The high-energy collision of two black holes
  We study the head-on collision of two highly boosted equal mass, nonrotating black holes. We determine the waveforms, radiated energies, and mode excitation in the center of mass frame for a variety of boosts. For the first time we are able to compare analytic calculations, black hole perturbation theory, and strong field, nonlinear numerical calculations for this problem. Extrapolation of our results, which include velocities of up to 0.94c, indicate that in the ultra-relativistic regime about (14\pm 3)% of the energy is converted into gravitational waves. This gives rise to a luminosity of order 10^-2 c^5/G, the largest known so far in a black hole merger. 
U. Sperhake, V. Cardoso, F. Pretorius, E. Berti, J. A. Gonzalez