  Shortly after the discovery of the Kerr metric in 1963, it was realized that
a region existed outside of the black hole's event horizon where no time-like
observer could remain stationary. In 1969, Roger Penrose showed that particles
within this ergosphere region could possess negative energy, as measured by an
observer at infinity. When captured by the horizon, these negative energy
particles essentially extract mass and angular momentum from the black hole.
While the decay of a single particle within the ergosphere is not a
particularly efficient means of energy extraction, the "collision" of multiple
particles can reach arbitrarily high center-of-mass energy in the limit of
extremal black hole spin. The resulting particles can escape with high
efficiency, potentially serving as a probe of high-energy particle physics as
well as general relativity. In this paper, we briefly review the history of the
field and highlight a specific astrophysical application of the collisional
Penrose process: the potential to enhance annihilation of dark matter particles
in the vicinity of a supermassive black hole.
