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Does the definition of locality in the QM sense include the prohibition of retrocausality?
Can you give a yes/no answer?Given that this is B-level, I have to ask if you know what those words mean. For example, how do you express locality mathematically? And if you don't, how can we provide a B-level answer?
In any proper journal article the authors will define what they mean by words of this sort, precisely because there is no single universally understood definition. Unless and until you do that, the answer to this question is going to be some variant of "It depends".Does the definition of locality in the QM sense include the prohibition of retrocausality?
The proof of the Bell theorem contains an assumption that there is no retrocausality. The transactional interpretation of QM assumes that it is precisely retrocausality that resolves the associated quantum puzzles.Does the definition of locality in the QM sense include the prohibition of retrocausality?
Sorry if I'm a bit fuzzy in my formulation.
Well, maybe if I put it this way:You apparently haven't grasped the fact that it is the fuzziness that makes it impossible to give a definite answer to your question. That's not going to change no matter how many times you rephrase it.
For that purpose you need to read an actual proof of the Bell theorem, because such a proof contains a precise formal definition of locality.Well, maybe if I put it this way:
Bell showed that "Local Realism" can not be maintained as part of QM. So what does the term "local" mean in this context? It is often explained as: "Spooky action at a distance", but I don't know what is ment by that.
Bell showed that "Local Realism" can not be maintained as part of QM. So what does the term "local" mean in this context?
Is that what you mean? Or need I do the math?that the result of a measurement on one system be unaffected by operations on a distant system with which it has interacted in the past
I can only find this definition
In this paper, the locality assumption is "the vital assumption" after Eq. (1).@PeterDonis I used this text, which is searchable. I think it is the same one. I searched for "local" and got four matches.
It's called the Reichenbach's common cause principle.If ##A## is correlated with ##B##, then it must be the case that one of the following is true:
If you make this assumption (@Demystifier probably knows the philosophical term for it),
- ##A## influences ##B##
- ##B## influences ##A##
- There is some common cause ##C## that influences both.
But if there was such a law, I don't see how can see such a law be considered a local law.But there is nothing logically inconsistent about assuming that the radios are just emitting random noise, and one of the laws of the universe is that the random noise produced on one radio is always the same as that produced on the other.
For that treason, some defenders of local interpretation of QM go a step further, by denying the existence of correlation before the observation of correlation. For instance, if Alice measures spin of one particle in New york and, at the same time, Bob measures spin of the other particle in London, there is no correlation until someone (say Charlie) looks at both measurement results.
But if there was such a law, I don't see how can see such a law be considered a local law.
To add to this, I think that if you propose a FTL mechanism, you need to show, at least in principle, a scenario where there can be FTL transmission.Yes, I agree. But it's not logically necessary to posit a FTL mechanism for the correlations. You could just say that that's the way the universe works.
To add to this, I think that if you propose a FTL mechanism, you need to show, at least in principle, a scenario where there can be FTL transmission.
It is not logically necessary, I agree. But if you propose FTL you are proposing a lot, which goes against what is known so far, and I think that it is scientifically necessary to set up an experiment, even if it is a thought experiment.I don't see why that's necessary, either. If you have some variables which have FTL influences, I don't see that it's necessary that those variables be settable (which is what it would take to use them for transmission of signals).
Fine but then, by the same token, one can say that Bohmian particle trajectories are what they are, without a FTL mechanism. My point is that Bohmian mechanics is not more non-local than such a more conventional interpretation.Yes, I agree. But it's not logically necessary to posit a FTL mechanism for the correlations. You could just say that that's the way the universe works.
True, but is it more realistic? The particles have trajectories, but what about spin? My understanding is that in BM spin is not real i.e. no definite values at all times.Fine but then, by the same token, one can say that Bohmian particle trajectories are what they are, without a FTL mechanism. My point is that Bohmian mechanics is not more non-local than such a more conventional interpretation.
Yes, but there is a good physical reason to think of positions as more real than spins. Spin is measured by the Stern-Gerlach apparatus, and if you think a bit how it works, you will see that the only thing that is directly observed by this apparatus is a position of something.True, but is it more realistic? The particles have trajectories, but what about spin? My understanding is that in BM spin is not real i.e. no definite values at all times.
It seems that Bell's separability condition is logically something beyond locality. Without even mentioning locality, there is an assumption along the lines of:
If ##A## is correlated with ##B##, then it must be the case that one of the following is true:
Entanglement in quantum mechanics without FTL influences seems a lot like this possibility. You have distant particles, and certain measurements on them are correlated, but there is no common cause to the measurement outcomes.
- ##A## influences ##B##
- ##B## influences ##A##
- There is some common cause ##C## that influences both.
And the above would be option # 4 that is related to the viewpoint by experimental physicist Stephen Boughn :
He states that QM predicted spin correlations P(z.n) = - cos θ arises from a single particle wave function .https://arxiv.org/pdf/1703.11003.pdf
Given that this is B-level, I have to ask if you know what those words mean. For example, how do you express locality mathematically? And if you don't, how can we provide a B-level answer?
And the above would be option # 4 that is related to the viewpoint by experimental physicist Stephen Boughn :
He states that QM predicted spin correlations P(z.n) = - cos θ arises from a single particle wave function .With one electron measured with a double Stern - Gerlach apparatus with first one aligned in z direction and the second one aligned in the n direction positioned in upper arm of first detector. He shows that the correlation for +1 from first detector with +1 in second detector is again P(z,n) = cos θ. The sequences of indeterminate events in a single particle show an overall pattern. And in a spacelike separated experiment in a common rest frame the two patterns of both entangled particles taken together show a pattern in accord with predicted QM correlations
https://arxiv.org/pdf/1703.11003.pdf