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Golden Busemann Coordinate

Abstract

Golden null coefficients carry a nontrivial Busemann rapidity coordinate.

Theorem 1.1 (The golden null basis exposes Busemann rapidity).

Proof. Machine-checked in Lean as D5/S3/Observer/GoldenCoding/GoldenBusemannCoordinate.golden_busemann_coordinate (✓ std3). ∎

Source. Repository-derived.

Commentary.

The real Lorentz form uses the existing sign convention Q_phi(x,y)=x^2-xy-y^2. The golden ratio and its negative conjugate give two null vectors, and direct polarization reduces the form of a v_plus+b v_minus to -5ab.

On the branch a>0 and b<0, the ratio a/(-b) is positive. At unit Lorentz level the coefficient product is a(-b)=1/5, so the half-log definition agrees exactly with log(a sqrt(5)). Differences of this coordinate satisfy the Busemann cocycle law by telescoping.

Reciprocal golden-square scaling preserves the branch and adds 2 log(phi) to rapidity. The points with coefficients (1/sqrt(5),-1/sqrt(5)) and (2/sqrt(5),-1/(2sqrt(5))) both have Lorentz value one, but their rapidities are zero and log(2).

References