  We compute the scattering amplitude for classical black-hole scattering to
third order in the Post-Minkowskian expansion, keeping all terms needed to
derive the scattering angle to that order from the eikonal formalism. Our
results confirm a conjectured relation between the real and imaginary parts of
the amplitude by Di Vecchia, Heissenberg, Russo, and Veneziano, and are in
agreement with a recent computation by Damour based on radiation reaction in
general relativity.
