View Poll Results: What do observed violation of Bell's inequality tell us about nature?  
Nature is nonlocal  11  32.35%  
Antirealism (quantum measurement results do not preexist)  15  44.12%  
Other: Superdeterminism, backward causation, many worlds, etc.  8  23.53%  
Voters: 34. You may not vote on this poll 
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What do violations of Bell's inequalities tell us about nature? 
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#73
Feb1413, 10:54 AM

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PF Gold
P: 5,299

I, being mostly a nonrealist, reject his thesis. I find that Bohmian representations of such experiments as delayed choice entanglement swapping (DCES) are unsatisfactory. Those seem to me to require a nonrealistic interpretation of some kind. I realize that Bohmians do not agree however, but you can judge for yourself. 


#74
Feb1413, 11:24 AM

P: 733

2. It is hardly as clear as you imply that MWI is local. I know everybody claims this, but in so far as MWI has *only* the wave function in its ontology, and insofar as the wave function doesn't live in physical space (but instead some highdimensional configuration space), it seems that MWI doesn't posit any physically real stuff in ordinary physical space at all. And so I literally have no idea what it would even mean to say that it's local, i.e., that the causal influences that propagate around between different hunks of stuff in physical space do so exclusively slower than the speed of light. It's ... a bit like saying that Beethoven's 5th symphony is local. It's not so much that it's nonlocal, but just that it's not even clear what it could mean to make *either* claim. Incidentally, there is a really nice and interesting paper that suggests a way for MWI to posit some local beables, i.e., some physical stuff in 3space. The authors end up concluding (correctly I think) that this theory is actually nonlocal: http://arxiv.org/abs/0903.2211 


#75
Feb1413, 12:24 PM

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P: 1,958

Suppose you arrange a bunch of atoms into a solid brick wall. Then a Bohmian type of theory would predict that the wall would continue to exist for a good long time afterward, giving a reassuring sense of solidity. But now, you take a baseball (another clump of atoms arranged in a particular pattern) and throw it at the wall. What happens then? The question for a Bohmian type theory is what wave function are you using to compute trajectories? The full wave function describes not the actual locations of the particles of the ball and the wall, but a probability amplitude for particles being elsewhere. If, as you seem to agree, the wave function affects the particles, but not the other way around, then the fact that you've gathered atoms into a wall doesn't imply that the wave function is any more highly peaked at the location of the wall. So if it's the wave function that affects the trajectory of the ball, then why should the ball bounce off the wall? The principle fact that Bohmians use to show that Bohmian mechanics reproduces the predictions of quantum mechanics is that if particles are initially randomly distributed according to the square of the wave function, then the evolution of the wave function and the motion of the particles will maintain this relationship. That's good to know in an ensemble sense, but when you get down to a small number of particlessay one electronthe wave function may say that the electron has equal probabilities of being in New York and in Los Angeles, but the electron is actually only in one of those spots. So either the wave function has to be affected by the actual location (in a mechanism that hasn't been demonstrated, I don't think) or there has to be the possibility of an electron having a location that is in no way related to the wave function (except in the very weak sense that if the electron is at some position, then the wave function has to be nonzero at that position). So either you have to have a "wave function collapse" or some other way for the wave function to change that doesn't involve evolution according to the Schrodinger equation, or you have the possibility that the trajectories of physical objects are unaffected by the locations of other physical objects. Which is certainly contrary to experience. 


#76
Feb1413, 01:28 PM

P: 733

You raise a number of important and interesting points... far more interesting than the lab reports I should probably be grading instead of writing this! =)
That's the overview point. Now let me try to explain exactly how some of your comments exhibit this confusion about how to understand the "roles" of the two things, the wave and the particles... What about the actual/bohmian particle positions? Well, at t=0, the wall has some actual position in the support of its wf, and likewise for the ball. And then the actual configuration point just moves along with the moving/bouncing packet in configuration space. So the story you'd tell about the two particles in real space is: the wall particle just sits there the whole time, while the ball particle comes toward it, bounces off, and heads away. Now you want to ask: what happens if, instead of initially being in a (near) position eigenstate, the wall is initially in a superposition of two places? It's a good question, but if you think it through carefully, you'll find that the theory says exactly what anybody would consider the right/reasonable thing. So, just recapitulate the above, but now with the initial wf for the wall being a sum of two packets, one peaked at x=0 and one peaked at x=D. Now (I'm picturing all of this playing out in the xy plane, and hopefully you are too) the initial 2particle wave function in the 2D config space has *two* lumps: one at (x=0, y=L), and the other at (x=D, y=L). So then run the wf forward in time using the sch eq: the two lumps each propagate "upward" (i.e., in the ydirection). Eventually the first lump reaches the potential wall near (x=0,y=0) and bounces back down. Meanwhile the other lump continues to propagate up until it reaches the potential wall near (x=D,y=D) at which point it too reflects and starts propagating back down. So much for the wave function. What about the particles? The point here is that in bohm's theory the *actual configuration* is in one, or the other, of the two initial lumps. If (by chance) it happens to be in the first lump, then the story is *exactly* as it was previously  the other, "empty" part of the wave function (corresponding to the wall having been at x=D) is simply irrelevant. It plays no role whatever and could just as well have been dropped. On the other hand, if (by chance) the actual positions are initially in the second lump, then the story (of the particles) is as follows: the ball propagates toward the wall (which is at x=D) until the ball gets to x=D, and then it bounces off. That is, there is some fact of the matter about where the wall actually is, and the ball bounces off the wall just as one would expect it to. The only thing that could possibly confuse anybody about this is that they are thinking: but the wall really *isn't* in one or the other of the definite places, x=0 or x=D, it's in a *superposition* of both! Indeed, that's what you'd say in ordinary QM. And then you'd have to make up some story about how throwing the ball at the wall constitutes a measurement of its position and so collapses its wave function and thus causes it (the wall) to acquire a definite position, just in time to let the ball bounce off it. But all of this is unbohmian. In bohm's theory everything is just simple and clear and normal. The wall (meaning, the wall PARTICLE) is, from the beginning, definitely somewhere. Maybe we don't know, for a given run of the experiment, where it is, but who cares. It is somewhere. The ball bounces off this actual wall when it hits this actual wall. Simple. HOWEVER, there is a really cool and important thing about bohm's theory  you can meaningfully define a wave function of a *subsystem*. Take the wall/ball system above. The wave function is a function \psi(x,y). But we also have in the picture the actual wall position X and the actual ball position Y. So we can construct a mathematical object like \psi(x,Y)  the "universal" wave function, but evaluated at the point y=Y. This is called the "conditional wave function for the wall": \psi_w(x) = \psi(x,Y). And likewise, \psi_b(y) = \psi(X,y) is called the "conditional wave function of the ball". Now here's the amazingly beautiful thing. Think about how the conditional wave function of the wall, \psi_w(x), evolves in time. To be sure, it starts off having two lumps, one at x=0 and one at x=D. But if you think about how \psi(x,y) evolves in time (with the two lumps becoming *separated* in the ydirection, because one of them reflects earlier than the other), you will see that \psi_w(x) actually "collapses"  after all the reflecty business has run its course, \psi_w(x) will be *either* a onelump function peaked at x=0, *or* a onelump function peaked at x=D. Which one happens depends, of course, on the (random) initial positions of the particles. The point is  and this is really truly one of the most important and beautiful things about Bohm's theory  the theory actually *predicts* (on the basis of fundamental dynamical laws which are simple and clear and which say *nothing* about "collapse" or "measurement") that *subsystem* wave functions (these "conditional wave functions") will collapse, in basically just the kinds of situations where, in ordinary QM, you'd have to bring in your separate measurement axioms to make sure the wfs collapsed appropriately. So not only does bohm's theory make all the right predictions (contrary to what I think you are worrying), it actually manages to *derive* the weird rules about measurement that are instead *postulated* in ordinary QM. 


#77
Feb1413, 02:24 PM

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P: 1,958

If that's not the case, I would like to see a simple example worked out; for example, a Bohmian model of two pointmasses interacting through a harmonic oscillator potential. That seems simple enough that it could be worked out explicitly. Maybe I'll try myself. 


#78
Feb1413, 03:27 PM

P: 733

Notice that your wrong way of thinking it should work is actually kinda/sorta the way you might talk about it in the quantum potential formulation of the theory, which I don't like  partly because it invites this kind of thinking, that "really", the theory is just "classical physics but with an extra quantumish force". But it's not. That's really just a wrong and misleading way to try to understand it. 


#79
Feb1413, 04:20 PM

P: 79

I had been thinking that it would be pointless to make a local nonrealistic theory, since the question, following Einstein (and Bell) was if a local model with hidden variables can be compatible with QM? But a local nonrealistic (and necessarily nonviable because of explicit locality) theory could be used to illustrate that hidden variables, ie., the realism of LHV models, have nothing to do with LHV models' incompatibility with QM and experiment. Your coinflip model, insofar as it would incorporate a λ representing the coinflip, would be a hidden variable model. But because the coinflip won't change the individual detection probability, λ can be omitted. (?) We can do that with Bell's general LHV form also, because in Bell tests λ is assumed to be varying randomly and therefore has no effect on the individual detection probability  ie., rate of individual detection remains the same no matter what the setting of the polarizer, so the inclusion of a randomly varying λ is superfluous. (?) Bell only includes it (I suppose) because that's the question he's exploring. That is, it's because the inclusion of a λ term is a major part of an exercise aimed at answering whether a local hidden variable interpretation of standard QM is possible. In the course of doing that it's been shown as well that a local interpretation of QM is impossible. So, it should be clear that I agree with you (and Bell) that it's all about the locality condition. The problem is that a Belllike (general) local form necessarily violates 2 (an incompatibility that has nothing to do with locality), because Bell tests are designed to produce statistical (ie., outcome) dependence via the selection process (which proceeds via exclusively local channels, and produces the correlations it does because of the entangling process which also proceeds via exclusively local channels, and produces a relationship between the entangled particles via, eg., emission from a common source, interaction, 'zapping' with identical stimulii, etc.). Is this a possibility, or has Bell (and/or you) dealt with this somewhere? 


#80
Feb1413, 05:07 PM

P: 733

But I absolutely agree with the way you put it, about what the question is postEinstein. Einstein already showed (in the EPR argument, or some less flubbed version of it  people know that Podolsky wrote the paper without showing it to Einstein first and Einstein was pissed when he saw it, right?) that "realism"/LHV is the only way to locally explain the perfect correlations. PostEinstein, the LHV program was the only viable hope for locality! And then Bell showed that this only viable hope won't work. So, *no* local theory will work. I'm happy to hear we're on the same page about that. But my point here is just that, really, the best way to convince somebody that "local nonrealistic" theories aren't viable is to just run the proof that local theories aren't viable (full stop). But somehow this never actually works. People have this misconception in their heads that a "local nonrealistic" theory can work, even though they can't produce an explicit example, and they just won't let go of it. Since it so perfectly captures the logic involved here, it's worth mentioning here the nice little paper by Tim Maudlin http://www.stat.physik.unipotsdam.d...Bell_EPR2.pdf where he introduces the phrase: "the fallacy of the unnecessary adjective". The idea is just that when somebody says "Bell proved that no local realist theory is viable", it is actually true  but highly misleading since the extra adjective "realist" is totally superfluous. As Maudlin points out, you could also say "Bell proved that no local theory formulated in French is viable". It's true, he did! But that does not mean that we can avoid the spectre of nonlocality simply by reformulating all our theories in English! Same with "realism". Yes, no "local realist" theory is viable. But anybody who thinks this means we can save locality by jettisoning realism, has been duped by the superfluous adjective fallacy. 


#81
Feb1413, 10:05 PM

P: 376




#82
Feb1513, 09:10 AM

P: 461

well well said.... they counfuse real with counterfactual definiteness real come from Latin res, thing, object just that. values are just attributes of objects, quality, characteristics, attributes, values are just secondary aspects of objects, i.e properties of objects. reality: the state of things as they actually exist. 


#83
Feb1513, 10:03 AM

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PF Gold
P: 5,299

ttn, no need for us to debate the point again; this is just the opposition's placard. Although by looking at the poll results as of now, it looks like you are losing 612. 


#84
Feb1513, 11:34 AM

P: 733

To be sure, Einstein was wrong about something. In particular, he simply assumed that locality was true. Then, applying his perfectly valid *argument* "from locality to" preexisting values (or "hidden variables" or "realism" or CFD or whatever anybody wants to call it), he *concluded* that these preexisting values really existed. (Which of course in turn implies that the QM description, which fails to mention any preexisting values, is incomplete.) Now it turns out locality is false. So Einstein was wrong to assume it. The EPR argument can no longer be used as a proof for the existence of preexisting values since we now know that its premise (locality) is actually false! But none of this undermines in the slightest bit the validity of the argument "from locality to" these preexisting values. That is, it remains absolutely true that preexisting values are the only way to locally explain the perfect correlations  whether locality is true or not. Let me urge you to put up a better, or at least additional, placard. So far your placard amounts to "nuh uh". Your position, though, is clear. You think that, by saying there are no preexisting values, we can consistently maintain locality. That is, you do not accept that Einstein/EPR validly argued "from locality to" preexisting values. That is, you think that it is possible to explain the perfect correlations locally but without preexisting values. This is precisely why I issued "ttn's challenge" in my first post in this thread: please display an actual concrete (if toy) model that explains the perfect correlations locally without relying on preexisting values. To not do this is to confess that your position (your vote for (b) in the poll) is indefensible. 


#85
Feb1513, 12:41 PM

P: 461

amazing ! travis norsen, in person....
travis, do you believe in CFD ? 


#86
Feb1513, 01:02 PM

PF Gold
P: 676

In Ch. 4 of "Many Worlds?: Everett, Quantum Theory, and Reality" So , if nonrealism, then the issue of locality vs nonlocality seems kind of pointless since there doesn't appear to be any ontological issues. I mean what ontological difference would there be between the local vs nonlocal version of nonrealism? Anyway, that's how I understood it or I'm not getting it. As I posted previously, I think Gisin argues similarily here: http://arxiv.org/pdf/1012.2536v1.pdf And even a Bayesian argument seems hard to swallow because as Timpson notes: http://arxiv.org/pdf/0804.2047v1.pdf 


#87
Feb1513, 01:22 PM

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P: 5,299

It is unreasonable to require that only those observables which can be simultaneously measured have reality. I.e. that counterfactual observables do have reality. So in my book, every Bohmian is an antirealist. 


#88
Feb1513, 03:11 PM

P: 733

But the point is that there are very few people who actually seriously think there's no physical reality at all. (This would be solipsism, right? Note that even the archquantumsolipsist Chris Fuchs denies being a solipsist! Point being, very few people, perhaps nobody, would openly confess to thinking there's no physical reality at all.) And yet there are at least 12 people right here on this thread who say that Bell's theorem proves that realism is false! What gives? Well, those people simply don't mean by "realism" the claim that there's a physical world out there. They mean something much much much narrower, much subtler. They mean in particular something like: "there is a fact of the matter about what the outcome of a measurement was destined to be, before the measurement was even made, and indeed whether it is in fact made or not." That is, they mean, roughly, that there are "hidden variables" (not to be found in QM's wave functions) that determine how things are going to come out. Of course, in the context of Bell's theorem, what really matters is just whether endorsing this (latter, noninsane) type of "nonrealism" gives us a way of avoiding the unpalatable conclusion of nonlocality. At least 12 people here think it does! And yet none of them have yet addressed the challenge: produce a local but nonrealist model that accounts for the perfect correlations. (Note, even if somebody did this, they'd still technically need to show that you can *also* account for the *rest* of the QM predictions  namely the predictions for what happens when the analyzers are *not* parallel  before they could really be in a position to say that local nonrealism is compatible with all the QM predictions. My challenge is thus quite "easy"  it only pertains to a subset of the full QM predictions! And yet no takers... This of course just shows how *bad* nonrealism is. If you are a nonrealist, you can't even account for this perfectcorrelations *subset* of the QM predictions locally! That's what EPR pointed out long ago...) 


#89
Feb1513, 03:14 PM

P: 733

But what you, Dr C, are missing above is that when Podolsky said something was "unreasonable", what he actually meant (and absolutely should have said instead!) was: "inconsistent with locality". But I've explained this so many times to you over the years, without getting through, there's really no point even trying again. 


#90
Feb1513, 05:05 PM

P: 724

We should all be thinking of reality as fields and particles as excitations of the fields, instead of crippled and incoherent classicallike models. Classicallike concepts like time, space, 'physical stuff', realism... could well be emergent. Just my unprofessional view(backed by some of the great names in physics).
In the same way that we can not even in principle predict the behavior of certain large collections of bodies from the behavior of just one constituent(e.g. a flock of birds), it seems equally impossible to predict the behavior of a large ensemble of particles from looking at just one electron or proton. Hence why it could be totally impossible to understand the reality of chairs and tables by looking at just quantum mechanical rules and axioms. The fundamental aspect of the emergent system is its capacity to be what it is while being completely unlike any other version of what it is. And we are just beginning to approach problems in this direction  we also have to embrace the emergence of life from nonlife and consciousnesss from nonconsciousness among other similar phenomena(like the possible emergence of a reality from a nonreality  these 3/life, consciousness and physical stuff/ account for all that can be observed in the universe). Emergence is an observational fact and sounds much less abusrd than many of the other ideas put forward here. PP. Since none of my conscious thoughts can at present be modelled and framed in purely classical/physical terms, shouldn't we also be proposing hidden variables for explaning the reality of the paragraph i wrote above? 


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