The significance and implications of a recent extension of Wigner’s friend thought experiment has been discussed and debated. The main aim of this online workshop is to help settle the controversial issues related to the suggested experiment.
Workshop Date: Sunday, October 21, 2018 to Wednesday, October 31, 2018
Advisory Board: Lajos Diósi, Arthur Fine, Gordon N. Fleming, Olival Freire Jr., Sheldon Goldstein, Robert B. Griffiths, Hans Halvorson, Richard A. Healey, Basil J. Hiley, Don Howard, Peter J. Lewis, Roger Penrose, and Maximilian Schlosshauer.
Based on the successful previous workshops, this online workshop will be more selforganized. Every participant, after logging in, may create a topic in the workshop forum on his own, which gives a concise introduction to his ideas to be discussed. Then other participants can leave comments and participate in the discussions by text chat in the forum.
All IJQF members are welcome.

editor replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
Hi Richard,
Many thanks for your further clarification! Here is my responses:
> 3. I don’t understand how you have derived your alternative value for E(b,c) in the special case c=b, a=d. Can you explain?
In Alice’s frame, we have the quantum links $c —> d —> a —>b$. We can then derive $E(b, c)$ from the correlation functions $E(c, d) = −cos(c…[Read more]

Renato Renner posted an update in the group 2018 Workshop on Wigner’s Friend 4 years, 4 months ago
I tried to address the criticism in a talk that I gave at ETH Zurich: https://www.video.ethz.ch/speakers/its/2018/autumn/colloquium.html
I hope that the view that I presented there clarifies some of the issues that were raised here. 
Richard Healey replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
Shan,
1. It is important to notice that a quantum state assignment on a fixed spacelike hyperplane (like the hyperplane t*^3) may itself be made with respect to different inertial frames (say, Alice’s and Bob’s). Quantum states on different spacelike hyperplanes (like t^3 and t*^3) are not related by a boost transformation. So a derivation of…[Read more]

Richard Healey replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
Ruth,
A modern version of the SternGerlach experiment uses a hotwire detector (see, for example, http://web.mit.edu/8.13/www/JLExperiments/JLExp18.pdf ).
In this case, potassium atoms pass through an SG magnet, thereby entangling their spin and translational quantum states (not collapsing the spin state)! Interaction with the hot wire likely…[Read more] 
Mark Stuckey replied to the topic Response to Frauchiger and Renner in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
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 W…[Read more]

Ruth Kastner replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
Thanks very much, Richard.
Regarding the interpretation of probability, it seems that this is still an open question. I don’t think the Born probability is restricted to being about semantic objects such as claims. It can be directly about a system’s actualized properties (at least it certainly can in TI).
But in any case: I recognize that…[Read more] 
editor replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
Hi Richard,
I think Alice’s reasoning is right. My worry is still that the inequality (32) should be defined (and can also be calculated) in one frame such as Alice’s frame. But in this frame it seems that QM does not require the relation E(b, c) = −cos(b − c), since the result of Carol has been erased by Alice and replaced by Alice’s resul…[Read more]

Richard Healey replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
Ruth,
In my view quantum theory may be applied to predict probabilities for certain magnitude claims, each restricting a dynamical variable to a Borel subset of real numbers. When quantum theory is targeted on a quantum system, a quantum state is assigned to that system in order to apply the Born rule to yield these probabilities. The magnitude…[Read more]

Ruth Kastner replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
Richard, do I understand correctly that in your proposed approach, measurement outcomes have no relation to values of observables for the system? My question arises from your statement:
In particular, it makes no assumption concerning the actual spin values of the measured particles, either before or after the spin measurements. It assumes only…

Richard Healey replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
Shan,
You focus on an important part of the third argument.
In my paper I first considered the use of QM to predict the probabilistic correlation E(a,d) in equation (29), and then appealed to Lorentz symmetry to justify the analogous equation for E(b,c). So let’s consider the argument for equation (29).If Carol had performed no measurement (C…[Read more]

editor replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
PS. Moreover, when Bob and Carol are in different frames, it seems that the relation E(b, c) = −cos(b − c) cannot be consistently defined in QM, since the measured states for them, which extends to two spacelikeseparated regions, are not the same.

editor replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
Hi Richard,
Thanks a lot for your further explanation! I now understand your third argument more clearly. I think the potential issue is that you used results from two different frames in the same inequality (32). Concretely speaking, I think in Alice’s frame corr(b, c) is not equal to E(b, c) = −cos(b − c). It seems that QM does not require thi…[Read more]

Ruth Kastner replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
Thanks, Richard for this clarification.
Of course, I think your analysis can also perfectly well apply to an ontic role for the quantum state.
Under an epistemic view of the quantum state, then it seems to me that if one posits that measurements really do have definite outcomes, then an implicit hidden variable view must be lurking in the…[Read more] 
Richard Healey replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
The third argument makes no assumption of hidden variables. In particular, it makes no assumption concerning the actual spin values of the measured particles, either before or after the spin measurements. It assumes only that each measurement has a definite physical outcome, which may correspond to a light flashing red rather than green (for…[Read more]

editor replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
Hi Richard,
Thanks for your kind explanation!
I understand that the inequality (31) is derived from the hidden variable assumption in Fine’s paper. But the statistical correlations between pairs of *actual experimental outcomes* does not necessarily satisfy the inequality. It is the assumption that these outcomes directly reflect the values of…[Read more]

Ruth Kastner replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
This is an interesting situation: assessments of the bearing and validity of the FR proof greatly depend on what quantum theory itself is taken to be. For example, Editor has referred to two different forms of QT:
(1) QT with collapse postulate — call it “QTCP”
(2) QT without collapse postulate — call it “QTNCP”
…and has noted, based on…[Read more] 
Richard Healey replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
As stated in the first sentence of the paragraph in which equation (31) appears, it is a central assumption of this third argument that every spin measurement performed by A,B,C and D has a definite, physical outcome. Consistent with that assumption, the measurements by A, B destroy all
 records
of C’s and D’s definite, physical outcomes. So none…[Read more]

editor replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
Hi Richard,
I think there are two concerns about your third argument. The first is that in order to derive the Bell inequality (31), one needs a locality assumption for the factorization, e.g. of the term corr(c,d), which is the same as that in the original Bell inequality. The second concern is that in your thought experiment, it seems that the…[Read more]

Mark Stuckey joined the group 2018 Workshop on Wigner’s Friend 4 years, 4 months ago

Richard Healey replied to the topic Is there an inconsistent friend? in the forum Workshop on Wigner’s Friend 2018 4 years, 4 months ago
Shan,
You say
” it seems to me that Richard’s result about the Limits of Objectivity is not valid. This result is derived from the third argument in his paper. I think the argument is based on the implicit assumption of locality, like Bell’s theorem, and one should drop this locality assumption, not the objectivity of outcomes.”Where do you thi…[Read more]
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