2019 International Workshop: Beyond Bell’s theorem

Testing the reality of Wigner’s friend’s experience

This topic contains 6 replies, has 3 voices, and was last updated by Avatar Eric G. Cavalcanti 3 months, 1 week ago.

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  • #5532
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    Howard Wiseman
    Participant

    https://arxiv.org/abs/1907.05607

    Does quantum theory apply to observers? A resurgence of interest in the long-standing Wigner’s friend paradox has shed new light on this fundamental question. Brukner introduced a scenario with two separated but entangled friends. Here, building on that work, we rigorously prove that if quantum evolution is controllable on the scale of an observer, then one of the following three assumptions must be false: “freedom of choice”, “locality”, or “observer-independent facts” (i.e. that every observed event exists absolutely, not relatively). We show that although the violation of Bell-type inequalities in such scenarios is not in general sufficient to demonstrate the contradiction between those assumptions, new inequalities can be derived, in a theory-independent manner, which are violated by quantum correlations. We demonstrate this in a proof-of-principle experiment where a photon’s path is deemed an observer. We discuss how this new theorem places strictly stronger constraints on quantum reality than Bell’s theorem.

     

    #5653
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    Federico Comparsi
    Participant

    “However, we emphasise once more that those correspond to observed events, and note that we make no assumption
    about hidden variables predetermining all measurement outcomes.”

    Regarding the first part of the quoted sentence, I would like to note that from Alice’s perspective those events observed by Charlie are not more real than hidden variables (or than Bob’s measurement choices and outcomes) until Alice become conscious of them. So it seems to me that it’s not a thing particularly interesting to emphasise.

    Regarding the second part of the quoted sentence, I think that this is not a step beyond Bell’s theorem, but it’s a step back, since the existence of the information for predetermining all measurement outcomes is a consequence of EPR argument, from which Bell’s theorem starts, as remembered in my paper for this workshop. This fact suggests that abandoning “macroreality” is not a valid option for saving locality.

    #5657
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    Eric G. Cavalcanti
    Participant

    Hi Federico,

    Thanks for the comments.

    Regarding the first part of the quoted sentence, I would like to note that from Alice’s perspective those events observed by Charlie are not more real than hidden variables (or than Bob’s measurement choices and outcomes) until Alice become conscious of them“.

    Your comment seems to already deny one of the assumptions that go into what we call “Local Friendliness”, namely the assumption of Observer-Independent Facts (OIF): that any event that is real to one observer is real to all observers. Some theories reject that assumption, and some theories do not (but must reject one of the other assumptions in light of the violation of the LF inequalities).

    I would also say that most physicists believe that they can keep that assumption–even while rejecting the existence of hidden variables more generally–and so they have something interesting to learn from our theorem.

    But I gather that what you mean is that if Charlie’s observations are real, then they must be (real) hidden variables (for Alice). That may be true, but again, we do not need to make that assumption. And importantly, what we are emphasising is that we do not need to assume that all measurement outcomes (even those that are not measured by Charlie) are predetermined by hidden variables.

    In other words, OIF is not the assumption that “unperformed experiments have predetermined results”. It is the (much weaker, and widely held) assumption that “performed experiments have absolute–i.e. observer-independent–results”.

    I think that this is not a step beyond Bell’s theorem, but it’s a step back, since the existence of the information for predetermining all measurement outcomes is a consequence of EPR argument, from which Bell’s theorem starts, as remembered in my paper for this workshop“.

    The EPR argument has its own set of assumptions, which have been debated for over 80 years. Whether or not predetermination of all measurement outcomes follows from the assumptions of the EPR argument is completely irrelevant to our argument, because we do not make the assumptions that EPR did. Furthermore, our assumptions DO NOT imply predetermination. Our assumptions are weaker than those of EPR, and weaker than those of Bell, and our conclusions are therefore stronger than both of those.

    It is pretty straightforward to show that our theorem has precisely one less assumption than a popular way of formulating the derivation of Bell inequalities. That is, our theorem assumes:

    Observer-Independent Facts (a.k.a. Macroreality)
    Locality (a.k.a. Parameter Independence)
    Freedom of Choice (a.k.a. No Superdeterminism / No retrocausality)

    To derive a Bell inequality one needs to make another assumption, for example, Outcome Independence. (Macroreality is not usually listed as an assumption, but it is needed, see e.g. [1] for a derivation in which it is explicitly included).

    Now, Shimony argued (and that’s a widely popular view) that a resolution of Bell’s theorem that could maintain “peaceful coexistence” with relativity was to reject Outcome Independence, while maintaining all of the other assumptions above. In particular, keeping Parameter Independence allowed for maintaining “no action-at-a-distance”. On the other hand, Shimony argued that the violation of Outcome Independence–dubbed “passion at a distance”–was a mild, acceptable form of nonlocality, since it was uncontrollable nonlocality.

    What our theorem shows is that this is no longer an option in light of violation of LF inequalities. Thus it is a strictly stronger conclusion than what can be reached via Bell’s theorem.

    To emphasise this point, another research programme that attempts to resolve Bell’s theorem in the same philosophical direction as “passion at a distance” is the program of quantum causal models. All frameworks for quantum causal models to date uphold all of the three assumptions that go into our theorem, and do not provide any mechanism by which the Local Friendliness inequalities could not be violated (as is done, for example, by collapse theories). Therefore our result also introduces a problem for quantum causal models, where Bell’s theorem clearly doesn’t–indeed QCMs were designed to circumvent Bell’s theorem! Again, this demonstrates that our conclusions are strictly stronger.

    This fact suggests that abandoning “macroreality” is not a valid option for saving locality“.

    It certainly is an option to save Locality (as explicitly defined in our paper) by abandoning Macroreality, or Observer-Independent Facts. This is what is done in Everett, relational QM, QBism, etc. Those are constructive examples that give the same empirical predictions as standard QM. Therefore, clearly, it can be done.

    If by “locality” you mean something else (I suspect you mean something similar to Bell’s notion of Local Causality), then I agree that that notion cannot be maintained by abandoning Macroreality. Please mentally substitute “Parameter Independence” whenever you see the word “Locality” in our paper. If you object to the idea that one can maintain Parameter Independence while abandoning Macroreality, I’m happy to discuss. If your criticism is simply that you do not agree that we call this notion “Locality”, then I am less interested in that debate.

    [1] Howard M. Wiseman, Eric G. Cavalcanti, “Causarum Investigatio and the Two Bell’s Theorems of John Bell”, https://arxiv.org/abs/1503.06413

    #5665
    Avatar
    Federico Comparsi
    Participant

    Hi Eric, thanks for the reply.

    #) “I gather that what you mean is that if Charlie’s observations are real, then they must be (real) hidden variables (for Alice). That may be true, but again, we do not need to make that assumption”

    Independently by the fact that you explicitly make that assumption, it’s nevertheless surely true.

    #) “If you object to the idea that one can maintain Parameter Independence while abandoning Macroreality, I’m happy to discuss. “

    I’m objecting exactly that, because the reality of the spatiotemporal events S, a, b, A, B (S is the process of emission of particles in the source) is a necessary condition to define speed of influences and so locality in physical space. In fact this is an implicit assumption of EPR-Bell argument/theorem.

    So if your theory reject this kind of “reality” assumption you cannot say neither that it’s local neither that it’s non-local (as in the many-world/mind interpretation). It’s pretty the same for retrocausal models. If you admit the possibility that the future can influence the past, then it’s not clear what’s the meaning of locality in physical space. For this reason I personally like to interpret this kind of models as superdeterministic models (conspiracy on global initial conditions).

    #5669
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    Eric G. Cavalcanti
    Participant

    Hi Federico,

    The reality of the spatiotemporal events S, a, b, A, B (S is the process of emission of particles in the source) is a necessary condition to define speed of influences and so locality in physical space. In fact this is an implicit assumption of EPR-Bell argument/theorem“.

    I agree that it is an implicit assumption in the EPR/Bell arguments. However, your claim that it is not possible to define speed of influences and locality without that assumption is a proof by lack of imagination. If you read the definition of Locality in our paper, I see no fundamental difficulty of understanding it from the perspective of a theory in which Macroreality is false, like Everett, relational quantum theory, or QBism. That assumption can be interpreted so as to be true in all of those theories.

    And regardless of whether or not you think that it is ultimately the best alternative, the fact remains that our theorem makes less assumptions than Bell’s, and therefore the violation of the LF inequalities leads to strictly stronger constraints on possible theories.

    #5671
    Avatar
    Federico Comparsi
    Participant

    “I see no fundamental difficulty of understanding it from the perspective of a theory in which Macroreality is false, like Everett, relational quantum theory, or QBism”

    Hi Eric, since I see many difficulties to give an operational notion of speed of influences in physical space without taking S,a,b,A,B as real events occurring in space, could you give me some references where these interpretation account for that? Thanks.

    #5673
    Avatar
    Eric G. Cavalcanti
    Participant

    could you give me some references where these interpretation account for that?

    This debate has developed in most detail in the case of the Everett interpretation. For example, Deutsch and Hayden, for example, have argued that a notion of locality can be maintained in Everett, and the notion of locality that can be maintained is essentially Parameter Independence:

    https://arxiv.org/pdf/quant-ph/9906007.pdf

    Here a more recent paper with reply to critics.

    https://royalsocietypublishing.org/doi/pdf/10.1098/rspa.2011.0420

    Now I don’t subscribe to all that Deutsch and Hayden claim there (e.g. I don’t think they show full appreciation for the subtleties involving Bell’s theorem), but I think they are at least partly right to claim that there exists a notion of locality that can be maintained in Everett (and it is not Bell’s notion of Local Causality, but a weaker notion of “no action-at-a-distance”).

    So if you want to continue claiming that this is impossible, you have to, at least, show where Deutsch and Hayden are wrong. And to simply point out that they are not maintaining Local Causality is not sufficient, since we already know that. You have to be open to try to understand that they are talking about another notion of Locality.

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