High-energy scattering in gravity and supergravity

We investigate features of perturbative gravity and supergravity by studying scattering in the ultra-Planckian limit, and sharpen arguments that the dynamics is governed by long-distance physics. A simple example capturing aspects of the eikonal resummation suggests why short-distance phenomena and, in particular, divergences or nonrenormalizability do not necessarily play a central role in this regime. A more profound problem is apparently unitarity. These considerations can be illustrated by showing that known gravity and supergravity amplitudes have the same long-distance behavior, despite the extra light states of supergravity, and this serves as an important check on long-range dynamics in a context where perturbative amplitudes are finite. We also argue that these considerations have other important implications: they obstruct both probing the conjectured phenomenon of asymptotic safety through a physical scattering process, and ultra-Planckian scattering exhibiting Regge behavior. These arguments sharpen the need to find a nonperturbative completion of gravity with mechanisms which restore unitarity in the strong gravity regime.