Relativity seminar

The quest for best-case scenarios of energy extraction from black holes

Although Penrose process remains a textbook example of a mechanism to extract energy from black holes, it has several practical limitations. A major problem is that it requires a particle to split into two fragments with very high relative velocity. This issue can be circumvented either by considering particles influenced by electromagnetic interaction or by switching to particle collisions instead of decays. Regarding the collisional Penrose process, a lot of attention has been devoted to idealised, best-case scenarios of test particle collisions with arbitrarily high centre-of-mass collision energies in the vicinity of extremal black holes. Bañados, Silk and West (BSW) first described such a possibility for particles with fine-tuned angular momentum orbiting a maximally rotating black hole. However, even in this simplified setup, the energy that can be extracted from the black hole turned out to be subjected to unconditional upper bounds. On the other hand, no such bounds were found for the electrostatic version of the BSW effect discovered by Zaslavskii. Since this variant occurs for radially moving charged particles close to extremally charged black holes, it is also impractical for astrophysical purposes. In this talk, I will present a way to solve this impossible dilemma by considering a unification of the two versions of the BSW effect and showing that there are no bounds on the extracted energy whenever both the black hole and the escaping particles are charged, regardless of the magnitude of the black-hole charge. Therefore, this generalisation can serve as an idealised model for energy extraction from astrophysical black holes, which can spin fast, but can have only a tiny charge induced via interaction with external magnetic fields.