Thanks, Jerry: that’s interesting.

You say:

I am guessing that your answer is “no” and that the only allowed possibilities are that Alice sees “+1” both times and that Bob sees “-1” both times, or visa versa.

For the spacelike separated case you describe, “no” is indeed my answer.
You are right to distinguish superluminal influence from superl…[Read more]

Thanks, Jerry.

A little clarification:
You say

Healey and Gao agree Alice expects the result “+1” only 50% of the time

.

In her situation prior to each of her individual measurements Alice expects each of its possible results to be equally likely. But in her situation prior to the whole sequence of her future measurements Alice expects eit…[Read more]

In my previous reply I said that when the Born Rule is applied from an agent situation indexed to a spacetime point p on Bob’s worldline that is timelike later than Bob’s outcome (such as point 3 in Figure 1) it yields chance 0 or 1 for each of Alice’s measurement outcomes in regions a – e. So from that agent situation, each of Alice’s outcomes is…[Read more]

The Born Rule should always be applied not at a time (that would already introduce a preferred frame in relativity) but from what I have elsewhere called an agent situation—a physically specified situation that may or may not actually be occupied by an agent. In this Gedankenexperiment we may take an agent situation to be adequately specified by…[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]

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]

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]

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]

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]

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]

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]

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]

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]