  Elastic collisions in the transplanckian region, where the center-of-mass
energy is much larger than the fundamental gravity mass scale, can be described
by linearized general relativity and known quantum-mechanical effects as long
as the momentum transfer of the process is sufficiently small. For larger
momentum transfer, non-linear gravitational effects become important and,
although a computation is lacking, black-hole formation is expected to dominate
the dynamics. We discuss how elastic transplanckian collisions can be used at
high-energy colliders to study, in a quantitative and model-independent way,
theories in which gravity propagates in flat extra dimensions. At LHC energies,
however, incalculable quantum-gravity contributions may significantly affect
the experimental signal.
