  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.
