Schwarzschild black hole, per Gullstrand-Painlevé (1921) line-element \[ d s^2 = - \, d t_{\rm ff}^2 + ( d r - v \, d t_{\rm ff} )^2 + r^2 d o^2 \ , \quad v = - \sqrt{2 G M \over r} \ . \]

Space flowing into a Schwarzschild black hole.


  Gullstrand and Painlevé