High School Does Locality in Quantum Mechanics Exclude Retrocausality?

[morrobay]

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

 

[stevendaryl]

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.

 

[stevendaryl]

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.

 

[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?

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:
https://www.physicsforums.com/threads/cluster-decomposition-in-qft.547574/

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

 

[RUTA]

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.

 

[ObjectivelyRational]

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

Others had asked for more specificity of what you mean by "locality" I wonder what you mean by "retrocausal"?

The concept, from its word forms, depends on a concept of "causal".

What do you mean when you refer to causal or causality? Take state A transforming into state B over time. Usually "A causes B" means something like "IF A had not occurred B would not have been the result" or perhaps "the outcome B necessarily depended upon A as one its causes" (which is another instance of "but for" A B would not necessarily have been the result. Observe, one never claims B was caused by A, if B was not necessarily the result of A, but could have been the result of C, or D.

At first blush "retrocausal" (here take A in the future as "causing" B in the present) seems to point at a meaning something like "If A will not occur in the future, B cannot be the result in the present" or "but for A occurring in the future, B would not occur in the present", or "the effect B of the present, would not be, but for the cause A in the future", or "B in the present is a necessary effect of A, the cause, in the future".

For A at a future time to cause B at an earlier time there is an element of necessary connection or a "but for" requirement, i.e. but for A (future) B (now) would not be the result, A caused (er will cause... and is causing) B.

The question that arises is, if A of the future causes B of the present, does that require that A must occur in the future (determinism) since we see B now, and there is a necessary connection. If there is a necessary connection how does this differ from identifying that B (now) causes A (later)? Would B's existence now stand as a barrier to any action or actor in preventing A happening later? i.e. Would your attempt to thwart A simply "somehow" result in your creating A? i.e. is A preordained or fated by the existence of B WHEN A and B are retrocausally related?

What would happen if one set up an apparatus to measure B when it was caused by a future A, and use that measurement to cause non-A before A comes into existence?Just wondering what you mean by "retrocausal" and its relation to "causality".

 

[DrChinese]

Just wondering what you mean by "retrocausal" and its relation to "causality".

I can't provide any kind of reasonable definition of retrocausal beyond this simple one, which is obviously flawed but at least helps to put the temporal element into the equation:

If the future is a factor affecting correlations now, it's retrocausal. I simply ignore the "causal" part or the word to make sense of it, and therefore there is no relationship to "causality". Some might prefer the label "time symmetric". In such theories, locality - the causal cone - extends both forward and backward in time. This precisely matches the limits of what occurs in nature. I.e. entanglement does not occur further than what can be cobbled together by such "extended" local connections. So the pure unlimited non-locality implied by Bohmian theory does not exist in retrocausal theories. Yet there are some "non-local" effects to be studied.

A major problem is that even "retrocausal" theories do not provide an explanation that is "more complete" than orthodox theory. RUTA's Relational BlockWorld does not purport to be retrocausal (even though I often refer to it as such, much to his chagrin). He calls it "acausal" because elements of both the past and the future are factors in quantum outcomes. Regardless of the term you use to label such theories, there is still no clear cut delineation of what is the cause vs. what is the effect. So I have no disagreement with that label either.

 

[bhobba]

What would happen if one set up an apparatus to measure B when it was caused by a future A, and use that measurement to cause non-A before A comes into existence?

There are a number of outs. But the usual one is whatever you are thinking about being retro-casual can't send information so you can do what you suggest.

Thanks
Bill

 

[entropy1]

What would happen if one set up an apparatus to measure B when it was caused by a future A, and use that measurement to cause non-A before A comes into existence?

It is because of that issue that I can't shake the feeling that retrocausality and acausality imply superdeterminism.

The question that arises is, if A of the future causes B of the present, does that require that A must occur in the future (determinism) since we see B now, and there is a necessary connection. If there is a necessary connection how does this differ from identifying that B (now) causes A (later)?

You could say that if A (future) causes B (now), then, if B wouldn't occur now, that A couldn't occur in the future. It seems to me a kind of handshake between A and B, in which A an B require each other to determine which happens and which not, that is, if you could manipulate the outcomes of A and B a little. This is: A → B, not(B) → not(A). Once it becomes known that B does not occur, the future is fixed, if A implies B.

 

[ObjectivelyRational]

It is because of that issue that I can't shake the feeling that retrocausality and acausality imply superdeterminism.

You could say that if A (future) causes B (now), then, if B wouldn't occur now, that A couldn't occur in the future. It seems to me a kind of handshake between A and B, in which A an B require each other to determine which happens and which not, that is, if you could manipulate the outcomes of A and B a little. This is: A → B, not(B) → not(A). Once it becomes known that B does not occur, the future is fixed, if A implies B.

I didn't mean to confuse the issue by proposing possible meaning for a term you used. I, not being you, was merely guessing.

Is this last thing what you had in mind when you used the concept "retrocausal"?

One thing I notice about your definition is that unlike the normal concept of "causal", interacting with a result B (after A caused B) has no effect on A. That is the causal chain is unidirectional. You seem to have a bi-directionality built into your "retrocausal"... is this true for your concept?

 

[entropy1]

I didn't mean to confuse the issue by proposing possible meaning for a term you used. I, not being you, was merely guessing.

Ok.

Is this last thing what you had in mind when you used the concept "retrocausal"?

One thing I notice about your definition is that unlike the normal concept of "causal", interacting with a result B (after A caused B) has no effect on A. That is the causal chain is unidirectional. You seem to have a bi-directionality built into your "retrocausal"... is this true for your concept?

I think that if the outcomes of A and B can be manipulated (a bit), that given a causal relationship, bidirectionality is or can be a result, yes.

 

[ObjectivelyRational]

How do you define and distinguish the difference between, the aspect of reality which is:

A causing B (one direction)
and

the aspect of reality which is

B causing A (the other direction)

such that there is bidirectional causality.

 

[entropy1]

I haven't scrutinized that, but I suppose that I would anwer something like that two observers become 'coupled' in the sense that the cause is distributed between them.

 

[bhobba]

How do you define and distinguish the difference between, the aspect of reality which is:

Leaving aside the issue of what reality is - the answer is simply common-sense convention. Feynman's retro-casual view of EM is valid - the usual treatment found in textbooks is just simpler. I formally studied math - not physics and we went into mathematical modelling and models a lot. Its the usual non stated assumption made. We might challenge it if we had to - but so far no need has been found.

Thanks
Bill

 

[RUTA]

It is because of that issue that I can't shake the feeling that retrocausality and acausality imply superdeterminism.

Superdeterminism is causal, there’s some “super force” that guides particles where they “need to go.” Acausal, as the name implies, means causality is not an issue. Acausal explanation is not about forces acting on objects causing them to move here or there, acausal explanation could be a constraint on a 4D (spacetime) pattern, e.g., stationary action principle. Einstein’s equations of general relativity constitute a 4D constraint as well. You can’t solve for the spacetime metric on the LHS without the stress-energy tensor (SET) on the RHS. But, you can’t supply force, momentum, and energy for the SET without knowing how to make spatiotemporal measurements, i.e., you need the metric. So, Einstein’s equations are a 4D self-consistency constraint between what you mean by spatiotemporal measurements and what you mean by force, momentum, and energy.