Does realism imply locality or vice versa?

[Simon Phoenix]

Here is Bell's definition of locality in his original paper: "result of measurement on one system be unaffected by operations on a distant system with which it has interacted in the past".
I do not see how you arrived at your conclusion unless you take "measurable" as synonym for "affected".

Erm, let's see: "result of measurement on one system be unaffected by operations on a distant system"

Which says that the results of measurement are unaffected - which means that there are no measureable consequences, doesn't it?

Yes, I think so based on Bell inequality violations.

Then I'm afraid you think wrong

 

[RUTA]

Here is Maudlin's argument as it pertains to the EPR claim.

If I disagree with EPR’s conclusion, i.e., that QM is incomplete, I have only two ways to deny its validity. One, claim that QM formalism is wrong (in its prediction about the S2 outcome given the S1 outcome). Two, claim there is a FTL influence on S2 given the outcome S1. The option of rejecting “realism” is not on the table. Here is why.

He’s looking at this in time-evolved fashion. After S1, we know S2 before the measurement is made (assuming same measurement on both). Realism in this case is simply to believe that claim (as he argues in his article). Since that claim comes from QM, if you believe the QM formalism is correct, you cannot reject that claim (of realism).

Now to finish the argument that the EPR claim implies reality is nonlocal, we simply accept that the QM formalism is correct and complete. That comes from the fact that we didn’t know S2 before making measurement S1. So, if there is a fact of the matter for S2 before S1, then QM is incomplete because it says we don’t know S2. If not, reality is nonlocal (in SR sense) because S1 is spacelike related to S2 and must be responsible for S2 becoming definite. You can see that “realism” just isn’t an issue in this view.

Here Maudlin is using the term “nonlocal” to mean spacelike separated, causally related events. Price would argue that the SR nonlocality can be avoided by making timelike connections and allowing causes to propagate backwards in time. I argue that Price’s retrocausality does not avoid the PROBLEM of spacelike causal relationships in the least, which is that effects will precede causes in some frames of reference if the events are spacelike separated. The complaint about that is precisely what retrocausality posits, i.e., information comes from the future to inform the present. So, Maudlin is correct.

Maudlin's argument tacitly assumes there really are two independent systems as evidenced by the spatial separation of the measurements S1 and S2. That means S1 must somehow be communicated to S2. But this is where you may disagree, S1 and S2 are just two parts of a single "entity." Thus, there is no causal relationship needed between S1 and S2, so no FTL nonlocality, and QM formalism is correct and complete. It comes down to his tacit dynamical view which, in leading him to reject realism (per his dynamical interpretation thereof), doesn’t even permit consideration of independence.

 

[zonde]

Erm, let's see: "result of measurement on one system be unaffected by operations on a distant system"

Which says that the results of measurement are unaffected - which means that there are no measureable consequences, doesn't it?

Yes. But your claim was sort of the other way around: as there are no measurable consequences 'here' of operations done 'there' it means that the results of measurement 'here' are unaffected by operations done 'there'.

 

[fanieh]

Realism is missing when there are no predetermined outcomes of measurements. Keep in mind that EPR's "perfect correlations" imply predetermination, but Bell implies the opposite.

If the outcomes are NOT predetermined, there are several other possibilities: a) the outcomes are determined at time of measurement, which implies non-local (instantaneous) interactions of some kind; or b) the future affects the past. Please note that for b) you can preserve locality (there would need to be some form of time symmetry). This is done by having the observers become part of the context of the measurement process. There still is no instantaneous effect, although it might look otherwise superficially.

Is there no standard usage of the word "Non-locality"? Demystifier just commented that "If realism is missing, then wave function is not real. If so, then collapse is also not real, so non-locality is also not real." So if outcomes are NOT predetermined, realism is missing, so non-locality is also not real.. but why did you use the word "non-local" in your paragraph above?

Should we follow the convention of DrChinese or Demystifier.. and which general physicists follow?

Although this may seem crazy at first: all entangled systems do trace out locally if you allow backward-in-time connections. This is especially notable when you attempt to create a spacetime diagram of entanglement swapping. +/- c is a limiting factor for these setups, and direct FTL action (between Alice and Bob) does not *appear* to describe anything on these.

Of course, whenever you talk about dropping realism vs. dropping locality, you are wading into the interpretations of QM. And there is no experimental evidence to select one of those over another at this time.

 

[Denis]

Realism is missing when there are no predetermined outcomes of measurements. Keep in mind that EPR's "perfect correlations" imply predetermination, but Bell implies the opposite.

No. EPR does not imply predetermination without strong Einstein causality (Einstein locality not only for signals, but also for hidden but real causal influences). This is the "without in any way disturbing" part of the EPR criterion.

Of course, whenever you talk about dropping realism vs. dropping locality, you are wading into the interpretations of QM. And there is no experimental evidence to select one of those over another at this time.

No. All this can be discussed without even mentioning QT at all. We have the experimental facts - BI are violated for space-like separated measurements - and the theorems which allow to derive the BI.

It is important to discuss the various definitions of realism and causality which would allow to derive the BI, and those which are insufficient. But above notions are important from the start, they are important already for classical mechanics, even to reject astrology, given that the decisive argument against astrology is not that the observed correlations are weak (at least the readers of astrological predictions think otherwise), but that it refuses to give any realistic and causal explanations for the hypothetical influences.

QT increases the probability that the BI are indeed violated, because it predicts a violation itself. And the only point where QT interpretation becomes relevant is the point that there exists at least one real and causal interpretation of it - which is relevant only as a rejection of the argument that QT somehow shows that realism or have to be given up anyway.

Is there no standard usage of the word "Non-locality"?

What is standard is to distinguish a weak notion of Einstein locality, which is only about the impossibility to send FTL signals, and the strong notion of Einstein locality, which forbids even hidden causal influences FTL.

What is also unfortunate standard is the "simplification" of naming "Einstein locality" simply "locality", which forces us to name completely local theories, which simply have higher limits for the speed of information transfer, "non-local" because they violate Einstein locality.

So, it would be meaningful to distinguish at least three notions: weak (signal) Einstein locality, strong (realistic) Einstein locality, and simply locality.

 

[Zafa Pi]

Maybe I was not sufficiently clear. I think that actual physics is most likely non-local. There is a possibility that it is local if some reasonable assumptions are not true, but I think that it is not very likely that those reasonable assumptions are not true.

Bell proof is a mathematical theorem which, like any other mathematical theorem, rests on certain unproved assumptions. It is always legitimate to question the unproved assumptions, even if the assumptions seem very reasonable. For various ways to save locality by rejecting certain reasonable but unproved assumptions see
https://arxiv.org/abs/1703.08341 Sec. 5.3.

Maudlin was quite definite, physics and the world are non-local.
You, however, seem a little wishy washy, "likely non-local"?? The odds are two to one the world is non-local?? I don't think your avatar would approve.
I wonder if Maudlin was being sentimental?

 

[Simon Phoenix]

Is there no standard usage of the word "Non-locality"?

It's probably a good idea to see where the various concepts get inserted into the actual derivation - it's much clearer. I love words and have a regrettable tendency to overuse them, but they can lead one astray

So imagine the set up where we have Alice, Bob and some source as follows

Alice <---------- source -----------> Bob

Alice and Bob each have a measurement device to measure properties of whatever it is the source produces. The outcomes are just 0 and 1 (or yes/no, or +/-, whichever convention you prefer) - it's just a binary outcome. If we let $A$ stand for the results of Alice and $B$ stand for the results of Bob we have that $A \in \left\{ 0,1 \right\}$ and $B \in \left\{ 0,1 \right\}$

Now Alice and Bob also have dial by which they can adjust the setting on their respective measurement devices. We'll suppose each of them have only 2 settings so that Alice can pick $a$ or $a'$ and Bob can pick $b$ or $b'$

So we could do experiments and collect enough data to construct a joint probability distribution $P(A,B)$. But that's not really getting what we want - we have to remember that we have different settings and so what we're really interested in are the distributions for different settings - hence we're really interested in the conditional distributions $P(A,B|a,b)$, $P(A,B|a',b)$, $P(A,B|a,b')$, and $P(A,B|a',b')$. These are all measureable quantities - there's no 'magic' here, we're just recording device settings and seeing whether our measuring machine goes 'ping' or 'pong'.

Analysing the results we see that there is some correlation and so we'd like to construct some kind of model. A correlation cries out for explanation - so we assume there are some extra variables that explain the correlation - these are the so-called 'hidden' variables (which is a bit of an unfortunate nomenclature, but it's the one that's stuck). These are given the symbol $\lambda$ and this single symbol is a shorthand for what could be a set of variables which might be discrete continuous, or functions, or even wavefunctions, it really doesn't matter in the slightest - they're just the things that explain why the results are correlated.

So if there are some variables we've not accounted for so far, we'd best put them in our model and write $P(A,B | a,b,\lambda)$ so our measured distribution is going to depend on the experimental settings and these extra variables. All is sweet smelling in the state of Denmark at the moment.

We then make the critical observation that if our variables $\lambda$ account for all of the correlations then any residual fluctuation in our results must be statistically independent so that : $$P(A,B | a,b,\lambda) = P(A | a,b,\lambda)P(B | a,b,\lambda)$$All very reasonable so far. Now we make the assumption of locality in the following sense. We assume the distribution of the results $A$ is not conditioned upon the settings at Bob, and vice versa, so that we can now write $$P(A,B | a,b,\lambda) = P(A | a,\lambda)P(B | b,\lambda)$$It's this step where the 'locality' assumption gets inserted and we can see it's quite specific.

I often make the mistake of saying 'non-local hidden variables' - and I've done so in this thread, but as Bell points out the hidden variables can be 'non-local' - which all gets very confusing. What has to remain local though is the effect of switching device settings - the hidden variables cannot be such that information about device settings is transferred from place to place in a non-local (FTL) fashion. Loosely then, there can be no mechanism by which the system (device plus measured thingy) at Alice 'knows' about the setting of Bob, and vice versa.

It's very important to note that no assumptions have been made about the nature of the source or what it produces - it's just some unspecified thing that produces some hoojamaflips which might be fields or particles, or something else - and, critically, no quantum mechanics at all has intruded here. We just have some source event that leads to measuring devices going 'ping' or 'pong'.

Where does the notion of 'realism' come in then? Well that's a bit more subtle. Essentially in the derivation it is tacitly assumed that different measurement settings can be meaningfully included in the same expression so that something like $P(A | a,\lambda)$ and $P(A | a',\lambda)$ can be meaningfully manipulated in the same expression. This is fine and valid provided we assume counterfactual definiteness (that is, 'realism'). This is tantamount to the assertion that things have objective properties independent of measurement - essentially a cornerstone of classical physics.

Everything is now set up and it's just straightforward (but ingenious) manipulations of probability distributions to arrive at the celebrated inequality. The assumptions are clear (although as Dr Chinese has beautifully pointed out above there are actually a couple of other implicit assumptions that I've not mentioned in the above - like the 'no-conspiracy' assumption, for example).

I don't know whether this clarifies things, or just muddies things even more - I've taken most of it from Bell's masterful exposition in his Bertlmann's socks paper - and if the above is unclear I urge you to read that - it really is the best explanation I've ever seen. I can't hope to match Bell's clarity and insight but I hope that I've been clear enough to show where the notions of 'locality' and 'realism' actually impact the analysis.

 

[fanieh]

I'm just going to re-iterate what Demystifier has said here.

Mathematical theorems are usually of the form of an IF . . .THEN statement. So IF we have $A$ and $B$, THEN $x$ follows.

In the context of Bell's work we have something like (and crudely),

IF (realism, locality) THEN (correlation functions bounded by inequality)

where I'm just focusing on these two properties and excluding things like superdeterminism.

QM predicts correlation functions that don't have to be bounded by this inequality, therefore QM cannot be equivalent to a theory constructed from quantities which have the properties 'realism' AND 'locality'.

It is therefore possible (in principle) to find a theory for which the correlations are not bounded by the inequality by :
(i) dispensing with 'realism' but keeping 'locality'
(ii) dispensing with 'locality' but keeping 'realism'
(iii) dispensing with both 'locality' and 'realism'

your (ii) is Bohmian Mechanics.. but what interpretations can describe (i) dispensing with 'realism' but keeping 'locality' and especially (iii) dispensing with both 'locality' and 'realism'?

It is NOT possible, in principle, to find a theory that agrees with the QM predictions for the correlation functions constructed from variables/quantities which possesses BOTH of the properties 'realism' and 'locality'.

Note that we're only talking about the violation of the bound on the correlation functions here - just because we construct some theory (eg by dispensing with 'locality') that has a violation for a particular experiment (eg the singlet state of QM) does not imply that it will also agree with other predictions of QM, or even the predictions of QM for other entangled systems (eg the GHZ state)

 

[Zafa Pi]

No. All this can be discussed without even mentioning QT at all. We have the experimental facts - BI are violated for space-like separated measurements - and the theorems which allow to derive the BI.

Right on! But I doubt @DrChinese would disagree.

QT increases the probability that the BI are indeed violated, because it predicts a violation itself.

Better would be QT predicts violation as well, and chuck the increased probability. But as you say it's not needed.

which is relevant only as a rejection of the argument that QT somehow shows that realism or have to be given up anyway.

?? Would you please clarify by correcting grammar.

What is standard is to distinguish a weak notion of Einstein locality, which is only about the impossibility to send FTL signals, and the strong notion of Einstein locality, which forbids even hidden causal influences FTL.

To me signals are information, and strong locality forbids such at FTL. Isn't what you want for weak locality is the inability to to make the entangled correlations from separated measurements? Is there a better word than signal? Spookiness, for example, is more in keeping with Einstein.

 

[Simon Phoenix]

your (ii) is Bohmian Mechanics.. but what interpretations can describe (i) dispensing with 'realism' but keeping 'locality' and especially (iii) dispensing with both 'locality' and 'realism'?

Well Bohmian mechanics is an example of a theory of the kind (ii) - not sure if there are any others. I also have no idea whether theories like (i) and (iii) have been constructed. Ultimately the message I take away from Bell's fabulous work is that 'classical' thinking isn't going to work - an idea already well-established before Bell, but Bell beautifully pinned it down and made it accessible to experimental proof. The gears and wheels and levers and fields of classical physics - all the notions of a 'clockwork' universe - these are simply not up to the job and Bell managed to tell us precisely why and described how to validate that experimentally - spectacular stuff

If we want to have a theory in which the quantities involved in that theory (like momentum, spin, etc) have some objective existence independent of measurement, then the only theory we have recourse to is one in which somehow the information about device settings gets transferred FTL from place to place (or appears to do so). That could be Bohmian mechanics, or some backward in time signals - or some cosmic conspiracy - but whatever it is we've already left Kansas a long time ago.

 

[fanieh]

Well Bohmian mechanics is an example of a theory of the kind (ii) - not sure if there are any others. I also have no idea whether theories like (i) and (iii) have been constructed. Ultimately the message I take away from Bell's fabulous work is that 'classical' thinking isn't going to work - an idea already well-established before Bell, but Bell beautifully pinned it down and made it accessible to experimental proof. The gears and wheels and levers and fields of classical physics - all the notions of a 'clockwork' universe - these are simply not up to the job and Bell managed to tell us precisely why and described how to validate that experimentally - spectacular stuff

If we want to have a theory in which the quantities involved in that theory (like momentum, spin, etc) have some objective existence independent of measurement, then the only theory we have recourse to is one in which somehow the information about device settings gets transferred FTL from place to place. That could be Bohmian mechanics, or some backward in time signals - or some cosmic conspiracy - but whatever it is we've already left Kansas a long time ago.

the (iii) or dispensing with both 'locality' and 'realism' is the better choice.. because it is opposite to Newton and more modern.. but I can't quite decide what convention of our two non-local masters, Demystier or DrChinese to follow.. in Demystifier context.. if you dispense with realism.. you already dispense locality.. so the phase "dispensing with both 'locality' and 'realism' is redundant.. "dispensing with realism" is enough. But for DrChinese, even if realism is dispense with, the words non-locality can still be used.. which of these do Sean Carrol, Weinberg or others follow? majority wins

 

[Demystifier]

I don't think your avatar would approve.

Why do you think so? I think he would, because he would not completely accept a proof based on a questionable assumption.

 

[Denis]

?? Would you please clarify by correcting grammar.

I'm not a native English speaker, but I will try to say it in other words.

The fact that there is a realistic causal interpretation of QM is relevant only because sometimes arguments against realism are proposed which rely on claimed properties of QM. For example, people who know only the Copenhagen interpretation may argue that quantum theory has already shown that classical realism has to be rejected.

To me signals are information, and strong locality forbids such at FTL. Isn't what you want for weak locality is the inability to to make the entangled correlations from separated measurements? Is there a better word than signal? Spookiness, for example, is more in keeping with Einstein.

"Signal locality" is an established phrase, and means the impossibility to send signals FTL. I think this is clear enough. It is clearly a weaker notion than the "Bell locality" used in the BI proof, so naming them "weak" vs. "strong" seems also unproblematic. All I want is to have clear notions, which do not cause confusion.

 

[RUTA]

A less technical description of locality and realism can be found in Mermin's work. I reproduced his argument concerning the Hardy experiment here https://www.physicsforums.com/insig...elayed-choice-no-counterfactual-definiteness/ . Just scroll down to the paragraph starting with "The Hardy experiment is shown and explained in Figure 2 (reference therein)." The entire explanation is only one paragraph long.

 

[DrChinese]

No. EPR does not imply predetermination without strong Einstein causality (Einstein locality not only for signals, but also for hidden but real causal influences). This is the "without in any way disturbing" part of the EPR criterion.

Of course EPR elements of reality imply predetermination. Of every possible outcome! But that makes the assumption there is no observer dependency. They thought any other view was unreasonable.

 

[Denis]

Of course EPR elements of reality imply predetermination. Of every possible outcome! But that makes the assumption there is no observer dependency. They thought any other view was unreasonable.

Sorry, no. You obtain predeterminism only if you believe that the choice of Alice what to measure does not disturb, in any way, Bob's part of the way. If you believe that it does, the EPR criterion of reality is not applicable to this configuration, thus, gives you nothing. Only EPR combined with Einstein causality implies predetermination. The result of the measurement can be undetermined, say, depend on the hidden parameters of the measurement device too, and the interaction with Alice' device then distorts what happens with Bob's particle. The details of this interaction can be described like in dBB theory.

A less technical description of locality and realism can be found in Mermin's work. I reproduced his argument concerning the Hardy experiment here https://www.physicsforums.com/insig...elayed-choice-no-counterfactual-definiteness/ . Just scroll down to the paragraph starting with "The Hardy experiment is shown and explained in Figure 2 (reference therein)." The entire explanation is only one paragraph long.

Seen the whole thing about the Blockworld view. What could you recommend as the best reference to this BW metaphysics? "Best" in the sense of using the best justification of it in the most radical (most fatalistic) way?

 

[Simon Phoenix]

but I can't quite decide what convention of our two non-local masters, Demystier or DrChinese to follow

Follow either of them and you won't go far wrong :-)

But you don't have to 'follow' anybody here - you're only forced to make a choice if you want a theory which models the conditional probabilities using these extra, so-called hidden, variables. If you make that choice first, then you're faced with the issues of 'locality' and 'realism'.

Bell's theorem says is that IF we want to explain the correlations using some extra variables THEN the resulting theory can't be both 'local' AND 'realistic', if we wish agreement with the predictions of QM. Therefore QM cannot be replaced with a locally realistic hidden variable theory.

Note that this does NOT mean that QM must therefore be one of the other kinds of hidden variable theory (non-local, realistic, for example). It certainly could be (as the Bohmian treatment shows us), but we don't have to think of QM as a hidden variable theory at all.

Remember that the first step in the analysis is not the assumption of 'locality' or 'realism', but the assumption that there exist these extra variables.

Just stick with standard QM then you don't have to worry.

 

[fanieh]

Follow either of them and you won't go far wrong :-)

But you don't have to 'follow' anybody here - you're only forced to make a choice if you want a theory which models the conditional probabilities using these extra, so-called hidden, variables. If you make that choice first, then you're faced with the issues of 'locality' and 'realism'.

Bell's theorem says is that IF we want to explain the correlations using some extra variables THEN the resulting theory can't be both 'local' AND 'realistic', if we wish agreement with the predictions of QM. Therefore QM cannot be replaced with a locally realistic hidden variable theory.

Note that this does NOT mean that QM must therefore be one of the other kinds of hidden variable theory (non-local, realistic, for example). It certainly could be (as the Bohmian treatment shows us), but we don't have to think of QM as a hidden variable theory at all.

Remember that the first step in the analysis is not the assumption of 'locality' or 'realism', but the assumption that there exist these extra variables.

Just stick with standard QM then you don't have to worry.

After thinking it over. Demystifier was the correct one. Here's why. If no realism, no locality.. so nothing to be non local about. Therefore DrChinese must change the phase "non-local correlations" to "beyond Einstein locality corrections" to avoid more confusion. Won't you agree DrChinese?

What are the thinking of other non PF physicists?

 

[Simon Phoenix]

After thinking it over. Demystifier was the correct one. Here's why. If no realism, no locality.. so nothing to be non local about. Therefore DrChinese must change the phase "non-local correlations" to "beyond Einstein locality corrections" to avoid more confusion. Won't you agree DrChinese?

I can't speak for Demystifier, or DrChinese, but I certainly disagree with you here

It's clear from the analysis that the 'locality' here is referring to the condition imposed that changes of the device settings (assumed to be independent changes - so changes of the independent variables $a$ and $b$ that occur in the conditional probability $P(A,B | a,b)$ and the resulting marginals) do not affect the results obtained at some remote location.

As Bell pointed out we could have the variables $\lambda$ being 'non-local' (in a wider sense) and still be bounded by the inequality, provided we still maintain the locality condition imposed on the changes of device setting.

I see no reason why relaxing the condition of realism (properties exist independent of measurement), implies non-locality. I see no reason why relaxing the condition of locality (device settings affect remote results) implies non-realism.

But I agree there is confusion over the use of the word 'local' - which means something very specific in Bell's analysis - and sometimes (as I have done) we use the word 'local' to mean slightly different things even within the same sentence! That's life - it happens - it's them pesky words and their interpretations which plague physics and mathematics, where the same words can get used to mean different things in different contexts.

 

[Zafa Pi]

Why do you think so? I think he would, because he would not completely accept a proof based on a questionable assumption.

If 1 + 1 = 3 then I am the Pope. Perfectly valid theorem with a questionable hypothesis. I'm sure B.R. would agree.
But saying, "I think that actual physics is most likely non-local." is kind of weird. Could you elaborate?

 

[Denis]

I see no reason why relaxing the condition of realism (properties exist independent of measurement), implies non-locality. I see no reason why relaxing the condition of locality (device settings affect remote results) implies non-realism.

Indeed, above things would be nonsensical. If there is a connection, it would be the reverse one: relaxing realism would allow stronger locality, relaxing locality would allow realism.

But in fact the situation is the following:

If we throw away realism, weak "signal" locality remains. Strong, Einstein locality is, then, anyway meaningless, given that it is about real causal influences, but we have abandoned realism.

But weak "signal" locality is not endangered at all. It holds anyway, with or without realism. So giving up realism gains nothing.

So, the situation can be easily summarized: Reality is not Einstein-causal. To save the fundamental(ist) variant of relativistic symmetry - that everything has to be Lorentz-covariant - we have to reject realism.

 

[Demystifier]

But saying, "I think that actual physics is most likely non-local." is kind of weird. Could you elaborate?

I think I already did. See Sec. 5.3 in my paper
http://lanl.arxiv.org/abs/1703.08341
Each entry is a possible local interpretation of QM, and each entry contains a "but" which makes it quite unreasonable. Yet, neither of those "buts" is as unreasonable as your 1+1=3.

By the way, my avatar would not agree that 1+1=3 is obviously wrong. He needed 379 pages to prove it.
https://en.wikipedia.org/wiki/Principia_Mathematica

 

[RUTA]

Seen the whole thing about the Blockworld view. What could you recommend as the best reference to this BW metaphysics? "Best" in the sense of using the best justification of it in the most radical (most fatalistic) way?

My Insights series has arguments for Wilczek's "God's-eye" or block universe view of physical reality. If you're already familiar with the standard argument for block universe from SR, then skip to number 3 of that series https://www.physicsforums.com/insig...lications-part-3-general-relativity-big-bang/. At the top of that article there are links to articles 4 and 5. My colleagues and I have an entire book coming out with Oxford UP making this case. That's all I can say about it here, as it's not relevant to this thread. I just wanted to share Mermin's account of the Hardy experiment on realism versus locality which happened to be in that series.

 

[Zafa Pi]

I think I already did. See Sec. 5.3 in my paper
http://lanl.arxiv.org/abs/1703.08341
Each entry is a possible local interpretation of QM, and each entry contains a "but" which makes it quite unreasonable. Yet, neither of those "buts" is as unreasonable as your 1+1=3.

By the way, my avatar would not agree that 1+1=3 is obviously wrong. He needed 379 pages to prove it.
https://en.wikipedia.org/wiki/Principia_Mathematica

Blaylock claims MWI is local.
I didn't say 1 + 1 = 3 was wrong, that is indeed very deep as was shown by Whitehead. I said it was questionable.
You said physics was most likely nonlocal, but you never gave the odds.

 

[Zafa Pi]

but what interpretations can describe (i)

According to Blaylock MWI.

 

[Zafa Pi]

"Signal locality" is an established phrase, and means the impossibility to send signals FTL. I think this is clear enough. It is clearly a weaker notion than the "Bell locality" used in the BI proof, so naming them "weak" vs. "strong" seems also unproblematic. All I want is to have clear notions, which do not cause confusion.

I was searching on line for a couple hours and I find a quagmire of definitions of local and non-local whether physics is one or the other, which interpretations are one or the other, if realism or CFD is given up only then do we have non-locality, or maybe non-locality is always there. So many disagree with one another and everybody is sure. For example Blaylock and Maudlin.

Can't find a a clear definition of signal locality. Perhaps you could define you terms or provide links that are specific

 

[morrobay]

You intended to say if realism or CFD is given up then you can have locality.* Realism in the context of perfect (anti) correlations when detectors are aligned that infers pre defined, deterministic properties.
That then are applied when detectors are not aligned in deriving the inequality.
*Local relativistic causality(no FTL signal) that QM is consistent with.
The question I have is how do the linked cluster principle and consistent histories (both local and realistic) define a non locality,sometimes called quantum non locality (not FTL) that cause the correlations in a spacelike separated experimental setting.
As In following diagram (credit stevendaryl) where Alice and Bob are spacelike separated.

 

[Denis]

...if realism or CFD is given up only then do we have non-locality, or maybe non-locality is always there.

Can't find a a clear definition of signal locality. Perhaps you could define you terms or provide links that are specific

Never forget that realism and CFD are very different things (but often confused). CFD has to be given up, because of Bell as well as Kochen-Specker, and there is nothing to discuss. Realism does not have to be given up, with dBB as a counterexample.

Signal locality means that there are no statistical effects which distribute a local change with velocity FTL, so that the non-Einstein locality cannot be used to send signals FTL.
I would say Valentini's paper https://arxiv.org/abs/quant-ph/0106098 is specific enough, and proves even an interesting theorem about signal locality.

 

[morrobay]

The author in this paper also derives the same correlation function with locality (no faster than light signal)
from the conservation laws.
P(a,b)_con = P(a,b)_qm = - cos θ
https://arxiv.org/pdf/quant-ph/0407041.pdf

 

[Simon Phoenix]

I was searching on line for a couple hours and I find a quagmire of definitions of local and non-local whether physics is one or the other, which interpretations are one or the other, if realism or CFD is given up only then do we have non-locality, or maybe non-locality is always there. So many disagree with one another and everybody is sure.

Yes indeed, and it's rather easy to tie one's philosophical knickers in all sorts of knots worrying about the nuances of all the terms that get bandied about; locality, Einstein locality, Bell locality, causality, Einstein locality, signal locality, CFD, non-contextuality, realism and so on ad nauseam. I suppose it gives the philosophically minded something to do on a long winter's evening and keeps them off the streets.

I am, of course, being overly unkind here in order to make a point. The words, divorced from the analyses that spawned them, can so easily lead one astray. That's why I personally feel it's so important to ground oneself in the actual specific analysis and see what the terms actually mean - instead of trying to wade through pages of overblown rhetoric that obfuscate rather than elucidate.

So my advice is to take a derivation of a BI that you like and pick it apart. My own favourite is Bell's wonderful analysis of the CHSH inequality that is contained in his Bertlmann's socks paper. Bell's original inequality is in fact a special case of the more general CHSH version. It's the ideas and the physics in the analysis, rather than what we call these things, that is important I feel. So, sure, one author might use the term 'locality' whereas another might call the same thing 'Bell locality', or even 'Einstein causality', but underlying it all are the same ideas that are more clearly expressed in symbols. In an ideal world it would be nice if we could all agree on the same nomenclature, or even use nomenclature consistently - but we're all human and often try to cut corners and use shorthand.

So let's take the CHSH version and see what is involved.
(1) we have an experiment that allows us, after sufficiently many runs, to measure to a good approximation the joint conditional distributions $P(A,B | a,b)$
(2) we wish to attempt to explain the correlations and so we assume that there are some extra variables or quantities, all lumped into the symbol $\lambda$ for convenience, that explain the correlations in the following sense $P(A,B | a,b, \lambda) = P(A | a,b, \lambda)P(B | a,b, \lambda)$. The physical meaning here is that these extra variables, $\lambda$, account for the observed correlation so that any residual fluctuation in the measured quantities must be independent

But look at $P(A | a,b, \lambda)$ here, for example. It's telling us that the distribution of results obtained by Alice is dependent on the setting chosen by Bob at some remote location. How can that be? It would seem to run counter to our intuition and, if it were true, if we could also control the variables $\lambda$ it would certainly allow us to construct a FTL signalling scheme where we could transmit real information faster than we could send the information using light.

In order to avoid this unwelcome possibility (at least) the further assumption is made that :
(3) $P(A,B | a,b, \lambda) = P(A | a,b, \lambda)P(B | a,b, \lambda) = P(A | a, \lambda)P(B | b, \lambda)$.

The last step on the RHS is where the 'locality' considerations get imposed. Physically we're saying that the distribution of results at Alice depends only on Alice's setting, and the hidden variables - and do not depend on the choice of device setting at some remote place. Physics would be very strange indeed if we allowed this to be so - if the experiments we do here depend on whether someone a million miles away has twiddled with his knob or not we'd never be able to trust the results we obtained in our laboratory. Intuitively we can see that the only way for this to happen is if information about a device setting at some remote location was communicated (in some fashion) to our lab. Somehow our own system (device + measured object) must 'know' about some remote knob twiddling. That definitely doesn't seem right does it?

Apart from the purposes of communication and using consistent terminology does it really matter whether we term condition (3) 'locality', or 'Bell locality', or 'the anti knob-twiddling postulate'? The meaning and intent is crystal clear in the analysis - it can become less clear when we abstract this idea out of the analytical context and try to reason about it. But as long as we continually reference back to our grounded experiment (hypothetical or otherwise) and the assumptions we use in order to construct a plausible model, then everything should be clear enough - even if different people use the same words in subtly different ways.

 

[Zafa Pi]

Never forget that realism and CFD are very different things (but often confused). CFD has to be given up, because of Bell as well as Kochen-Specker, and there is nothing to discuss. Realism does not have to be given up, with dBB as a counterexample.

Yes, Blaylock also says CFD and realism are different, but comes to the opposite conclusion. I don't see the difference that well.

Signal locality means that there are no statistical effects which distribute a local change with velocity FTL, so that the non-Einstein locality cannot be used to send signals FTL.

"distribute"? Does this mean that the correlations that occur in separated measurements on entangled entities take a while to be seen?
Can't find "non-Einstein locality".
If you find this boring you need not respond and I will not feel hurt. There have been many threads on this topic and I'm a bit jaded. I have my own very elementary way of doing a Bell type result (close to CHSH) and what the lab shows and only mention QT at the end as a theory that predicts the lab results.

 

[Zafa Pi]

Yes indeed, and it's rather easy to tie one's philosophical knickers in all sorts of knots worrying about the nuances of all the terms that get bandied about; locality, Einstein locality, Bell locality, causality, Einstein locality, signal locality, CFD, non-contextuality, realism and so on ad nauseam. I suppose it gives the philosophically minded something to do on a long winter's evening and keeps them off the streets.

Hear hear! Unfortunately it doesn't keep them off the internet. Thanks for the response.

So my advice is to take a derivation of a BI that you like and pick it apart. My own favourite is Bell's wonderful analysis of the CHSH inequality that is contained in his Bertlmann's socks paper. Bell's original inequality is in fact a special case of the more general CHSH version. It's the ideas and the physics in the analysis, rather than what we call these things, that is important I feel. So, sure, one author might use the term 'locality' whereas another might call the same thing 'Bell locality', or even 'Einstein causality', but underlying it all are the same ideas that are more clearly expressed in symbols. In an ideal world it would be nice if we could all agree on the same nomenclature, or even use nomenclature consistently - but we're all human and often try to cut corners and use shorthand.

As you can see from post #66 I long ago have taken your advise. My target is a freshman math or physics major and any technical term is only parenthetical.

I think that the presentation in Nielsen & Chuang is easier than the one you give and mine is easier/more elementary than theirs, though longer.

 

[Denis]

Yes, Blaylock also says CFD and realism are different, but comes to the opposite conclusion.

I will not comment Blaylock, for reasons connected with the netiquette. But, ok, look at the following quote:

Following this reasoning, some scientists insist that Bell’s inequality rests only on the assumption of locality, and that counterfactual definiteness, which is implied in the definition of “elements of reality,” is inferred rather than assumed. This line of thought neglects to realize that the single-reality assumption is already built into the definition of EPR’s “elements.” Multi-reality interpretations such as many worlds provide a contrasting viewpoint.

These "some scientists" include Bell, who wrote

It is important to note that to the limited degree to which determinism plays a role in the EPR argument, it is not assumed but inferred. What is held sacred is the principle of 'local causality' -- or 'no action at a distance'. ... It is remarkably difficult to get this point across, that determinism is not a presupposition of the analysis.

And is CFD implied in the "definition" of the EPR "elements of reality"? There is not really a definion there, instead, there is a criterion, the EPR criterion of reality. It is:

If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity.

So, it tells us nothing about the existence of some elements of reality, except in the case that we (for whatever reason) assume that we can make this prediction without in any way disturbing a system. And this clearly presupposes Einstein locality, in some version which forbids such a disturbance.

Regarding the possibility that elements of reality could depend on non-local effects, they concluded: “No reasonable definition of reality could be expected to permit this.”

Not really. The context is the following:

This makes the reality of P and Q depend upon the process of measurement carried out on the first system, which does not disturb the second system in any way. No reasonable definition of reality could be expected to permit this.

This obviously already takes as already given that the process "does not disturb the second system in any way". It is a comment about the possibility that, despite this, the reality of P and Q depends on this.

"distribute"? Does this mean that the correlations that occur in separated measurements on entangled entities take a while to be seen?

Observation can never tell us that information is distributed with infinite velocity. What looks like infinite for us may appear finite but large in reality.

Can't find "non-Einstein locality".

Usually people use "nonlocal" instead. But the term nonlocality is misleading, because local theories with speed of information transfer greater then c would have to be named nonlocal, which is a little bit Orwellian.

 

[N88]

...
Usually people use "nonlocal" instead. But the term nonlocality is misleading, because local theories with speed of information transfer greater then c would have to be named nonlocal, which is a little bit Orwellian.

How about this clarification? "Local theories with speed of information transfer greater then c would best be named Einstein-nonlocal."

 

[Zafa Pi]

I will not comment Blaylock, for reasons connected with the netiquette.

Lol

These "some scientists" include Bell, who wrote

"It is important to note that to the limited degree to which determinism plays a role in the EPR argument, it is not assumed but inferred. What is held sacred is the principle of 'local causality' -- or 'no action at a distance'. ... It is remarkably difficult to get this point across, that determinism is not a presupposition of the analysis."

I find this a bit vague, but if HE is saying that CFD is not employed in his original argument then HE is correct (to a "limited degree") in saying, "It is remarkably difficult to get this point across", Because, I, for one, disagree with HIM. I see clearly where HE stealthily introduced CFD into HIS proof. I've been in two previous debates in this forum on this matter, and several others in different contexts. I invite you to heap on me all the nonetiquette abuse that your heart desires, I would find it an honor. Just watch out for the monitors.

And is CFD implied in the "definition" of the EPR "elements of reality"? There is not really a definion there, instead, there is a criterion, the EPR criterion of reality.

What is the definition of speeding on Montana highways? / What is the criterion for speeding on Montana highways?
I you wish to quibble over the difference, feel free, but I won't.

"it is not assumed but inferred." J.B.
After checking the historical record Alice inferred Dr. Bob was wrong. / After checking the historical record Alice assumed Dr. Bob was wrong.
I think that Alice thinks Dr. Bob is wrong. I think determinism plays a role in the EPR argument, to the limited degree of above -40^o.

Yes indeed, and it's rather easy to tie one's philosophical knickers in all sorts of knots worrying about the nuances of all the terms that get bandied about

I got entangled knots in my quantum knickers.