Acceleration of particles by black holes -- general explanation

TL;DR

This paper provides a simple, general explanation for how particles can be unbound and accelerated by black holes, relating it to the behavior of vectors near the horizon and illustrating with Reissner-Nordstrom and rotating black hole metrics.

Contribution

It introduces a unified, straightforward explanation for particle acceleration by black holes based on the scalar product of four-velocity and horizon generators, applicable to various black hole types.

Findings

The scalar product tends to zero in special cases, enabling unbounded acceleration.

The effect is related to the marginal satisfaction of the 'motion forward in time' condition.

Examples with Reissner-Nordstrom and rotating black holes confirm the explanation.

Abstract

We give simple and general explanation to the effect of unbound acceleration of particles by black holes. It is related to the fact that the scalar product of a timelike vector of the four-velocity of an ingoing particle and the lightlike horizon generator tends to zero in some special cases, so the condition of "motion forward in time" is marginally satisfied. In this sense, an ingoing particle with special relation between parameters imitates the property of infinite redshift typical of any outgoing particle near the future horizon of a black hole. We check this assertion using the Reissner-Nordstrom and rotating axially-symmetric metrics as examples.