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The usual argument goes that QM demands giving up either locality or realism or both. Without getting into the statistics side of things which has been thrashed to death here, one real puzzle for me at least is the implicit notion of FTL 'signalling' as possible explanation for correlations if one retains realism but gives up non-locality. But how could any signalling between members of an 'entangled pair' plausibly work - even for time-like separation scenarios where causality is taken to hold?

Any physically real signal must surely embody thus require expenditure of energy-momentum by the signalling particle(s). Yet afaik there is never any talk of a concomitant back reaction on signalling particles that would alter said particle dynamics accordingly. Why is that? It seems especially troublesome to postulate 'signals' between photons - given photons are themselves normally the signalling medium.

Further, how could any signalling not, at least statistically, reasonably be allowed to propagate other than as a 'dumb' spherical wavefront? With the attendant rapid attenuation in intensity (at least as 1/r) surely rendering such useless as causative agent of QM correlations except over small spatial separations.

Apparently then one need implicitly assume a 'smart' signalling strategy where somehow each 'signal' knows exactly where to track the other particle. And further, since distance cannot, as per last point, be a factor, such 'signals' must not diminish with distance. Which implies propagation via some truly weird 1-D 'transmission-line' linking the entangled pair to any arbitrary separation. And presumably, additional to any signal(s) themselves, such trailed-out 'transmission-line' requires zero expenditure of energy-momentum resources to create! Or does one postulate a pre-existing, super dense network of 1-D 'transmission-lines' that somehow have no physical effect other than to efficiently and on demand transmit 'signals' between entangled particles?

Wow - just WOW! Am I being just too naive? Missing something obvious? Can someone point to where any of these evident issues have been sensibly addressed in the literature? If not, why is 'signalling' ever admitted as possibility at all?

Oh dear, my window for editing was rather brief. So, just to post an errata wrt above post:
"if one retains realism but gives up non-locality" should have read "if one retains realism but gives up locality"

Does non-locality and/or non-realism make any physical sense? Of course not. Those that think QM means one or the other has to be possible just don't have a very good imagination. Fortunately thanks to Joy Christian, we have a model that explains strong correlations in a classical local realistic way.

Now, I suppose signalling could make physical sense if the "connection" was by something we don't know about yet. From what we do know, it is not physically possible and since we have a viable explanation, the probability that signalling could be due to something we don't know about is close to zero.

FrediFizzx wrote:Does non-locality and/or non-realism make any physical sense? Of course not.

Agreed, as elsewhere stated - 'magic literally at the heart of reality' implied. More on that below.

Those that think QM means one or the other has to be possible just don't have a very good imagination.

Or maybe an overly active one.

Fortunately thanks to Joy Christian, we have a model that explains strong correlations in a classical local realistic way.

Too bad that after so much debate a meaningful concession from at least one of the key protagonists here has not eventuated. I certainly have a strong philosophical leaning to Joy's position, but certain scenarios involving contextuality give me cause to frown - e.g. Ghosh-Mandel experiments, p171 onward here:
http://www.jnu.ac.in/Faculty/rghosh/Tex ... xample.pdf
That reference is just there for example not discussion this thread. If anyone wants to start one on that kind of issue then feel free.

Now, I suppose signalling could make physical sense if the "connection" was by something we don't know about yet. From what we do know, it is not physically possible and since we have a viable explanation, the probability that signalling could be due to something we don't know about is close to zero.

Right but even if one accepts it could somehow exist, how so with evidently zero energy-momentum cost and zero perturbative back-reaction on signalling particles?

Then again, no more troublesome perhaps than say Bohmian pilot-waves, assumed physically real yet devoid of energy-momentum content, able to one-way react on particles but not vice versa, and instantaneously update throughout the universe, notwithstanding SR/GR considerations. Or Everett many-worlds, local and realistic at the price of an unending exponentially exploding rate of branching 'worlds'. Just how the branching physically occurs is a mystery it seems, as is the issue of where a postulated absolutely perfect linearity somehow avoids a rational expectation of such scenario quickly yielding a saturation/pile-up crisis grinding everything to a halt.

Q-reeus wrote:

FrediFizzx wrote:Fortunately thanks to Joy Christian, we have a model that explains strong correlations in a classical local realistic way.

Too bad that after so much debate a meaningful concession from at least one of the key protagonists here has not eventuated. I certainly have a strong philosophical leaning to Joy's position, but certain scenarios involving contextuality give me cause to frown - e.g. Ghosh-Mandel experiments, p171 onward here:
http://www.jnu.ac.in/Faculty/rghosh/Tex ... xample.pdf
That reference is just there for example not discussion this thread. If anyone wants to start one on that kind of issue then feel free.

Oh good; make me do some research. Perhaps this paper will alleviate your concerns.
http://arxiv.org/pdf/1312.6357.pdf
"Nonclassical effects in two-photon interference experiments: event-by-event simulations"
I quote from the paper, "In this paper, we demonstrate that the second-order intensity interference with visibility 1/2 in a HBT experiment with two independent single photon sources and with visibility 1 in the Ghosh-Mandel experiment can be entirely explained in terms of this event-based model, that is in terms of a locally causal, modular, adaptive, classical (non-Hamiltonian) dynamical system. Hence, there is no need to invoke quantum theory to explain the observations and the commonly accepted criterion of the nonclassical nature of light needs to be revised."

Now, I suppose signalling could make physical sense if the "connection" was by something we don't know about yet. From what we do know, it is not physically possible and since we have a viable explanation, the probability that signalling could be due to something we don't know about is close to zero.

Q-reeus wrote:Right but even if one accepts it could somehow exist, how so with evidently zero energy-momentum cost and zero perturbative back-reaction on signalling particles?

Then again, no more troublesome perhaps than say Bohmian pilot-waves, assumed physically real yet devoid of energy-momentum content, able to one-way react on particles but not vice versa, and instantaneously update throughout the universe, notwithstanding SR/GR considerations. Or Everett many-worlds, local and realistic at the price of an unending exponentially exploding rate of branching 'worlds'. Just how the branching physically occurs is a mystery it seems, as is the issue of where a postulated absolutely perfect linearity somehow avoids a rational expectation of such scenario quickly yielding a saturation/pile-up crisis grinding everything to a halt.

Well, I'm sticking with Einstein. "Spooky action at a distance" is just plain physical nonsense.

FrediFizzx wrote:Oh good; make me do some research. Perhaps this paper will alleviate your concerns.
http://arxiv.org/pdf/1312.6357.pdf

Ah yes... the fine and subtle art of motivating others to do ones leg-work . Anyway thanks for that link. And note there has been a raft of similar follow-on articles by Hans de Raedt et al. True to earlier remark, will not comment further here on that very illuminating article other than to say it represents a sunny break through an otherwise gloomy overcast sky.

Well, I'm sticking with Einstein. "Spooky action at a distance" is just plain physical nonsense.

Indeed.

Q-reeus wrote:

Now, I suppose signalling could make physical sense if the "connection" was by something we don't know about yet. From what we do know, it is not physically possible and since we have a viable explanation, the probability that signalling could be due to something we don't know about is close to zero.

Right but even if one accepts it could somehow exist, how so with evidently zero energy-momentum cost and zero perturbative back-reaction on signalling particles?

Then again, no more troublesome perhaps than say Bohmian pilot-waves, assumed physically real yet devoid of energy-momentum content, able to one-way react on particles but not vice versa, and instantaneously update throughout the universe, notwithstanding SR/GR considerations. Or Everett many-worlds, local and realistic at the price of an unending exponentially exploding rate of branching 'worlds'. Just how the branching physically occurs is a mystery it seems, as is the issue of where a postulated absolutely perfect linearity somehow avoids a rational expectation of such scenario quickly yielding a saturation/pile-up crisis grinding everything to a halt.

It seems to me that the rational way forward is to accept that quantum theory is fundamentally *different* from classical theory. It allows some things to be done, which classically are impossible; but it also forbids many things to be done, which classically can be done. Since it is so different, it means that there are fundamental differences in what are the fundamental ingredients of physical theory. I think that quantum theory brings in randomness at the ground level. As a fundamental physical feature of the universe. Something which cannot be explained by reduction to other features. Not an emergent feature. This in contrast to randomness as we otherwise know it in physics.

Now unfortunately the main components of classical physics are also hard-wired in our brains by evolution. They are called "systems of core knowledge" in neuro-linguistics. The basic brain modules which allow a new-born to process new sensory inputs and build up a "model" of the world. Going outside these systems therefore goes deeply against the grain. What we call "understanding" is just "representing within a baby classical physics model of the world". Hence if nature, if reality, does not fit to that baby model of the world, we are never going to "understand" it. We can *understand* relativity theory, but not quantum theory.

On top of the limitations of our "embodied cognition" there are also extremely strong cultural prejudices. Aristotle said that every effect has a cause. Even the Buddha said it. Randomness, unpredictability, is felt as chaos, as being evil. It is connected to strong taboos. To religion. In order to see the future we try to see meaning in random patterns of tea-leaves or the entrails of slaughtered animals. Gambling is evil (forbidden by many religions) but attractive. The pre-socratic philosophers however saw randomness as something creative, not something threatening; something to be valued, not something to be feared or banned. Possibly the transition from valuing randomness to deep fear of it goes with the transition from early matriarchal society to later patriarchal society. That's what technology has done for us: given the males dominance over the females. A society built on rigid hierarchical principles sees chance as a huge threat, instead of as a constant source of opportunity. (Chance drives evolution! Chance drives sex!).

There is no need to bring in "signalling" to explain some of the features of quantum physics, provided we are prepared to allow something else into our world-view which so far we have instinctively shunned: irreducible, fundamental, randomness. A slogan version of Bell's theorem (not the math inequality, but the metaphysical deductions from that entirely elementary bit of calculus) is "QM is incompatible with locality + realism + no-conspiracy". http://en.citizendium.org/wiki/entanglement_(physics). So if we accept QM (or more precisely: if we accept that certain of its predictions are pretty close to being correct, including that the background situation in which they are predicted is realizable) one must reject one of the three.

Most people don't want to reject locality (just the Bohmians managed to convince themselves it is OK). No one wants to accept super-determinism of a such a conspiratorial type that it makes a total mockery of science, except perhaps Gerhard 't Hooft (probably because of the Calvinistic background culture he was raised in). In all of the rest of physics, there is no evidence for non-locality. In all of the rest of physics, there is no evidence for conspiracy. So Occams' razor tells us to reject "realism". Note that realism is a bad name, since the concept is actually a kind of idealism: realism means assuming the reality of outcomes of unperformed experiments. It means assuming the reality of a hidden layer behind what does manifestly exists. It seems it just gets us into trouble. Just what Bohr was saying all along.