 Kerr ergoregion

For a Kerr black hole, let $K={\partial }_t$ be the stationary Killing vector field normalized at infinity. The exterior Kerr ergoregion is where $g(K,K)>0$, so the tangent to motion following $K$ alone would be a spacelike vector. A physical observer there cannot remain at fixed angular coordinates relative to infinity.

For nonzero spin its outer stationary-limit surface isIt lies outside the outer event horizon except where they meet on the rotation axis. A causal curve can escape this exterior Kerr ergoregion; forced co-rotation does not make it the black-hole interior. The conserved Killing energy $E=-p\cdot K$ can be negative there, enabling the Penrose process, while locally measured future-particle energy remains positive.^[ref][ref]