Speaker
Description
The \emph{classical double copy} is a correspondence between solutions of
Yang--Mills theory and gravity that extends the color-kinematics duality,
originally discovered in the context of scattering amplitudes, to fully
classical field configurations.
Its most familiar exact realization is the Kerr--Schild double copy, which
identifies the Schwarzschild black hole as the gravitational counterpart of
the Coulomb field of a static point charge.
However, this identification turns out to be only a special case of a richer
structure.
Working within the perturbative classical double copy framework of Goldberger
and Ridgway --- which systematically maps color-charged sources in Yang--Mills
theory to gravitational sources in Einstein gravity coupled to a massless
scalar field --- we show that the single copy of a static Coulomb
configuration does \emph{not} reproduce the Schwarzschild solution.
Deviations appear already at second perturbative order, and the gravitational
solution that emerges order by order is the
\textbf{Janis--Newman--Winicour (JNW) spacetime}: a static, spherically
symmetric, asymptotically flat solution of Einstein gravity coupled to a
massless scalar field, labelled by a parameter $\beta$ controlling the ratio
of scalar to gravitational charge.
The Schwarzschild metric is recovered only in the limit $\beta\to 0$, when
the scalar field is absent; for $\beta\neq 0$ the geometry represents a
naked singularity endowed with a non-trivial dilaton.
JNW is therefore the most general classical double copy of the Coulomb field,
with Schwarzschild as a special case.
This result was previously established by Kim et al.\ (JHEP 02, 2020) using
a combination of perturbative methods and Double Field Theory (DFT), a
formalism rooted in string theory.
Our work reaches the same conclusion working entirely within standard General
Relativity, without invoking DFT or any string-theoretic structure, showing
that the correspondence is already fully captured at the level of classical
gravity.
To characterize the physical content of this double copy, we compute the
scattering angle and radial action for a geodesic in the JNW background,
working perturbatively in the large-impact-parameter regime.
These observables interpolate smoothly between the pure-scalar and
Schwarzschild limits as $\beta$ is varied, and provide a concrete
gravitational-wave-relevant characterization of the JNW geometry as a
testing ground for the classical double copy beyond Schwarzschild.