Ultrarelativistic Black Hole Formation

We study the head-on collision of fluid particles well within the kinetic energy dominated regime (γ=8 to 12) by numerically solving the Einstein-hydrodynamic equations. We find that the threshold for black hole formation is lower (by a factor of a few) than simple hoop conjecture estimates, and, moreover, near this threshold two distinct apparent horizons first form postcollision and then merge. We argue that this can be understood in terms of a gravitational focusing effect. The gravitational radiation reaches luminosities of 0.014c⁵/G, carrying 16±2% of the total energy.