Energy extraction from Kerr black holes by rigidly rotating strings
  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. 
Shunichiro Kinoshita, Takahisa Igata, Kentaro Tanabe
gr-qc astro-ph.HE hep-th