- #71
martinbn
Science Advisor
- 3,658
- 1,857
Then it is all lost in translation (from maths to English) because it seems to me to be the opposite.PeroK said:Well, IMO, that is precisely what @DrChinese is saying!
Then it is all lost in translation (from maths to English) because it seems to me to be the opposite.PeroK said:Well, IMO, that is precisely what @DrChinese is saying!
I cannot follow. Maybe that is a question of terminology? Nonlocal correlations are pretty exactly the thing @vanhees71 proposes. Every serious full quantum field theory has these built in automatically. Mermin once wrote the hilarious tongue-in-cheek quote:PeroK said:Well, IMO, that is precisely what @DrChinese is saying!
Consider:PeroK said:Well, IMO, that is precisely what @DrChinese is saying!
DrChinese said:the synchronization of the distant end components clearly cannot occur without "spooky action at a distance" as demonstrated by the cited experiment.
Seems pretty opposite to me.Cthugha said:The authors do not intend to demonstrate spooky action at a distance in this paper. The authors aim to debunk this idea.
Perhaps it's best that @DrChinese confrms whether he thinks the cited paper debunks or supports superluminal "influences".PeterDonis said:Consider:
Seems pretty opposite to me.
@DrChinese is the one who cited the paper; what I quoted from him is from his post citing the paper. So I think we already have his opinion.PeroK said:Perhaps it's best that @DrChinese confrms whether he thinks the cited paper debunks or supports superluminal "influences".
I didn't know this experiment, but indeed the paper does in no way contradict my point of view. The "nonlocality" they are referring to is rather meaning the long-ranged correlations due to entanglement. This has nothing to do with faster-than-light signal propagation and a contradiction to microcausality, i.e., the mathematically precise meaning of "locality" in relativistic QFT.DrChinese said:I never said there was a causal signal faster than 10000 c. I would assume you already knew of this experiment by a top team, but assuming not:
https://arxiv.org/abs/0808.3316
"Testing spooky action at a distance"
"In science, one observes correlations and invents theoretical models that describe them. In all sciences, besides quantum physics, all correlations are described by either of two mechanisms. Either a first event influences a second one by sending some information encoded in bosons or molecules or other physical carriers, depending on the particular science. Or the correlated events have some common causes in their common past. Interestingly, quantum physics predicts an entirely different kind of cause for some correlations, named entanglement. This new kind of cause reveals itself, e.g., in correlations that violate Bell inequalities (hence cannot be described by common causes) between space-like separated events (hence cannot be described by classical communication). Einstein branded it as spooky action at a distance.
"A real spooky action at a distance would require a faster than light influence defined in some hypothetical universally privileged reference frame. Here we put stringent experimental bounds on the speed of all such hypothetical influences. We performed a Bell test during more than 24 hours between two villages separated by 18 km and approximately east-west oriented, with the source located precisely in the middle. We continuously observed 2-photon interferences well above the Bell inequality threshold. Taking advantage of the Earth’s rotation, the configuration of our experiment allowed us to determine, for any hypothetically privileged frame, a lower bound for the speed of this spooky influence. For instance, if such a privileged reference frame exists and is such that the Earth’s speed in this frame is less than 10−3 that of the speed of light, then the speed of this spooky influence would have to exceed that of light by at least 4 orders of magnitude."
As I mentioned, this experiment demonstrates that the selection of a measurement basis at one place (call that Alice) affects the outcome at a distant portion of the entangled system (call that Bob). The speed of the change cannot be less than 10,000 c per the experiment, and the experiment does NOT specify the time direction of the "spooky action at a distance". There is no known signal, nor can any causal element be deduced from this entanglement experiment (other than the fact that the subsystems Alice and Bob synchronize much faster than c would allow).
We are now over 60 replies into a thread started just a few hours ago. In case anyone forgot, the OP's question was: "are there any suggestions that entanglement might imply some sort of faster than light signaling between the entangled particles?" I said YES, and this experiment confirms those suggestions (subject to the caveats I provided). Keep in mind that before a measurement, the full entangled system cannot be said to be 2 individual systems (although there is spatial extent). However, the synchronization of the distant end components clearly cannot occur without "spooky action at a distance" as demonstrated by the cited experiment.
In both of these analyses, the hypo-
thetical superluminal influence was termed the speed of quantum
information to stress that it is not classical signalling. We shall use
this terminology, but we emphasize that this is only the speed of a
hypothetical influence and that our result casts very serious doubts
on its existence.
[\QUOTE]
To my mind, “superluminal influence” is an unfortunate term because of its unphysical connotations. From “Quantum entanglement” by Ryszard Horodecki, Pawel Horodecki, Michal Horodecki and Karol Horodecki (Rev. Mod. Phys. 81, 865 – Published 17 June 2009):PeroK said:... superluminal "influences"
My understanding is that this lower limit is for a hypothetical (and highly questionable) quantum information mechanism. This would be in contrast to what I perceive as the default position:DrChinese said:Although they don't state this, I believe their experiment also points to a lower bound for the "speed of quantum information" (I would use the word "nonlocality" instead of "information"). From the paper: "The violation of the Bell inequality at all times of the day allows one to calculate the lower bound for the speed of quantum information for any reference frame."
And, in this default position on entanglement there is no component of the theory where the term "speed" or "FTL" would be appropriate. There is simply no sense in which anything (however you describe it) is being exchanged between entangled particles.Lord Jestocost said:“Nature on its fundamental level offers us a new kind of statistical non-message-bearing correlations, which are encoded in the quantum description of states of compound systems via entanglement.
The general form of these claims doesn't make sense to me. If the measurement events are spacelike separated, the FTL "speed" of anything between them is frame dependent. I can make it 10,000 c or 100,000 c or instantaneous by choosing an appropriate frame, but I can also make it, say, 10 c or 1.1 c or 1.00000000000000000001 c by choosing an appropriate frame. There is no invariant that I can see that would correspond to the "speed" being talked about.DrChinese said:The cited experiment does just that, a lower limit of 10,000 c
This is where their assumptionPeterDonis said:but I can also make it, say, 10 c or 1.1 c or 1.00000000000000000001 c by choosing an appropriate frame
comes in, to allow then to get "a lower limit of 10,000 c".DrChinese said:given certain constraints ("that the Earth's speed in this frame is less than 10^-3 that of the speed of light")
And that's another questionable aspect of looking for a signaling mechanism: the need for a preferred, universal frame.gentzen said:This is where their assumption
comes in, to allow then to get "a lower limit of 10,000 c".
In other words, they are only considering a small subset of all possible reference frames. Which leads to the obvious next question: why should I care about whatever they think they're showing, since it's frame dependent and so has no physical meaning?gentzen said:This is where their assumption
comes in, to allow then to get "a lower limit of 10,000 c".
Because the implication is that relativity is wrong!PeterDonis said:In other words, they are only considering a small subset of all possible reference frames. Which leads to the obvious next question: why should I care about whatever they think they're showing, since it's frame dependent and so has no physical meaning?
PeterDonis said:The general form of these claims doesn't make sense to me. If the measurement events are spacelike separated, the FTL "speed" of anything between them is frame dependent. I can make it 10,000 c or 100,000 c or instantaneous by choosing an appropriate frame, but I can also make it, say, 10 c or 1.1 c or 1.00000000000000000001 c by choosing an appropriate frame. There is no invariant that I can see that would correspond to the "speed" being talked about.
Why is that the implication? Why isn't the implication that their method of analysis is wrong since it attributes physical meaning to frame-dependent quantities?PeroK said:Because the implication is that relativity is wrong!
As I understand, they decided to analyse the idea of a quantum information signaling and make the requisite assumptions and see what conclusions they could draw. And, this led to at least a certain scepticism that the mechanism is viable. And possibly the researchers felt they had debunked the whole idea.PeterDonis said:Why is that the implication? Why isn't the implication that their method of analysis is wrong since it attributes physical meaning to frame-dependent quantities?
"The requisite assumptions" can't in themselves be inconsistent with relativity because there is a relativistic theory, QFT, that correctly predicts the correlations. The assumption of a preferred frame does not seem to me to be "requisite" to explain the correlations; it seems to me to be an additional assumption driven only by the particular method of analysis chosen. The fact that that additional assumption is inconsistent with relativity seems to me to be far more likely to be an issue with the assumption and the method of analysis that drove it than with relativity itself, particularly in view of the fact that, as above, we can correctly predict the correlations with a relativistic theory.PeroK said:they decided to analyse the idea of a quantum information signaling and make the requisite assumptions and see what conclusions they could draw
PeterDonis said:"The requisite assumptions" can't in themselves be inconsistent with relativity because there is a relativistic theory, QFT, that correctly predicts the correlations.
No, I'm not assuming that. It's perfectly possible to have an "explicitly nonlocal/nonlrelativistic interpretation" that still acknowledges and accounts for the fact that, in domains where relativistic effects are significant, our best current experimental data says that those relativistic effects are present. For example, the version of Bohmian Mechanics that @Demystifier has published is explicitly nonlocal and nonrelativistic, but it still has Lorentz invariance as an emergent symmetry that is expected to be observed in all of our experimental tests to date.DrChinese said:You assume that all explicitly nonlocal/nonrelativistic quantum theories/interpretations are wrong (of which Bohmian Mechanics is just one).
But if that's the case, how can any such interpretation possibly be ruled out by an experiment that confirms those same predictions for Bell tests?DrChinese said:Those generally make identical predictions for Bell tests as does QFT (which has many other advantages),
But if there is any such interpretation, it would make predictions for Bell tests different from the predictions of standard QM/QFT. But you said above (in what I quoted) that these interpretations make identical predictions for those tests to standard QM/QFT. So what, exactly, is being ruled out by this experiment?DrChinese said:IF there exists a universal preferred reference frame, AND the Earth's motion relative to it is within a threshold, THEN: a Bell test like the one they ran will in fact show differences in some directions (which are not predicted by garden variety QM).
PeterDonis said:But if that's the case, how can any such interpretation possibly be ruled out by an experiment that confirms those same predictions for Bell tests?But if there is any such interpretation, it would make predictions for Bell tests different from the predictions of standard QM/QFT. But you said above (in what I quoted) that these interpretations make identical predictions for those tests to standard QM/QFT. So what, exactly, is being ruled out by this experiment?
How have proponents of those theories/interpretations responded?DrChinese said:The purpose of the cited paper was to demonstrate a little known consequence of some of the outlier candidate/competing theories. Thus showing that there are some predictions which are not identical.
Of course, within non-relativistic QM there is no such apparent tension between causality and the assumption that there's instantaneous influence between far-distant objects, and thus there's nothing to be debated in this direction within non-relativistic QM and even interpretational extensions of it like Bohmian mechanics. GRW is another case, because that's a new theory beyond QM. There's not the slightest hint for its necessity though.DrChinese said:You assume that all explicitly nonlocal/nonrelativistic quantum theories/interpretations are wrong (of which Bohmian Mechanics is just one). I'm sure you are aware of some others (such as GRW). Those generally make identical predictions for Bell tests as does QFT (which has many other advantages),
So, why have you claimed the opposite before?DrChinese said:That was their point, without them saying so directly. IF there exists a universal preferred reference frame, AND the Earth's motion relative to it is within a threshold, THEN: a Bell test like the one they ran will in fact show differences in some directions (which are not predicted by garden variety QM). It doesn't, so one should rule those out by experiment. Of course, defenders of those alternate candidate theories will attempt to explain away the experiment or further modify the theory...
Surely this is interpretation dependent and therefore not obvious. Many interpretations would not frame quantum correlations as evidence of nonlocal influences.DrChinese said:Obviously: Alice's selection of a measurement basis causes (by assumption only) distant Bob's later measurement to be synchronized with Alice's results. This occurs FTL, possibly instantaneously, and further the causal direction is ambiguous. Put a different way: the relative timing of Alice's and Bob's measurements in a Bell test are irrelevant (this is consistent with any interpretation). Whatever influence occurs to make this happen, is labeled "quantum nonlocality" in the more modern usage of the term, and this is generally accepted as well.
This statement cannot be found in professional scientific literature, which this thread is supposed to be about.vanhees71 said:If by nonlocal influences you mean faster-than-light causal influences then any interpretation which assumes them is in plain contradiction with the mathematical facts of microcausal relativistic QFTs!
I cannot cite one now, but I am sure I have seen published papers that state or suggest that the causal structure as described by relativity is incorrect. My impression is that is most Bohmians even if they don't say it out loud.Demystifier said:This statement cannot be found in professional scientific literature, which this thread is supposed to be about.
Sure, I have written it myself, but such papers emphasize that it is not in contradiction with "the mathematical facts of microcausal relativistic QFTs". The idea of such papers is that relativistic QFT is right but incomplete. Completeness is often an additional tacit assumption in standard QFT literature, but completeness is not a mathematical fact.martinbn said:I cannot cite one now, but I am sure I have seen published papers that state or suggest that the causal structure as described by relativity is incorrect. My impression is that is most Bohmians even if they don't say it out loud.
Of course it can. The microcausality condition is precisely imposed to avoid faster-than-light signal propagation.Demystifier said:This statement cannot be found in professional scientific literature, which this thread is supposed to be about.
That's true, but signal is the key word. Your original statement was wrong because it didn't contain the word signal. Standard QFT forbids FTL propagation of signals, but it doesn't forbid FTL propagation of influences. I know that you don't distinguish signal from influence, but professional scientific literature does.vanhees71 said:The microcausality condition is precisely imposed to avoid faster-than-light signal propagation.