Soon after the discovery of the Kerr metric , Penrose realized that superradiance can be exploited to extract energy from black holes .
The original idea ( involving the breakup of a single particle ) yields only modest energy gains .
A variant of the Penrose process consists of particle collisions in the ergoregion .
The collisional Penrose process has been explored recently in the context of dark matter searches , with the conclusion that the ratio \eta between the energy of post-collision particles detected at infinity and the energy of the colliding particles should be modest ( \eta \lesssim 1.5 ) .
Schnittman [ Schnittman ( 2014 ) ] has shown that these studies underestimated the maximum efficiency by about one order of magnitude ( i.e. , \eta \lesssim 15 ) .
In this work we show that particle collisions in the vicinity of rapidly rotating black holes can produce high-energy ejecta and result in high efficiencies under much more generic conditions .
The astrophysical likelihood of these events deserves further scrutiny , but our study hints at the tantalizing possibility that the collisional Penrose process may power gamma rays and ultra-high-energy cosmic rays .