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The hidden variables in RBW are the (graphical) spacetimesource element and the adynamical global constraint. We take a God’s eye (4D) view, so our hidden variables don’t “bring about events.” Per Geroch,
“There is no dynamics within space-time itself: nothing ever moves therein; nothing happens; nothing changes. In particular, one does not think of particles as moving through space-time, or as following along their world-lines. Rather, particles are just in space-time, once and for all, and the world-line represents, all at once, the complete life history of the particle.”
So, in the God’s eye view, we’re just trying to explain the 4D patterns, e.g., the relative frequency of occurrence for spin up and down outcomes in the many trials of the 3-particle GHZ experiment. Thus, each spacetimesource element relating source (emission event) to sink (specific detector events) in the 4D experimental configuration has a probability amplitude providing its frequency of occurrence in the overall 4D pattern of outcomes for that particular experiment. The adynamical global constraint provides a rule for computing that probability amplitude in the context of the path integral formalism (think lattice gauge theory, since the spacetimesource element is graphical). I wouldn’t call this probability amplitude a “wave function,” since it’s computed via the path integral using future boundary conditions (specific outcomes), not the SE. But, that’s semantics. There is a conceptual overview in the Introduction of the paper (9 pp).