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> contrary to the previous results, allows for joint violation of them. What does this mean? Wouldn't this be expected? Why wouldn't previous results have demo

> contrary to the previous results, allows for joint violation of them. What does this mean? Wouldn't this be expected? Why wouldn't previous results have demonstrated this?

eigenket 4y ago
Detecting Bell-nonlocality requires two parties doing measurements on separated systems. Contextuality (in the formulation used here) is detected by measuring two out of a possible five measurements on a single particle. Quantum mechanics says you can't measure arbitrary observables simultaneously, so you can only choose two out of five rather than the whole set of five. A previous work combined the nonlocality and contextuality setup and showed that you couldn't detect the contextuality on one particle and the nonlocality between the two particles simultaneously. However the previous work assumed you only use the results of one of the two contextuality measurements you do as part of your nonlocality detection. I.e. the previous work had both measurements on particle A being used to detect contextuality, and used the results of one of those combined with the results of a measurement on particle B to detect nonlocality. In this work they show that if you lift this assumption, so the nonlocality is now being detected by both measurements on A as well as the measurement on B, you can detect the nonlocality and contextuality simultaneously.
- naasking 4y ago
  > In this work they show that if you lift this assumption, so the nonlocality is now being detected by both measurements on A as well as the measurement on B, you can detect the nonlocality and contextuality simultaneously Which makes me think that Bell was right that Bohmian mechanics places the real problem with QM, non-locality, front and centre where it can't be ignored.
  - eigenket 4y ago
    While I disagree (quite strongly) with the claim that non-locality is a problem with QM (on the contrary it seems like an extremely interesting and useful property), I would like to know why you think this paper has anything specifically interesting for Bohemian mechanics rather than any other interpretation.
    - naasking 4y ago
      It's a problem for QM because it establishes an obvious tension with relativity that can't be ignored. The reason this has more relevance for Bohmian mechanics is because that interpretation makes the non-locality explicitly manifest in the core ontology, where other interpretations try to eliminate the non-locality via other means, eg. Copenhagen eliminates counterfactual definiteness, many worlds eliminates collapse, etc. Edit: This means Bohmian mechanics has an unresolved tension with relativity, which is why it hasn't yet been extended to a field theory.
      - raattgift 4y ago
        > obvious tension with relativity Quantum nonlocality is perfectly compatible with special relativity. The spacetime of the standard model of particle physics is exactly the flat spacetime of special relativity. Far from being ignored, the compatibility between special relativity and quantum mechanics has been investigated and demonstrated experimentally over the past century. In modern graduate courses this is best seen by studying the relevant group theory. You can read more about the SO(3,1)+ Lorentz group (which also applies to the Maxwell-Heaviside field equations for electromagnetism and the Dirac equation) at e.g. <https://en.wikiversity.org/wiki/Representation_theory_of_the_Lorentz_group#Non-technical_introduction_to_representation_theory https://en.wikiversity.org/wiki/Representation_theory_of_the...>. These are also excellent lecture notes: <https://arxiv.org/abs/math-ph/0602018 https://arxiv.org/abs/math-ph/0602018> and <https://www.ippp.dur.ac.uk/~krauss/Lectures/StandardModel/StandardModel.pdf https://www.ippp.dur.ac.uk/~krauss/Lectures/StandardModel/St...>
        naasking 4y ago
        > Quantum nonlocality is perfectly compatible with special relativity... ...if you accept the ontology of certain interpretations of QM. Bohmian mechanics is based on a different ontology, which is why Bohmian mechanics currently has to posit a preferred foliation of spacetime in order to square with relativity. This is a little ugly, to put it mildly. Edit: and the point I was making re:non-locality being the real problem, is because there are still plenty of unresolved issues between QM and general relativity, and I think these will come down to ontological incompatibilities, and so a different ontology may provide the insight needed to make a breakthrough.

> contrary to the previous results, allows for joint violation of them. What does this mean? Wouldn't this be expected? Why wouldn't previous results have demonstrated this?

eigenket 4y ago

Detecting Bell-nonlocality requires two parties doing measurements on separated systems. Contextuality (in the formulation used here) is detected by measuring two out of a possible five measurements on a single particle. Quantum mechanics says you can't measure arbitrary observables simultaneously, so you can only choose two out of five rather than the whole set of five. A previous work combined the nonlocality and contextuality setup and showed that you couldn't detect the contextuality on one particle and the nonlocality between the two particles simultaneously. However the previous work assumed you only use the results of one of the two contextuality measurements you do as part of your nonlocality detection. I.e. the previous work had both measurements on particle A being used to detect contextuality, and used the results of one of those combined with the results of a measurement on particle B to detect nonlocality. In this work they show that if you lift this assumption, so the nonlocality is now being detected by both measurements on A as well as the measurement on B, you can detect the nonlocality and contextuality simultaneously.

naasking 4y ago
> In this work they show that if you lift this assumption, so the nonlocality is now being detected by both measurements on A as well as the measurement on B, you can detect the nonlocality and contextuality simultaneously Which makes me think that Bell was right that Bohmian mechanics places the real problem with QM, non-locality, front and centre where it can't be ignored.
- eigenket 4y ago
  While I disagree (quite strongly) with the claim that non-locality is a problem with QM (on the contrary it seems like an extremely interesting and useful property), I would like to know why you think this paper has anything specifically interesting for Bohemian mechanics rather than any other interpretation.
  - naasking 4y ago
    It's a problem for QM because it establishes an obvious tension with relativity that can't be ignored. The reason this has more relevance for Bohmian mechanics is because that interpretation makes the non-locality explicitly manifest in the core ontology, where other interpretations try to eliminate the non-locality via other means, eg. Copenhagen eliminates counterfactual definiteness, many worlds eliminates collapse, etc. Edit: This means Bohmian mechanics has an unresolved tension with relativity, which is why it hasn't yet been extended to a field theory.
    - raattgift 4y ago
      > obvious tension with relativity Quantum nonlocality is perfectly compatible with special relativity. The spacetime of the standard model of particle physics is exactly the flat spacetime of special relativity. Far from being ignored, the compatibility between special relativity and quantum mechanics has been investigated and demonstrated experimentally over the past century. In modern graduate courses this is best seen by studying the relevant group theory. You can read more about the SO(3,1)+ Lorentz group (which also applies to the Maxwell-Heaviside field equations for electromagnetism and the Dirac equation) at e.g. <https://en.wikiversity.org/wiki/Representation_theory_of_the_Lorentz_group#Non-technical_introduction_to_representation_theory https://en.wikiversity.org/wiki/Representation_theory_of_the...>. These are also excellent lecture notes: <https://arxiv.org/abs/math-ph/0602018 https://arxiv.org/abs/math-ph/0602018> and <https://www.ippp.dur.ac.uk/~krauss/Lectures/StandardModel/StandardModel.pdf https://www.ippp.dur.ac.uk/~krauss/Lectures/StandardModel/St...>
      - naasking 4y ago
        > Quantum nonlocality is perfectly compatible with special relativity... ...if you accept the ontology of certain interpretations of QM. Bohmian mechanics is based on a different ontology, which is why Bohmian mechanics currently has to posit a preferred foliation of spacetime in order to square with relativity. This is a little ugly, to put it mildly. Edit: and the point I was making re:non-locality being the real problem, is because there are still plenty of unresolved issues between QM and general relativity, and I think these will come down to ontological incompatibilities, and so a different ontology may provide the insight needed to make a breakthrough.