  The three-point amplitude is the key building block in the on-shell approach
to scattering amplitudes. We show that the classical objects computed by
massive three-point amplitudes in gauge theory and gravity are Newman-Penrose
scalars in a split-signature spacetime, where three-point amplitudes can be
defined for real kinematics. In fact, the quantum state set up by the particle
is a coherent state fully determined by the three-point amplitude due to an
eikonal-type exponentiation. Having identified this simplest classical solution
from the perspective of scattering amplitudes, we explore the double copy of
the Newman-Penrose scalars induced by the traditional double copy of
amplitudes, and find that it coincides with the Weyl version of the classical
double copy. We also exploit the Kerr-Schild version of the classical double
copy to determine the exact spacetime metric in the gravitational case.
Finally, we discuss the direct implication of these results for Lorentzian
signature via analytic continuation.
