DrChinese
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But what else could you say? You could say they can't be explained by Bell-local theories, but that's a tautology. Bell's theorem has quite a few assumptions, as does the ontological models framework.No disagreement, but I hope we don't have to say that every time...
Well, I guess my vote would be... Quantum Nonlocality.But what else could you say?
Well, we have Witten, who wrote quite a detailed review paper on entanglement in QFT (Rev.Mod.Phys. 90, 045003 (2018), https://arxiv.org/abs/1803.04993). Reinhard Werner and others also frequently emphasize that the story is more complicated than one usually assumes. Indeed semi-popular papers rarely make use of anything more complicated. They would be pretty dumb to do so. Let me give more details in the next response.He is also saying QFT is local microcausal. I admittedly do not follow the distinction between "local causal" and "local microcausal". However, if I don't follow that distinction, I doubt many others do either unless they are knee deep in QFT. The term "microcausal" does not show up in papers on entanglement, ergo I assume it is not relevant. In fact, I would say as a rule that elements of QFT (as opposed to older QM) are not usually referenced in papers on entanglement.
Well, let me put it this way: Physics is the art of making models that make predictions that match reality (as quantified by experiments). So of course any effect should be considered within its model or framework. If one uses standard QM, which is not relativistically invariant anyway, it is quite natural to consider nonlocality and consider entanglement as a property of the states. It is the natural way of looking at entanglement in QM. In QFT, entanglement is already a property of the algebra of observables (see e.g. Witten's review above) and not just of the states. It is quite natural to consider different mechanisms and terminology.2. I agree with everything you say here. So apparently the point missed is: whether it is non-locality or non-realism/contextuality that rules, the effect is called Quantum Nonlocality in the literature and it is a generally accepted feature in the quantum world. Attempting to mask this by calling it "nonlocal correlations that result from local microcausality" goes against the grain of almost any publication, either lay or scientific. Just this year, an entire book was written on this so I guess we should call them up and tell them to retitle it "Local Microcausality". So I would say it is very misleading to label it "local microcausality" when the Bell options are to reject locality or to reject realism/contextuality.
Although I think what's sometimes missing in these accounts is that dropping determinism is not enough to get nonclassical correlations, you also have to drop the existence of countertfactuals.
Rejecting assumptions about unmeasured variables goes not give way out of Bell type inequalities.The standard meaning in Quantum Foundations, that variables unmeasured have values.
Chapter 6 of Peres's monograph "Quantum Theory: Concepts and Methods" discusses it and it is used very explicitly in his proof of Bell's theorem. It's not an assumption called out in the original Bell proof, but it is the assumption Copenhagen rejects so it is important to recognize. He says famously "Unperformed experiments have no results"
I do not mean (and I want to empasize this as it is what people seemed to think it means in previous discussions) the trivial fact that unperformed experiments did not happen.
I don't see how this statement can be taken seriously.Essentially, it boils down to Mermin's tongue-in-cheek statement (American Journal of Physics 66, 753-767 (1998) , https://arxiv.org/abs/quant-ph/9801057):
"My complete answer to the late 19th century question “what is electrodynamics trying to tell us” would simply be this:
Fields in empty space have physical reality; the medium that supports them does not.
Having thus removed the mystery from electrodynamics, let me immediately do the same for quantum mechanics:
Correlations have physical reality; that which they correlate does not."
What do you mean by "joint choice of measurement bases for Alice and Bob"? Choices of measurement bases are made by Alice and Bob at two spacelike separated events. You need nonlocality influencing choice of measurment settings (!) or superdeterminism to have someting like that.There is a joint choice of measurement bases for Alice and Bob and QFT (and every correct theory) yields the correct results for this combination of measurements. Within this framework it does not matter which measurement comes first and one does not have to assume any causal influence. It of cause does not rule out such an influence either, but there is no need to assume one.
Basically you are saying that Many local Worlds permits violations of Bell's inequality? Or I didn't understood you correctly?Rejecting a common sample space clearly permits violations of Bell's inequality, it's what QM actually does where there is no Gelfand homomorphism mapping all four CHSH variables into one sample space.
The Schrodinger picture does have a state ##|\psi(t)\rangle## for spatially extended system. It is not manifestly Lorentz invariant, but there is a Lorentz-invariant version based on many-time formalism.Neither formulation has states for spatially extended systems
But at least Minkowski spacetime has an analytic continuation to an Euclidean space, right? This means that path integral QFT in the absence of gravity is well defined. QFT in curved spacetime has other problems too, but the full theory of quantum gravity is expected to solve those one day.Note the path integral is only well-defined in a Riemannian space, not in Lorentzian spacetimes. Since some spacetimes have no analytic continuation to a Riemannian space there is no path integral in general.
Nice to see that there is a popular journalist exposition of my work. Thanks!**Our own @Demystifier published an article
Huh? experimental correlations on detectors do not fall from the sky. You can measure spin correlations, momentum correlations, OAM correlations time-bin correlations or whatever. The message is quite clear. Having a state with well-defined correlations does not imply that the individual correlated quantites are well defined. Well-defined values of spin correlations do not imply that the individual spin values are well-defined. In fact, quite the opposite is true as these are usually complementary quantities. See, e.g. Phys. Rev. A 62, 043816 (2000) or Phys. Rev. A 63, 063803 (2001).I don't see how this statement can be taken seriously.
In Bell experiments correlations correlate detection events given measurement settings. Would you say following Mermin that either or both detection events and measurement settings do not have physical reality?
I do not get your post. Why do you bring LHV models into play? This thread is not about LHV models. By joint choice I mean just that: a set of detector settings. Ordinary standard QM already gives the correct predictions for every possible detector setting Alice and Bob might use. It does so irrespective of which interpretation of QM you might use as long as it is consistent with QM, which means that it cannot be local realistic. QFT as the more demanding theory contains ordinary QM in the non-relativistic limit and of course also already gives the correct predictions for every possible detector setting Alice and Bob may use. I do not see how this could be even controversial. The math of QFT also does so irrespective of how you want to interpret QFT as long as your interpretation is consistent with QFT. This again means no local realism. However, as QFT is relativistically invariant already, it would be somewhat awkward to sacriface this feature by going for a non-local interpretation. At least unless you assume that there is some need for potentials with an infinite number of derivatives in some even deeper theory or something like that.What do you mean by "joint choice of measurement bases for Alice and Bob"? Choices of measurement bases are made by Alice and Bob at two spacelike separated events. You need nonlocality influencing choice of measurment settings (!) or superdeterminism to have someting like that.
Well, if you know the choice of measurement basis for either Alice or Bob at the moment when entangled particles are produced you can replicate correlations with LHV, no doubt about that.
Definitely it's well defined in Minkowski spacetime. It's just an interesting fact as such.But at least Minkowski spacetime has an analytic continuation to an Euclidean space, right? This means that path integral QFT in the absence of gravity is well defined. QFT in curved spacetime has other problems too, but the full theory of quantum gravity is expected to solve those one day.
No, I'm saying the lack of a common sample space/context permits violation of Bell's inequalities. This isn't really anything to do with Many Worlds.Basically you are saying that Many local Worlds permits violations of Bell's inequality? Or I didn't understood you correctly?
It is standard and correct to say that relativistic QFT is "local microcausal". "Microcausality" means no superluminal communication.He is also saying QFT is local microcausal.
Thanks for clarifying that.It is standard and correct to say that relativistic QFT is "local microcausal". "Microcausality" means no superluminal communication.
Just to add to that, it should be said that we already know that "no signalling" is an assumption that is too weak to represent reality. It is well known that in theory one can consider states that are non-signaling as defined by relativistic causality and still not realizable in quantum mechanics. We know that e.g. from the seminal paper by Popescu and Rohrlich.After all: the issue here is "spooky action at a distance" via entanglement, which does not offer FTL signalling. On the other hand, I guess it makes sense to point out that a relativistic formulation of QM explicitly requires signal locality. Which also answers the OP.
Well, you was the one who quoted Mermin who said correlations fall from the sky. If the quote is viewed in context it might become clear Mermin meant that correlated quantities are not well defined, that way it makes more sense actually.Huh? experimental correlations on detectors do not fall from the sky. You can measure spin correlations, momentum correlations, OAM correlations time-bin correlations or whatever. The message is quite clear. Having a state with well-defined correlations does not imply that the individual correlated quantites are well defined. Well-defined values of spin correlations do not imply that the individual spin values are well-defined. In fact, quite the opposite is true as these are usually complementary quantities. See, e.g. Phys. Rev. A 62, 043816 (2000) or Phys. Rev. A 63, 063803 (2001).
And the alternative in your viewpoint is ... ?However, as QFT is relativistically invariant already, it would be somewhat awkward to sacriface this feature by going for a non-local interpretation.
But that cannot be correct. The measurements are spacelike separated, so they are made simultaneously. Neither measurement can occur "before" the other and create a cause and effect relationship because they cannot be time ordered. You would have to reject relativity. Apparently, what people must be arguing about is whether relativity is correct.Weinberg phrases the answer as: "Of course, according to present ideas a measurement in one subsystem does change the state vector for a distant isolated subsystem..."
Can you explain what do you mean by "sample space/context" because it seems that you attach different meaning to "sample space" than the one used in probability theory.No, I'm saying the lack of a common sample space/context permits violation of Bell's inequalities. This isn't really anything to do with Many Worlds.
No, relativity does not become incorrect if you add preferred reference frame to it. Only consensus interpretation of relativity becomes invalid.But that cannot be correct. The measurements are spacelike separated, so they are made simultaneously. Neither measurement can occur "before" the other and create a cause and effect relationship because they cannot be time ordered. You would have to reject relativity. Apparently, what people must be arguing about is whether relativity is correct.
He does quite the opposite. I linked the full article. It is of course using basic language, but I do not see how your statement relates to Mermin's.Well, you was the one who quoted Mermin who said correlations fall from the sky. If the quote is viewed in context it might become clear Mermin meant that correlated quantities are not well defined, that way it makes more sense actually.
So? Who claimed that it does? My post was in response to the assumption that the QFT version outlined above is local realistic. It is obviously not as assuming that something has well-defined correlations does not mean that the correlated quantities on their own have well-defined values or are local realistic hidden variables. That is all.But still this approach does not resolve Bell inequality question as Bell type inequality is provable without any reference to hypothetical quantities relying only on measurement settings and detections: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.47.R747 (see part II Bell inequalities for n<100%)
If I had to bet money, I would always go for non-realism/contextuality or whatever you would like to call it. From my point of view, during the last few years, studies of uncertainty relations (which is what "non-realism" usually boils down to) with respect to entanglement have been the most relevant studies to advance the field, such as the paper linked in my last post or Science 330, 1072 (2010) (https://arxiv.org/abs/1004.2507). Of course, other people will have a different opinion about what is significant. That is perfectly fine.And the alternative in your viewpoint is ... ?
I'm using it in the same sense as in probability theory. See Chapter 6 of Streater's "Lost Causes in Theoretical Physics". He has a good explanation of it.Can you explain what do you mean by "sample space/context" because it seems that you attach different meaning to "sample space" than the one used in probability theory.
Wikipedia: In probability theory, the sample space of an experiment or random trial is the set of all possible outcomes or results of that experiment.
There's quite a few of these results now, obtaining the Tsirelson bound from some principle. Others are information causality or Cabello's exclusion principle. @RUTA has a pretty good derivation in terms of conservation of angular momentum and discrete outcomes.Just to add to that, it should be said that we already know that "no signalling" is an assumption that is too weak to represent reality. It is well known that in theory one can consider states that are non-signaling as defined by relativistic causality and still not realizable in quantum mechanics. We know that e.g. from the seminal paper by Popescu and Rohrlich.
We also know that it is locality applied to uncertainty relations (or rather: a general formulation of uncertainty and a certain version of locality termed relativistic independence that in a nutshell boils down to being unable to tinker with local uncertainty relations from a distance) that exactly give the bounds: https://advances.sciencemag.org/content/5/4/eaav8370 (should be open access).
No one is questioning relativity per se. Experiments support it. (And as a point worth mentioning, the equations of relativity are time symmetric anyway. So that could potentially explain the appearance of quantum nonlocality, although that is speculation at this time.) But there seems to be a gray area when it comes to entangled systems: they have spatial extent but act as if that extent doesn't constrain the system as might be otherwise expected.But that cannot be correct. The measurements are spacelike separated, so they are made simultaneously. Neither measurement can occur "before" the other and create a cause and effect relationship because they cannot be time ordered. You would have to reject relativity. Apparently, what people must be arguing about is whether relativity is correct.