What do violations of Bell's inequalities tell us about nature?

DennisN

I don't know, but I could quote Isaac Newton;
which is a sort of caveat to his law of universal gravitation (his law implies that gravitational force is transmitted instantaneously, which we now understand is not correct). This quote is of course about gravitation, not quantum entanglement. But my point is that many people find it hard (incl. me) to accept any kind of action at a distance without any mediator/medium in between and/or without any mechanism which describes it in more detail. And if the action seems to be instantaneous, it's even worse (considering the finite value of the speed of light). That pretty much sums up my own problems with action at a distance .

(I saw bohm2 already had replied to this while I was writing my reply)

Nugatory

I find myself wondering which of realism and locality is more "natural" to our thinking, more easily accepted at an intuitive level.

I'm inclined to think that it's realism:
- A cat and a bird are outside watching one either right now... I am quite confident that the biochemical computers that guide their behavior are programmed to analyze the situation in purely realistic terms. I doubt that this bias would change if either were to develop greater capacity for abstract thought.
- People are discouragingly willing to accept magical non-local explanations such as astrology. These non-local magical explanations are generally realistic; the astrologers don't question whether the moon and the planets are there when no one is looking.
- Few people are disturbed by the truly egregious non-locality of Newtonian gravitation; and I expect that most laypeople find Schrodinger's cat more disturbing/confusing/"wrong" than gravitational action at a distance.

Interesting though (at least to me) is that the poll results are running the other direction...

danR

I don't see how 'action at a distance' applies to entanglement in quantum world, even by analogy, where/(if) there is no 'action' or 'distance'. Of course ultramicroscopic particles are subject to other properties dependent on space and time. They are 4-space dependent, but quantum-wise non-local. Or to put it less prejudicially (since 'non-local' has the connotation of being somehow defective, deviant, odd), quantum-entanglement has only one locale.

Of course, there are spins that are not entangled, but I could speculate further that all spin-baggage, correlated or not, is permanently stuck in some cosmic LaGuardia airport.

Nugatory

true enough... but also worth noting that Newton is something of an outlier here. For every person who has shared Newton's (and many other thinkers') discomfort with action at a distance, probably thousands of people have cheerfully accepted and swallowed the notion.

DennisN

True. I have once been one of those thousands of people . But I changed.

Maui

Materialism would be the old mechanistic concept of reality but this is beside the point. The point is not why there could potentially be non-locality but why there is locality. When you answer that question from the point of view of qm(since this is the quantum theory forum!), then we can know why under certain circumstances non-locality could be observed. People seem to forget(even in this forum) that reality is quantum mechanical and not classical. If you treat classical mechanics as fundamental(not emergent) you get action at a distance, nonlocality, tunneling through barriers, many worlds, backward causation, objects spinning in two directions at the same time and other wonderful phenomena. And people go on to extrapolate all the time the reality of tables and chairs to the quantum realm as if they are somehow interchangeable or compatible.

bohm2

In hindsight, I'm not sure Gisin's argument that Newton's non-locality is more "radical" is accurate. For instance, quantum non-locality would not also have to be FTL (instantaneous) but would also have to be unattenuated and discriminating as Maudlin and others note:

The quantum connection is unattenuated:
The quantum connection is discriminating:
Quantum non-locality & Relativity
http://www.amazon.com/Quantum-Non-Lo.../dp/0631232214

That's a good point.

danR

Locality is simply entailed by the original expansion of 4-space, the condensation of matter, and the fractionation of the forces. There's no reason to require that every attribute of the original entity was dragged along with the emergence of locality.

PS: I've now mangled the quotes thoroughly, but hope y'all can sort it out.

rubi

The "Shut up and calculate" choice is definitely missing. Unless there are some observable differences between different interpretations of QM or unless they make calculations easier, it's a waste of time to think about it. I also don't want to be counted to option #3.

However, if i had to choose between non-locality and anti-realism, i would choose anti-realism, because i don't really see why realism is so desirable apart from the fact that otherwise, one has to give up his beliefs and prejudices about nature that originate from the naive assumption that we can extrapolate the laws of the macroscopic world to the microscopic world as well.

On the other hand i'd rather not give up locality, because that would mean that some events in the andromeda galaxy or even outside the observable universe could in principle influence events on earth, unless you impose some strong limitations on the non-locality in your theory (and if you do so, then you'd have to justify them somehow). That would make physics entirely pointless, because it would mean that our equations would have to depend on parameters that can't be measured here on earth. So even if the world were non-local, it's reasonable to assume that it's not, in order to even be able to write down equations that are of any use.

danR

Limitations are precisely what are involved. Spin correlation of co-generated photons, for example. Since quantum entanglement is not a mediator of any of the forces, I would want to know what the influence would be.

If Bob measures ↓ here and Alice measures ↑ at the Andromeda galaxy, they are only measuring a single down-up attribute shared (somewhere in nowhere-ville) by two co-generated particles.

DrChinese

Of course that is not a requirement for entanglement, that they are co-generated. They don't even need to interact by conventional means - or even have interacted at all if entanglement swapping is considered.

nanosiborg

That QM predicts (and experiment confirms) that quantum entanglement is unaffected by distance is based on the classical conservation laws and empirically based optics principles (eg., Malus' Law). The quantum entanglement correlations (and the idea that distance isn't a factor) aren't unexpected or 'weird' given what's been ported from classical physics and wave optics to the quantum theory.

This is essentially correct, except for the bolded part (and also that the 'private arrangement' need not be between just two particles). In a typical optical Bell test involving paired photons, measurement at one end need not be affecting the photon at the other end in order to produce the observed correlations. There just needs to have been a relationship produced between the motional properties of paired photons. The production of such an entanglement doesn't require that the photons have interacted or that they have a common source.

It's true that ...
... but it's not true, as DrChinese has pointed out, that ...
The motional properties of entangled particles need only to have undergone some sort of similar modification which produces a measurable relationship between their resulting motions.

In light of the contributions of the classical conservation laws and classical wave optics to the QM treatment of polarization entangled photons, it's maybe a bit misleading to say that ...
The difference between the sorts of relationships that can be produced in classical preparations and those that can be produced in quantum preparations is one of degree. But the principle is essentially the same. A common origin, interaction, or imparting a common or related motional property to spatially separated particles produces statistical dependence and predictable correlations ... with the underlying fine tuning of quantum entanglement correlations remaining something of a mystery.

Regarding the question of why there is locality, this is similar to the question of why disturbances in media expand more or less omnidirectionally (depending on the properties of the medium in which the disturbance is produced), in that they both might well be unanswerable questions. That is, they both might be irreducibly fundamental properties of physical reality, and as such would form part of the axiomatic structure of a comprehensive theory. Which is sort of the place that the principle of local action, along with causal determinism, has in contemporary physical science. These are (at least tacitly held) assumptions that are required for physical science to have any unambiguously communicable meaning.

The metaphysical speculations about nonlocality, etc. remain just that. If violations of Bell inequalities actually informed regarding nature, well, that would be great. Unfortunately, they don't.
But that doesn't make Bell's theorem 'short-sighted', as another current thread asked. Bell's analysis provides a very clear answer to the question he was asking. Namely, are QM-compatible LHV models of quantum entanglement possible? The answer, mathematically proven, is no, they aren't. If you take Bell's formulation to be generalizable, and I do, then QM-compatible LHV models of quantum entanglement are definitively ruled out. Beyond that, violations of Bell inequalities tell us nothing about nature. If that doesn't do it for you, then you might be talking round and round about this stuff, and getting nowhere, for a really long time.

stevendaryl

Here's a wild idea that's probably nonsensical, but I wonder if anyone has investigated it: Kaluza-Klein theory introduced the trick of having extra spatial dimensions that are unobservable because they are wrapped into tiny little circles. I'm wondering if there is some topology that can be constructed using extra dimensions, so that, essentially, every point in space is the same, very short, distance from every other point, if one travels in the hidden dimensions. For illustration, imagine a flat sheet of paper, crumpled into a ball and compressed to a tiny volume. Travel within the plane of the paper is unaffected by the crumpling, but the crumpling allows a "short-cut" between any two points, by traveling perpendicular to the plane of the paper.

So I'm wondering if there is a way to understand the "instantaneous" quantum interactions of Bohm theory as interactions that only seem instantaneous because they only travel a short distance.

stevendaryl

Well, following the Bohm interpretation of quantum mechanics, the weird statistics is explained through action at a distance via an instantaneous "quantum potential" term in the equations of motion.

nanosiborg

Interesting stevendaryl, but I think that whatever you're getting at is way over my head.

In line with danR's statement, I don't think that instantaneous action at a distance is understandable. There's no mechanics, no propagation, no time for any sort of physical interaction. I view it as basically a collection of terms that function as a placeholder for our ignorance and refer to something that happens in the mathematics of a theory.

But it sounds like you might be able to fashion some sort of novel mathematical contrivance or other. Not that that would provide any understanding either, but then mathematical contrivances (and placeholders) don't have to. They just need to help facilitate the calculation of accurate quantitative predictions.

nanosiborg

But the statistics aren't weird. They're understandable through the QM incorporation and application of classical laws.

stevendaryl

They seem pretty weird to me. When you are measuring, for instance, the projection of the spin of an electron on the z-axis, for example, I think it's understandable that the result may be nondeterministic. The measurement process may interact with the electron in an uncontrollable way, and so a deterministic prediction might not be possible. But if that electron is part of an electron-positron twin pair, then it's weird to me that you can tell with absolute certainty that if you measure spin-up in the z-direction, then whoever checks the spin of the positron will find spin-down in the z-direction.

That's the weirdness of quantum randomness--not the randomness by itself, but the combination of randomness with a kind of certainty of the distant correlations.