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 
Register to reply 
What do violations of Bell's inequalities tell us about nature? 
Share this thread: 
#91
Feb1513, 06:46 PM

P: 733

The first thing I'd say is: who cares? If the topic is Bell's theorem, then it simply doesn't matter. CFD *follows* from locality in the same way that "realism" / hidden variables do. That is: the only way to locally (and, here crucially, nonconspiratorially) explain even the perfect correlations is with a "realistic" hiddenvariable theory with predetermined values for *all* possible measurements, i.e., a model with the CFD property. So... to whatever extent somebody thinks CFD needs to be assumed to then derive a Bell inequality, it doesn't provide any kind of "out" since CFD follows from locality. That is, the overall logic is still: locality > X, and then X > inequality. So whether X is just "realism" or "realism + CFD" or whatever, it simply doesn't make any difference to what the correct answer to this thread's poll is. So, having argued that it's irrelevant to the official subject of the thread, let me now actually answer the question. Do I believe in CFD? I'm actually not sure. Or: yes and no. Or: it depends on a really subtle point about what, exactly, CFD means. Let me try to explain. As I think everybody knows, my favorite extant quantum theory is the dBBB pilotwave theory. So maybe we can just consider the question: does the pilotwave theory exhibit the CFD property? To answer that, we have to be very careful. One's first thought is undoubtedly that, as a *deterministic* hidden variable theory, of course the pilot wave theory exhibits CFD: whatever the outcome is going to be, is determined by the initial conditions, so ... it exhibits CFD. Clear, right? On the other hand, I've already tried to make a point in this thread about how, although the pilotwave theory assigns definite preexisting values (that are then simply revealed in appropriate measurements) to particle positions, it does *not* do this in regard to spin. That is, the pilotwave theory is in an important sense not "realistic" in regard to spin. And that starts to make it sound like, actually, at least in regard to the spin measurements that are the main subject of modern EPRBell discussions, perhaps the pilotwave theory does *not*, after all, exhibit CFD. So, which is it? Actually both are true! The key point here is that, according to the pilotwave theory, there will be many physically different ways of "measuring the same property". Here is the classic example that goes back to David Albert's classic book, "QM and Experience." Imagine a spin1/2 particle whose wave function is in the "spin up along x" spin eigenstate. Now let's measure its spin along z. The point is, there are various ways of doing that. First, we might use a set of SG magnets that produce a field like B_z ~ B_0 + bz (i.e., a field in the +z direction that increases in the +z direction). Then it happens that if the particle starts in the upper half of its wave packet (upper here meaning w.r.t. the zdirection) it will come out the upper output port and be counted as "spin up along z"; whereas if it happens instead to start in the lower half of the wave packet it will come out the lower port and be counted as "spin down along z". So far so good. But notice that we could also have "measured the zspin" using a SG device with fields like B_z ~ B_0  bz (i.e., a field in the zdirection that *decreases* in the +z direction). Now, if the particle starts in the upper half of the packet it'll still come out of the upper port... *but now we'll call this "spin down along z"*. Whereas if it instead starts in the lower half of the packet it'll still come out of the lower port, but we'll now call this *spin up along z*. And if you follow that, you can see the point. Despite being fully deterministic, what the outcome of a "measurement of the zspin" will be  for the same exact initial state of the particle (including the "hidden variable"!)  is not fixed. It depends on which *way* the measurement is carried out! Stepping back for a second, this all relates to the (rather weird) idea from ordinary QM that there is this a correspondence between experiments (that are usually thought of as "measuring some property" of something) and *operators*. So the point here is that, for the pilotwave theory, this correspondence is actually manytoone. That is, at least in some cases (spin being one of them), many physically distinct experiments all correspond to the same one operator (here, S_z). But (unsurprisingly) distinct experiments can have distinct results, even for the same input state. So... back finally to the original question... if what "CFD" means is that for each *operator*, there is some definite fact of the matter about what the outcome of an unperformed measurement would have been, then NO, the pilotwave theory does *not* exhibit CFD. On the other hand, if "CFD" means that for each *specific experiment*, there is some definite fact of the matter about what the outcome would have been, then YES, of course  the theory is deterministic, so of course there is a fact about how unperformed experiments would have come out had they been performed. This may seem like splitting hairs for no reason, but the fact is that all kinds of confusion has been caused by people just assuming  wrongly, at least in so far as this particular candidate theory is concerned  that it makes perfect sense to *identify* "physical properties" (that are revealed or made definite or whatever by appropriate measurements) with the corresponding QM operators. This is precisely what went wrong with all of the socalled "no hidden variable" theorems (KochenSpecker, etc.). And it is also just the point that needs to be sorted out to understand whether the pilotwave theory exhibits CFD or not. The answer, I guess, is: "it's complicated". That make any sense? 


#92
Feb1513, 07:00 PM

P: 724

The notion of 'particles' is oxymoronic. If microscopic entities obey Heisenberg’s uncertainty principle, as we know they do, one is forced to admit that the concept of “microscopic particle” is a selfcontradictory concept. This is because if an entity obeys HUP, one cannot simultaneously determine its position and momentum and, as a consequence, one cannot determine, not even in principle, how the position of the entity will vary in time. Consequently, one cannot predict with certainty its future locations and it doesn't have the requisites of classical particles like exact position and momentum in spacetime. What is the reason why an entity of uncertain nature but evidently nonspatial should obey classical notions like locality at all times?



#93
Feb1513, 07:13 PM

P: 128

ttn: regarding MWI, I am aware of the difficulties with the pure WF view, but what do you think of Wallace and Timpson's Space State time realism proposal?
It seems David Wallace is the only one every MWI adherent refers to when asking the difficult questions. He just wrote a huge *** book on the Everettian interpretation and argues for solving the Born Rule problem with decisiontheory. He argues that the ontological/preferred basis issue is solved by decoherence + emergence. Lastly he posits the Space State realism 


#94
Feb1613, 12:02 AM

PF Gold
P: 674

http://www.physicsforums.com/showthread.php?t=167320 Since I hate writing stuff in my own words since others write it down so more eloquently the necessary contextuality present in the pilotwave model is summarized in an easily understandible way (for me) here also: http://plato.stanford.edu/entries/ko...ker/index.html So, while the KS theorem establishes a contradiction between VD + NC and QM, the qualification above immunizes pilotwave/deBroglie/Bohmian mechanics from contradiction. 


#95
Feb1613, 04:15 AM

P: 79

I like the way Maudlin writes also. Thanks for the link. In the process of rereading it. But yes I see that the sample space at either end is always (+,) no matter what. At least in real experiments. In the ideal, iff θ is either 0° or 90°, then a detection at one end would change the sample space at the other end. But the sample space of what's registered by the detectors isn't the sample space I was concerned about. There's also the sample space of what's transmitted by the filters, and the sample space ρ(λ) that's emitted by the source. It's how a detection might change ρ(λ) that I was concerned with. Is the following quote what you're saying is a better way to say what you think I'm saying but is wrong?: "Bell assumes locality and shows that this implies a certain limit on the correlations; the experiments show that the correlations are stronger than the limit allows; therefore we conclude that nature is nonlocal." Or are you saying that that's the correct way of saying it? Or what? I think the way I'd phrase it is that Bell codified the assumption of locality in a way that denotes the independence (from each other) of paired events at the filters and detectors. Bell proved that models of quantum entanglement that incorporate Bell's locality condition cannot be compatible with QM. It is so far the case that models of quantum entanglement that incorporate Bell's locality condition are inconsistent with experimental results. I don't yet understand how/why it's concluded that nature is nonlocal. 


#96
Feb1613, 05:07 AM

P: 733




#97
Feb1613, 05:12 AM

P: 733

And, of course, separately: Bell's theorem rules out local theories. The pilotwave theory is not a local theory. People who voted for (b) in the poll evidently get these two theorems confused. They try to infer the conclusion of KS, from Bell. 


#98
Feb1613, 05:17 AM

P: 733




#99
Feb1613, 07:52 AM

Sci Advisor
P: 1,914

In my experience, whenever things are philosophically murky, and people are stuck into one or more "camps", it sometimes helps to ask a technical question whose answer is independent of how you interpret things, but which might throw some light on those interpretations. That's what Bell basically did with his inequality. They may not have solved anything about the interpretation of quantum mechanics, but certainly afterwards, any interpretation has to understood in light of his theorem.
Anyway, here's a technical question about ManyWorlds. Supposing that you have a wave function for the entire universe, [itex]\Psi[/itex]. Is there some mathematical way to interpret it as a superposition, or mixture, of macroscopic "worlds"? Going the other way, from macroscopic to quantum, is certainly possible (although I'm not sure if it is uniqueprobably not). With every macroscopic object, you can associate a collection of wave packets for the particles making up the object, where the packet is highly peaked at the location of the macroscopic object. But going from a microscopic description in terms of individual particle descriptions to a macroscopic description in terms of objects is much more complicated. Certainly it's not computationally tractable, since a macroscopic object involves unimaginable numbers of particles, but I'm wondering if it is possible, conceptually. 


#100
Feb1613, 08:10 AM

P: 79




#101
Feb1613, 02:56 PM

P: 733

For more details, see any contemporary treatment of MWI, e.g., the David Wallace book that was mentioned earlier. (Incidentally, I just ordered myself a copy!) 


#102
Feb1613, 04:11 PM

P: 733




#103
Feb1613, 05:08 PM

P: 79

I take Bell's formulation as general, and assume that the QM treatment of quantum entanglement will always agree with experiment. So, insofar as Bell locality and QM have been mathematically proven to be incompatible, then there's no possible viable local theory of quantum entanglement. But consider that Bell tests are designed to produce statistical dependence by the entanglement creation process (eg., common emitter, interaction of the particles, common 'zapping' of separated particles, etc.) and the data pairing process, both of which proceed along exclusively local channels. Then consider that the locality condition codifies statistical independence. I'm just wondering if there's anything significant enough about that inconsistency so that it, and not nonlocality, might be the effective cause of the inconsistency between local theories and experiment. 


#104
Feb1613, 05:25 PM

P: 733

On the other hand, if you don't have anything definite in mind  if it's just "well what if there's some illicit assumption smuggled in there? prove that there isn't such a thing!"  then that would be quite silly and would certainly leave nothing to discuss. 


#105
Feb1713, 04:24 AM

P: 79

If the locality condition codifies statistical independence in addition to codifying locality, then the question becomes: is the inconsistency between the statistical independence codified by the locality condition and the statistical dependency necessitated by the experimental design significant enough that this inconsistency is the effective cause of the inconsistency between the predictions of models incorporating the locality condition and experimental results? . 


#106
Feb1713, 05:42 AM

Sci Advisor
Thanks
P: 3,432

That doesn't make it ipso facto wrong, just gives us a starting point for considering whether, in any given experiment, the experiment might not completely preclude locality. 


#107
Feb1713, 06:20 AM

P: 79




#108
Feb1713, 06:41 AM

P: 733




Register to reply 
Related Discussions  
Why are Bell's inequalities violated?  Quantum Physics  97  
What does this example say about the applicability of Bell's inequalities?  Quantum Physics  38  
How strong is the evidence for Bell inequality violations?  General Physics  0  
How strong is the evidence for Bell inequality violations?  General Physics  0  
Explaining EPR after Bell's inequalities  Quantum Physics  8 