Causal angular velocity bounds in Kerr spacetime
= Causal angular velocity bounds in Kerr spacetime
Outside a subextremal <Kerr black hole>, regular <causal curves> obey $(-g_{t\phi}-\sqrt D)/g_{\phi\phi}\le d\phi/dt\le(-g_{t\phi}+\sqrt D)/g_{\phi\phi}$, where $D=g_{t\phi}^2-g_{tt}g_{\phi\phi}>0$. Positive radial and polar metric terms establish this necessary bound. In the open <Kerr ergoregion> both endpoints are positive for $a>0$, forcing co-rotation.