Everyone,
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

Thnx for the detailed response, Richard. Let me see if I totally understand it.

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]

Hi Mark,

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]

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 c…[Read more]

Hi Richard,
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]

Hi Richard,

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]

@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…[Read more]

Thanks Richard, let me address your comment that:

… 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]

Dear Ruth,

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]

Dear Shan,

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)=0 for any a, b.

I think these two…[Read more]

Thnx, Richard. I figured that was the answer, but I wanted to make sure before I fashioned a response (forthcoming).

Thanks, Ruth. Your pointed out a potential issue. My argument does not reply on Alice’s result being erased, but relies on the state of the particles being recovered before Bob’s measurement. Shan

Mark: of course, I’ve contended and continue to contend that the disease infecting conventional approaches to QM is that nobody can define ‘measurement’. This leaves adherents of these traditional approaches to simply help themselves to measurement results. In particular, I would have to respectfully differ with Richard’s comment:

The argument…

[Read more]

Hi Richard,

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]

Mark,

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]

Shan,

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]

No one in the topic on Frauchiger and Renner (FR) “Quantum theory cannot consistently describe the use of itself” (2018) answered this question, so I’ll post it here.

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]

Thanks very much Richard.
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]