No Friendly Observer: The Local-Friendliness Ledger

Every laboratory notebook rests on one quiet assumption: that what was seen in the room is a fact for everyone outside the room. In 2018 Frauchiger and Renner showed that three natural demands on reasoning agents — that quantum theory applies at every scale (Q), that agents reasoning with the same theory do not contradict each other (C), and that a measurement has one outcome for the agent who makes it (S) — cannot all hold together. In 2020 Bong and collaborators proved a theory-independent theorem: any theory in which observed events are absolute, influences are local, and experimental choices are not pre-correlated with the world must obey Local Friendliness inequalities that quantum correlations violate, and a photonic experiment violated a genuine one. This paper states both theorems at the strength their authors printed, enters the two photonic receipts with their numbers (Proietti et al. 2019: S = 2.416 ± 0.075 against a bound of 2, from 1,794 six-photon events; Bong et al. 2020: genuine LF inequality, bound 0, maximum quantum value 1.345, first LF violation at state parameter 0.87 after a Bell-without-LF separation at 0.80–0.81), prices the three exits — surrender absoluteness of observed events, surrender locality, or surrender free choice — plus the fourth door of denying the friend was an observer, and prints in full the live seam: in both experiments the friend was a photon, and whether a photon's path is an observer is the very thing under dispute. Grade: MENU STANDS — THREE EXITS PRICED, FRIEND UNPROVEN. This paper stands outside my work on the vacuum catastrophe; it draws nothing from that work and repairs nothing in it, and it touches no claim of that program.