Can locality be retrocausal?

• B
• entropy1

entropy1

Does the definition of locality in the QM sense include the prohibition of retrocausality?

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?

bhobba
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?
Can you give a yes/no answer?

No I can't because I can't figure out what you actually mean by your question.

Demystifier
I looked up 'quantum mechanics locality', and did a few similar searches, but I can't find a definition of QM locality, so I don't know how to describe or formulate it. I mean the locality as it is ment by the Bell-inequality. If you need an I-thread I will ask to make this thread one.

Does the definition of locality in the QM sense include the prohibition of retrocausality?
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".

entropy1
Ok, suppose an event E lies in the same lightcone as an event C, that occurs earlier in time. Then C can causally affect E, as long as E lies in the same lightcone as C. Suppose that we call this "C locally causes E". Now my question is: if we consider retrocausality to be theoretically possible, then, if E would retrocause an event R that lies in the same lightcone as E, but earlier in time, would this be called "local"?

Sorry if I'm a bit fuzzy in my formulation.

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.

bhobba and entropy1
Sorry if I'm a bit fuzzy in my formulation.

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.

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.
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.

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.
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.

Bell showed that "Local Realism" can not be maintained as part of QM. So what does the term "local" mean in this context?

Have you read Bell's paper? He gives an explicit definition of his concept of "locality".

Ok, I will check that out.

I can only find this definition:
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
Is that what you mean? Or need I do the math?

Demystifier
@PeterDonis I used this text, which is searchable. I think it is the same one. I searched for "local" and got four matches.

@PeterDonis I used this text, which is searchable. I think it is the same one. I searched for "local" and got four matches.
In this paper, the locality assumption is "the vital assumption" after Eq. (1).

entropy1
Ok. I tried to read through this paper at least two times in the past, but I find the math pretty complicated. I will give it another try.

entropy1
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:
1. ##A## influences ##B##
2. ##B## influences ##A##
3. There is some common cause ##C## that influences both.
If you make this assumption (@Demystifier probably knows the philosophical term for it), then it follows that quantum mechanics has FTL influences. Conversely (or contrapositively, maybe), if you assume that there are no FTL influences, then it follows that quantum mechanics violates the above principle/assumption.

So what is the status of that assumption? It doesn't seem logically necessary, but it seems that an assumption along those lines is behind every scientific investigation. The fact that two things are correlated is considered grounds to investigate would could cause the correlation.

An example: If you find two radios, you turn them on, and you find out that they are playing the same sequence of songs and news announcements, then you assume that either they are playing a recording, and the two have copies of the same recording, or else there is a radio station broadcasting to both of them. You assume there is a common cause for the correlation.

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. 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.

Demystifier
If ##A## is correlated with ##B##, then it must be the case that one of the following is true:
1. ##A## influences ##B##
2. ##B## influences ##A##
3. There is some common cause ##C## that influences both.
If you make this assumption (@Demystifier probably knows the philosophical term for it),
It's called the Reichenbach's common cause principle.

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.
But if there was such a law, I don't see how can see such a law be considered a local law.

For that reason, 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.

Last edited:
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.

What is the punishment for treason against the scientific method?

Demystifier
But if there was such a law, I don't see how can see such a law be considered a local law.

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.

martinbn
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.

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.

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).

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).
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.

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.
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.

bhobba
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.
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.

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.
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.

Anyway, it is not essential that everything observed must be real. For instance beauty is not real, it is only in the eyes of the beholder. What is essential for a non-solipsistic view of the world is that something is real, and that this something can explain all the observations.

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:
1. ##A## influences ##B##
2. ##B## influences ##A##
3. There is some common cause ##C## that influences both.
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.

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

Last edited:
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

Thanks for the reference. I'm in the middle of reading it. However, that particular point is not the least bit surprising. I think everybody knew that. The point of considering entangled particles is that in the case of a single particle, there is nothing particularly mysterious about a "collapse" interpretation. You interact with an electron to measure its spin along a certain axis, and afterward, it is in an eigenstate of spin along that axis. That's not too weird from a classical point of view. You can interpret it (which was the original way of interpreting the uncertainty principle) as the measurement disturbing the state of the electron. The significance of an entangled pair is that if measuring one particle forces the other into a eigenstate, then that can't be interpreted as the measurement disturbing the particle, unless the disturbance involves faster than light influences.

I've said this several times before about quantum nonlocality. It seems to me that there is a pretty straightforward notion of "locality" under which quantum mechanics is nonlocal.

You have two experimenters well-separated in space: Alice and Bob. Alice is performing her experiment, confined to some region of space ##V_A##. Her experiment is completed by some time ##T_A##. Bob is performing his experiment, confined to another region of space, ##V_B##. His experiment is completed by some time ##T_B##.

In trying to predict what result Bob will get, you can look at conditions in the region ##V_B##, and all other regions that may have influenced his experiment, which according to the principle of locality is the backwards light-cone of the points in ##V_B## at time ##T_B##. Based on a knowledge of those conditions, you could make some (possibly probabilistic) prediction about Bob's result. Now, if I also told you Alice's result, you would change your prediction about Bob's result. So there is more information about Bob's result than is available locally to Bob. That information is nonlocal. I don't know what else you could call it. It's not local, so it's nonlocal.

Of course, a classical correlation is nonlocal in the same sense. If I have a deck of cards and shuffle it, and give one to Bob and one to Alice, and they depart to a large distance before looking at it, then the probability that you would assign to the outcome that Bob's card is an ace will be different if you know that Alice's card was an ace. So that kind of information is nonlocal, classically, as well. The difference is simply that classically, nonlocal correlations are always "implemented" by local state information. That is not the case quantum-mechanically. So in the classical case, the correlation can be interpreted as subjective---due to a lack of knowledge about the complete situation for Bob. That interpretation is not available quantum-mechanically.

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?

It's actually quite deep and not at the B level - its really part of QFT rather than QM and is associated with the so called cluster decomposition property:

If you read my posts about bell and locality I always bring up defining locality in QM really means defining it in QFT and that is not an easy task. As the link says its best to exclude correlated systems which EPR is. This makes the whole thing even murkier. My solution is its a non-issue. EPR is excluded from its proper definition - but others hold a different view. Under my view all EPR is, is showing that QM has different correlation statistics than classically - it nothing mysterious, defying locality, or anything like that. But oithers, as I said, disagree. For some its an absolutely critical issue.

Thanks
Bill

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

You can turn Boughn's point into an argument that QM correlations for the spin singlet state follow statistically from conservation of angular momentum Europhys.Lett. 69 (2005) 489-495 https://arxiv.org/abs/quant-ph/0407041. Of course, if you're still thinking classically, this won't solve the mystery, i.e., what is the mechanism responsible for conservation of angular momentum? We know for example that instruction sets per Mermin won't work. Boughn doesn't pretend to resolve that mystery.

bhobba