Thermal interpretation and Bell's inequality

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  • #26
DarMM
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No, the fact that QM violates the Bell inequalities shows that nature is either nonlocal, or retrocausal, or many-worlds, or whatever the short version of "not all variables come from a common sample space" is.
Complimentarity or failure of unicity.
 
  • #27
DrChinese
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The move that I am enquiring about is at the top of p.198; from the first eqn there to the second eqn there.
Although not so labeled, this is where the Realism assumption is added. The Realism assumption is that there is a counterfactual c, in addition to a and b. So just as a and b are separable: a and c are separable; and b and c are separable as well. Separable meaning: they are independent; essentially saying locality applies.
 
  • #28
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Bell assumes no retrocausality, no nonlocal effects, all variables come from a common sample space and that there is a single world.

QM violates the resulting INequalities, thus one of these is wrong. [EDIT]
I like this because it brings a possible point of difference into clearer focus.

#1. From my POV: Bell does not assume that all relevant correlations arise from test results obtained in the same instance; see #2 next.

#2. He recognises #1 as a physical fact of the experiment that he is studying.

#3. So he rightly makes #2 a condition on the use of his first equation; (1).

#4. However. When he uses (1) -- as he says he does -- to, move from the first equation atop p.198 to the second: he breaches (1).

#5. The breach in #4 arises from the point discussed in Post-15 above.

#6. That is: in breaching* a physical matter-of-fact, his inequality is liable to be violated by QM; and it is.

#7. Now you say: "QM violates the resulting inequalities, thus one of these assumptions [that you list above] is wrong."

#8. Whereas I propose that "QM violates the resulting inequalities because Bell violates a well-recognized physical matter-of-fact of the setting that he analyses."

#9. Is this an agreeable proposal?

#10. The corollary being that, when we avoid this breach, we can derive valid inequalities: doubly valid because they agree with QM, and with locality.

HTH.

* Busting up one instance to create an invalid new one: invalid because we know that it breaches QM.
 
  • #29
PeterDonis
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When he uses (1) -- as he says he does -- to, move from the first equation atop p.198 to the second: he breaches (1).
How?

#5. The breach in #4 arises from the point discussed in Post-15 above.
I don't see how. Post #15 says nothing about the reasoning using (1) to go from the first equation at the top of p. 198 to the second being incorrect.
 
  • #30
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What does "Einstein-local"mean?
I understand "Einstein-local" to mean "No influence propagates superluminally".
 
  • #31
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Although not so labeled, this is where the Realism assumption is added. The Realism assumption is that there is a counterfactual c, in addition to a and b. So just as a and b are separable: a and c are separable; and b and c are separable as well. Separable meaning: they are independent; essentially saying locality applies.
Is "counterfactual" the right word to use re c ? I take c to be another unit-vector in 3-space.
 
  • #32
PeterDonis
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I understand "Einstein-local" to mean "No influence propagates superluminally".
That's still not quite precise, because "influence" is vague.

If we're talking about non-relativistic QM, then there is no restriction on how fast anything can propagate; but non-relativistic QM is only an approximation.

If we're talking about relativistic QM, i.e., QFT, then we have what is called "signal locality", which means that actual information (as opposed to "influences") cannot propagate faster than light; the only way to tell what happened at some other measurement that is spacelike separated from the measurement you just made is to wait for the person who made the other measurement to send you their results by ordinary light-speed or slower means. Even if you and the other person measured entangled particles, you can't tell that from your own measurement alone; only from the correlation between the two, which you can only find out by getting the other person's results by ordinary light-speed or slower means, can you determine that there were correlations that violated the Bell inequalities. Violating the Bell inequalities is what is usually termed "nonlocality", but as you can see, this nonlocality is perfectly consistent with signal locality (and also with microcausality, discussed next).

QFT also has something called "microcausality", which means that spacelike separated measurements must commute, i.e., their results must be independent of the order in which they happen. This is simply because if the measurements are spacelike separated, there is no invariant order in which they happen; the order depends on your choice of reference frame. This is sometimes also seen as a version of "locality", but that might not be the best way to look at it, because, as noted above, it is perfectly consistent with correlations between spacelike separated measurements that violate the Bell inequalities.

I'm not sure which, if any, of the above correspond to what you mean by "Einstein locality".
 
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  • #33
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That's still not quite precise, because "influence" is vague.

If we're talking about non-relativistic QM, then there is no restriction on how fast anything can propagate; but non-relativistic QM is only an approximation.

If we're talking about relativistic QM, i.e., QFT, then we have what is called "signal locality", which means that actual information (as opposed to "influences") cannot propagate faster than light; the only way to tell what happened at some other measurement that is spacelike separated from the measurement you just made is to wait for the person who made the other measurement to send you their results by ordinary light-speed or slower means. Even if you and the other person measured entangled particles, you can't tell that from your own measurement alone; only from the correlation between the two, which you can only find out by getting the other person's results by ordinary light-speed or slower means, can you determine that there were correlations that violated the Bell inequalities. Violating the Bell inequalities is what is usually termed "nonlocality", but as you can see, this nonlocality is perfectly consistent with signal locality (and also with microcausality, discussed next).

QFT also has something called "microcausality", which means that spacelike separated measurements must commute, i.e., their results must be independent of the order in which they happen. This is simply because if the measurements are spacelike separated, there is no invariant order in which they happen; the order depends on your choice of reference frame. This is sometimes also seen as a version of "locality"
, but that might not be the best way to look at it, because, as noted above, it is perfectly consistent with correlations between spacelike separated measurements that violate the Bell inequalities.

I'm not sure which, if any, of the above correspond to what you mean by "Einstein locality". [Emphasis added.]
Many thanks for this helpful detail.

I would like to be consistent with relativistic QM and microcausality. So it seems to me that "signal locality" would be bound by Einstein-locality : more clearly defined as "No beable propagates superluminally".

Then, it seems to me and I would hope: "nonlocality" would have no physical relevance or significance; being relegated to a synonym for Einstein's "spooky action at a distance" and an antonym for his "principle of separability" -- at least in my dictionary.

Would such a position give you any difficulties?
 
  • #34
PeterDonis
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it seems to me that "signal locality" would be bound by Einstein-locality : more clearly defined as "No beable propagates superluminally".
I don't think you can say that because the existence of correlations that violate the Bell inequalities suggests that there are nonlocal "beables" that would violate your statement here. In other words, such correlations suggest that there has to be something connecting spacelike separated measurements, even if that something can't be used to send actual information faster than light (i.e., signal locality holds).

"nonlocality" would have no physical relevance or significance
How can that be since it's directly observable? We have experimentally confirmed that you can get correlations that violate the Bell inequalities, and that's what "nonlocality" means.
 
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  • #35
DrChinese
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Is "counterfactual" the right word to use re c ? I take c to be another unit-vector in 3-space.
Does it exist alongside a and b, which are what we can actually measure on an entangled pair? If you say that we can measure 2, but other settings also exist (such as c) which could have been measured: that is the definition of a counterfactual. Certainly in classical terms, a very reasonable assumption. But QM does not make predictions about measurements that are counterfactual. So yes, c is a counterfactual (unit vector).
 
  • #36
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#10. The corollary being that, when we avoid this breach, we can derive valid inequalities: doubly valid because they agree with QM, and with locality.
That is not correct. There are 2 assumptions in Bell, locality/separability being one of them. Bell inequality violation indicating that at least one is incorrect. It does not say which one(s).
 
  • #37
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I don't think you can say that because the existence of correlations that violate the Bell inequalities suggests that there are nonlocal "beables" that would violate your statement here. In other words, such correlations suggest that there has to be something connecting spacelike separated measurements, even if that something can't be used to send actual information faster than light (i.e., signal locality holds).



How can that be since it's directly observable? We have experimentally confirmed that you can get correlations that violate the Bell inequalities, and that's what "nonlocality" means.
But aren't the Bell inequalities based on naive-realism?

By which I mean views like this:
Realism is the opinion that objects have values for observable properties: and that these values exist prior to measurement and independent of the choice of measurement (ie, they are noncontextual).
I ask because [if true] such would appear (to me) to be so naive as to be dismissed out of hand? And to thus have no relevance to locality?

PS: What is your definition, please, of the realism that underpins Bell's analysis?

Thanks
 
  • #38
PeterDonis
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aren't the Bell inequalities based on naive-realism?
They're based on the assumptions Bell specifically gave in his paper. Those assumptions are stated in math, not ordinary language. What ordinary language people choose to describe those assumptions is a matter of words, not math or physics.

What is your definition, please, of the realism that underpins Bell's analysis?
I don't have one. I haven't used the word "realism", and I am not going to try to give a definition of a vague ordinary language term that has nothing to do with the actual math or physics.
 
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  • #39
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That is not correct. There are 2 assumptions in Bell, locality/separability being one of them. Bell inequality violation indicating that at least one is incorrect. It does not say which one(s).
Please see my last response, Post #37: Isn't a naivish realism also an assumption in Bell's work?

How do you define Bell's "realism" please?
 
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  • #40
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They're based on the assumptions Bell specifically gave in his paper. Those assumptions are stated in math, not ordinary language. What ordinary language people choose to describe those assumptions is a matter of words, not math or physics.



I don't have one. I haven't used the word "realism", and I am not going to try to give a definition of a vague ordinary language term that has nothing to do with the actual math or physics.
Thank you. I like this approach, and appreciate you taking such a clear position. Especially as it seems to me that realism [a term to me so confusedly used] "of some sort" gives rise to this question:

Could you explain please (in your terms) "the physics" (shall we call it) behind Bell's move from the first equation on p.198 [of his famous essay] to the second equation?

PS: I see that he uses his eqn (1), and thus (I presume) its conditions about "instances". But his "physics" must allow him to somehow merge the 2 instances in p.198's first eqn to deliver different instances in p.198's second eqn. Thanks.
 
  • #41
DarMM
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Could you explain please (in your terms) "the physics" (shall we call it) behind Bell's move from the first equation on p.198 [of his famous essay] to the second equation?
Do you mean the two different forms of the integral prior to the inequality?

It's just a simple reordering of the terms given they only have values ##\pm 1##
 
  • #42
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(X). Do you mean the two different forms of the integral prior to the inequality?

(Y). It's just a simple reordering of the terms given they only have values ##\pm 1##
Yes to (X) and the difference between the first and second integral on p.198. But I think (Y) is invalid.

To be clearer: I am seeking to understand the "physics" that Bell has in mind when using his eqn (1) to move from the 2 instances in p.198's first eqn to the 2 instances in p.198's second eqn. It seems to me that Bell's key "physics" assumption -- and where he departs from the related physical-reality that he is studying; since his departure is violated by QM -- can only be introduced in this move: everything else is plain mathematics.

You are correct that these instances can only deliver ##\pm 1##. But please note that his eqn (1) from p.196 is qualified by the sentence that introduces it. So the EPRB correlations -- that Bell is working with when he uses his eqn (13) in the derivation of his inequality -- would not hold if A and B are put together from different instances simply by a reordering.
 
  • #43
DarMM
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I don't get what you mean, he's just using the fact that they equal ##\pm 1## and a bit of simple algebra.
 
  • #44
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I don't get what you mean, he's just using the fact that they equal ##\pm 1## and a bit of simple algebra.
OK, thanks, let me try again. Let's call the 2 top equations on p.198: (14a) and (14b).

So, to me, (14a) is true; it is simply a definition of Bell's terms.

But (14b) looks false to me; and this view is backed by the fact that it (by plain mathematics) leads to Bell's famous eqn (15): which IS false under QM.

So I'm trying to understand: what is Bell up to -- mentally, physically, mathematically, if it helps --- when he moves from (14a) to (14b)?

For to me, whatever he's done in that single step, it is not correct under QM or EPRB nor mathematically: AND I still cannot see any reasonable way to make it work (though several advisors here have surely tried).
 
  • #45
PeterDonis
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Could you explain please (in your terms) "the physics" (shall we call it) behind Bell's move from the first equation on p.198 [of his famous essay] to the second equation?
It's not physics, it's math. Bell does not give any specific physical interpretation of his math. He just does the math.

whatever he's done in that single step, it is not correct under QM or EPRB
Of course not; the whole point of the paper is to show that the mathematical model he is using cannot match the predictions of QM. As I've already pointed out multiple times. I cannot understand why you keep saying his model doesn't match QM as if it were a problem, when it's precisely the point of his paper.

nor mathematically
It's perfectly correct mathematically; I've already demonstrated that in post #14.

this view is backed by the fact that it (by plain mathematics) leads to Bell's famous eqn (15): which IS false under QM
This argument is wrong, and I don't understand why you would even make it. Once more: the fact that Bell's mathematical model makes predictions which do not match QM is the entire point of the paper. Bell is not trying to construct a mathematical model that will match the predictions of QM. He is trying to show that a certain class of mathematical models, the ones that satisfy his assumptions, cannot match the predictions of QM. That is the entire point of the paper.

Please read the above again and again until it sinks in. This whole thread is basically going in circles because you have not grasped this basic point.
 
  • #46
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It's not physics, it's math. Bell does not give any specific physical interpretation of his math. He just does the math.



Of course not; the whole point of the paper is to show that the mathematical model he is using cannot match the predictions of QM. As I've already pointed out multiple times. I cannot understand why you keep saying his model doesn't match QM as if it were a problem, when it's precisely the point of his paper.



It's perfectly correct mathematically; I've already demonstrated that in post #14.



This argument is wrong, and I don't understand why you would even make it. Once more: the fact that Bell's mathematical model makes predictions which do not match QM is the entire point of the paper. Bell is not trying to construct a mathematical model that will match the predictions of QM. He is trying to show that a certain class of mathematical models, the ones that satisfy his assumptions, cannot match the predictions of QM. That is the entire point of the paper.

Please read the above again and again until it sinks in. This whole thread is basically going in circles because you have not grasped this basic point.
Sorry for my slowness; I share your frustration. Did you reply to post #15. That will help me to see how Bell's mathematical model departs from QM and nature. Thanks.
 
  • #47
PeterDonis
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Did you reply to post #15.
Yes, in post #21. To repeat my reply there: QM (and any interpretation of QM, including TI) has absolutely nothing to say about Bell's math--the whole derivation in the paper, not just the equations at the top of p. 198, the whole thing. Bell's entire derivation has nothing whatever to do with QM. He is just making some mathematical assumptions and deriving their consequences. That derivation has nothing to do with QM and QM says nothing about it. The only role QM plays in any of this is that its predictions are different from the ones Bell gets from the mathematical model he constructs. That's it.
 
  • #48
PeterDonis
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QM (and any interpretation of QM, including TI) has absolutely nothing to say about Bell's math
Perhaps it might help if I try to pose a different question, one that I think you should be asking instead of the ones you are asking:

Why do people think Bell's derivation is such a big deal?

I think the reason is that, before Bell wrote his paper, practically all physicists had implicitly assumed, without really thinking about it, that any physical theory would be capable of being written mathematically in a way that would satisfy Bell's assumptions. In particular, the two that have been pointed out in this thread: that probabilities of correlations would always factorize, as in Bell's equation (2) (note that the A probability in the integral there does not depend on the ##\vec{b}## setting, and vice versa), and that it would always be possible to introduce "counterfactual" items into the equations, such as the vector ##\vec{c}## that is introduced in Bell's equation at the top of p. 198 (I call this vector "counterfactual" because it is different from either of the measurement settings that are actually measured), and still derive valid predictions from them.

But Bell showed that any mathematical model that satisfies those assumptions cannot match the predictions of QM, and since the predictions of QM are correct for these experiments (as has now been verified many times), physicists now are forced to grapple with the fact that the implicit assumptions they had been making about how valid physical theories could be expressed mathematically are wrong. And physicists find it very difficult to imagine how they could be wrong--how Nature could work in such a way that a model that satisfies such seemingly simple and innocuous assumptions cannot match actual experimental results.

All this is true, but it does not mean that QM itself has anthing to say about Bell's mathematical model and its consequences, other than the obvious fact that QM's predictions are different. Bell's model is simply a different theory from QM (or rather a general class of theories all of which are different from QM), which we now know is falsified by Nature. That's all there is to it.
 
  • #49
DrChinese
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Please see my last response, Post #37: Isn't a naivish realism also an assumption in Bell's work?

How do you define Bell's "realism" please?
EPR defined "elements of reality". Then they asked whether those elements were simultaneously real if they could not be simultaneously measured. They felt it unreasonable to require them to simultaneously measured to be real. In other words, they assumed counterfactual definiteness. I would call that realism. I don't call this "naive" as I don't know what useful definition of realism exists which is "less naive".
 
  • #50
PeterDonis
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they assumed counterfactual definiteness
Which, just to relate this back to the math in Bell's paper, is the assumption that introducing the extra vector ##\vec{c}## at the top of p. 198 is valid, even though that vector does not describe either of the measurements that are actually made.

The other assumption, that the joint probability factorizes as in Bell's equation (2), is the one that is usually called "locality".
 

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