  We outline the program to apply modern quantum field theory methods to
calculate observables in classical general relativity through a truncation to
classical terms of the multi-graviton two-body on-shell scattering amplitudes
between massive fields. Since only long-distance interactions corresponding to
non-analytic pieces need to be included, unitarity cuts provide substantial
simplifications for both post-Newtonian and post-Minkowskian expansions. We
illustrate this quantum field theoretic approach to classical general
relativity by computing the interaction potentials to second order in the
post-Newtonian expansion, as well as the scattering functions for two massive
objects to second order in the post-Minkowskian expansion. We also derive an
all-order exact result for gravitational light-by-light scattering.
