What Is an Element of Reality?

[ttn]

I had the impression (but I can be wrong) that if you take the hidden variables in Bohm for real (and you have to, if you consider them part of the reality description), that LOCAL probability distributions of these hidden variables can have expectation values which change according to what happens elsewhere, so that these probability distributions of these hidden variables are not local in the sense of relativity (in that we can send information that way, if only we had local access to these hidden variables).
It is in *that* sense that I thought that Bohm was non-local.

Yes, that's exactly right. But (and this is becoming something of an anthem on my part..) it's just the same for QM. If you could actually discover somehow what the wave function for some entity next to you was, you would be able to use this information to send messages. Send your friend one of the boxes with "half a particle" in it. The value of the wf over by your friend is 1/sqrt(2). [or something like that... technically I'm talking about the mod of the wf integrated over the volume of the box, but who cares about that detail.] But as soon as you open your box and either find or don't find the particle there, the value of the wf over by your friend will immediately change to either zero or one (respectively). And if he had access to that change -- if he knew that the value of the wf in his box had suddenly jumped, he'd know that you had just opened your box. Hence, information transfer.

Of course, everybody knows that you can't just "learn the value of the wf at some point". So you can't actually use this underlying non-locality in orthodox QM to transmit information. But if this kind of argument gets QM off the hook, it ought to get bohmian mechanics off the hook too. They're really equivalent -- both are theories about some quantity/quantities (wave functions only for QM, wf's plus particle positions for Bohm) which are affected nonlocally by various fiddling that can be done at distant locations. And if only you had access to the exact local state (as indicated by the local values for the quantities your theory is *about*) you could use this nonlocality to transmit information and thus get into all sorts of hot water with relativity. But, in both theories, you *don't* have access to the exact local state, so you are *prevented* from using the nonlocality to transmit information, and hence (by the "info" type definition of locality) both theories turn out to be *local*. But this has a very uncomfortable, conspiratorial feel to it, which people have no trouble expressing when it comes to Bohm. They all say more or less what you said above: "come on, the underlying *physics* in Bohm's theory is blatantly nonlocal -- so the fact that this nonlocality is washed out and can't be put to use is irrelevant." For some reason people aren't as willing to say what is, I think, obviously and equally true of QM: "the underlying *physics* of QM is blatantly nonlocal [specifically, the collapse postulate] -- the fact that this is washed out and can't be put to use is irrelevant."

By the way, the kind of statements you are making here about Bohm's theory -- that it is obviously nonlocal if you take it seriously -- is exactly what bothered Bell about Bohm's theory at first. It is, I suspect, part of why he was motivated to come up with clean mathematical condition by which one could judge deep/fundamental locality [or what he called "local causality"]. Remember, Bohm's theory is *local* by the standard of info transfer, so *some* clean way of expressing its "obvious" nonlocality is needed. What he came up with -- "Bell Locality" -- does the job beautifully. It's because Bohmian mechanics violates this condition that we all feel good about saying: "ahh, OK, so despite the fact that you can't send messages FTL in Bohm's theory, it really is nonlocal behind the scenes." But then you notice that orthodox QM violates this same condition -- something which people remain far less comfortable about, but which is painfully obvious nevertheless.

 

[ttn]

That's where we differ.
IF you consider this theory as "Bell Local" it is obviously deterministic, in that for each pair of bells emitted, you are in case A (cube left, piramid right) or you are in case B (cube right, piramid left). If you are in case A, all the probabilities are 1 or 0, and if you are in case B, idem. So if the case is determined, everything is deterministic. Now, if you think you have the right to put the "case" into the "complete description of nature" then I have also the right to say that this complete description of nature determines all outcomes with certainty, and that's what I call a deterministic theory.
Whether this CASE information is accessible in principle to us, observers, or not (in which case it is a "hidden variable") doesn't change anything: if you consider it part of a complete description, it "is there".

Yes, you're entirely right about this. My mistake. I shouldn't have said that picking between cases A and B in some "irreducibly stochastic" way made the theory genuinely stochastic. It doesn't, for just the reasons you give.

I don't think this changes anything significant, though. I still maintain that it's possible to have a genuinely stochastic theory that either does or does not satisfy Bell Locality -- i.e., the Bell Locality condition makes perfect sense applied to genuinely stochastic theories -- i.e., that condition isn't somehow uniquely applicable to deterministic theories.

We already have on the table an example of a genuinely stochastic theory that, I think, we've agreed violates Bell Locality. (namely, QM) So maybe it would help to make up an example of a genuinely stochastic theory that is Bell Local. Would that help?? I'm actually a bit confused now about what you're even claiming, so maybe this won't help at all. In fact, I'm pretty sure it won't since it's so damn trivial. But, for what it's worth, here's an example of a genuinely stochastic theory that is consistent with Bell Locality:

Alice and Bob shake hands, walk to opposite sides of the room, and then each flips a fair coin (or some other event we're willing to pretend is irreducibly random). The joint probability for Alice and Bob both getting heads factors: 50% for Bob times 50% for Alice = 25% for two heads. Bell Locality is respected.

Stupid, huh? Admittedly so, but it's an example of applying Bell Locality to a stochastic situation. Maybe you'll think what's special about this example is that there are actually no correlations at all between the two sides. If so, modify the scenario in another admittedly stupid way: say Alice and Bob each have two coins in their pockets, a two-headed coin and a regular heads/tails coin. After separating, Alice and Bob each independently decide, with irreducibly random probability, whether to flip their H/H coin or their H/T coin. Say there is a 99% chance each time that they'll choose the H/H coin, and only a 1% chance that they'll decide to flip the regular H/T coin. So... a large fraction of the time, Alice and Bob both end up with a "heads" outcome.

I think it is obvious that Bell Locality is still 100% respected. Yet the correlation coefficient

P(H,H) + P(T,T) - P(H,T) - P(T,H)

is not zero.

So it isn't merely the lack of correlations between separated events that permits one to apply Bell Locality to stochastic situations.

I dunno, somehow I doubt any of this will help move the conversation forward. Maybe you could remind me/us what exactly you object to in applying Bell Locality to stochastic theories (in particular, what precisely you object to in my claim that QM, so long as you believe that the wf is a complete description of the system, violates Bell Locality)...

 

[ttn]

Bell said that EPR's argument - which also tried to define what an element of reality was - did not actually work as they had pictured it.

That is *definitely* not true! Bell was emphatic that the EPR argument *had indeed* established that, if complete, QM itself was nonlocal. This was the first part of his two-part argument that nature violates Bell-Locality. (The second part is, of course Bell's Theorem: you can't get rid of the apparent nonlocality of QM by rejecting the completeness doctrine, i.e., by building a hidden variable theory.)

Perhaps you are confusing the proposition that EPR actually argued for (QM is either incomplete or nonlocal) with the conclusion they (naturally, at the time) drew from this: since locality is true, QM must be incomplete. That is, EPR showed that, for QM, locality --> incompleteness. Then as a separate premise, they postulated: locality. Combining these obviously gives the conclusion: incompleteness.

Bell's later work undermines the "separate premise: locality" but in no way undermines the important dilemma that EPR argued for, namely, "locality --> incompleteness." Indeed, as I said, Bell continued to cite EPR as having provided the first half of the argument which proves that locality fails, period (whether or not one subscribes to completeness).

The problem being that their assumption - elements of reality exist independent of the measurement - was flawed.

This was hardly an *assumption* of EPR! They proved (under the assumption of locality) that these pre-measurement elements of reality must exist.

 

[DrChinese]

That is *definitely* not true! Bell was emphatic that the EPR argument *had indeed* established that, if complete, QM itself was nonlocal. This was the first part of his two-part argument that nature violates Bell-Locality. (The second part is, of course Bell's Theorem: you can't get rid of the apparent nonlocality of QM by rejecting the completeness doctrine, i.e., by building a hidden variable theory.)

Perhaps you are confusing the proposition that EPR actually argued for (QM is either incomplete or nonlocal) with the conclusion they (naturally, at the time) drew from this: since locality is true, QM must be incomplete. That is, EPR showed that, for QM, locality --> incompleteness. Then as a separate premise, they postulated: locality. Combining these obviously gives the conclusion: incompleteness.

...

This was hardly an *assumption* of EPR! They proved (under the assumption of locality) that these pre-measurement elements of reality must exist.

No, not so! Bell may have said various things, same Einstein, but their work speaks for itself. Bell's Theorem does not rest upon locality, and neither does EPR, and in both of these papers locality is barely mentioned. Replace the word "locality" with "causality" (which is I think is close to your Bell locality) and we are in the same ballpark.

EPR claimed that if the result of a measurement could be predicted in advance, then the observable must correspond to an element of reality and that that observable was in fact predetermined. Bell explored this idea too.

While you are talking about the locality of the observable, I am talking about the reality of the observable. EPR said: "Indeed, one would not arrive at our conclusion if one insisted that two or more physical quantities can be regarded as simultaneous elements of reality only when they can be simultaneously measured or predicted."

Bell showed that we should insist on the more restrictive definition of reality; i.e. that quantum attributes are not objectively real if we can't measure or predict them. Bell represented the reality condition within his theorem explicitly - see his (2) with \lambda. Under this criteria, then, EPR does not prove that QM is incomplete by admission of EPR because they assumed that these elements of reality existed.

EPR never claimed that they proved that QM was non-local if complete, although I can see why that would be a logical deduction IF you didn't know about Bell. After all, they considered predetermined "elements of reality" to be a given. Now that we know this is questionable, evertyhing looks different.

 

[ttn]

No, not so! Bell may have said various things, same Einstein, but their work speaks for itself. Bell's Theorem does not rest upon locality, and neither does EPR, and in both of these papers locality is barely mentioned.

Bell's Theorem does not rest upon locality? Are you kidding? Read any of Bell's papers -- I think you'll find (a) that the theorem assumes that the theories the theorem is about satisfy the Bell Locality condition and (b) Bell spends a lot of time and energy arguing for this condition. See especially the article "La Nouvelle Cuisine", re-printed as the final chapter in the new 2nd edition of Speakable and Unspeakable.

Re: Einstein, you are right in one sense: the actual EPR paper barely mentions the locality issue. But Podolsky wrote that paper, and Einstein wrote in a letter (later in '35) to Schroedinger that he thought the point he considered crucial (namely, the completeness-locality dilemma) had been "smothered by the formalism" in Podolsky's paper!

Here are Einstein's words from the "Reply to Criticisms" essay in the Schilpp volume:

"By this way of looking at the matter it becomes evident that the paradox [EPR] forces us to relinquish one of the following two assertions:
1. the description by means of the \psi-function is complete.
2. the real states of spatially separated objects are independent of each other."

For more detail on this point, see the first few chapter of Arthur Fine's wonderful book, "The Shaky Game." One notable line: "It is important to notice that the conclusion Einstein draws from EPR is not a categorical claim for the incompleteness of quantum theory. It is rather that the theory poses a dilemma between completeness and separation; both cannot be true." The paper "Einstein's Boxes" in the Feb. '05 American Journal of Physics also discusses this issue in some detail.

Bell showed that we should insist on the more restrictive definition of reality; i.e. that quantum attributes are not objectively real if we can't measure or predict them.

This sounds nothing like the Bell I know and love.

Under this criteria, then, EPR does not prove that QM is incomplete by admission of EPR because they assumed that these elements of reality existed.

EPR proved that QM, if complete, is nonlocal.
Bell proved that if QM is *not* complete, the resulting hidden variable theory has to be nonlocal.
Combined, these two arguments prove that nature is nonlocal. *That* is what Bell proved -- at least, it is what I think he proved... which wouldn't count for much except that this matches what Bell himself thought he proved.

EPR never claimed that they proved that QM was non-local if complete, although I can see why that would be a logical deduction IF you didn't know about Bell. After all, they considered predetermined "elements of reality" to be a given. Now that we know this is questionable, evertyhing looks different.

Yes, the actual EPR paper obscured the importance of the locality issue, and generally failed to make clear what Einstein (later) did -- that the real point of EPR was (supposed to be!) that there is a dilemma, for QM, between completeness and locality. You seem to think EPR just *assumed* the existence of the elements of reality they needed to show that QM was incomplete. Wouldn't that make their argument trivially circular/empty? I don't think it was empty at all. They didn't just assume the desired conclusion; they showed that it followed from the locality assumption -- an assumption which was indeed, as you say, a logical one until/unless one knows about Bell. After Bell, you realize that you're stuck with a nonlocal theory regardless of your position re: completeness. See quant-ph/0408105 for further details on this.

 

[DrChinese]

Bell's Theorem does not rest upon locality? Are you kidding? Read any of Bell's papers -- I think you'll find (a) that the theorem assumes that the theories the theorem is about satisfy the Bell Locality condition and (b) Bell spends a lot of time and energy arguing for this condition. See especially the article "La Nouvelle Cuisine", re-printed as the final chapter in the new 2nd edition of Speakable and Unspeakable.

Re: Einstein, you are right in one sense: the actual EPR paper barely mentions the locality issue. But Podolsky wrote that paper, and Einstein wrote in a letter (later in '35) to Schroedinger that he thought the point he considered crucial (namely, the completeness-locality dilemma) had been "smothered by the formalism" in Podolsky's paper!

Here are Einstein's words from the "Reply to Criticisms" essay in the Schilpp volume:

"By this way of looking at the matter it becomes evident that the paradox [EPR] forces us to relinquish one of the following two assertions:
1. the description by means of the \psi-function is complete.
2. the real states of spatially separated objects are independent of each other."

The EPR paper and Bell's 1964 follow up say it all:

I. EPR proves: "...either (1) the quantum-mechanical description given by the wave function is not complete or (2) when the operaters corresponding to two physical quantities do not commute the two quantities cannot have simultaneous reality".

II. Bell proves both cannot be true: (1) QM is incomplete (as represented by the \lambda in his formulas; and (2) the predictions of QM are correct. To quote: "The paradox of Einstein, Podolsky and Rosen was advanced as an argument that quantum mechanics was not complete but should be supplemented by additional parameters... In this note that idea will be formulated mathematically and shown to be incompatible with the statistical predictions of quantum mechanics."

III. Accepting both EPR and Bell as correct (as I do), as well as Aspect, you must conclude that:

a) Aspect et al proves that the predictions of QM are correct (please Cat stay out of this discussion as we are not interested in debating this).
b) If QM is correct, then Bell (2) is true; therefore Bell (1) is false.
c) If Bell (1) is false, then EPR (1) is also false as they are equivalent by design.
d) If EPR (1) is false, then EPR (2) is true.

IV. Ergo: Aspect + Bell + EPR -> Reality fails ("when the operates corresponding to two physical quantities do not commute the two quantities cannot have simultaneous reality"). This is the logical result of the chain, and you can clearly see that locality is not a factor by examining the formalisms.

A close look at the arguments of EPR (as you have seen) and Bell, you will see that whether QM is local or non-local is not a factor in any way. The only requirement Bell mentions is that "the requirement of locality, or more precisely 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." However, this requirement is not actually represented in any way in Bell's formalism (that I can see - perhaps you can find a *formula* which embodies this). The only assumption Bell actually makes is that there is an A, B and C when we can measure only 2 at a time. In other words, his conclusion is correct and his derivation is correct; but his description strays a bit in ways that do not affect his work in any way.

Put another way in my own words: so what if there are hidden variables across the universe when t=0? There still cannot be an A, B and C which are simultaneously real at t=0. Therefore, it is the measurement at t=T which creates the reality. The location of the hidden variables is not a factor, there is no A, B and C regardless.

 

[ttn]

A close look at the arguments of EPR (as you have seen) and Bell, you will see that whether QM is local or non-local is not a factor in any way. The only requirement Bell mentions is that "the requirement of locality, or more precisely 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." However, this requirement is not actually represented in any way in Bell's formalism (that I can see - perhaps you can find a *formula* which embodies this).

Are you kidding?? How about the requirement that the joint probabilities factor, as expressed, e.g., in Bell's equation (14) [of "On the E-P-R paradox"]. The discussion in his later papers is much clearer: check out, e.g., section 4 of "Bertlmann's socks...", or the very extensive and detailed discusison in "La Nouvelle Cuisine."

Here is a nice statement (from "Bertlmann's socks...", one of his later papers, after he had had lots of time to get his thinking straight on exactly what he had proved):

"Let us summarize once again the logic that leads to the impasse. The EPRB correlations are such that the result of the experiment on one side immediately foretells that on the other, whenever the analyzers happen to be parallel. If we do not accept the intervention on one side as a causal influence on the other, we seem obliged to admit that the results on both sides are determined in advance anyway, independently of the intervention on the other side, by signals from the source and by the local magnet setting. But this has implications for non-parallel settings which conflict with those of QM. So we *cannot* dismiss intervention on one side as a causal influence on the other." (pg 149-50 of Speakable...)

I don't see any possible way of interpreting this, other than the one I have been advocating here. Bell is saying: under the assumption of locality ("if we do not accept the intervention on one side as a causal influence on the other") we are led to conclude that there exist local hidden variables determining the outcomes. [that is the EPR argument!] But as he goes on to point out, this assumption (that there exist local hv's) leads to a contradiction with the experimentally observed results. [that is Bell's theorem.]

In other words, the only way of trying to interpret QM as a local theory (namely, by dropping the completeness assumption and trying for a local hidden variable theory) does not work. You cannot get rid of the nonlocality.


I'd like to keep this as positive as possible, but your anti-realist comments are really inexcusable. There is just no reasonable way of believing that somehow the upshot of EPR/Bell is that it's impossible to believe in realism or elements of reality or whatever. Bohmian mechanics exists. It is an unambiguous counterexample to any such claims.

 

[DrChinese]

I don't see any possible way of interpreting this, other than the one I have been advocating here. Bell is saying: under the assumption of locality ("if we do not accept the intervention on one side as a causal influence on the other") we are led to conclude that there exist local hidden variables determining the outcomes. [that is the EPR argument!] [1]But as he goes on to point out, this assumption (that there exist local hv's) leads to a contradiction with the experimentally observed results. [that is Bell's theorem.]

In other words, the only way of trying to interpret QM as a local theory (namely, by dropping the completeness assumption and trying for a local hidden variable theory) does not work. You cannot get rid of the nonlocality.

I'd like to keep this as positive as possible, but your anti-realist comments are really inexcusable. There is just no reasonable way of believing that somehow the upshot of EPR/Bell is that it's impossible to believe in realism or elements of reality or whatever. Bohmian mechanics exists. It is an unambiguous counterexample to any such claims.

Anti-realist comments are "inexcusable"? Say that to Einstein and Bell, not me. I am quoting him (EPR): "...either (1) the quantum-mechanical description given by the wave function is not complete or (2) when the operators corresponding to two physical quantities do not commute the two quantities cannot have simultaneous reality". That is what the paper is all about, and Einstein clearly took it - as obviously you do - that (2) is false. (That, by the way, is not the same as your EPR argument above.) I realize your opinion is different than mine, but these are the actual relevant words from the actual paper, and not an out of context comment made later. The fact is, Einstein didn't miss a trick as his actual words allowed for him to be wrong about (1) and right about (2) and therefore right - again - in the end when it mattered. He publicly supported the (1) position and yet - here is it again - (2) is the anti-realist position you disdain.

Bell (14): P(a,b)=-\int d\lambda p(\lambda) A(a,\lambda) A(b,\lambda)

Perhaps you can explain how this has anything to do with the location of the hidden variables. On the other hand, Bell's realist assumption follows on the very next line... "It follows that c is another unit vector" and thereafter there is a, b and c. This is the explicit labeling of attributes that do not commute; and that we now know does not have simultaneous reality. In his paper, Bell states: "the quantum mechanical expectation value cannot be represented, either accurately or arbitrarily closely, in the form (2)" which is

P(a,b)=\int d\lambda p(\lambda) A(a,\lambda) B(b,\lambda)

That means to me that there are NO hidden variables ANYWHERE. So how could *you* argue otherwise? I am sure that in most ways our position is more alike than different.

I agree with your deduction [reference 1 above] that an observation on one side is causally connected to the results on the other. And the reason I believe that has nothing to do with whether QM is local or non-local! I believe that because I believe in the QM formalism and that is what it says is the most complete specification of the system possible. Therefore, I agree with the conclusion (2) of EPR, and I specifically deny (1). That conclusion is 100% in keeping with EPR, Bell and Aspect and I would challenge you to deny that is a logical deduction from the facts (see again III my preceding post for a recap).

So if you want to say that "proves" QM is non-local, then I say fine. If someone else says that conclusion is not part of the formalism of QM, then I say fine to that too. But if you try to tell me that there is simultaneous reality to non-commuting quantum attributes, I say... prove it by experiment. (You can't.)

 

[vanesch]

Yes, that's exactly right. But (and this is becoming something of an anthem on my part..) it's just the same for QM. If you could actually discover somehow what the wave function for some entity next to you was, you would be able to use this information to send messages. Send your friend one of the boxes with "half a particle" in it. The value of the wf over by your friend is 1/sqrt(2). [or something like that... technically I'm talking about the mod of the wf integrated over the volume of the box, but who cares about that detail.] But as soon as you open your box and either find or don't find the particle there, the value of the wf over by your friend will immediately change to either zero or one (respectively).

Ah we're home
You think of "collapse of the wavefunction". Well, let me tell you something: EVERYBODY AGREES that collapse of the wavefunction in this way would be bluntly non-local. So I fully agree with you that such a thing is just as ugly non-local as Bohm ! And it is one of the reasons many people don't like it. ( (There is also another reason that I find even more severe: that is that we don't know what physical process could ever lead to such a collapse)
But in an MWI-like view of QM THERE IS NO SUCH COLLAPSE AT A DISTANCE.
So if Bob "could locally look at your part of the wavefunction" nothing special would happen when Alice "looks at her part of the wavefunction"
And if they see the wavefunction, they wouldn't see any result of a measurement. It is only because of a property of observers that apparently they have to choose a result that they 1) obtain a result and 2) experience some randomness in that result. But the wavefunction itself nicely continues to evolve in all its splendor, whether you have looked or not (well, except for your OWN part of the wavefunction, which gets smoothly entangled, locally, with what you are measuring and of which you have to pick one branch).

That's what I've been trying to tell you.
In the "internal information sense":

Bohm is non-local
Copenhagen QM is non-local
MWI QM is local

In the "external information sense"
Bohm is local
Copenhagen and MWI QM are local

In the "Bell local sense"
Bohm is nonlocal
Copenhagen and MWI QM are non local

cheers,
Patrick.

 

[vanesch]

I dunno, somehow I doubt any of this will help move the conversation forward. Maybe you could remind me/us what exactly you object to in applying Bell Locality to stochastic theories

I do not object to applying Bell locality to stochastic theories, I tell you that it is a criterium *designed* on the basis of deterministic theories, and that stochastic theories that by coincidence obey it, can (that's exactly the MEANING of Bell Locality) simply be turned into deterministic local hidden variable theories, so that ALL the randomness comes about from the lack of knowledge of local variables, which, if we would know them, determine all outcomes with certainty.

Bell Locality is a criterion that says: from *this* theory, it is possible to make a local, deterministic hidden variable theory.

That's why I consider it as a too severe criterion to judge locality on.


cheers,
Patrick.

 

[DrChinese]

Ah we're home
You think of "collapse of the wavefunction". Well, let me tell you something: EVERYBODY AGREES that collapse of the wavefunction in this way would be bluntly non-local. So I fully agree with you that such a thing is just as ugly non-local as Bohm !

cheers,
Patrick.

Well said! (And the same "non-local" collapse happens in single particle experiments too, not just in EPR setups. In EPR setups, we see it more clearly.)

 

[vanesch]

"Let us summarize once again the logic that leads to the impasse. The EPRB correlations are such that the result of the experiment on one side immediately foretells that on the other, whenever the analyzers happen to be parallel. If we do not accept the intervention on one side as a causal influence on the other, we seem obliged to admit that the results on both sides are determined in advance anyway, independently of the intervention on the other side, by signals from the source and by the local magnet setting. But this has implications for non-parallel settings which conflict with those of QM. So we *cannot* dismiss intervention on one side as a causal influence on the other." (pg 149-50 of Speakable...)

Well, again, we've switched vocabulary, but Bell is wrong on this issue. "causal influence" in my book, corresponds to information transfer. It is because he cannot get rid of the old paradigm of deterministic theories in which of course ALL correlations between A and B are related to 1) direct causal influence or 2) indirect "common cause" causal influence.

The reason for this, in a deterministic paradigm, is that we only know of one mechanism to have randomness, namely the lack of information we have of internal degrees of freedom. And then it is clear that upon the observation of correlations between the observed randomness of A and B, you have somehow to "transport these internal degrees of freedom", or, directly from A to B (1), or you had to transport them from a common origin C to A and to B (2). And Bell cannot get out of that view. (1) is non-local, and (2) is Bell Locality. He cannot conceive the possibility that these correlations "just are", and are not related to a lack of knowledge. After all, it has been the paradigm that has been with us for all of classical physics. Einstein was apparently a bit smarter: he believed in this paradigm too, but understood that it could be different (but he didn't want to accept it "God doesn't play dice").

But if you now switch to another paradigm, which is the one of fundamentally stochastic theories, "god does play dice" and the fundamental quantity this time is the probability distribution (the n-point correlation function ; usually in our examples 2-point correlations are sufficient) then you do not need to assume that all randomness is related to lack of knowledge of internal degrees of freedom. And thus you do not need anymore to conclude that correlations can only come about through 1) or through 2). It is by forcing such a fundamentally stochastic theory in the deterministic paradigm that you end up drawing conclusions about locality or about the age of your mother in law.

The deterministic paradigm (randomness only comes about by incomplete knowledge of internal degrees of freedom) comes under many terms:
"complete state" (the internal degrees of freedom), "realism", Bell Locality, ...

cheers,
Patrick.

 

[Cat]

That is a logical flaw. You want it both ways. You refuse to accept it as conclusive evidence when folks stop looking; and you see it as supporting your position when they are looking!

Of course it's not "conclusive evidence" if they stop looking! It just means that they've given up the search for common-sensical explanations for the time being.

From your logic, it makes no sense to repeat an experiment, either! (Presumably that would mean that the experimented does not accept the initial results.) There are a lot of reasons to do experiments, even ones in which the essential results are not in question.

You're right in a way, in that I'm so convinced the world is both real and local that I'll eat my hat if Grangier's team's experiment manages to infringe the CHSH test!

The proposal in question is:

R. García-Patrón Sánchez, J. Fiurácek , N. J. Cerf , J. Wenger , R. Tualle-Brouri , and Ph. Grangier, “Proposal for a Loophole-Free Bell Test Using Homodyne Detection”, Phys. Rev. Lett. 93, 130409 (2004)
http://arxiv.org/abs/quant-ph/0403191

Though the result as it stands is a forgone conclusion, the experiment could, I think, be modified so as to settle the matter of how the detection loophole works once and for all. As it stands, they propose to treat as the + results all voltage differences greater than zero, and - all negative ones. Since there will always (except on a set of measure zero) be some voltage difference they will have no non-detections. [Even if I've misinterpreted their intentions slightly here, they've got the "event-ready" detectors to ensure that they define their sample before analysing the results, so it will be, by definition, "fair".]

But they could instead look at the raw voltages and digitise these, using (as in Aspect's experiments) some minimum threshold voltage to decide what to count. They could then explore what happens as the threshold is altered. When it is zero, we have effectively "perfect" detectors; when very high we have very low efficiency ones and lots of non-detections. What then happens to the CHSH test statistic? Under local realism, this is predicted to increase and eventually infringe the inequality, as per Pearle's 1970 argument.

Cat

 

[ttn]

I am quoting him (EPR): "...[...]"

As I noted here earlier, Einstein did not write the EPR paper. Podolsky did. And Einstein was rather disappointed with how the paper turned out. This is a well-documented historical fact. So you can't quote the EPR paper and assert that this is revealing the views of Einstein. I quoted a passage that was actually written by Einstein in which he states with complete clarity that he thought the point of EPR -- the point that was unfortunately "smothered" in Podolsky's text -- is that there is a *dilemma* between locality and completeness. Both cannot be true for QM. This is hardly an "out of context comment made later." This was rather Einstein's attempt to set the historical record straight given that the author had, in Einstein's own written opinion, flubbed the argument in the EPR paper.

Re: Bell, I suppose we will have to agree to disagree. I simply don't understand how you can claim that what he proved has nothing to do with (i.e., is not openly premised on) locality. The whole point of the theorem is that a theory in which the outcomes of measurements are pre-determined by some kind of "instruction set" in the particles -- AND THAT RESPECTS THE BELL LOCALITY CONDITION -- cannot agree with experiment. i.e., local hidden variable theories are ruled out.

BUT NONLOCAL HIDDEN VARIBLE THEORIES ARE NOT RULED OUT. That is why the existence of Bohmian mechanics doesn't cause the universe to disappear in a puff of logic.

That means to me that there are NO hidden variables ANYWHERE.

Bell was for about 20 years pretty much the only living human being who actively pursued, lectured on, wrote about, and studied Bohmian mechanics. To claim that he believed he had proved that hidden variables theories as such are impossible, is thus rather odd.

I agree with your deduction [reference 1 above] that an observation on one side is causally connected to the results on the other. And the reason I believe that has nothing to do with whether QM is local or non-local!

Then you must be confused about what Bell proved. Bell's theorem shows that, if you try to "complete" QM by adding local hidden variables, the theory you get cannot both respect the Bell Locality condition and agree with experiment. So, as lots of people say, if you want a local theory, you'd better stick with QM and its completeness doctrine, and not go down the hidden variables road. But that strategy obviously presupposes that QM itself is local -- otherwise, saying "you should stick with QM and not pursue hidden variable theories, on pain of nonlocality" just makes no sense.

And the final piece: Bell states openly that, he thinks, nonlocality is a fact, period -- that it's *not* something which merely afflicts hv theories. As he says, you *cannot* dismiss the operations on one side as causal influences on the other. How can he believe this? What else would he need to have to believe to make this claim given the above paragraph? Obviously he would have to think that orthodox QM was *also* nonlocal. IF it wasn't, there'd be no grounds for claiming that all possible alternatives -- i.e., nature -- were nonlocal.

I believe that because I believe in the QM formalism and that is what it says is the most complete specification of the system possible.

Perhaps you could explain why you believe the completeness doctrine. Bell's theorem is no argument in its favor, since QM itself is just as nonlocal as the hidden variable theories you'd want to dismiss on Bellian grounds. And if anything has come out very clearly in this thread, it's that Bohmian mechanics (a real honest to god hidden variable theory that reproduces all the predictions of QM) and regular QM are exactly parallel when it comes to their various senses of locality and nonlocality. So how could there possibly be a conclusive argument in favor of the completeness doctrine? I have never heard one, but I would certainly like to if it exists.

 

[vanesch]

BUT NONLOCAL HIDDEN VARIBLE THEORIES ARE NOT RULED OUT. That is why the existence of Bohmian mechanics doesn't cause the universe to disappear in a puff of logic.

It is indeed trivial to associate to every stochastic theory a deterministic nonlocal hidden variable theory, which gives you exactly the same outcome.
A stochastic theory is fully determined when the n-point correlation functions are given as a function of all the parameters of free choice that are given to the user(s), such as the choice of the polarizer, activating or not a laser etc...
It is of course possible to set up a hidden variable theory with as random variable an n-tuple of numbers which has the same n-point correlation function as a function of the parameters, call these random variables "hidden variables" which determine the outcomes in a strict 1-1 way and I'm done.

So there's no point in saying that such a theory is possible. It is always possible.

What is more interesting is to do what Bell did: to prove that a certain stochastic theory (in casu QM) predicts probabilities that cannot be generated by deterministic local hidden variable theories, where local means local in the internal information sense, which, together with the deterministic part, leads to the Bell locality condition ; which leads to the Bell inequalities.

So the choice is between respect of the internal information locality or determinism.

Given the fact that a theory like QM is on the outside information-local, I prefer to sacrifice determinism, because I would consider sacrificing internal information-locality as a kind of conspiracy (why does the internal machinery not respect it, but does the outside user not notice it ?).

cheers,
Patrick.

EDIT: just to repeat: what I mean by "information-local" is that the probability distribution of all quantities pertaining to something local at A cannot depend on all FREE CHOICE parameters which are fixed elsewhere at B. If it were, I have an information channel that allows me, by doing experiments at A, to know what was the message, sent by B (by making use of his free choice of parameters).
What I mean by "external information-local" is what I said above, with real, executable measurements. What I mean by "internal information-local" applies moreover to a super-creature that has access to all local hidden variables at A even if some strange principle forbids me to turn them into real experiments.
But the free choice of settings at B remains fundamental.

I repeat again that it is *this* locality which is required by relativity in order to avoid the paradox of receiving as a message, what will be my free choice later (so that I can make another choice and lead to a paradox). This is the reason why I stick to it. If it weren't for this property, I wouldn't give a damn about locality.

 

[ttn]

Ah we're home

Yes, I think so! How refreshing to have a high-level, rather heated discussion about an important and controversial issue, that actually ends with mutual understanding and agreement! Practically unprescedented!

You think of "collapse of the wavefunction". Well, let me tell you something: EVERYBODY AGREES that collapse of the wavefunction in this way would be bluntly non-local. So I fully agree with you that such a thing is just as ugly non-local as Bohm ! And it is one of the reasons many people don't like it. ( (There is also another reason that I find even more severe: that is that we don't know what physical process could ever lead to such a collapse)
But in an MWI-like view of QM THERE IS NO SUCH COLLAPSE AT A DISTANCE.
So if Bob "could locally look at your part of the wavefunction" nothing special would happen when Alice "looks at her part of the wavefunction"
And if they see the wavefunction, they wouldn't see any result of a measurement. It is only because of a property of observers that apparently they have to choose a result that they 1) obtain a result and 2) experience some randomness in that result. But the wavefunction itself nicely continues to evolve in all its splendor, whether you have looked or not (well, except for your OWN part of the wavefunction, which gets smoothly entangled, locally, with what you are measuring and of which you have to pick one branch).

I pretty much agree with all this. I think there are some difficult questions for the MWI type view regarding what exactly it would even mean to talk about "if they see the wavefunction..." Terms like "they" and "the wavefunction" start to get slippery when there are a bunch of parallel universe copies of everything (and a still-not-very-clear way of telling the difference between two distinct branches and one branch with a superposition in it). But it seems clear to me, and I'm happy to grant, that in some sense (i.e., in some way that perhaps still requires some polishing around the edges) MWI is able to maintain locality.

In the "internal information sense":

Bohm is non-local
Copenhagen QM is non-local
MWI QM is local

In the "external information sense"
Bohm is local
Copenhagen and MWI QM are local

In the "Bell local sense"
Bohm is nonlocal
Copenhagen and MWI QM are non local

Yes, exactly. I wish I had more to say, but you encapsulated it beautifully. So I'll just copy your statements for everyone to look at one more time.



cheers,
Patrick.[/QUOTE]

 

[ttn]

I do not object to applying Bell locality to stochastic theories, I tell you that it is a criterium *designed* on the basis of deterministic theories, and that stochastic theories that by coincidence obey it, can (that's exactly the MEANING of Bell Locality) simply be turned into deterministic local hidden variable theories, so that ALL the randomness comes about from the lack of knowledge of local variables, which, if we would know them, determine all outcomes with certainty.

Bell Locality is a criterion that says: from *this* theory, it is possible to make a local, deterministic hidden variable theory.

That's why I consider it as a too severe criterion to judge locality on.

That is very clarifying, thank you. I now understand what you've been saying.

I'm still not certain that this point is true, however. I think you are making a rather nontrivial claim here -- that any stochastic theory that satisfies Bell Locality can be "built" out of an underlying deterministic local theory. If true, I think that is a very interesting point. Do you have any sense of how/whether this could be rigorously proved? Or maybe (since I'm just now getting your point for the first time and haven't really thought about it a lot yet) it's more trivial than I am sensing, i.e., nothing so fancy as a formal proof is really needed. Thoughts?

 

[ttn]

Well, again, we've switched vocabulary, but Bell is wrong on this issue. "causal influence" in my book, corresponds to information transfer.

It's not that Bell is wrong, but merely that you prefer different terminology. When Bell says "causal influence" in that quote, he is not talking about (what he might call) "mere information transfer" but about Bell Locality -- which he regarded as a good test for causal influences (even ones that couldn't be used to build telephones).

It is because he cannot get rid of the old paradigm of deterministic theories in which of course ALL correlations between A and B are related to 1) direct causal influence or 2) indirect "common cause" causal influence.

I would say (and I suspect Bell would say the same thing, but who really knows) that the requirement for (persistent, lawlike) correlations to involve either direct causal connection or a common cause has nothing to do with determinism per se. Non-deterministic theories can still support causal connections and common causes, and Bell would be perfectly happy if one of these ended up being true. The issue, Bell says, is *local causality*, not determinism.

If your claim from the other post is true -- if any such Bell Local non-deterministic theory can be straightforwardly converted into a deterministic theory -- that would be an interesting twist here. But I'm not sure it would change anything -- Bell could still quite reasonably claim merely to be forbidding nonlocal causation, and if the only way nature can figure out to respect local causality is with deterministic theories, too bad for nature!

The reason for this, in a deterministic paradigm, is that we only know of one mechanism to have randomness, namely the lack of information we have of internal degrees of freedom. And then it is clear that upon the observation of correlations between the observed randomness of A and B, you have somehow to "transport these internal degrees of freedom", or, directly from A to B (1), or you had to transport them from a common origin C to A and to B (2). And Bell cannot get out of that view. (1) is non-local, and (2) is Bell Locality. He cannot conceive the possibility that these correlations "just are", and are not related to a lack of knowledge.

I don't think that's right, at least as a statement of Bell's motivation. What made him uncomfortable (as you say "he cannot conceive") was that the correlations couldn't be explained without requiring that events *here* depend on events *over there* in a way that can't be screened out by a common cause in the shared past. On its face, that is a requirement for *locality*, not determinism. If it turns out that only deterministic theories satisfy this locality requirement, I see that as in some sense an accident. Bell's primary intention was to require *local causality*, period.

But if you now switch to another paradigm, which is the one of fundamentally stochastic theories, "god does play dice" and the fundamental quantity this time is the probability distribution (the n-point correlation function ; usually in our examples 2-point correlations are sufficient) then you do not need to assume that all randomness is related to lack of knowledge of internal degrees of freedom. And thus you do not need anymore to conclude that correlations can only come about through 1) or through 2). It is by forcing such a fundamentally stochastic theory in the deterministic paradigm that you end up drawing conclusions about locality or about the age of your mother in law.

I don't think Bell had any strong objections to god playing dice. He just figured, given the empirical success of relativity theory, if god is going to be playing dice he evidently better roll separate dice independently at different locations -- the outcomes of his dice rolls shouldn't depend on, e.g., the color of the dice rolled, or the outcome, at space-like separated dice-rolling events.

I think this is a prima facie reasonable thing to impose on theories (stochastic or otherwise) if you take relativity's prohibition on superluminal causation seriously. It is not simply a way of "smuggling in" some pre-existing bias for determinism.

 

[DrChinese]

1. As I noted here earlier, Einstein did not write the EPR paper. Podolsky did. ...

2. Re: Bell, I suppose we will have to agree to disagree. I simply don't understand how you can claim that what he proved has nothing to do with (i.e., is not openly premised on) locality. The whole point of the theorem is that a theory in which the outcomes of measurements are pre-determined by some kind of "instruction set" in the particles -- AND THAT RESPECTS THE BELL LOCALITY CONDITION -- cannot agree with experiment. i.e., local hidden variable theories are ruled out.

3. BUT NONLOCAL HIDDEN VARIBLE THEORIES ARE NOT RULED OUT. That is why the existence of Bohmian mechanics doesn't cause the universe to disappear in a puff of logic.

4. Perhaps you could explain why you believe the completeness doctrine. Bell's theorem is no argument in its favor, since QM itself is just as nonlocal as the hidden variable theories you'd want to dismiss on Bellian grounds. And if anything has come out very clearly in this thread, it's that Bohmian mechanics (a real honest to god hidden variable theory that reproduces all the predictions of QM) and regular QM are exactly parallel when it comes to their various senses of locality and nonlocality. So how could there possibly be a conclusive argument in favor of the completeness doctrine? I have never heard one, but I would certainly like to if it exists.

1. Einstein's name is on the paper and it is generally accepted in the literature. What else do I need to say? I provided the quotes and they speak for themselves. For anyone who wants to read the original paper, and see for themselves whether the formalism is about locality or reality: EPR, Bell and Aspect: The original references.

2. The entire question I am raising is: is there any evidence that there is independent reality to observations not actually made? (That would exactly correspond to EPR's (2) which is the non-communting operators) And the answer is NO, there is no such evidence whatsoever.

3. The question I have is: does BM state that there is independent reality of unmeasured observables? I don't think it says this, but perhaps we could discuss this in more detail. As I understand BM (which is woefully little), it specifies a mechanism by which the non-local effects can occur.

4. I simply believe the completeness doctrine because of the wall that has been bumped up against in attempting to locate a more complete specification of the system. I also believe that electrons have no internal structure. Do you think that is unjustified too?

I believe that theories which are indistinguishable in their predictions from other theories are "ad hoc" and therefore of no utility. Should that change, I would happily reconsider my position.

 

[vanesch]

Do you have any sense of how/whether this could be rigorously proved? Or maybe (since I'm just now getting your point for the first time and haven't really thought about it a lot yet) it's more trivial than I am sensing, i.e., nothing so fancy as a formal proof is really needed. Thoughts?

Let us give it a try. We assume Bell Locality, so:

Let "L" be the state in a stochastic theory describing the stochastic variables A and B such that we have:

P(( A,B) = (A1,B2) ;a,b,L) = P(A=A1;a,L) x P(B=B1; b,L)

So there are two functions p1(A1,a,L) giving the first factor and p2(A2,b,L) giving the second factor.
We will, for the sake of argument, assume that a and b can only take on a finite number of values, so a can be a1, a2, a3, a4... a26 and b can be b1, b2, b3, ... b87.

Let me now introduce two extra "hidden variable tuples" u(a) and v(b) which have an independent probability distribution given as follows:
P(u(a) = A1) = p1(A1,a,L)
and P(v(b) = B1) = p2(B1,b,L)

Note that u(a) stands for 26 different real random variables, and v(b) stands for 87 different real random variables, and that each individual component within a tupel is an independent random variable.

Next, let K be the state in a deterministic theory which is everything in L, plus u(a) and v(b), seen as extra hidden variables.
We haven't talked about the probability distribution of L, because we are taking each individual case of L separately. So we should now consider each individual case of u(a) and v(b).

We next define a new "probability law" for our new theory K. For a given complete state description K, (an L, plus a specific value of u(a) and v(b)) we define new probabilities:
P(A = A1 ; a, K) = 1 if u(a) == A1 and = 0 if not
P(B = B1 ; b, K) = 1 if v(b) == B1 and = 0 if not

Note that these probabilities are still "local": the right hand side, in the first case, only depends on a, A1 and K and in the second case only on b, B1 and K.

P( (A,B) = (A1,B1) ; a, b, K) = P(A = A1 ; a, K) x P(B = B1 ; b, K)

We impose Bell Locality here.

Now, if we lack knowledge of the value of u(a) and v(b), you can easily find out that if u(a) and v(b) are drawn according to the distributions we specified for them, the ensemble probabilities will fall back on those given by the stochastic "L" theory. So u(a) and v(b) in theory K, as hidden variables, with the given distributions, generate the stochastic theory L.
We also see that by definition, Bell Locality is satisfied.

And we see that all probabilities in K theory are 1 or 0, hence it is a deterministic theory.

Hence, Patrick's theorem: "A stochastic theory satisfying Bell Locality is equivalent to a deterministic hidden variable theory satisfying Bell Locality".

Hey, I didn't know the proof was that easy, I just felt it in my bones that it had to be that way

cheers,
Patrick.

 

[ttn]

So there's no point in saying that such a theory is possible. It is always possible.

This may be true, but even if it is always possible, I think one shouldn't minimize Bohm's achievement in actually creating such a thing. For one thing, people had allegedly (!) proved that this *wasn't* possible for QM, so Bohm deserves credit for (a) trying and (b) showing by construction of counterexample that the proofs were bogus. In addition, even if it is possible in principle always to construct such a theory, it seems unlikely that the theory so constructed would turn out to be so natural. Bohmian mechanics is. My point is just that, as a particular example of a way to fill out a nonlocal stochastic theory with an underlying deterministic nonlocal dynamics, Bohmian mechanics is far more *interesting* than the tone of your comment suggests.

What is more interesting is to do what Bell did: to prove that a certain stochastic theory (in casu QM) predicts probabilities that cannot be generated by deterministic local hidden variable theories, where local means local in the internal information sense, which, together with the deterministic part, leads to the Bell locality condition ; which leads to the Bell inequalities.

So the choice is between respect of the internal information locality or determinism.

This starts to sound suspiciously like the inconsistency I thought we agreed was bad. Sure, it's nice to know that the QM predictions can't be generated by a deterministic (or stochastic!) local hidden variable theory (w/ "local" = "internal info sense of local"). But to cast the resulting choice as between "respect of the internal info locality or determinism" is to imply that QM itself respects "internal info locality". But as we agreed previously, it doesn't. So (leaving aside MWI) one is forced to accept that viable theories cannot respect "internal info locality". There is no choice of interest there. There is also no choice of interest w.r.t. "signal locality" -- no theory on the table allows superluminal communication.

There *is* a genuine choice between stochastic and deterministic, e.g., between orthodox QM and Bohmian mechanics. But it is a choice without a price -- that is, it's not like choosing Bohmian mechanics means you have to give anything up.

So I truly don't understand why you would say that the choice is between "internal info locality" and "determinism".


Oh yeah, a couple of comments on why I said above that Bell's Theorem rules out both deterministic *and stochastic* hv theories that obey Bell Locality. (I still can't tell for sure if you disagree with this??) Leaving aside our discussion of whether Bell Locality is the appropriate way to impose "local causality" on a theory, and just taking for granted for the sake of this point that it is, I think it is clear that Bell's inequality applies to Bell-Local-Stochastic theories just as much as it applies to Bell-Local-Deterministic theories. After all, the whole derivation is in terms of probabilities like P(A|a,b,B,L), etc. In a deterministic theory, all these P's would be either zero or one (since we are conditionalizing on "L"). But this assumption is never made in the derivations of the inequality. That is, the inequality still holds even if the P's are just regular old probabilities, i.e., if the theory is genuinely stochastic (but still Bell Local). So there you go. Of course, you have claimed that any genuinely stochastic Bell Local theory can be trivially filled out by an underlying deterministic dynamics. Perhaps; I'm not convinced, but maybe that's true. But any way, regardless, Bell's Theorem as stated does surely apply to Bell-Local stochastic theories. So no such theory is empirically viable, given Aspect et al. So it is terribly misleading to suggest that the choice we face post-Bell is between (a) deterministic nonlocal theories and (b) stochastic local theories. That kind of statement would make Bell roll over in his grave!

Given the fact that a theory like QM is on the outside information-local, I prefer to sacrifice determinism, because I would consider sacrificing internal information-locality as a kind of conspiracy (why does the internal machinery not respect it, but does the outside user not notice it ?).

We've been here before. QM *also* suffers from this kind of "conspiracy" -- it is nonlocal in the Bell or "internal info" sense, but local "on the outside". It is exactly parallel to Bohmian mechanics on both counts. So why talk of "preferring to sacrifice determinism"? Nothing -- literally nothing -- is *saved* by making this sacrifice. That doesn't necessarily prove you ought to choose the deterministic theory, but surely it shows that there's no *reason* for rejecting the deterministic theory. And as I've said about a bajillion times now, that all I really want to argue for here.

 

[vanesch]

This may be true, but even if it is always possible, I think one shouldn't minimize Bohm's achievement in actually creating such a thing. For one thing, people had allegedly (!) proved that this *wasn't* possible for QM, so Bohm deserves credit for (a) trying and (b) showing by construction of counterexample that the proofs were bogus. In addition, even if it is possible in principle always to construct such a theory, it seems unlikely that the theory so constructed would turn out to be so natural. Bohmian mechanics is. My point is just that, as a particular example of a way to fill out a nonlocal stochastic theory with an underlying deterministic nonlocal dynamics, Bohmian mechanics is far more *interesting* than the tone of your comment suggests.

I hope you understood that I am of the opinion that the only viable ways to view QM are:
1) as purely a generator of probabilities, and we shouldn't attach any physical meaning to the formalism (I'm not in favor of that because it brings your physical intuition to a grinding halt, but I have to admit it is a logical possibility)
2) an MWI like view which I favor.

I agree with you that Copenhagen QM is an ugly theory, which is not only ridden with a lot of internal inconsistencies, but is also bluntly non-local in its mechanism, except of course in its probability predictions.

I think in the building of a theory, one should more stick to general principles than to any other criterion. One such principle is information-locality ; it is the essential principle of SR combined with causality. Another one is the superposition principle ; it is the essential principle of QM. No great principle demands for determinism and it turns out that the first two make determinism impossible.

So we have a paradigm which is build upon information-locality and the superposition principle, and which will turn out to be essentially stochastic.

Within that paradigm, we try to set up a specific theory, and we do now what we want, but we do not infringe on the principles of the paradigm we are working in. So no bricolage in the internal mechanism of a theory that infringes on the principles we've set forth, even if we think of extra stuff to protect us from detecting it (such as *hidden* variables).

Copenhagen QM is bricolage of course, _except_ if we do not consider it as a theory in which the formalism corresponds to anything physical, but just as a generator of probabilities, in which case you don't have to take the collapse of the wavefunction seriously: it is just a mathematical trick to generate probability functions. There are so many things wrong with taking Copenhagen QM as a description of any reality that infringing on information locality in its internal workings is only one defect. It also infringes on the superposition principle ! So it does everything wrong if you take the wavefunction description as something "real". But it works just fine if you consider it as a tool that cranks out probability distributions.

Bohm is just as well bricolage because it wants to introduce (hidden ) determinism, but sacrifices one of the great principles, namely information locality, in its internal workings. It doesn't even consider the superposition principle. But it works just fine if you consider it as a tool that cranks out probability distributions.

However, MWI-like QM DO respect information locality and the superposition principle. That's why I think it is the natural view on quantum theory. It contains fundamentally stochastic elements (namely the imposed choices of the branch of the observer), but it sticks to the basic philosophy of the paradigm laid out.

This starts to sound suspiciously like the inconsistency I thought we agreed was bad. Sure, it's nice to know that the QM predictions can't be generated by a deterministic (or stochastic!) local hidden variable theory (w/ "local" = "internal info sense of local"). But to cast the resulting choice as between "respect of the internal info locality or determinism" is to imply that QM itself respects "internal info locality". But as we agreed previously, it doesn't. So (leaving aside MWI) one is forced to accept that viable theories cannot respect "internal info locality". There is no choice of interest there. There is also no choice of interest w.r.t. "signal locality" -- no theory on the table allows superluminal communication.

But why do you leave the only natural option, namely MWI, aside ?

EDIT:
Quantum theory seen as a stochastic theory of which we do not add any reality to the formalism, but just a calculational trick to let us obtain probabilities of outcomes of measurements, respects information locality. That is "external information locality" because there is no internal mechanism postulated so there's nothing to test "internal information locality" against.

Quantum theory, seen in an MWI formulation, where one DOES give a kind of reality to the wavefunction, also respects internal information locality.

The thing that does not, is when one assumes that measurements "collapse the wavefunction" and that this is some kind of physical phenomenon. In _that_ case, this internal mechanism doesn't respect "internal information locality". That's why one shouldn't do it.

There *is* a genuine choice between stochastic and deterministic, e.g., between orthodox QM and Bohmian mechanics. But it is a choice without a price -- that is, it's not like choosing Bohmian mechanics means you have to give anything up.

With Bohmian mechanics you construct a hybrid. You want determinism and then you have to hide it. So, determinism, IS it, or ISN'T it a fundamental principle on which you want to build your theory ? If it is, I don't know why we have to hide it, and if it isn't, I don't know why you try to put it inside.
Only, you HAVE to hide the determinism, because otherwise you CAN do FTL signalling. But what does it mean, hidden determinism ?
I'm not going to comment orthodox QM, I already told you it is just as ugly, EXCEPT as a generator of statistics. It is then on the same level as Bohm, and I don't see why you should even consider Bohm, given that you don't win anything (but I agree with you that you don't loose anything either: you've anyway lost everything else already in Copenhagen QM !). The two are viewed as two calculational procedures to arrive at the only physical quantities of interest: probabilities of measurement outcomes. Maybe some calculations are easier in Bohm's formulation than in the Hilbert state space formulation ; but I doubt that.

So I truly don't understand why you would say that the choice is between "internal info locality" and "determinism".

Well, because I think that information locality is one of the pillars of SR and QM. Determinism isn't. So if there is one thing I would like to stick to, it is information locality, and I can construct a quantum theory that respects that and does have a kind of description of reality (MWI approach) or one that just gives you probabilities (abstract probability calculation, with the calculational technique of your choice: Hilbert or Bohm, whichever leads to the result in the smallest amount of CPU time).

Oh yeah, a couple of comments on why I said above that Bell's Theorem rules out both deterministic *and stochastic* hv theories that obey Bell Locality. (I still can't tell for sure if you disagree with this??)

Sure, I agree with it.

Leaving aside our discussion of whether Bell Locality is the appropriate way to impose "local causality" on a theory, and just taking for granted for the sake of this point that it is, I think it is clear that Bell's inequality applies to Bell-Local-Stochastic theories just as much as it applies to Bell-Local-Deterministic theories. After all, the whole derivation is in terms of probabilities like P(A|a,b,B,L), etc. In a deterministic theory, all these P's would be either zero or one (since we are conditionalizing on "L"). But this assumption is never made in the derivations of the inequality. That is, the inequality still holds even if the P's are just regular old probabilities, i.e., if the theory is genuinely stochastic (but still Bell Local). So there you go. Of course, you have claimed that any genuinely stochastic Bell Local theory can be trivially filled out by an underlying deterministic dynamics. Perhaps; I'm not convinced, but maybe that's true. But any way, regardless, Bell's Theorem as stated does surely apply to Bell-Local stochastic theories. So no such theory is empirically viable, given Aspect et al. So it is terribly misleading to suggest that the choice we face post-Bell is between (a) deterministic nonlocal theories and (b) stochastic local theories. That kind of statement would make Bell roll over in his grave!

I agree with most of what you say, but I consider it not the right criterion, and I reiterate my claim that Bell's theorem is based upon Bell Locality, which is a condition that is INSPIRED by deterministic thinking, namely that in order to have a correlation, you need a direct causal influence, or an indirect "common cause" influence. This by itself comes from the fact that we consider that the ONLY randomness we're willing to accept is lack of knowledge of internal variables.
Part of my proof you already have: a stochastic theory satisfying Bell Locality is replacable by a deterministic theory satisfying Bell Locality.
So you can read this that we only allow for stochastic theories which can be deterministically explained by lack of knowledge of the complete state.

This is a reformulation that Bell Locality is the requirement that the only form of randomness allowed in a theory, is through incomplete knowledge of some parameters in a state description, which, if we would know them, would determine every outcome with certainty.

We've been here before. QM *also* suffers from this kind of "conspiracy" -- it is nonlocal in the Bell or "internal info" sense, but local "on the outside". It is exactly parallel to Bohmian mechanics on both counts. So why talk of "preferring to sacrifice determinism"? Nothing -- literally nothing -- is *saved* by making this sacrifice. That doesn't necessarily prove you ought to choose the deterministic theory, but surely it shows that there's no *reason* for rejecting the deterministic theory. And as I've said about a bajillion times now, that all I really want to argue for here.

That's because you're comparing to Copenhagen QM. But that's indeed a very ugly theory. Try to compare to MWI like QM. You'll be delighted

cheers,
Patrick.

 

[DrChinese]

Re: Bell, I suppose we will have to agree to disagree. I simply don't understand how you can claim that what he proved has nothing to do with (i.e., is not openly premised on) locality. The whole point of the theorem is that a theory in which the outcomes of measurements are pre-determined by some kind of "instruction set" in the particles -- AND THAT RESPECTS THE BELL LOCALITY CONDITION -- cannot agree with experiment. i.e., local hidden variable theories are ruled out.

BUT NONLOCAL HIDDEN VARIBLE THEORIES ARE NOT RULED OUT. That is why the existence of Bohmian mechanics doesn't cause the universe to disappear in a puff of logic.

...

And the final piece: Bell states openly that, he thinks, nonlocality is a fact, period -- that it's *not* something which merely afflicts hv theories. As he says, you *cannot* dismiss the operations on one side as causal influences on the other. How can he believe this? What else would he need to have to believe to make this claim given the above paragraph? Obviously he would have to think that orthodox QM was *also* nonlocal. IF it wasn't, there'd be no grounds for claiming that all possible alternatives -- i.e., nature -- were nonlocal.

OK, I think I can come pretty close to a position that we can see eye to eye on. Repeating a portion of an argument made in an earlier post and adding a bit:

I. EPR proves: "...either (1) the quantum-mechanical description given by the wave function is not complete or (2) when the operators corresponding to two physical quantities do not commute the two quantities cannot have simultaneous reality".

II. Bell proves both cannot be true: (1) QM is incomplete (as represented by the \lambda in his formulas; and (2) the predictions of QM are correct. To quote: "The paradox of Einstein, Podolsky and Rosen was advanced as an argument that quantum mechanics was not complete but should be supplemented by additional parameters... In this note that idea will be formulated mathematically and shown to be incompatible with the statistical predictions of quantum mechanics."

III. Accepting both EPR and Bell as correct (as I do), as well as Aspect, you must conclude that:

a) Aspect et al proves that the predictions of QM are correct (please Cat stay out of this discussion as we are not interested in debating this).
b) If QM is correct, then Bell (2) is true; therefore Bell (1) is false.
c) If Bell (1) is false, then EPR (1) is also false as they are equivalent by design.
d) If EPR (1) is false, then EPR (2) is true.

I.e. my position, that "when the operators corresponding to two physical quantities do not commute the two quantities cannot have simultaneous reality".

IV. EPR also said that if relativistic causality (what maps, I believe, to your Bell locality condition) is assumed, then EPR (2) is false. (I don't see this stated too well in the paper itself, so I will see if I can locate some additional material which ties this loose end up as well as provide a quote.)

Bell locality -> Reality (of non-commuting observables)

If we agree that is a valid deduction of EPR, then the contranegative is also true:

~Reality -> ~Bell locality

Since we know from III.d. above that EPR (2) is in fact true (~Reality), then we deduce that relativistic causality cannot be true. Ergo, by your definition, non-locality is demonstrated.

 

[vanesch]

I would say (and I suspect Bell would say the same thing, but who really knows) that the requirement for (persistent, lawlike) correlations to involve either direct causal connection or a common cause has nothing to do with determinism per se. Non-deterministic theories can still support causal connections and common causes, and Bell would be perfectly happy if one of these ended up being true. The issue, Bell says, is *local causality*, not determinism.

Let us elaborate a bit because it is the essential point in our differences in viewpoint I think.

My claim is that causality only has a meaning as "information transfer". This can be "internal information transfer" also, even if we cannot perform real experiments in the lab because the internal quantity we're talking about is not directly accessible (such as a hidden variable) ; but one thing is necessary to be able to send information, and that is making free choices at the sending end. Upon my decision of acting at A, if something happens at B or not determines if there is information transfer and hence a causal link.
Some semantics: my "choice at A" _causes_ "an effect at B". In order to cause something, I have to have a choice in causing it, otherwise I just see it as a "description of what is happening" and not of "what causes what".
Let us call this view on causality "information - causality".
From "information - causality" follows then naturally "information - locality".
I told you why I think that is the right definition, it comes from a paradox you can obtain in SR if you don't stick to it.

You could also define a "correlation causality" and it leads to "Bell Locality".
"Correlation causality" states that you can only have statistical correlations if there is a direct dependence of the result at A on the result at B (in a statistical sense) or if they have both a common origin (state L). Bell Locality is the mathematical expression of this causality if we assume that the direct influence cannot take place ("locality"), that the only link between the two factors is through L (common cause).

But I don't see any requirement in special relativity to require Bell locality.

I will now try to find the link between "information locality" (required by SR) and Bell Locality (required by, eh, what ? We'll see :-).

My second claim is that "Bell Locality" is the above notion, applied to an underlying deterministic model ; that the notion that a "correlation implies a direct causal link or an indirect common cause link" finds its origin in a deterministic underlying model.
I think it is THIS point which is hard to get by, because THIS is the real paradigm shift needed to let go determinism. And I think it was this paradigm shift that Bell couldn't conceive, namely that you could have correlations which were NOT implying a direct causal link or an indirect common cause link.
I don't know what I can do more than reiterate Patrick's theorem
"Any stochastic theory satisfying Bell locality leads to a deterministic theory satisfying Bell locality".
I think it is a small step to show:
"From Bell locality follows information locality."

Indeed, the factorized form of P(A,B ; a,b) = P(A ; a) x P(B ; b) means that the choice of a cannot influence the probability of B.


I guess what I still should try to prove is that from information-local determinism follows Bell locality.

So now we have, from determinism, that P(A,B ; a,b,K) equals 1 or 0 ; so do the individual probabilities P(A ; a, b, K) and P(B ; a, b, K) ;
and from information locality follows that P(A ; a, K) and P(B ; b, K) do not depend on the "other" choices b and a respectively.

This means, in fact, that A = A(a) and B = B(b): for a given value (choice) of a, there is ONE A value that is the outcome, with certainty ; all other A values have probability 0.

So P(A(a), B(b) ; a,b,K) = 1 = P(A(a) ; a,K) x P(B(b) ; b,K)

So at least for the P=1 values, we can write the product form.
But this is also true for the P=0 values, because if A1 != A(a) OR B1 != B(b), then P(A1, B(b) ; a,b,K) = 0 = P(A1 ; a,K) x P(B(b) ; b, K) (namely 0 x 1)
and idem for the two other cases A(a), B1 and A1,B1.

So we have that determinism and information-locality leads to Bell locality.
So I came to a full circle:

(1)From Bell locality follows Bell local determinism. (Patrick's theorem)
(2) From Bell locality follows information locality
(3) From information locality and determinism follows Bell locality

Together:

BELL LOCALITY <===> information locality and determinism

QED

cheers,
Patrick.

 

[ttn]

Hey, I didn't know the proof was that easy, I just felt it in my bones that it had to be that way

Very good! I jusk skimmed the proof, but it looks like what you were saying was pretty trivial, now that I understand better what it actually meant. Basically, so long as a stochastic theory respects Bell-Locality, you can always just postulate new hidden variables which determine the outcomes: just pick the probability distribution over the outcome-determining hv's to match whatever the original stochastic theory said the probabilities were. Duh!

I still want to say this doesn't change anything, though. This argument shows that all Bell Local theories either are or can be made into deterministic theories. As you have made clear elsewhere, you think that part of what it means to really jettison the deterministic paradigm and accept deep, irreducible stochasticity in a theory, is to accept not only that specific local outcomes can't be explained by any previous state of the world but that, more generally, correlations between separated events also can't be explained by any previous state of the world. Something like: if you're going to accept outcomes just "popping" into existence, you should also accept *correlations* just popping into existence. Is that a fair statement of your position? (If so, it exactly matches what I remember Arthur Fine arguing in "Do Correlations Need to be Explained?", the article I mentioned a while back... Maybe you'd enjoy checking it out.)

I don't have any particularly clean argument against this; I concede it's an internally consistent position. But I still don't see how one can fail to be bothered by unexplained, lawlike correlations between distant events. You can talk all you want about god rolling dice, but as I said earlier, I don't think relativity theory ought to permit god to, in effect, roll the same dice simultaneously at two distant locations. Of course, he's god, so he can do whatever he wants -- but if he does this, I think we have to call a spade a spade and say that god is exerting a nonlocal (stochastic) causality, even if the nonlocality is "well hidden" or "washed out" or "behind the scenes" or "not useful for sending messages" or whatever.

But probably there is nothing else interesting to say about this. What matters is what we agreed on before: Copenhagen and Bohm are both non-local in the various stricter senses (Bell-Locality or "internal info locality") and both local in terms of "external info transfer". And to the extent that MWI makes sense and/or is well-defined it is probably more local than either of those other two alternatives.

 

[ttn]

I hope you understood that I am of the opinion that the only viable ways to view QM are:
1) as purely a generator of probabilities, and we shouldn't attach any physical meaning to the formalism (I'm not in favor of that because it brings your physical intuition to a grinding halt, but I have to admit it is a logical possibility)
2) an MWI like view which I favor.

I agree with you that Copenhagen QM is an ugly theory, which is not only ridden with a lot of internal inconsistencies, but is also bluntly non-local in its mechanism, except of course in its probability predictions.

and then later

Bohm is just as well bricolage because it wants to introduce (hidden ) determinism, but sacrifices one of the great principles, namely information locality, in its internal workings. It doesn't even consider the superposition principle. But it works just fine if you consider it as a tool that cranks out probability distributions.

I'm not sure what you mean by saying Bohm "doesn't even consider the superposition principle". Wave functions in Bohmian mechanics are solutions of Schroedinger's equation (or whatever), just like orthodox QM. Bohm doesn't forbid or jettison superpositions!

Also, a subtle equivocation has I think snuck into your comment here. You talk about "information locality" being one of the cornerstones of SR and hence a guiding principle for building theories/interpretations. From your comments elsewhere it is clear that what you actually mean by that is what you once clarified as "external info locality", i.e., no transmission of information superluminally. But when you criticize Bohm for sacrificing "information locality in its internal workings" this either equivocates on "info local" or just plain doesn't make sense. Bohm's theory is local in the "external info" sense but nonlocal in the "internal info" sense. Just like regular QM on both counts.

I think you understand this perfectly well, but a lot of people are terribly confused, so let me repeat it for the benefit of others who are reading. If what you mean by "local" is "information cannot be transferred superluminally" then orthodox QM and Bohmian mechanics are equally local. If, on the other hand, what you mean is "the internal guts of the theory obey Bell's local causality constraint" then both orthodox QM and Bohmian mechanics are nonlocal. And keeping that straight will, I think, help prevent dubious statements like "Unlike regular QM, Bohm's theory is nonlocal, which puts it in conflict with SR, which means we shouldn't really take it seriously." That argument just doesn't hold water unless you equivocate like mad about the meaning of "local" -- i.e., unless you cheat!

But why do you leave the only natural option, namely MWI, aside ?

As I think I said a long time ago in this thread, I just think MWI is too crazy to take seriously. You talk about faithfully respecting certain principles (superposition, relativity, ...) to guide one in interpreting or building theories. Well, one of the principles that is to me even more fundamental than the ones you mention is scientific realism. I just can't take seriously something claiming to be a theory of physics that is in fact a form of solipsism. I know others disagree, which is why I prefer to just leave that issue aside and focus on, say, Bohm vs. QM, which we can speak about without it turning into a pointless debate about philosophy.

With Bohmian mechanics you construct a hybrid. You want determinism and then you have to hide it. So, determinism, IS it, or ISN'T it a fundamental principle on which you want to build your theory ? If it is, I don't know why we have to hide it, and if it isn't, I don't know why you try to put it inside.
Only, you HAVE to hide the determinism, because otherwise you CAN do FTL signalling. But what does it mean, hidden determinism ?
I'm not going to comment orthodox QM, I already told you it is just as ugly, EXCEPT as a generator of statistics. It is then on the same level as Bohm, and I don't see why you should even consider Bohm, given that you don't win anything (but I agree with you that you don't loose anything either: you've anyway lost everything else already in Copenhagen QM !). The two are viewed as two calculational procedures to arrive at the only physical quantities of interest: probabilities of measurement outcomes. Maybe some calculations are easier in Bohm's formulation than in the Hilbert state space formulation ; but I doubt that.

You seem to have forgotten the main point in favor of Bohm -- that it gets rid of all the "unprofessional vagueness and ambiguity" of Copenhagen. In particular, there is no measurement problem in Bohm's theory. So, putting it a different way, Bohm's theory is actually well defined as a theory. Surely that counts in its favor relative to regular QM.

 

[ttn]

I. EPR proves: "...either (1) the quantum-mechanical description given by the wave function is not complete or (2) when the operators corresponding to two physical quantities do not commute the two quantities cannot have simultaneous reality".

I'm not really all that interested in arguing about this here. I think it says a lot that (1) Einstein didn't write the EPR paper you are quoting, (2) Einstein explicitly stated that he thought that paper had failed to make clear the real point that was important to him, and (3) Einstein later explicitly stated what he thought that real point was. If you disagree, so be it. I would just mention once again Arthur Fine's book "The Shaky Game" -- the first few chapters cover EPR and some of the surrounding history that Fine uncovered in great detail and are extremely clarifying re: understanding and untangling the bizarre structure of the actual EPR paper. Check it out!

 

[ttn]

So we have that determinism and information-locality leads to Bell locality.
So I came to a full circle:

(1)From Bell locality follows Bell local determinism. (Patrick's theorem)
(2) From Bell locality follows information locality
(3) From information locality and determinism follows Bell locality

Together:

BELL LOCALITY <===> information locality and determinism

OK... and what, in your opinion, is the significance of this? I think it was obvious before that Bell Locality was a stronger condition than "no signalling". So this clarifies exactly how it is stronger. I guess you'll want to say that this shows that Bell's error in thinking of locality as Bell Locality was that he was going beyond what was actually required by relativity (which you claim is "no signalling") and secretly smuggling in the additional requirement of determinism. And that, of course, is a bad thing since it merely reflects "classical bias" or an inability to drop the old paradigm and get with the times or whatever.

This is an interesting argument, and I think it is very cool to have shown that Bell Locality is equivalent to the conjunction of "no signalling" and "determinism."

However, your interpretation of this result hangs on a crucial premise: namely, "no signalling" is what relativity *really* requires. That is not obvious. Surely relativity requires something *at least as strong as* "no signalling" but many people believe it requires something more, something stronger. For example, Bell eloquently asked:

"Do we then have to fall back on 'no signalling' faster than light' as the expression of the fundamental causal structure of contemporary theoretical physics? That is hard for me to accept. ... ...the 'no signalling' notion rests on concepts which are desperately vague, or vaguely applicable. The assertion that 'we cannot signal faster than light' immediately provokes the question: Who do we think *we* are? ..."

Even if you disagree with this and believe that *all* relativity requires is that we not be able to transmit information faster than light, you surely must admit that Bell on pretty reasonable ground for raising these questions. Relativity is supposed to be about the fundamental structure of spacetime. It would be frankly bizarre if what it imposed on that structure was somehow intimately bound up with human activities like "signalling" and building telephones and whatnot. It seems like the requirements of relativity ought to be more fundamental -- it ought to forbid any kind of causal influence outside the light cone, even if (for whatever reason) it is one that can never be used by humans to transmit information. That just makes sense. And as soon as you start thinking that way, you will come to believe, like Bell, that "Bell Locality" is what relativity *really* requires, not merely "no signalling."

And that means that your parsing of Bell Locality into "no signalling" plus determinism doesn't have the kind of implications you are suggesting. It has, really, no implications... it merely bring out the fact that, in order to have a serious (i.e., non-MWI ) theory that is consistent with experiment, you have to back off significantly from the relativity-motivated idea of (strong) local causality, and retreat to something that is strange, vague, and extremely superficial. And I think, in such a situation, one ought to simply concede that relativity (taken *seriously* as a statement about the *fundamental* causal structure of spacetime) is just wrong. That is, one should begin to take seriously the possibility that there is more structure in spacetime than is attributed to it by relativity, e.g., the "foliations" introduced in the context of relativistic Bohmian theory.

(I'm sure that will get your blood boiling!...)

 

[vanesch]

I'm not sure what you mean by saying Bohm "doesn't even consider the superposition principle". Wave functions in Bohmian mechanics are solutions of Schroedinger's equation (or whatever), just like orthodox QM. Bohm doesn't forbid or jettison superpositions!

The superposition principle says:
if L1 is a "complete state of nature" and if L2 is "a complete state of nature", then there are infinitely many other "complete states of nature" described by a L1 + b L2. If I understand Bohm a bit, this only applies to the "wave function" part, but not to the "guiding wave" part, no ?

From your comments elsewhere it is clear that what you actually mean by that is what you once clarified as "external info locality", i.e., no transmission of information superluminally. But when you criticize Bohm for sacrificing "information locality in its internal workings" this either equivocates on "info local" or just plain doesn't make sense. Bohm's theory is local in the "external info" sense but nonlocal in the "internal info" sense. Just like regular QM on both counts.

How many times do I have to repeat this that MWI QM does not suffer from this problem ? It is only a problem for Copenhagen QM (which is in any case, when considered as describing something physical, wrong - yes, probably Bohm is not as ugly as Copenhagen QM, I agree with that, in the same way that Frankenstein is not as ugly as the Living Dead )

I think you understand this perfectly well, but a lot of people are terribly confused, so let me repeat it for the benefit of others who are reading. If what you mean by "local" is "information cannot be transferred superluminally" then orthodox QM and Bohmian mechanics are equally local. If, on the other hand, what you mean is "the internal guts of the theory obey Bell's local causality constraint" then both orthodox QM and Bohmian mechanics are nonlocal. And keeping that straight will, I think, help prevent dubious statements like "Unlike regular QM, Bohm's theory is nonlocal, which puts it in conflict with SR, which means we shouldn't really take it seriously." That argument just doesn't hold water unless you equivocate like mad about the meaning of "local" -- i.e., unless you cheat!

Absolutely !

As I think I said a long time ago in this thread, I just think MWI is too crazy to take seriously. You talk about faithfully respecting certain principles (superposition, relativity, ...) to guide one in interpreting or building theories. Well, one of the principles that is to me even more fundamental than the ones you mention is scientific realism. I just can't take seriously something claiming to be a theory of physics that is in fact a form of solipsism. I know others disagree, which is why I prefer to just leave that issue aside and focus on, say, Bohm vs. QM, which we can speak about without it turning into a pointless debate about philosophy.

I can understand that viewpoint but I think it is misguided. I don't know who said "we all agree that your theory is crazy. The debate is on if it is crazy enough".
The amount of solispsim in MWI is in fact rather modest, you know. It only relates to what *you* observe, and honestly, you should acknowledge that that is a very private affair. It doesn't deny the existence of others either. Only, the person you talked to yesterday is not "the same" as the one you're talking to today, but a clone with exactly the same memory and physical body, which has a new "I experience" (while the "I experience" of the person you saw yesterday is now somewhere else, forever separated: admit the romantic drama in all this ).

No, seriously, I went through a lot of effort to make you see what you intuitively call "scientific realism" is "underlying determinism", in that if you would know all the nitty gritty details nature is hiding for you, you would know everything with certainty. You might stop your theoretical description short of that, and allow for a so-called "essential stochastic process", but your requirements are such that it still allows for underlying determinism.

You seem to have forgotten the main point in favor of Bohm -- that it gets rid of all the "unprofessional vagueness and ambiguity" of Copenhagen. In particular, there is no measurement problem in Bohm's theory. So, putting it a different way, Bohm's theory is actually well defined as a theory. Surely that counts in its favor relative to regular QM.

Absolutely ! Copenhagen QM is a mess. But...
When you say that there is no measurement problem in Bohm, how does this happen then ? Because there IS an objective difference between Copenhagen QM and MWI for instance: it is the physical process that determines measurement. In Copenhagen QM, this system CANNOT be considered to be in a superposition,and in MWI that's what you do. So there is a difference _in principle_ because, with enough care and technology, you COULD make the measurement instrument interfere with itself in MWI, and not in Copenhagen. So on which side does Bohm then flip ? Could I, or could I not, in principle, make a measurement instrument interfere with itself ?
When is a physical process a measurement ? I don't know enough about Bohm to realize this.

cheers,
Patrick.

 

[vanesch]

I still want to say this doesn't change anything, though. This argument shows that all Bell Local theories either are or can be made into deterministic theories. As you have made clear elsewhere, you think that part of what it means to really jettison the deterministic paradigm and accept deep, irreducible stochasticity in a theory, is to accept not only that specific local outcomes can't be explained by any previous state of the world but that, more generally, correlations between separated events also can't be explained by any previous state of the world. Something like: if you're going to accept outcomes just "popping" into existence, you should also accept *correlations* just popping into existence. Is that a fair statement of your position?

I should let you talk to yourself. You explain my views better than I do myself.

But probably there is nothing else interesting to say about this. What matters is what we agreed on before: Copenhagen and Bohm are both non-local in the various stricter senses (Bell-Locality or "internal info locality") and both local in terms of "external info transfer". And to the extent that MWI makes sense and/or is well-defined it is probably more local than either of those other two alternatives.

yup

cheers,
Patrick.