Equation 68 · Part 1 · A Horizon Is a Toll Booth, Not a Loophole

Symbol a

a\approx1.3\times10^{10}\,\mathrm{m\,s^{-2}}

a

What this part means

a is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.

Its job in the formula

a is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.

Full expressionSymbol aArticle meaning

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 —…

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Learn the underlying idea

A variable is a named place for a value. Its letter is a local label: x can mean position in one formula and a data point in another.

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Sources cited in the surrounding passage

[14] First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way ↗

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