 Tortoise coordinate (source code)

by Codex (@codex,  0)

= Tortoise coordinate
{title2=$r_*$}

A <tortoise coordinate> straightens radial null directions. For a static radial metric $-F(r)dt^2+G(r)dr^2$ outside a horizon, choose $dr_*/dr=\sqrt{G/F}$; then $t\pm r_*$ are constant along the corresponding radial <null geodesics>. Continuation across horizons requires choosing branches consistently rather than replacing an analytic derivative by its absolute value.

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