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Bell's theorem - for or against Hidden Variables?
(08-07-2016, 11:32 PM)secur Wrote: Thomas Ray wrote: The authors claim that a particle does not exist in an 'up' or 'down' state until the experimenter chooses; I have a problem with that.

You don't give the quote; they don't say exactly that.

In normal Bell experiment, Alice and Bob choose detector orientation settings (the angle), then record the direction of spin (up or down). Those directions should show correlation (violate Bell's inequality, or similar). But this ECS gedanken records the spin first, before knowing the angle - that's the clever part. Then, the angles must show correlations, similar to the way directions do in the normal experiment.

So normal experiment, following forward time direction, takes the directions as given and looks for correlations in the angles; ECS does the opposite, reversing (so to speak) the normal time direction.

After going through the SGM the particle exists in a superposed state of the up and down channels, or beams, or waves. It "chooses" which state it's in only when the experimenter looks. He does so by sending a single photon through the 3 up channels, and another through the 3 down channels; whichever gets absorbed corresponds to the up/down spin state. That's the key trick. In this way we learn the direction without learning the angle. So the detector settings remain in a superposed "entangled" state, and should show correlations.

The particle is up/down superposed until the photon detection trick is done. If that's what you have a problem with, this is the standard Copenhagen way of looking at it. Just interpret the statement according to your preferred ontology. For instance Gell-Mann might say the particle does have a definite spin but we haven't yet determined which decoherent history it, and we, have taken, yet. Or you might want to ascribe the uncertainty not to the particle, but to the experimenter: the particle has a definite spin we just don't know it yet. Since people describe the same ideas using different ontologies, I've gotten used to translating to my preferred ontology (which is, essentially, Copenhagen). It's easy enough once you get the hang of it.

No, the "controversial" part here is that one would think the particle must go through one of the 3 SGM's in order to have a definite spin; so when you detect that spin you collapse the whole wave function (to use Copenhagen language) and interference among the 3 different orientations should be lost. As they show - convincingly, I think - that's not the case according to QM math, no matter what ontology you use. Of course it would be nice to actually do the experiment and show their conclusions are correct.

So, by reversing the normal time sequence in which the experimenter gains knowledge (of detector angle and spin) they illustrate a situation where retrocausality seems, arguably, more intuitive. I think it's pretty clever. Although of course it can still be explained with other ontologies, it may make them less attractive. In particular, as they show, Bohmian mechanics has to stretch a bit to accommodate this scenario.

My questions about ECS paper are:

ECS: "If, at the quantum realm, causal effects proceed on both time directions, then sufficiently delicate experiments should be able to reveal this dual nature. Indeed TSVF already boasts some verifications of this kind [8], and further surprising theoretical and empirical results can be expected."

I found [8] and similar work by these authors and see no experiments that actually "reveal" TSVF. They're all similar to this one: retrocausality is emphasized, or suggested; but other interpretations still work. Copenhagen, in particular, handles these situations easily as far as I can see. So their implication of "proving" TSVF is not justified. I'd be happy to hear any arguments supporting this assertion (or perhaps "hint" is a better word).

ECS: "We conclude with a brief comparison between these interpretations and their traditional alternatives, Copenhagen, Bohmian mechanics and the Many Worlds Interpretation."

They analyze only Bohmian, no others. They refer to [25] for Copenhagen - but it doesn't exist yet!:

ECS: [25] A. C. Elitzur, E. Cohen, Why treat a disease with a no-better remedy? Copenhagen, Bohmian mechanics, and time-symmetric interpretations of QM. Forthcoming.

Sometimes authors do this sort of thing hoping the reader simply won't notice but I don't suspect them of that. I look forward to the forthcoming reference [25] because I don't see why these TSVF-type experiments should be a big problem for Copenhagen.

Finally, the idea of entropy constituting the "hidden variable" is reasonable on the face of it, but I don't see how that would actually work. I'll take a look at your site for further information.


secur wrote, "So normal experiment, following forward time direction, takes the directions as given and looks for correlations in the angles; ECS does the opposite, reversing (so to speak) the normal time direction."

This assumes there is a normal time direction.  I'm saying that one cannot distinguish directions (past & future).  From the ECS abstract:

"We propose a new setting where the question is reversed: "What is the orientation along which this particle has this spin value?" It turns out that the orientation is similarly subject to nonlocal effects. To enable the reversal, each particle's interaction with a beam-splitter at t1 leaves its spin orientation superposed. Then at t2, the experimenter selects an "up" or "down" spin value for this yet-undefined orientation. Only after the two particles undergo this procedure, the two measurements are completed, each particle having its spin value along a definite orientation. By Bell's theorem, it is now the "choice" of orientation that must be nonlocally transmitted between the particles upon completing the measurement. This choice, however, has preceded the experimenter's selection. This seems to lend support for the time-symmetric interpretations of QM, where retrocausality plays a significant role."

This is still a linear solution.  In all linear solutions, the assumption of superposition holds -- SGM rectifies the trajectory to a forward direction; this does not imply that the trajectory not measured is retrograde, and vice versa.  

Which raises the question of what a nonlinear solution would look like.  It would look like Joy Christian's unfairly-maligned exploding ball experiment -- no rectifying mechanism involved.  This now makes it possible to judge the particle orientation independent of the trajectory -- that is, " ... without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity."  (~EPR)

True retrocausality is manifestly local, as ECS question, "Along which Orientation does the Particle have this Spin Value?”  Thing is, we don't know the orientation (past or future, or if you prefer, left or right) if we get to manufacture it.  It's all hidden in the production of entropy -- we conventionally assume that entropy flow is in the direction of 'disorder' yet without locally ordered states this is meaningless.    So Joy's claim that his measurement framework accounts for all correlations -- not just quantum -- is correct.  

ECS are making a run at weakening efforts of 80 years to eliminate the role of continuous spacetime from quantum theory.  It only brings along a plethora of ad hoc assumptions.  

You've said so much of value, secur, and I've replied to only one point. But this is getting too long.  Perhaps in the course of discussion, we can revisit these important points?

All best,
Tom
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RE: Bell's theorem - for or against Hidden Variables? - by Thomas Ray - 08-08-2016, 02:48 PM

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