2018 Workshop on Wigner’s Friend

Response to Frauchiger and Renner

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    Dustin Lazarovici

    Dear all,

    thanks for the opportunity to participate. My colleague Mario Hubert and I have written a comment on the paper of Frauchiger and Renner (2018) “Quantum theory cannot consistently describe the use of itself”. We believe that their “no-go theorem” doesn’t actually show anything of interest. In particular, if the proposed thought experiment is analyzed correctly, no inconsistent predictions arise from a precise “single-world” quantum theory without collapse law such as Bohmian mechanics.

    There are some other interesting aspects to the thought experiment that we try to highlight, but those are not really new.

    We are happy to receive your comments and feedback.

    You can download our paper here: https://dustinlazarovici.com/comment_renner_new.pdf




    A good paper, Dustin. Your analysis is consistent with the analyses of others, including those of Tony and Richard, which are listed in the “References related” post. Shan

    Aurelien Drezet

    Dear Friends, I discovered the work by Frauchiger and Renner yesterday.
    I didn’t read the literature on the topics but I wrote a kind of comment which I put on arxiv one hour ago. Being a Bohmian my view is that actually this is just a rephrasing of Hardy’s paradox involving nonlocality between agents. The problem is not more paradoxical that it was before when Hardy’s wrote his fantastic article. For me the nicest part of Hardy’s work is about Lorentz invariance and it pushed people like Durr and coworkers to develop a hypersurface Bohm dirac model which I simply love.
    Now, involving massive thing is not a problem for a Bohmian you will have a lot of empty waves and nonlocal interactions but no paradox.
    The preprint will be available on arxiv tomorrow

    Mark Stuckey

    I read Frauchiger and Renner (FR) “Quantum theory cannot consistently describe the use of itself” (2018) and I’ve read several responses in this workshop, but I have a question that has not been answered.

    FR talk about a measurement of |h> – |t> by Wbar on the isolated lab Lbar. What does this measurement mean? If Lbar is a quantum system for Wbar, then all possible Hilbert space bases obtained via rotation from the basis |h>,|t> correspond to some physical measurement and the eigenvalues correspond to the physical measurement outcomes. I understand what such rotated bases and outcomes for spin measurements mean in terms of up-down results for relatively rotated SG magnets. Would someone please describe the measurement process and outcomes corresponding to the Wbar measurement of |h> – |t> on Lbar? Clearly, it’s not merely “opening the door and peaking inside,” as that would simply be a measurement in the original |h>,|t> basis. Right?

    Thnx in advance for the answer,
    Mark Stuckey

    Dustin Lazarovici

    @editor: Thank you, we quoted A. Sudbery’s paper in ours, but weren’t aware of R. Healey’s response.

    @Aurelien: I agree with you and highly recommend your paper. We made similar points in our paper, though maybe too briefly. Frauchiger and Renner admit that their thought experiment is modeled on Hardy’s paradox, so we didn’t stress too much that it’s not really new.

    : Indeed, I think these “extended Wigner’s friend” measurements are practically impossible, since they require a reversel of decoherence of macroscopic quantum states. Frauchinger and Renner, in their paper, have a section on how a more or less analogous experiment could be performed, but I didn’t study that carefully enough to comment.

    Renato Renner

    Some of the commenters are claiming that our thought experiment is “just a rephrasing of Hardy’s paradox”. As mentioned in our paper, our construction indeed invokes ideas due to Hardy, as well as, by the way, ideas from Wigner and Deutsch.

    However, in contrast to Hardy-type (as well as Bell-type) arguments, our paradox does not rely on concepts such as “locality” or “free choice”. (In fact, in our setup, the agents do not make any choices at all.) Rather, our argument is based on the idea that any universally valid theory should have the property that “the theory can be used to describe users of the same theory”.

    I am not sure whether the commenters really meant that this requirement is equivalent to the assumptions that enter Hardy’s argument. But if they think so, I would appreciate if they could explain in more detail how they translate, for instance, Hardy’s locality assumption, or the assumption of free choice, to our setup.

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