What is Non-locality: Definition and 57 Discussions

In theoretical physics, quantum nonlocality refers to the phenomenon by which the measurement statistics of a multipartite quantum system do not admit an interpretation in terms of a local realistic theory. Quantum nonlocality has been experimentally verified under different physical assumptions. Any physical theory that aims at superseding or replacing quantum theory should account for such experiments and therefore must also be nonlocal in this sense; quantum nonlocality is a property of the universe that is independent of our description of nature.
Quantum nonlocality does not allow for faster-than-light communication, and hence is compatible with special relativity and its universal speed limit of objects. Thus, quantum theory is local in the strict sense defined by special relativity and, as such, the term "quantum nonlocality" is sometimes considered a misnomer. Still, it prompts many of the foundational discussions concerning quantum theory, see Quantum foundations.

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  1. Tolga T

    B Bell's Inequalities and the issue of non-locality

    QM is compatible with Bell's inequalities clearly shows that no deterministic program can give similar results for entangled particles. In other words, there is no deterministic algorithm that mimics QM that the particles already follow before the measurement. So far, so good. But of course...
  2. R

    I New Nobel Aspect contradicts himself on action at a distance bel

    Alain Aspect got a Noble Physics prize for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science. According to this article https://phys.org/news/2022-10-quantum-entanglement-spooky-science-physics.html Aspect is quoted as...
  3. koroshii

    Finding a unitary operator for quantum non-locality.

    My trouble might be from how I interpret the problem. Alice and Bob are entangled. After Alice makes the measurement both of their states should collapse to one of these states with a certain probability. (Unless my understand of how entanglement is wrong.) The way I am understand the question...
  4. bhobba

    I EPR and Non-Locality - For and Against

    Hi As most people I think know I do not think that QM requires non-locality of any kind. The reason is it is a limiting case of QFT which since it combines SR and QM it can not violate the assumptions it is built on. Specifically we have the cluster decomposition property...
  5. L

    I Objective Wave Function and Non-locality

    In interpretations where the wave function represents something real, like Many worlds, Copenhagen with objective wave function and spontaneous objective collapses. I'd like to understand which of them has true non-locality. First. Is Many Worlds not having true non-locality due to the...
  6. Silvio Macedo

    I Wave function of particle / quantum field in space, also in time?

    Quantum fields have wave functions that determine a particle position in space. It solves non-locality, double-slit paradox, tunnel effect, etc. What if the wave function is also in time? Won't it solve the breaking of causality at quantum level? (Delayed Choice/Quantum Eraser/Time) Not much...
  7. Auto-Didact

    A Non-locality: sheaf cohomology

    Here are a few wonderful papers which describe non-locality and contextuality in detail using a combination of sheaf theory, graph theory and algebraic topology. Abramsky et al. 2011, The Sheaf-Theoretic Structure Of Non-Locality and Contextuality Abramsky et al. 2015, Contextuality...
  8. E

    B Why would non-locality solve the time paradox in dcqe?

    I've been trying to work out exactly what part of the experiment I'm fundamentally not getting. The experiment basically says that no information is being sent back in time because the coincidence counters are needed to isolate the signal photons and expose the interference pattern. But the...
  9. Deepblu

    I Why Bohmian Mechanics needs non-locality?

    I always think about entanglement as pure conservation of energy and conservation of angular momentum. In fact I see that only conservation of energy is non-local, and that quantum objects have nothing to do with non-locality, for example you can entangle 2 atoms that have never interacted with...
  10. Derek P

    I Exploring Non-locality in Many-Worlds Interpretation (MWI)

    In another thread, @Nugatory asked "how exactly does it turn out that both spacelike-separated observers end up in the same world without some appeal to non-locality?" Worlds are defined by their observed macroscopic values. They separate through decoherence. It is a physical process. Since...
  11. S

    A Pilot Wave Theory & Non-Locality

    So Heisenberg's Uncertainty says that we can't know both the position and the velocity of a particle accurately, because measuring one will disturb the particle enough that it's no longer possible to accurately measure the other as it was. So one or the other has to remain unknown to us. This...
  12. Dennis Plews

    A Non-locality and The First Law of Thermodynamics

    I'm sure I read somewhere in my physics books that non-locality is required in order to maintain the integrity of the 1st Law of Thermodynamics. Is that correct? If so, that seems to require one or more additional spatial dimensions via which the total amount of energy is maintained. What...
  13. M

    B Here is a good video that explains non-locality simply

    [Mentor's note: YouTube videos are not generally an acceptable reference under the PhysicsForums rules. Many are oversimplified, seriously misleading, or just plain wrong, and no one who is serious about understanding quantum mechanics should be trying to learn from them. We've left this...
  14. A

    I Holographic principle, non-locality, and Lorentz invariance

    Hi all, Some recent comments from the forums here led me to do a bit of reading on the holographic principle, and to a posting on "The Reference Frame" by Lubos Moti about the (likely lack of) 'holographic noise' in the experiment by Craig Hogan at Fermilab...
  15. Dennis Plews

    I The information paradox and non-locality, ER = EPR

    I heard recently that Dr. Maldecena recently commented to Dr. Susskind, regarding the black hole information paradox, that ER = EPR. Can anyone illuminate this for us mere mortals? It seems to link all information via non-locality, which has interesting implications.
  16. M

    I Holographic Principle discussion in Sean Carroll's book

    am reading Sean Carroll's pop-science book 'From Eternity to Here' and am having trouble connecting the links in his discussion of the Holographic Principle. At the outset, I would ask that you try to answer in terms of Carroll's discussion and without moving into concepts much more advanced...
  17. R

    I Is this Quantum non-locality set-up right?

    I'm not sure if this set-up has been put forth already but I'm curious what people think of it. It is about ascertaining the correlation of the polarization state of a pair of photons(A and B) by two observers considered inertial (x and y)that await at the same sufficient distance at opposite...
  18. Igael

    A Is non-locality a necessary consequence of the Bell theorem?

    What is the difference between quantum mechanics and realism ? quantum mechanics states on statistics while the hardy assumption of EPR is that hidden variables may describe exactly the outcomes of each individual test. Bell refutes the last idea. But, he didn't need to refute the case where...
  19. M

    Quantum non-locality and vacuum polarization

    Quantum particles are not localized before they are observed, as shown with the Young double slit experiments and those with entangled particles. On the other side, vacuum is filled with virtual particles. Are the non-localized particles responsible for the virtual particles? or only for a part...
  20. entropy1

    Non-locality: (FTL) 'influence' at all?

    I have a basic question about the concept of non-locality in QM. I'll explain as I understand it (correct me if I'm missing the essence): The so called 'non-local' property of two entangled photons (for instance), suggests that the measurement of one of the pair 'communicates' with the other...
  21. zonde

    Quantum mechanics is not weird (locality and non-locality weirdness)

    Let's be fair, it's not true. Pure states are the ones that correspond to exact physical states. And it is not intuitive that exact physical states should transform continuously. Our belief about outcome can transform continuously but belief does not correspond to pure state.
  22. P

    How does QFT handle non-locality?

    [Split off from https://www.physicsforums.com/threads/photon-energy-and-light-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...
  23. D

    Non-locality and Counterfactual definiteness

    I understand that through Bell's theorem both locality and counterfactual definiteness cannot exist within physics. However shouldn't the double slit experiment give physicists a clue that losing counterfactual definiteness is actually the best way of interpreting the problem? The double slit...
  24. Mentz114

    Can synchronization emulate non-locality ?

    I remember reading some time ago of a hypothesis ( I can't find it now) that quantum superpositions could be modeled by oscillating states. So some two valued attribute could be (say) 0 for half the time and 1 for other half ( the value flip-flops). Individual measurements at random (ish)...
  25. entropy1

    Are photons passing a polarization filter randomly?

    This may be a dumb question, but maybe someone can help me out: Consider a pair of entangled photons A and B, fired at respectively Alice and Bob who both let it go through a polarisation filter at different angles. Now Alice establishes that half of the photons get through her filter, and half...
  26. entropy1

    Perceived contradiction in non-locality principle

    I have had some criticism on a post of mine in another topic. Since I don't want to pollute that thread with my own discussion, and since I am a layman and am really curious about the answer, I'll pose my question here. Consider two polarisation-entangled photons A and B measured by Alice and...
  27. Jimster41

    Phonons and classical non-locality?

    I just discovered the concept of phonons and the story of Brian Josephson (from reading "Sync" by Steven Strogatz). What the h...? So Phonon's are collective excitations of classical stuff made from QM ensembles, excitations made possible by the QM properties of the ensemble members? Are...
  28. .Scott

    Given T-symmetry, what limits non-locality?

    A video on time reversal inspired me to attempt a version of Conway's "life" that would share QM's T-symmetry. If you have never hear of Conway's life, it is described here: http://en.wikipedia.org/wiki/Conway's_Game_of_Life My thought was that instead of the two colors (black and white) in...
  29. bohm2

    Non-locality versus Action-at-a-distance

    I thought this was an interesting paper as it argues that while some type of non-locality is likely a necessary feature of QM, it might not necessitate action-at-a-distance (even within an ontological interpretation). The basic argument appears to rest on a some distinction between locality...
  30. ChrisVer

    Spinless Salpeter equation non-locality

    A straightforward attempt to generalize the Schrodinger equation to a relativistic equation, is obtained by taking the Eigenvector/value equation: H |a_k> = E_k |a_k> with a hamiltonian of the form H= T+V , In the relativistic case we have T=\sqrt{p^2+m^2} , and taking also (for my purposes)...
  31. W

    Does Relativity Allow for Non-Locality in the Universe?

    It is my understanding (perhaps I am wrong) that relativity implies that all frames of reference are equal. For example, if there are two uniformly (no acceleration ) moving objects ( and no others, an empty universe) then one can say object "A" is moving and "B" is still. It is equally true to...
  32. B

    Has this approach to non-locality been explored?

    Non-locality is based on the assumption that the act of observing causes an effect instantaniously elsewhere. It occurred to me that we can't be certain that one thing(observing) is causing the other(non-local wave function collapse). Another solution might be that both events are...
  33. nomadreid

    Non-locality and the Bekenstein bound?

    On one side, the amount of information is bounded above for any fixed volume of space: this would seem (?) to indicate that information content is local. Yet physical states are not necessarily local, as non-local entanglement shows. So how do you have local information content of a non-local...
  34. C

    Space as not empty, non-locality, conceptual implications?

    Hi there! I understand the tendency of physicists to stick to the math, and the logic itself, and to often avoid attempting to conceptualize a process or law, but that's kinda the opposite of what I'd like to do here. I don't mind at all if you refer to the theory, math or logic, and id...
  35. S

    Entanglement and Bell's theorem. Is the non-locality real?

    The results of measurements of phase entangled particles together with Bell's theorem provide pretty convincing evidence that the Universe contains non-local interactions. Yet I'm lead to wonder. Let's imagine the usual idealised experimental scenario, where there is an emitter of...
  36. C

    Is there a consensus on Bell's Theorem and its implications?

    Hi. I have read about Bell's Theorem in a couple of popular science books, namely 'Quantum Reality' by Nick Herbert and the recent book by John Gribbin. However, I am still struggling to understand the overall idea. Could anyone explain the concept? Is it taught at university at all...
  37. A

    Contextuality and Non-Locality

    Every state that is contextual is nonlocal. ...and the inverse, is every state that is nonlocal is contextual ?
  38. R

    Scalar Boson and Quantum Non-locality

    It's impossible that the higgs boson was the only scalar boson in nature. Could quantum-nonlocality be mediated by scalar boson or connected with scalar field? How do you discount or refute this?
  39. I

    Big Bang Non-Locality: Evidence for an Infinite Universe?

    As I understand it the big bang happened "everywhere". It was not an explosion of matter, but a rapid and accelerating expansion of the space between matter. This is overcome by local gravity (solar systems / galaxies). If this assumption is correct, is there anything preventing or any theory...
  40. S

    Consensus about Non-Locality & Spacetime

    What's the present most popular consensus about non-locality and spacetime? Is it since the wave function is not something physical, there is nothing there in spacetime to be non-local about. So let's just extinguish the concept of physicality this means we just treat wave function and spacetime...
  41. D

    Does spacelike separation have anything to do with non-locality?

    The cause-effect relationship of timelike events is fairly self-evident. But I was wondering, do non-local effects of QM "synchronise" events that are separated along spacelike intervals? Could there be some kind of co-dependent arising, such that if event X happens in one region of the...
  42. StevieTNZ

    No non-locality after all in QM?

    At least, according to this paper - http://arxiv.org/ftp/quant-ph/papers/0206/0206190.pdf - nonlocality isn't real in QM. Thoughts?
  43. V

    How do you understand Non-Locality ?

    How do you understand "Non-Locality"? Some people state that "non-locality" is a wrong word to describe it because there is nothing to be non-local about.. meaning properties don't exist before measurement... that is, the particles don't exist before measurement.. so how can you say the...
  44. M

    Difference Between Non-Seperability and Non-Locality?

    I am taking a philosophy of quantum mechanics class and my teacher told me that if we believe in non-separability than locality is true. However, I do not see how this is possible. Non-separability and non-locality appear to be the same thing. Non-seperability is the fact that events...
  45. L

    Exploring Non-Locality in dBB Theory: Insights from EPR and Bell's Inequalities

    Where does non-locality come from in dBB? I've heard that when dealing with multiple particles, dBB is a non-local theory. The standard knowledge from studying Bell's inequalities is that any hidden-variable theory must be either non-local or non-realist. I'm ok with non-realist theories, but...
  46. zonde

    Pilot wave without non-locality

    I am trying to analyze EPR photon polarization experiment with pilotwaves. We create H and V photons in both channels of PDC source. They have associated H and V pilotwaves. Now we analyze composed system of H and V photons and their pilotwaves with PBS. If PBS is rotatated so that it is in...
  47. N

    Alain Aspect's Experiment: Investigating Non-locality and Entangled Photons

    Alain Aspect's experiment: In the Alain Aspect experiment, the two entangled photons will indeed both go through their respective polarizers. I'm hip with that. I'm also hip with Aspect's diligence in making it a "two-channel" testing apparatus. What I don't understand is why should...
  48. S

    Easy (I think) quantum non-locality question for the experts

    Hello All, I've recently become interested in learning more about the concept of quantum non-locality, and have a quick question I hoped someone might be able to shed some light on for me. My question is this: How do we know that Einstein was wrong that some "local hidden variable"...
  49. A

    Quantum Non-Locality Subspace Radio

    http://www.mjyoung.net/misc/quantum.htm read this and tell me if it works or if its a crock, it sounds good, but all I've had is high school Physics
  50. P

    Why Couldn't Einstein Accept Non-Locality?

    If General Relativity is background-independent, and, as Einstein himself said, general covariance "takes away from space and time the last remnant of physical objectivity," why do you think it was so hard for him to accept the possibility that quantum mechanics also could be non-local? Are...
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