Born’s Exponent and Gleason’s Theorem: Placing the Geometric Reading Beneath the Stronger Derivation — and the Qubit, Where Gleason Is Silent.
Papers 53 and 68 read the exponent 2 of Born’s rule as a sesquilinear “2”: p = |ψ|² = ⟨ψ|P|ψ⟩, the same quadratic form as the Hopf (Bloch) map. This paper asks the honest question — is “why 2” derived or merely assumed? — and places the geometric reading beneath the rigorous royal-road result. Gleason’s theorem (1957) shows that on a Hilbert space of dimension ≥ 3, any non-contextual, countably additive measure on the projection lattice has the form μ(P) = Tr(ρP); for a pure state this is Born’s rule, exponent 2 included, DERIVED from assumptions strictly weaker than assuming |ψ|². Hence a clear hierarchy: Gleason (and, for all dimensions including the qubit, the POVM extension of Busch 2003) is the derivation; Papers 53/68 are a geometric picture — subordinate as a derivation, complementary as a picture. The one place the picture is not subsumed: projective Gleason requires dim ≥ 3 and FAILS for the qubit (dim 2), exactly the Hopf case of Paper 68 — restored only by POVM-Gleason, and precisely where the geometric picture earns its keep. Finally, Gleason grounds Paper 68’s “census of twos” by forcing the sesquilinear form (non-contextuality). Neither Gleason nor this paper solves the measurement problem; the distinct roots stay distinct.
A note on the aim of this work. This is a structural interpretation and an act of intellectual humility: it defers to Gleason’s theorem and keeps only the geometry that survives beneath it. It is not a new derivation of the Born rule and does not solve the measurement problem. Nothing here is intended to contradict existing results.
On the making of this work: The ideas and content of this work stem from the author’s own considerations. Assistance from an AI (a large language model) was used for structuring, English translation, and checking the algebra. Any remaining errors or misinterpretations are solely the author’s. Feedback and corrections are sincerely appreciated.
Reference
Yamagishi, Y. (2026). Born’s Exponent and Gleason’s Theorem: Placing the Geometric Reading Beneath the Stronger Derivation (and the Qubit, Where Gleason Is Silent) [Paper 71]. Zenodo.
DOI: 10.5281/zenodo.21650577
ORCID: 0009-0006-8587-3450