Have a look at this Physics Forums Insight to see our take on Wigner’s friend.

Richard,

When you get a chance, let us know if we have correctly characterized your view in that PF Insight. I can make changes anytime.

Mark

]]>The state given by your Eq 13 applies to any of the three possibilities for the definite, single outcomes recorded by Xena and Yvonne in one world, i.e., heads- or tails- or tails+, respectively, prior to Zeus and Wigner making their measurements.

Eq 13 says that if…[Read more]

]]>No, I don’t agree. I regard retrocausation here as a desperate and unnecessary response to the situation you present. (Though I’m happy to entertain this as a conceptual possibility in other contexts and for other reasons.)

Equation (13) and its equivalents represent probabilistic correlations between the outcomes of possible m…[Read more]

]]>However, in contrast to Hardy-type (as well as Bell-type) arguments, our paradox does not rely on c…[Read more]

]]>I much prefer your presentation of FR in Quantum Theory and the Limits of Objectivity (2018), so I will refer to that. Looking at your Eq (13) and understanding that there exists an objective fact of the matter about what Xena and Yvonne have recorded for their measurements (h or t and + or -, respectively), it seems unavoidable that…[Read more] ]]>

In your post, you said: “To correctly calculate the correlation between his outcome and the outcome of Alice’s actual measurement, Bob must assign a state at a time (in his frame) before the superobserver’s intervention.” Could you explain why? Thanks!

Shan

PS. Gijs Leegwater has just posted a paper in arXiv (ht…[Read more]

]]>@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…[Read more]

]]>… if there were no true magnitude claims then an application of the Born rule would have nothing to which to assign probabilities!

The above statement/position depends on the notion of ‘magnitude claim’. That’s a linguistic entity, and I think we earlier agreed that QM doesn’t demand or depend…[Read more]

]]>Let me comment on something you said in your last post:

**Richard noted earlier (if I understand correctly) his assumption that the existence of a measurement result in the world has no relation to whether anyone knows what it is, nor to any particular physical condition of the system under study. So it seems that when one party engages…**[Read more]

I quote two sentences from your draft paper:

In Bob’s frame, since after the superobserver’s reset measurement the states of Alice and the particles are the same as their initial states, the result of Bob’s measurement has no correlation with the result of Alice’s measurement. Then we have

E(a,b)=0for anya,b.

I think these two…[Read more]

]]>]]>The argument…

Thanks for your further clarification!

I think you did not understand my objection concerning the non-SR case. In the non-SR case, since the time order of spacelike separated events is invariant in different frames, you cannot derive the relation $E(b,c)=-cos(b-c)$ in Bob’s frame using your derivation in the SR case, since just l…[Read more]

]]>Wbar measures an observable *z* on a quantum system composed of **everything** in Fbar’s lab (including the quantum coin, Fbar herself, her measurement apparatus and recording devices, …). *z* is a two-valued observable with orthonormal eigenstates okbar, failbar. No-one, including Frauchiger and Renner, has any idea of how to measure this…[Read more]

Re 3. I think your derivation of $E(b,c)=4sin^2[(a-b)/2]cos^2[(a-b)/2]-1$ is incorrect. The derivation proceeds by separately considering two possible outcomes of Carol’s measurement and then summing over the associated probabilities, treated as exclusive and exhaustive. In effect, this is to treat Carol’s measurement as inducing a physical…[Read more]

]]>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…[Read more]

]]>Of course, in the transactional picture, once photons are detected/actualized (as is necessary to yield a current), collapse has occurred, and conserved quantities (such as angular momentum, spin, etc) have been transferred. (Photons are the mediator of em processes such as the creation of electron current; for details,…[Read more] ]]>

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]

]]>I hope that the view that I presented there clarifies some of the issues that were raised here. ]]>

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]

A modern version of the Stern-Gerlach experiment uses a hot-wire detector (see, for example, http://web.mit.edu/8.13/www/JLExperiments/JLExp18.pdf ).

In this case, potassium atoms pass through an S-G magnet, thereby entangling their spin and translational quantum states (not collapsing the spin state)! Interaction with the hot wire likely…[Read more]

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]

]]>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] ]]>

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]

]]>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]

]]>]]>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…

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]

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]

]]>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] ]]>

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]

]]>(1) QT with collapse postulate — call it “QTCP”

(2) QT without collapse postulate — call it “QTNCP”

…and has noted, based on…[Read more] ]]>

- records

of C’s and D’s definite, physical outcomes. So none…[Read more]

]]>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]

]]>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]

]]>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…[Read more] ]]>

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…[Read more] ]]>

The theorem claimed by Frauchiger and Renner in their recent publication in Nature Communications is different from the one they presented in their eprint arXiv:1604.07422, which was discussed in the references to this forum. Both theorems use the same extension of the Wigner’s friend experiment and claim to show that…[Read more]

]]>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…[Read more]

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