non-local action at a distance. But there is no absolute proof. Nor any suggestions that signals can propagate FTL.

The latest professional literature all pretty much says the same thing: that quantum mechanical experiments are evidence of nonlocality, which is often referenced as "quantum nonlocality" to distinguish it from classical nonlocality (which would presumably allow signaling).

There has been no meaningful change in this state of affairs (quantum theoretical predictions seemingly defying special relativity) since the advent of QM and the Einstein-Bohr debates in the 1930's. Even the latest theories make the same (as earlier) predictions regarding experimental outcomes for entangled systems of 2 distant particles.

 

[vanhees71]

As a plain statement of "fact", this is obviously false, since we can get different correlations by changing what measurements we choose to make. So the measurements also play a role in what we observe.

Of course, what we get depends on what we measure. Of course we are talking about correlations for outcomes of any possible measurement. These correlations are predicted, within QT, given the prepared state.

One could interpret your statement here as saying that standard QFT doesn't include any "influences" in its model--it just has state preparations and measurements, and the quantum fields that let us predict probabilities for measurement results based on state preparations--but that is just agreeing with @Demystifier.

Indeed, that's all there is in any QT, including standard QFT, and there's nothing else needed to test the validity (local realistic HV theory) or the violation (QT/QFT) Bell inequality.

Or one could interpret your statement as a claim along the lines of "no hidden variables"--nothing else is required to explain the correlations besides state preparations, quantum fields, and measurements--in which case we would move any further discussion of this subtopic to the intepretations subforum, since this kind of claim is a claim about what kinds of interpretations of the math are valid or reasonable, and many people do not agree with your position about this, and there is no way of testing it by experiment since all interpretations make the same predictions for all experimental results.

This has nothing to do with interpretation at all. There are no hidden variables in standard QFT, and still it describes the observed correlations in accordance with the observations (and local realistic HV theories don't).

 

[vanhees71]

There are substantial experimental suggestions that there exists a type of non-local action at a distance. But there is no absolute proof. Nor any suggestions that signals can propagate FTL.

The latest professional literature all pretty much says the same thing: that quantum mechanical experiments are evidence of nonlocality, which is often referenced as "quantum nonlocality" to distinguish it from classical nonlocality (which would presumably allow signaling).

There has been no meaningful change in this state of affairs (quantum theoretical predictions seemingly defying special relativity) since the advent of QM and the Einstein-Bohr debates in the 1930's. Even the latest theories make the same (as earlier) predictions regarding experimental outcomes for entangled systems of 2 distant particles.

I'd say Bell's work was a tremendous progress in comparison to EPR: It made the claims of EPR testable against QT by experiment, and it's now clear that QT describes the state of affairs correctly and local realistic HV theories don't. Of course you can endlessly debate, whether EPR and Bell's local realistic HV theories are the same thing ;-)).

 

[martinbn]

There are substantial experimental suggestions that there exists a type of non-local action at a distance. ...

Are they interpretation free?

 

[BWV]

I thought it was possible for entangled particles to exist on either side of a black hole event horizon, which would imply FTL?

 

[martinbn]

I thought it was possible for entangled particles to exist on either side of a black hole event horizon, which would imply FTL?

Assuming that they can, how does it imply FTL?

 

[BWV]

Assuming that they can, how does it imply FTL?

Could you find out the spin of the entangled partner within the black hole?

 

[martinbn]

Could you find out the spin of the entangled partner within the black hole?

Is that a question or a statement?