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The passage around this formula

Rotating (Kerr) holes complicate the static-observer story: inside the ergosphere, no observer can remain static relative to infinity at all, and a locally nonrotating (ZAMO) observer is instead dragged around the hole at an angular rate fixed by the metric, with a redshift-like relation between locally measured and asymptotic energy that generalizes $E_{\rm loc}$=$E_\infty$/$\sqrt f$ to the appropriate lapse function of Boyer-Lindquist coordinates [ 13 ] . The ergosphere is also where the Penrose process operates: a particle split inside the ergosphere can send one fragment outward carrying more energy than the original particle possessed, with the deficit paid by the hole’s own rotational…

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