Dear Anton Garrett,
I greatly enjoyed your essay [I use the same "conversational" vehicle in my essay, which I hope you will read.]
Literally thousands of comments have been spent on FQXi concerning Bell's theorem, which, as you state, "is about logic, not quantum mechanics". Bell's first statement defining the problem is his equation (1) in which he defines measurements A and B to have +/- unit values. The logical outcome is completely determined from this point!
Bell essentially asks for a "classical" explanation [the 'hidden variable'] while insisting on a "quantum" result. Stern-Gerlach did not find "quantum" results. Their deflection data is smeared over an upper "lip" and a lower "lip" which are arbitrarily called +1 and -1 to fit the naïve quantum picture of spin. But which picture? Pauli's, Dirac's, Feynman's? As you obviously spent time and effort on this topic I hope you might look at Spin: Newton, Maxwell, Einstein, Dirac, Bell.
Pauli conveniently chose half integral eigenvalues, which Bell uses unquestioningly, while Dirac, who many think more fundamental, derived a four component equation that is no longer an eigenvalue equation. Indeed, it is only "converted into" an eigenvalue equation by the Foldy-Wouthuysen transformation which smears the particle with spin over a region of space. Only after this integration do we arrive at a Dirac-based eigenvalue equation.
Perhaps if Bell had thought more deeply about spin he would've had more reservations than he did about this issue. Unfortunately, about the same time Bell developed his theorem, Feynman, deeply in love with the two slit experiment, decided to apply the analogy to Stern-Gerlach-as-two-slit and [of course!] the two state "wave function" worked. [What a surprise -- Pauli invented a workable 'wave-function' when he used O|+> = +|+> and O|-> = -|->.] Thus deBroglie's linear momentum-based wave function, with wavelength proportional to inverse momentum, compatible with experimental tests, was conceptually extended to angular momentum, with no logical justification for associating a wavelength with electron spin. [Spin waves in condensed matter or solid-state physics are not spin wave functions.] But, like Einstein, it is today verboten to question Feynman, so we are stuck with "wave functions" for spin analogous to wave functions for particles with momentum. This leads to superposition concepts for particles going through non-homogeneous fields that are entirely inappropriate but unquestioned, although never demonstrated.
Nino says: "I presume physicists... are now looking for a theory that predicts what happens each time you put a particle through successive Stern-Gerlach apparatuses."
Neo answers: "Actually we are not."
Actually we are. Or were. Two other physicists and myself [one received the National Medal of Technology at the White House in 2014] began this experiment in 2015. We produced fine healthy silver atomic beams but finally decided that single atom detectors were far beyond our resources.
If the first SG detector is used to prepare atoms from the oven in a particular state, say + (up), and the second SG detector is offset at angle theta from the first, then the deflection of the particle from the second device will be a factor of theta.
Here's the kicker: according to my theory (which does violate Bell's theorem classically) only + particles will be detected from the second SG device. According to Feynman's two slit spin analogy, the wave function will predict some - states will be found.
If spin is actually 3D, then the deflection of SG can be shown to depend on the angle between the spin and the field. This is what is actually seen in the SG data. But as you imply, no one wants to test this. Even suggesting it is to be drummed out of the corps.
But if spin is actually 3D, then the measured deflection is not +1 or -1 but is ~cos(theta). This conflicts with Bell's definition A,B= +/-1. Using real theta-based measurement results (i.e., deflection) it is easy to show the correlation cos(a.b). Using Bell's +1 or -1 constraints it is logically (not physically) impossible.
In Modern Classical Spin Dynamics see figure 6 on page 20 wherein the classical model overlays SG data almost exactly, and in Bell was simply wrong see page 6 where the energy-exchange model is shown to yield cos(a.b) while the Bell-constrained version cannot accomplish this.
So, to repeat, if one accepts Bell's requirement that measurements be +1 or -1 , instead of actual deflection seen in the SG data, then one is logically bound to fail. If one allows actual deflection data the classical model obtains the quantum correlation cos(a.b) violating Bell's theorem and removing even the suggestion of "entanglement".
This is further complicated by loose thinking, such as Nino's statement "but nevertheless only one of the detectors actually goes off." If this is applied to Stern-Gerlach, it means only that every particle is deflected up or down based on initial state, but does not imply A,B= +/-1. On the other hand, Bell is not tested with SG atoms but with photons which are detected (+1) or not (0).
Due to Feynman's beloved two-slit-spin-analogy, people consider atomic spin wave functions and photon wave functions to be the same, thus on/off photon detection results are conflated with the SG deflection results.
Confusion reigns.
Nino says "quantization is indeed a mental rather than a physical procedure." Bell forces classical physics into a quantized mold that is mental rather than physical. When this artificially constrained logical problem leads to the conclusion that classical physics cannot yield the measured correlation we invented "entanglement". This nonlocality that has ruled physics for fifty years is a farce, but one which cannot be challenged without forfeiting one's establishment position. The natives should be restless.
Congratulations on a very fine essay,
My very best regards,
Edwin Eugene Klingman