General limitations on trajectories suitable for super-Penrose process

TL;DR

This paper investigates the limitations of trajectories that can facilitate the super-Penrose process near extremal rotating black holes, revealing constraints on particle parameters and collision scenarios for unbounded energy extraction.

Contribution

It demonstrates that particles with generic parameters cannot result from previous collisions unless one initial particle has an extremely large mass, extending prior findings in Kerr black hole studies.

Findings

Particles with generic parameters cannot be produced from finite-mass collisions.

A particle with mass of order N^{-2} can be obtained, generalizing previous results.

Unbounded energy extraction requires specific collision conditions near extremal black holes.

Abstract

Collisions of particles near a rotating black hole can lead to unbound energies $E_{c.m.}$ in their centre of mass frame. There are indications that the Killing energy of debris at infinity can also be unbound for some scenarios of collisions near the extremal black hole horizon (so-called super-Penrose process). They include participation of a particle that (i) has generic (not fine-tuned) parameters and (ii) moves away from a black hole before collision. We show that for any finite particle's mass, such a particle cannot be obtained as a result of the preceding collision. However, this can be done if one of initial infalling particles has the mass of the order $N^{-2}$ that generalizes previous observations made in literature for radial infall in the Kerr background.