  This letter constructs, making use of the on-shell spinor-helicity formalism,
a possible ultraviolet completion of gravity following a "bottom-up" approach.
The assumptions of locality, unitarity and causality i) require an infinite
tower of resonances with increasing spin and quantized mass, ii) introduce a
duality relation among crossed scattering channels, and iii) dress all
gravitational amplitudes in the Standard Model with a form factor that closely
resembles either the Veneziano or the Virasoro-Shapiro amplitude in string
theory. As a consequence of unitarity, the theory predicts leading order
deviations from General Relativity in the coupling of gravity to fermions that
could be explained if space-time has torsion in addition to curvature.
