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How does QFT handle non-locality?

  1. Dec 28, 2015 #1
    [Split off from https://www.physicsforums.com/threa...ight-quantum-hypothesis.847848/#post-5329824]
    As we know, there is a serious contradiction between nonlocal indeterminacy of quantum theory and local reality of special relativity, specifically reflected in the superluminal propagation of quantum states of an entangled electron pair. This problem probably comes from the wave-function assumption: The wave function collapses instantly when a measurement is made. I wonder, how is the problem solved in QFT? The fields of electron and positron disappear instantly when they annihilate?
     
    Last edited by a moderator: Dec 29, 2015
  2. jcsd
  3. Dec 28, 2015 #2

    bhobba

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    There is no conflict - except in the half truths of the pop sci press.

    The principle QFT is based on is the so called cluster decomposition property which precludes correlations of the EPR type:
    https://www.physicsforums.com/threads/cluster-decomposition-in-qft.547574/
    'It is one of the fundamental principles of physics (indeed, of all science) that experiments that are sufficiently separated in space have unrelated results…'

    Its a 'simplistic' statement by Wienberg of the actual property, which is technically more subtle. But the bottom line is the same - EPR type correlations are precluded. And when you think about it its obvious they should be. Put a red slip of paper in an envelope, and a green slip in another. Send one to the other side of the universe and keep the other. Open one and you immediately know the other. Its exactly the same with EPR type correlations - they must be precluded if cluster decomposition is to make sense.

    Also relativity is not based on local reality - its based on space-time symmetries. But that is a whole thread in itself for the relativity sub-forum - not the QM sub-forum.

    Thanks
    Bill
     
    Last edited: Dec 28, 2015
  4. Dec 28, 2015 #3
    But Fuwa at al claims:
    ... for the first time, we demonstrate Einstein's "spooky action at a distance" ...
    See: http://www.nature.com/ncomms/2015/150324/ncomms7665/full/ncomms7665.html

    I assume that Nature Communications reflects main-stream science.
     
  5. Dec 28, 2015 #4

    bhobba

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    Yes - its acceptable - but doesn't make it correct.

    Its wrong - and obviously so once you understand QM. You see the formalism of QM does not include collapse - that's only something some interpretations have.

    When encountering papers that make grandiose claims like that best to do a search on what people say. Here is what I found:
    http://physics.stackexchange.com/qu...nt-really-demonstrated-wave-function-collapse
    'The paper doesn't explain how their predictions would differ from those of non-collapse theories. Since the paper doesn't even discuss what would be predicted without collapse, it is difficult to see how it could rule out quantum theory without collapse. Quantum theory without collapse explains all of the predictions commonly attributed to quantum theory with collapse:

    http://arxiv.org/abs/1212.3245.

    Variations on quantum theory that include collapse, such as the GRW theory, may or may not reproduce the predictions made in the paper, but as this is not discussed it is difficult to tell whether the results are even consistent with such a theory. As such, the title of the paper does not accurately describe its contents.'

    We have had a number of similar papers discussed here. Invariably they are a misunderstanding of what's called weak measurements.

    Thanks
    Bill
     
  6. Dec 28, 2015 #5

    vanhees71

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    Well, sometimes Nature tends to have quite spectacular headlines, and sometimes I'm a bit worried concerning their review politics. If I'd have had to review an article like this, I'd never let through such a title, let alone the abstract, which I'm able to read and find almost completely wrong! It reads like a bad pop science "explanation" but not like a scietific paper in a referees journal. I've no access to Nature Communications; so I can't say what really has been done; for sure not what's said in title and abtract, because if so, this would mean a clear disproof of very general principles of local microcausal relativistic quantum field theory. If this was really the case, I'd expect to have heard about this and a whole flood of new theory papers should have appeared on arXiv ;-)).
     
  7. Dec 28, 2015 #6
    Einstein's "spooky action at a distance" is the basis of quantum teleportation, which, reportedly, has been well verified experimentally, as Wang et al. claim:

    "As well as being of fundamental interest, teleportation has been recognized as an important element in long-distance quantum communication, distributed quantum networks and measurement based quantum computation. There have been numerous demonstrations of teleportation in different physical systems such as photons, atoms, ions, electrons and superconducting circuits."

    See: Quantum teleportation of multiple degrees of freedom of a single photon, https://www.researchgate.net/journal/1476-4687_Nature[/URL] (Impact Factor: 41.46). 02/2015; 518 (7540):516-9. DOI: 10.1038/nature14246; [URL]http://www.nature.com/nature/journal/v518/n7540/full/nature14246.html[/URL]
     
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  8. Dec 28, 2015 #7

    bhobba

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    Where are you getting this from? That's incorrect.

    Once and for all, there is no definite spooky action at a distance - it's simply something some interpretations have. All EPR is, is a correlation. Bell sorted it out ages ago:
    https://cds.cern.ch/record/142461/files/198009299.pdf

    Thanks
    Bill
     
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  9. Dec 28, 2015 #8
    So you should write a Comment to clarify the confusion.

    In addition, I never heard "experiments that are sufficiently separated in space have unrelated results" is a fundamental principle. This statement itself is very ambiguous; "how far separated in space" is "sufficiently separated"?
     
  10. Dec 28, 2015 #9

    mfb

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    Spacelike separated, so no light ray (or anything slower than light) can go from measurement event A to measurement event B or vice versa.
     
  11. Dec 28, 2015 #10

    bhobba

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    In SR signals cant go FTL. So separate them by a distance greater than it takes light to travel during the experiment and what is done in one system cant affect the other.

    This thread isn't about SR, but there is sufficient confusion about it in the pop-sci press seeing exactly what its about will likely help:
    http://www2.physics.umd.edu/~yakovenk/teaching/Lorentz.pdf

    The FTL stuff associated with EPR you have likely read about doesn't bypass this because it cant be used to send information so clocks can be synchronised.

    Thanks
    Bill
     
  12. Dec 29, 2015 #11
    1. "Experiments that are sufficiently separated in space have unrelated results" is your reasoning from special theory, and it cannot be sais to be a fundamental principle. In fact, you failed to provide a reference from master journals.

    2. What you are trying to say is that the instant propagation of quantum states is not consistent with special theory, as I mentioned in Post #12.

    3. Repeatedly, I would like to have the question of Post #12 for you: "As we know, there is a serious contradiction between nonlocal indeterminacy of quantum theory and local reality of special relativity, specifically reflected in the superluminal propagation of quantum states of an entangled electron pair. This problem probably comes from the wave-function assumption: The wave function collapses instantly when a measurement is made. I wonder, how is the problem solved in QFT? The fields of electron and positron disappear instantly when they annihilate?"

    Thanks a lot.
     
  13. Dec 29, 2015 #12

    bhobba

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    Its from THE master textbook on QFT - written by the master field theorist - Wienberg - Quantum Theory Of Fields
    https://www.amazon.com/The-Quantum-Theory-Fields-Volume/dp/0521670535

    Its the textbook those that aspire to mastery of QFT work towards. Its beyond my current level - although I own them.

    The level I am at is:
    https://www.amazon.com/Quantum-Field-Theory-Gifted-Amateur/dp/019969933X

    Quantum fields are there all the time. They reside in a Fock Space:
    https://en.wikipedia.org/wiki/Fock_space

    Its in a superposition of zero particles, one particle etc etc. There is one field for photons, one for electrons and positions etc etc. As in ordinary QM all that happens is the state changes.

    QED contains two interacting fields - the electron-positron field and the photon field. An electron-positron disappears the electron field changes and so does the photon field. Note - its not instantaneous but happens very very fast and like so much in QT modelled as a perturbation.

    Thanks
    Bill
     
    Last edited: Dec 29, 2015
  14. Dec 29, 2015 #13
    I do not have access to Weinberg book with the statement "Experiments that are sufficiently separated in space have unrelated results" as a fundamental principle, but I don't think it is the main-stream view, because Einstein's "spooky action at a distance" is the basis of quantum teleportation, which, reportedly, has been well verified experimentally, as Wang et al. claim:

    "As well as being of fundamental interest, teleportation has been recognized as an important element in long-distance quantum communication, distributed quantum networks and measurement based quantum computation. There have been numerous demonstrations of teleportation in different physical systems such as photons, atoms, ions, electrons and superconducting circuits." See: Quantum teleportation of multiple degrees of freedom of a single photon, Nature (Impact Factor: 41.46). 02/2015; 518 (7540):516-9. DOI: 10.1038/nature14246; http://www.nature.com/nature/journal/v518/n7540/full/nature14246.html

    There are thousands of papers in Physical Review and Nature, which support Einstein's "spooky action at a distance". Are they all wrong? Only Weinberg is correct? Why does Weinberg not write papers to rebut those "wrong" papers?

    Note: You probably misunderstood the meaning of "master" in my post. I was told that Physics Forum accepts citations from journals only in Master Journal List: http://ip-science.thomsonreuters.com/mjl/
     
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  15. Dec 29, 2015 #14

    bhobba

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    I gave the link to Bells paper that gives the detail of the situation. If any paper says different its WRONG - simple as that. While refereed papers are not perfect, by and large they are correct so what is more likely is you are misinterpreting it.

    Obviously quotes from well respected standard texts are acceptable references.

    Thanks
    Bill
     
  16. Dec 29, 2015 #15
    Last edited by a moderator: Dec 29, 2015
  17. Dec 29, 2015 #16
    I agree that "quotes from well respected standard texts are acceptable references", but I don't agree that they must be correct. This paper [Can. J. Phys. 93: 1470–1476 (2015)] picked out some mistakes in Weinberg's book, claiming: "It is found in the paper that the Landau–Lifshitz version of Laue’s theorem ... and Weinberg’s version of Laue’s theorem ... are both flawed, ...".
     
    Last edited: Dec 29, 2015
  18. Dec 29, 2015 #17
  19. Dec 29, 2015 #18

    bhobba

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    Again I know its wrong because of a theorem called Bells Theorem:
    http://www.johnboccio.com/research/quantum/notes/paper.pdf

    The theorem states: If QM is correct then you cant have both counter-factual definiteness and locality. You can have one or the other but not both. If you want locality then you cant have counter-factual definiteness. If you want counter-factual definiteness then you need FTL influences - but only if you want counter-factual definiteness. The paper claiming to have measured FTL influences is claiming to have shown that QM must have the property of counter-factual definiteness, which the formalism most definitely does not require. In other words they would have proven QM as it currently stands incomplete - which was Einstein's position. That would be BIG - no VERY BIG news earning the discoverer an instant Nobel prize.

    How did it get by the referees process - well you already have had examples of that.

    Again it almost certainly is a misunderstanding of so called weak measurements.

    I see a pattern in your posts. You are told by me and other science advisor's you have misconceptions. Instead of taking it on board you squirm and post this paper or that paper that supposedly supports your position. Some of those science advisors are high powered Phd's in physics who have written textbooks on it. I am not in that category, but if I said anything that was incorrect rest assured they would have picked me up on it instead of liking my posts.

    This is not the way to learn. If you keep it up I wont respond because its a waste of my time.

    Thanks
    Bill
     
    Last edited: Dec 29, 2015
  20. Dec 29, 2015 #19

    bhobba

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    Can I ask your level of training in QFT? Do you have the background to understand Weinberg? Its a very advanced text not for the faint of heart. Unless you are at that level then there is no way to tell who is correct. I am not at that level - Weinberg is currently beyond me.

    In science, especially in advanced esoteric areas, there is sometimes disagreement. But that in no way changes things that are very well known, things like Bells Theorem. If they are proved wrong it would be big news - not simply a disagreement on some fine point of advanced theory.

    Thanks
    Bill
     
  21. Dec 29, 2015 #20

    vanhees71

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    Well, but standard relativistic QFT is a very well-established part of physics and is in no way esoteric (it becomes only esoteric when one adds the esoteric ideas of some socalled "interpretations" of quantum theory, of which QFT is one specific formulation that go beyond the empirically testable and indeed very well tested minimal interpretation). The Standard Model describes all so far discovered particles with an astonishing precision (some quantities like the anomalous magnetic moment of the electron or the Lamb shift of the hydrogen-atom energy levels are of the most precise agreement between theory and experiment ever). Weinberg's books are just the most comprehensive treatment of relativistic QFT, starting from the very fundamental concepts as special-relativistic spacetime structure, the representation theory of the Poincare group, and S-matrix theory, showing that the assumption of locality, microcausality, and boundedness of the energy spectrum from below are sufficient conditions for the linked-cluster principle to hold. Without this principle it is hard to believe that natural science as we know it was possible at all. If every local experiment, accessible to human observation, would be strongly correlated with long-distant objects, we'd hardly have the control over our experimental setups as we have, which is obvious from the last 118 years of elementary particle physics (I count 1897, the year of the discovery of the electron as the first elementary particle (lepton) as the birth year of elementary-particle physics).

    QFT in this well-established sense, as any formulation of QT, includes the possibility of socalled entangled states which includes the possibility of strong correlations, going beyond the possible correlations within any local deterministic theory, between observables of long-distant parts of a single quantum system. The astonishing thing is not that there seem to be "spooky actions at a distance" (as is the case in that flavor of the Copenhagen interpretation that assumes a collapse as an additional hypthesis and as was rightfully pointed out in the famous Einstein-Podolsky-Rosen paper) but that the strong correlations are explainable by QFT and thus compatible with the locality of interactions and that the linked-cluster principle is not violated either. Particularly, within standard QFT there is no possibility to use entanglement for faster-than-light communication and, to the contrary, the empirical verification of the long-range correlations require the exchange of "classical information" in terms of the measurement protocols done at the distant places, which can be transmitted at most with the speed of light in vacuo.
     
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