Is the Kerr black hole a super accelerator?

TL;DR

This paper investigates whether Kerr black holes can act as unbounded particle accelerators and finds that, under certain conditions, they cannot effectively transfer energy to particles to produce infinite velocities.

Contribution

The study demonstrates that a single corrective collision in the equatorial plane prevents Kerr black holes from serving as super accelerators, limiting energy transfer.

Findings

Unbounded energy transfer requires infinitely energetic incoming particles.

Single corrective collision in the equatorial plane inhibits unlimited acceleration.

Mechanism for black hole particle acceleration is ineffective under studied conditions.

Abstract

A number of long-standing puzzles, such as the origin of ultrahigh-energy cosmic rays, could perhaps be solved if we found a mechanism for effectively transferring energy from black holes to particles and, correspondingly, accelerating the latter to (unboundedly, as long as we neglect the back reaction) large velocities. As of today the only such candidate mechanism in the case of the non-extreme Kerr black hole is colliding a particle that freely falls from infinity with a particle whose trajectory is subject to some special requirements to fulfil which it has to be suitably corrected by auxiliary collisions. In the present paper we prove that---at least when the relevant particles move in the equatorial plane and experience a single correcting collision---this mechanism does not work too. The energy of the final collision becomes unboundedly high only when the energies of the incoming particles do.