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arXiv · 2604.05103

Untwisting the double copy: the zeroth copy as an optical seed

Damien A. Easson·Michael J. Falato
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Abstract

We present a historical optical foundation for stationary vacuum Kerr--Schild spacetimes on a flat background and interpret it in modern double-copy language. In this setting, a complex optical seed \(\rho=-\theta-i\omega\), built from the expansion and signed twist of the Kerr--Schild congruence, is harmonic, while its inverse obeys an eikonal equation and reconstructs the congruence algebraically. Thus the local stationary geometry is organized by a single complex seed. In the overlap of the stationary Kerr--Schild and Petrov type--D Weyl double-copy framework, this seed furnishes a normalized representative of the zeroth-copy data, while its real part yields the Kerr--Schild profile and its gradient generates the single-copy gauge-field strength. The construction provides, without recourse to twistor methods, a spacetime realization of how a single complex seed builds the congruence, organizes the associated spacetime and gauge fields, and encodes the geometric content of the zeroth copy.

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Damien A. Easson, Michael J. Falato. 2026-04-06. Untwisting the double copy: the zeroth copy as an optical seed. https://doi.org/10.1007/jhep05(2026)249

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