  If two ultrarelativistic nonrotating black holes of masses $m_1$ and $m_2$
approach each other with fixed center-of-momentum (COM) total energy $E =
\sqrt{s} \gg (m_1+m_2)c^2$ that has a corresponding Schwarzschild radius $R =
2GE/c^4$ much larger than the Schwarzschild radii of the individual black
holes, here it is conjectured that at the critical impact parameter $b_c$
between scattering and coalescing into a single black hole, there will be an
inspiral of many orbital rotations for $m_1c^2/E \ll 1$ and $m_2c^2/E \ll 1$
before a final black hole forms, during which all of the initial kinetic energy
will be radiated away in gravitational waves by the time the individual black
holes coalesce and settle down to a stationary state. In the massless limit
$m_1 = m_2 = 0$, in which the black holes are replaced by classical massless
point particles, it is conjectured that for the critical impact parameter, all
of the total energy will be radiated away by the time the two particle
worldlines merge and end. One might also conjecture that in the limit of
starting with the massless particles having infinite energy in the infinite
past with the correct ratio of impact parameter to energy, the spacetime for
retarded time before the final worldline merger at zero energy will have a
homothetic vector field and hence be self similar. Evidence against these
conjectures is also discussed, and if it proves correct, I conjecture that two
massless particles can form any number of black holes.
