Energy extraction from Kerr black holes by rigidly rotating strings

TL;DR

This paper demonstrates that rigidly rotating strings can extract rotational energy from Kerr black holes, with the process depending on the string's angular velocity and the position of the effective horizon relative to the ergosphere.

Contribution

It introduces a novel mechanism for energy extraction using stationary Nambu-Goto strings in Kerr spacetime, analyzing conditions for efficiency and configurations.

Findings

Energy extraction requires the effective horizon to be inside the ergosphere.

The string's angular velocity must be less than the black hole's angular velocity.

Global configurations are analyzed for slow-rotation and extremal Kerr cases.

Abstract

In this paper, we show that a rigidly rotating string can extract the rotational energy from a rotating black hole. We consider Nambu-Goto strings stationary with respect to a corotating Killing vector with an uniform angular velocity $\omega$ in the Kerr spacetime. We show that a necessary condition of the energy-extraction process is that an effective horizon on the string world sheet, which corresponds to the inner light surface, is inside the ergosphere of the Kerr black hole and the angular velocity $\omega$ is less than that of the black hole $\Omega_\mathrm{h}$. Furthermore, we discuss global configurations of such strings in both of a slow-rotation limit and the extremal Kerr case.