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The passage around this formula

To make this concrete rather than asymptotic, take the mass of Sagittarius A* as measured by the Event Horizon Telescope, M$\approx$4$\times$10^6\,$M_\odot$ , giving a Schwarzschild radius $r_s$$\approx$1.18$\times$10^{10}\,$\mathrm m$ and a horizon-scale acceleration factor GM/$r_s^2$$\approx$3.80$\times$10^{6}\,$\mathrm{m\,s^{-2}}$ [ 14 ] . At a clearance of just one kilometer above the horizon — a distance smaller than a millionth of the horizon’s own radius — f$\approx$8.46$\times$10^{-8} and the required proper acceleration is already a$\approx$1.3$\times$10^{10}\,$\mathrm{m\,s^{-2}}$ , more than a billion Earth gravities. Even at a clearance of 1.8$\times$10^6\,$\mathrm{km}$ — about fifteen percent of the horizon’s own…

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