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the reach of black holes [Q&A]


Gautham of Poornaprajna College wonders what keeps black holes from disrupting the stability of the Universe and gobbling up everything?

It is a misconception to think that black holes are somehow more “powerful” at gravitational attraction than other objects. Black holes have mass, and they continue to obey the inverse square law at large distances. It is only when you get to within a few multiples of its event horizon (the distance at which it becomes impossible for light to escape) that General Relativistic effects become important. At distances much greater than the event horizon, gravitationally speaking, a black hole is indistinguishable from any other classical object.

How big is the event horizon? It is a sphere of radius G/c2=7.43 10-29 [cm/gm] per unit mass (in [gm]), where G=6.67 10-8 [cm3/gm/s2] is the Gravitational constant and c=3 1010 [cm/s] is the speed of light. For a black hole that is as massive as the Sun, 2 1033 gm, this is a sphere of radius ~1500 m. This is not the same as getting to within a mile of the center of the Sun — you must also have the entire mass of the Sun, all two thousand trillion trillion metric tons of it, to be squeezed in inside this sphere.