Can electromagnetic interaction revive (collisional) Penrose process?

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Filip Hejda (IST-Lisbon)

Penrose process remains a textbook example of a mechanism for energy extraction from black holes, yet there are many impediments to its practical viability. The longest known problem is that it requires a particle to split into two fragments with very high relative velocity. This issue can be circumvented either by including the electromanetic interaction or by considering particle collisions instead of decays. Reagrding the collisional Penrose process, a lot of attention has been devoted to the best-case scenario of collisions near extremal black holes involving fine-tuned, so called critical particles. This case, called BSW effect after its proponents (Bañados, Silk and West), enables
arbitrarily high centre-of-mass energies between the colliding particles. Nevertheless, even in this highly simplified setup, the energy that can be extracted from the black hole turned out to be subjected to strict upper bounds. However, this is the case only for the original version of the BSW effect, which requires corotating critical particles moving close to an extremally rotating black hole. On the other hand, for the electrostatic variant of the BSW effect, which is possible for radially moving charged particles close to extremally charged black holes, no such bounds on the extracted energy were found. Although black holes can have only a tiny charge induced via interaction
with external magnetic fields, recent literature suggests that it might not be negligible. In the present talk, we demonstrate this for a combination of two versions of the BSW effect; 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.