# On the Penrose process for rotating black holes

## TL;DR

This paper critically examines proposed implementations of Penrose's energy extraction process from rotating black holes, revealing limitations and quantifying the maximum energy that can be extracted, which falls short of theoretical limits.

## Contribution

It clarifies the conditions under which the Penrose process can occur and quantifies the maximum energy extractable from an extreme Kerr black hole.

## Key findings

- Energy extraction is limited to about 9.7% for an extreme Kerr black hole.
- Procedures with negative energy particles violate black hole area increase constraints.
- Gradual exchange method can extract energy without decreasing the black hole to a Schwarzschild state.

## Abstract

Penrose described a process that, in principle, could extract energy and angular momentum from a rotating black hole. Here we examine two procedures that were claimed to be capable of implementing the Penrose idea; both make use of a particle moving at the horizon. In one, the particle is swallowed, and in the other the particle and black hole gradually exchange energy and angular momentum. We show that if the particle has negative energy and negative angular momentum but no radial momentum both procedures violate the requirement that the area of a black hole not decrease. For the gradual exchange method, however, it appears that the Penrose process could proceed if the particle has positive energy and angular momentum, but nevertheless removes energy from the black hole. It does not, however, lead to a Schwarzschild black hole. For an extreme Kerr black hole it's mass decreases by at most 9.7%, well short of the theoretical limit for a reversible process of 1-1/sqrt{2} =29%.