  We elucidate how black holes form in trans-Planckian collisions. In the rest
frame of one of the incident particles, the gravitational field of the other,
which is rapidly moving, looks like a gravitational shock wave. The shock wave
focuses the target particle down to a much smaller impact parameter. In turn,
the gravitational field of the target particle captures the projectile when the
resultant impact parameter is smaller than its own Schwarzschild radius,
forming a black hole. One can deduce this by referring to the original argument
of escape velocities exceeding the speed of light, which Michell and Laplace
used to discover the existence of black holes.
