Reissner-Nordstrom spacetime ($r_{\pm }=M\pm \sqrt{M^2-Q^2}$)

The four-dimensional charged spherically symmetric electrovacuum solution has radial function $f=1-2M/r+Q^2/r^2$. For $0<|Q|<M$, its outer and inner horizon radii are $r_{\pm }=M\pm \sqrt{M^2-Q^2}$. A complete two-ended spatial bridge has a maximal Cauchy development ending at smoothly extendible inner Cauchy horizons in the exact solution.

In ingoing Reissner-Nordstrom spacetime, orient $-{\partial }_r$ to the future and normalize the other radial null vector to ${\partial }_v+(f/2){\partial }_r$. The spherical null expansion in ingoing coordinates gives $-2/r$ and $f/r$. Hence the strictly future trapped surfaces among the round spheres occur exactly between the distinct horizons. At either horizon the outgoing null expansion vanishes. Inside the inner horizon the same round spheres are not future trapped.