## Ask Ethan: When Do Black Holes Become Unstable?

“Is there a critical size for black hole stability? [A] 10

^{12}kg [black hole] is already stable for a couple of billion years. However, a [black hole] in the range of 10^{5}kg, could explode in a second, thus, definitely not stable… I guess there is a critical mass for a [black hole] where the flow of gained matter will equal to the Hawking evaporation?”

Wherever you have a black hole in the Universe, you have two competing processes. On the one hand, anything that crosses the event horizon, whether it’s normal matter, dark matter, or even pure energy, can never escape. If you fall in, you just add to the overall mass of the black hole, and grow it in size, too. But on the other hand, all black holes radiate away energy in the form of Hawking radiation, and that subtracts mass over time, shrinking your black hole. For all realistic-mass black holes, the rate-of-growth far outstrips the rate of mass loss, meaning they’ll grow for a very long time before they start to shrink.