B Why is it assumed communication through entanglement would be FTL?

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As far as I can tell nobody actually knows how quantum entanglement really works, and yet everyone assumes that quantum states are teleported faster than light. But what if there isn't any actual teleportation, because both sides still share the same position? Maybe in a dimension we cannot perceive, or maybe they form something like a micro-wormhole, or maybe entanglement itself creates our spacetime (10.1146/knowable-050319-1)?
So it would only be perceived as FTL, but is actually just using a shorter path.

Has anyone proven, or disproven, anything in this direction?
 
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Relativity tells you that the events (sender and receiver) are spacelike seperated and therefore cannot be time ordered. That means there are some frames in which the assumed receiver gets the message before it is sent.
 

DrChinese

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As far as I can tell nobody actually knows how quantum entanglement really works, and yet everyone assumes that quantum states are teleported faster than light. But what if there isn't any actual teleportation, because both sides still share the same position? Maybe in a dimension we cannot perceive, or maybe they form something like a micro-wormhole, or maybe entanglement itself creates our spacetime (10.1146/knowable-050319-1)?
So it would only be perceived as FTL, but is actually just using a shorter path.

Has anyone proven, or disproven, anything in this direction?
Experiments have set the *lower* bound (if it is not instantaneous) for such transmission in the vicinity of 10,000 c.

On the other hand: if there is a "shortcut" we can't see, no other quantum effect is known to use that. And it does not fit into any existing theoretical framework. (Of course you can just make up speculative ideas - maybe there are speedy invisible birds carrying quantum information - but the issue is that there is no scientific value to that. BTW speculation is banned here by forum rules.)

All that is out there are what are called interpretations of quantum mechanics which attempt to explain entanglement and other quantum phenomena. Many Worlds (MWI) and Bohmian Mechanics (BM) are 2 such, and there are more. I would recommend that you read up on those.

Interpretations of QM
 

atyy

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Entanglement cannot be used for faster than light communication.

However, entanglement and non-commuting observables in quantum mechanics show (with some assumptions) that things in the underlying reality are not restricted to travelling no faster than light. However, if there are things travelling faster than light, we cannot use them to send faster-than-light messages.

The relationship between EPR entanglement and wormholes is not part of any standard theory, but does appear in the research literature.
E. Sergio Santini, Might EPR particles communicate through a wormhole? Europhys.Lett.78:30005, 2007
Gregory S. Duane, Tunneling through bridges: Bohmian non-locality from higher-derivative gravity, Phys. Lett. A (2018)
Juan Maldacena, Leonard Susskind, Cool horizons for entangled black holes, Fortschritte der Physik, 2013
 

vanhees71

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Experiments have set the *lower* bound (if it is not instantaneous) for such transmission in the vicinity of 10,000 c.
AFAIK there's not a single experiment that proves this. Can you cite a proper peer-reviewed article claiming such a bold contradiction to all we know about relativistic quantum field theory?

There are no instantaneous interactions ever seen in any of the many Bell experiments performed with higher and higher precision. Everything is in full accordance with standard relativistic QFTs, where such a thing is mathematically impossible by construction. I don't know, why you keep claiming the opposite to cearly established mathematical facts of QFT, which is the most successful theoretical description of nature we have!
 
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AFAIK there's not a single experiment that proves this. Can you cite a proper peer-reviewed article claiming such a bold contradiction to all we know about relativistic quantum field theory?
10,000c! I would really like to see that experiment! Btw, Susskind has posted, on Youtube, some really good lectures on quantum entanglement. They are light on math but very insightful from the master. Search Youtube "Stanford physics Susskind entanglements"
 

PeroK

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10,000c! I would really like to see that experiment! Btw, Susskind has posted, on Youtube, some really good lectures on quantum entanglement. They are light on math but very insightful from the master. Search Youtube "Stanford physics Susskind entanglements"
Well, if they are really light on math, then perhaps @vanhees will be able to follow them!
 

DarMM

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AFAIK there's not a single experiment that proves this. Can you cite a proper peer-reviewed article claiming such a bold contradiction to all we know about relativistic quantum field theory?
It's not that the experiments found something violating relativity, it's that they have found that if there is a nonlocal deterministic process behind entanglement it has to operate at at least 10,000c to match experiment.

 
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DrChinese

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AFAIK there's not a single experiment that proves this. Can you cite a proper peer-reviewed article claiming such a bold contradiction to all we know about relativistic quantum field theory?
Sure, this from one of the top teams in this subject:


Testing spooky action at a distance
D. Salart, A. Baas, C. Branciard, N. Gisin, H. Zbinden
(Submitted on 25 Aug 2008)
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.
That would be 10,000 c. This type of experiment is the most relevant to what the OP was asking, I believe. Of course, spooky action at a distance could also be instantaneous IF there is something that is the cause, and something that is the effect. However, it is not clear that is the case from any existing experiment I am aware of.

PS I see DarMM beat me to the punch on this. :smile:
 

DrChinese

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There are no instantaneous interactions ever seen in any of the many Bell experiments performed with higher and higher precision. Everything is in full accordance with standard relativistic QFTs, where such a thing is mathematically impossible by construction. I don't know, why you keep claiming the opposite...
There is precisely one person I have ever encountered who asserts what you do above. That being... you. I could quote as many top scientists as you like - Weinberg being one I have quoted repeatedly - that agree 100% with me*. His opinion on "spooky action at a distance", which is more usually labeled as "quantum nonlocality" (a generally accepted element of QFT):

"Of course, according to present ideas a measurement in one subsystem does change the state vector for a distant isolated subsystem..."

That's the very definition of action at a distance, something demonstrated and documented in thousands of experiments. Of course, most physicists accept that an entangled system cannot be considered localized in the first place, in complete contradiction to your statements: "There is a troubling weirdness about quantum mechanics. Perhaps its weirdest feature is entanglement, the need to describe even systems that extend over macroscopic distances in ways that are inconsistent with classical ideas."


*And please be aware that my viewpoint is nearly 100% identical to the consensus of the many authors I read. I am not really in a position to have opinions that deviate from scientific consensus. And I would certainly identify those differences clearly if I expressed them, something you seem unwilling to do. I would say why you don't, except it would be rude to speak that way. As in the other threads we spar in, you will have the opportunity to have the last word. I will not respond further to you in this thread because it does not relate to the OP.
 

vanhees71

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Well, if they are really light on math, then perhaps @vanhees will be able to follow them!
Usually, I have problems to understand QT without the math. Susskinds "Theoretical Minimum" however has exactly the minimum of math needed for understanding. It's a masterpiece in the (semi-)popular science literature.
 

vanhees71

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It's not that the experiments found something violating relativity, it's that they have found that if there is a nonlocal deterministic process behind entanglement it has to operate at at least 10,000c to match experiment.

It's not that the experiments found something violating relativity, it's that they have found that if there is a nonlocal deterministic process behind entanglement it has to operate at at least 10,000c to match experiment.

This only underlines once more that the assumption of nonlocal deterministic processes is in clear contradiction with (quantum) electrodynamics. In this sense you can take this kind of Bell tests as another sensitive test of the (in this case special) reltaivistic space-time description.
 

vanhees71

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Sure, this from one of the top teams in this subject:


Testing spooky action at a distance
D. Salart, A. Baas, C. Branciard, N. Gisin, H. Zbinden
(Submitted on 25 Aug 2008)

That would be 10,000 c. This type of experiment is the most relevant to what the OP was asking, I believe. Of course, spooky action at a distance could also be instantaneous IF there is something that is the cause, and something that is the effect. However, it is not clear that is the case from any existing experiment I am aware of.

PS I see DarMM beat me to the punch on this. :smile:
But you take the wrong conclusions! Under the assumption of a non-local deterministic theory there's be the violation to the space-time model of special relativity, but that contradicts the empirical facts about its very validity, particularly the universality of the speed of light in vacuum. The only conclusion from this experiment (as from many others) thus can be that non-local deterministic models contradict fundamental physics, which is not the case for local (microcausal) relativistic QFT, which in turn describes the observed results of all Bell tests known today.
 

vanhees71

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There is precisely one person I have ever encountered who asserts what you do above. That being... you. I could quote as many top scientists as you like - Weinberg being one I have quoted repeatedly - that agree 100% with me*. His opinion on "spooky action at a distance", which is more usually labeled as "quantum nonlocality" (a generally accepted element of QFT):

"Of course, according to present ideas a measurement in one subsystem does change the state vector for a distant isolated subsystem..."

That's the very definition of action at a distance, something demonstrated and documented in thousands of experiments. Of course, most physicists accept that an entangled system cannot be considered localized in the first place, in complete contradiction to your statements: "There is a troubling weirdness about quantum mechanics. Perhaps its weirdest feature is entanglement, the need to describe even systems that extend over macroscopic distances in ways that are inconsistent with classical ideas."


*And please be aware that my viewpoint is nearly 100% identical to the consensus of the many authors I read. I am not really in a position to have opinions that deviate from scientific consensus. And I would certainly identify those differences clearly if I expressed them, something you seem unwilling to do. I would say why you don't, except it would be rude to speak that way. As in the other threads we spar in, you will have the opportunity to have the last word. I will not respond further to you in this thread because it does not relate to the OP.
Where is this quote by Weinberg from? In his Vol. I of QT of Fields he is obvioiusly still of another opinion taking the locality of interactions in terms of microcausaly as one way (in fact the only known way today) to realize the cluster-decomposition principle and also to make the S-matrix Poincare invariant. It's the very principle that there should be none spooky actions at a distance. The correlations described by entanglement are in full accord with this standard QFT and there's no spooky action at a distance. To the contrary it's ruled out by the very construction of this QFT as made explicit in Weinberg's books much more explicitly than in most other QFT textbooks.
 

atyy

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Where is this quote by Weinberg from? In his Vol. I of QT of Fields he is obvioiusly still of another opinion taking the locality of interactions in terms of microcausaly as one way (in fact the only known way today) to realize the cluster-decomposition principle and also to make the S-matrix Poincare invariant. It's the very principle that there should be none spooky actions at a distance. The correlations described by entanglement are in full accord with this standard QFT and there's no spooky action at a distance. To the contrary it's ruled out by the very construction of this QFT as made explicit in Weinberg's books much more explicitly than in most other QFT textbooks.
The constructions you refer to rule out faster than light communication, but do not rule out spooky action at a distance. @DrChinese is not saying anything controversial (modulo a couple of word choices), but you are - and you are wrong.
 

vanhees71

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How do you prove "spooky action at a distance" empirically, if "faster-than-light communication" is ruled out? At least in Einstein's original meaning both notions are synonymous.
 

atyy

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How do you prove "spooky action at a distance" empirically, if "faster-than-light communication" is ruled out? At least in Einstein's original meaning both notions are synonymous.
A simple way of constructing spooky action at a distance is to simply take the collapse or state reduction to be real. Whether the collapse is real or not is only a matter of interpretation within the orthodox interpretation, but neither is ruled out by the orthodox interpretation. Also, even if we allow the collapse to be real, it does not permit faster than light communication.
 

vanhees71

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This is semantics. You can assume a lot of phantasy. If it's not observable in principle, it's not part of science but of (sometimes enertaining) science fiction.
 

atyy

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This is semantics. You can assume a lot of phantasy. If it's not observable in principle, it's not part of science but of (sometimes enertaining) science fiction.
And are the operators that on which you enforce microcausality observable?
 

vanhees71

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Not necessarily. E.g., any fundamental half-integer spin field operator does not refer directly to observable quantities. What's always observable are energy, momentum, and angular-momentum densities, charge-current distributions, etc built thereof.
 

atyy

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Not necessarily. E.g., any fundamental half-integer spin field operator does not refer directly to observable quantities. What's always observable are energy, momentum, and angular-momentum densities, charge-current distributions, etc built thereof.
Are observables observable? Do self-adjoint operators exist in the lab by the same criterion you use to reject collapse as observable?
 

OCR

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Well. . . !!

There is precisely one person I have ever encountered who asserts what you do above.

That being... you.

That seems to narrow things down, a bit. . . . :DD








OK, even though I read everything here, yes everything, I probably shouldn't

post here, so. . .



Carry on . . 👌 . :oldbiggrin:

.
 

martinbn

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To me these discussions, here and in other thread, between @vanhees71 on one side and @DrChinese and/or @atyy on the other, seem to be entirely about language and way of expressing oneself. For instance when people say "the measurement here makes instantaneously a change there" they could mean all kinds of things, to me it is so vague that it is almost vacuous. What does it actually mean?

Let's take a specific question. The usual Bell scenario with an entangled pair of spin one half particles. A and B are going to measure along the z-axis only. In the given frame A measures first, then B. The agreement is that A can do one and only one of two things, either she measures or she does nothing. If needed we do it over many trials. Say in the first million particles she does one of the two options and on the second million the other option. B's task is to do whatever he wants to, and at the end he has to say on which million A measured and on which she didn't. Can he do that? My understanding is that he cannot, of course I might be wrong. But if I am not mistaken then what in the world does it mean that A's measurement causes a change in the state of B's particle if the two cases are indistinguishable for him, even if they do it millions of times?
 

vanhees71

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Are observables observable? Do self-adjoint operators exist in the lab by the same criterion you use to reject collapse as observable?
Observables are observable, which is why they are called observables. Neither self-adjoint operators nor Hilbert space vectors, nor real-valued classical fields or coordinates describing point particles in Newtonian physics "exist in the lab", or have you ever seen some in any lab you've visited? These are all descriptions of what we observe or can observe in the lab. It's done with all kinds of equipment, from a meter stick to measure macroscopic distances to ultrafine detectors to detect single quanta. Theoretical physics describe in mathematical terms what we observe or expect to observe given a certain experimental setup, no more no less. In the case of QT the predictions for what we shall observe is probabilistic, and as far as we know there's no other way to describe it, because nature is generically random in a very specific sense described by QT. At least nobody has been able to make another model and do construct any real-lab apparatus to prove this assumption wrong. At the same time QT (in its formulation as relativistic local QFT) is fully consistent with the relativistic space-time structure.

I think, we agree upon the clear mathematical statement that one cannot produce any faster-than-light communication within a relativistic local QFT. Where we don't agree is the status of the "collapse": While I interpret it as updating the description due to a measurement, including predictions about far-distant parts of the system for accordingly filtered partial ensembles with the correlations described by entanglement due to the initial preparation in the entangled state, you interpret it as physical instantaneous action at a distance creating these correlations at the moment the measurement is made to select the subensemble. While the former view is in full accordance with the mathematical construction of QFT, I don't see, how this can be claimed for the interpretation of the collapse as an instantaneous causal interaction between far-distant pieces of the can be made logically consistent. For me this interpretation is excluded by the construction of the local observables such that they obey the principle of microcausality. Whether or not the fundamental field operators represent local observables is completely irrelevant for this question. Most of these field operators do not represent local observables. Often they are not even self-adjoint nor gauge-invariant within gauge theories (as which the Standard Model is formulated) and thus don't represent observables. The observables are usually defined via what I'd call "the group-theoretical correspondence principle", i.e., via the representations of the observables as generators of symmetry transformations (Noether's theorem).
 

vanhees71

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To me these discussions, here and in other thread, between @vanhees71 on one side and @DrChinese and/or @atyy on the other, seem to be entirely about language and way of expressing oneself. For instance when people say "the measurement here makes instantaneously a change there" they could mean all kinds of things, to me it is so vague that it is almost vacuous. What does it actually mean?

Let's take a specific question. The usual Bell scenario with an entangled pair of spin one half particles. A and B are going to measure along the z-axis only. In the given frame A measures first, then B. The agreement is that A can do one and only one of two things, either she measures or she does nothing. If needed we do it over many trials. Say in the first million particles she does one of the two options and on the second million the other option. B's task is to do whatever he wants to, and at the end he has to say on which million A measured and on which she didn't. Can he do that? My understanding is that he cannot, of course I might be wrong. But if I am not mistaken then what in the world does it mean that A's measurement causes a change in the state of B's particle if the two cases are indistinguishable for him, even if they do it millions of times?
That's the perfect description! In my opinion it's completely right and to the point and nothing else than the minimal statistical interpretation. It's taking the content of the cluster decomposition principle which follows from the microcausality principle (which is sufficient but not necessary, though afaik nobody has ever constructed a successful QFT not obeying it) seriously: B cannot know what A has done to her spin at all. All he'll measure is that he has completely unpolarized spins (I guess the preparation of two spins in one of the four Bell states is the most accurate realization of unpolarized spins you can have from 1st principles). The same mutually holds for A herself.

Nevertheless due to the preparation in an entangled state there are observable correlations that are stronger than possible within local deterministic hidden-variable theories, which is the content of Bell's theorem about his inequality. To test this alternative (i.e., the local deterministic HV theories vs. relativistic QFT) you need to make an accurate measurement protocol about each single measurement events at both A's and B's place and then compare the outcomes. If both measure their spin in the same direction they'll find 100% correlation (for the spin-singlet if A find up, B necessarily finds down and vice versa). It doesn't matter, who measured his spin first or whether it's done at the same time (or to tell it in coordinate independent way: if the measurement events are space-like separated). This clearly demonstrates (by the ways in many real-world experiments on polarization entangled photon pairs) that A's measurement cannot be the cause for B's findings and vice versa. The correlations described by the entangled state are due to its preparation at the very beginning. Also the violation of Bell's inequality has been demonstrated. Also the possibility of entanglement swapping and delayed choice is in complete accordance with this "minimal statistical interpretation", and it's the only interpretation I know, which never leads to contradictions with the causality structure of relativistic spacetime.
 

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