What would happen if a black hole, the size of a coin, suddenly appeared in my pocket?

Short answer: You, and everyone around you, will die. But how you die depends on your definition of “size.”

Long answer: It depends on your definition of size. Is it a black hole with the mass of a coin? Or is it as wide as a coin?

Suppose a US nickel with the mass of about 5 grams magically collapsed into a black hole. This black hole would have a radius of 10-30 meters. For comparison, a hydrogen atom is about 10-10 meters. So the black hole compared to an atom is as small as an atom compared to the sun. Unimaginably small.

This black hole would have a similarly unimaginably short lifetime to decay by Hawking radiation – it would radiate away what little mass it has in 10-23 seconds. Its 5 grams of mass will be converted to 450 teraJoules of energy, which is comparable to the detonation of about 100,000 tonnes of TNT, and produce to an explosion roughly three times bigger than the atomic bombs dropped on Hiroshima and Nagasaki combined.

In this case, you die.


If the black hole has the radius of a common coin, then it will be considerably more massive. In fact, a black hole with the radius of a nickel would be slightly more massive than the earth. It would have a surface gravity a billion billion times greater than our planet currently does. The black hole would consume you before you even realized what’s happening. But the black hole wouldn’t stop with just you.

The black hole is now a dominant gravitational piece of the earth-moon-black hole of death system. If you watch a lot of sci-fi movies, you might think that the black hole sinks towards the center of the planet and will consume it from the inside out. In actuality, the earth will also move up onto the black hole, and begin to bob around as if it was orbiting the black hole, all while having swaths of mass eaten with each pass.


As the earth is eaten up from the inside it collapses into a scattered disk of hot rock, surrounding the black hole in a tight orbit. The black hole doubles its mass by the time it’s done feeding. The moon surprisingly survives, adjusting to a highly elliptical orbit.

The effects on the solar system are awesome, but moreso in the biblical sense of “awesome”, which more closely means terrifying. Tidal forces from the black hole would probably disrupt the near earth asteroids, maybe even parts of the asteroid belt, sending rocks careening through the solar system  – bombardment and impacts may become commonplace for the next few million years.

The planets are slightly perturbed but stay approximately on the same orbit. The black hole we used to call earth will now continue on orbiting the sun, in the earth’s place.
In this case, you also die.




asked by /u/ontheverge73

cover image credit: Kurz Gesagt



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