  In 2007 Pretorius and Khurana 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, Berti, Cardoso, and Pretorius performed numerical calculations that
led them to the contrary conclusion: "An extrapolation of our results to the
limit $\gamma \rightarrow \infty$ 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 $\gamma$, all but an arbitrarily small
fraction of the total energy can be radiated away.
