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…

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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]

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]

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]

Ruth,

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]

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]

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]

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]

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]

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…

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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]

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 spacelike-separated regions, are not the same.

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]