The weak equivalence principle asserts that all matter species couple identically to the spacetime metric at the kinetic-term level, tested by composition-dependent free-fall experiments to fractional precision 10^-15. This paper asks whether that universality stops at the kinetic term or extends to the next operator in the effective-field-theory expansion of the matter–metric interaction. The Equivalence Principle Extension (EPE) is the postulate that it does: at a specified matching scale, the coefficient ξ of the non-minimal curvature operator ξ R |Φ|² is universal across elementary scalar species. EPE fixes the universality of this leading coefficient at μ★, not its magnitude. Three empirical relations carry the content of the postulate at first curvature-response order: (i) for elementary scalars, the Ricci-coupled phase along a common trajectory is mass-weighted and species-invariant; (ii) for ordinary laboratory matter, matching the Standard Model Higgs operator ξH R H†H onto atomic rest masses yields the leading laboratory null QB Δϕ₀, ₑ − QA Δϕ₁, ₑ = 0 via the Higgs-response charge QA; (iii) on an inflationary quasi-de Sitter background, every scalar spectator obeys mₑff² = m² + 12 ξ ℏ² Hᵢnf² / c⁴. The three relations have different empirical status: the Ricci phase is a formal elementary-scalar relation, differential matter-overlap or source-modulated atom interferometry tests the matched QA null, and cosmology supplies a one-way scalar-isocurvature falsification channel together with model-dependent axion/PQ-sector consistency tests. The manuscript develops the EPE postulate, its laboratory and cosmological empirical channels, the matched effective field theory linking the elementary-scalar operator to composite matter via a Damour–Donoghue decomposition, an open-system Lindblad extension recovering the standard Blencowe gravitational-decoherence rate, and the relation of EPE to the Spacetime Pilot Wave (SPW) hypothesis as one possible ontological reading. This is the final manuscript version (v59), dated April 27, 2026.
Andrew A. Schoen (Wed,) studied this question.