 Charged-particle turning-point exclusion in Reissner-Nordstrom spacetime

For $0<|Q|<M$, the charged-particle radial equation in Reissner-Nordstrom spacetime implies that any radial turning point obeysSince $1+l^2/r^2>0$, it follows that $F\ge 0$. Thus no massive test particle, regardless of its electric charge, can have a radial turning point in $r_{-}<r<r_{+}$, where $F<0$. At a regular horizon crossing, a timelike vector cannot lie in the tangent space of a null hypersurface, so a massive particle has $\dot{r}\ne 0$ there as well. The algebraic inequalities $r\ge r_{+}$ or $r\le r_{-}$ are necessary conditions, not an existence theorem for radial turning points.