Can two ultrarelativistic objects lose almost all their energy to gravitational radiation?

Abstract

In 2007, Pretorius and Khurana [Black hole mergers and unstable circular orbits, Classical Quantum Gravity 24, S83 (2007).] did ``speculate that at threshold [at a critical impact parameter], all of the kinetic energy of the system [two ultrarelativistic black holes] is converted to gravitational waves, which can be an arbitrarily large fraction of the total energy.'' However, in 2012, Sperhake et al. [Universality, Maximum Radiation and Absorption in High-Energy Collisions of Black Holes with Spin, Phys. Rev. Lett. 111, 041101 (2013).] performed numerical calculations that led them to the contrary conclusion: ``An extrapolation of our results to the limit $\ensuremath{\gamma}\ensuremath{\rightarrow}\ensuremath{\infty}$ suggests that about half of the center-of-mass energy of the system can be emitted in gravitational radiation, while the rest must be converted into rest-mass and spin energy.'' Here I present arguments against this latter conclusion and in support of the earlier speculation that for sufficiently large $\ensuremath{\gamma}$, all but an arbitrarily small fraction of the total energy can be radiated away as gravitational waves.