Supermassive black holes, millions of times the mass of our Sun, eat stars for breakfast. But how dangerous would a small black hole be?

Could it swallow up the entire planet? Or would it simply rip apart anyone who got close to it?

Black holes are extremely dense. They aren’t really holes, but instead, huge amount of mass.

Black holes were formed when the Universe began. If it was possible for you to create one, you’d have to slam a lot of particles together in a tiny, tiny space.

In practice, if you were able to collapse all the particles of Mount Everest and create a black hole, that black hole would be just a few atoms across. Even then, you wouldn’t want to stand too close to it. For 10 meters (33 feet) around it, the gravitational pull of that tiny hole would be as strong as gravitational pull at the Earth’s surface.

So what trouble could a coin-sized black hole cause? The answer depends on how you define size. Would our hypothetical black hole be as wide as a coin? Or would it have the coin’s mass?

Scenario 1. The black hole with a diameter of a coin.

Looks pretty small, right? Well, because black holes are so dense, this one would be about the same mass as the Earth.

It would also have the gravitational pull one billion billion times greater than our planet does. But the Earth wouldn’t just fall into the black hole. Rather, it would orbit it while having chunks of the planet eaten with every pass.

Earth’s rotation would slow down this banquet, preventing the black hole from swallowing all of it. Whatever mass of Earth was left, would collapse into a disk of hot rock and start rotating around the black hole.

From space, this would look like an accretion disk – our spinning planet’s debris around the black hole’s event horizon. By that time, the black hole would have doubled in mass. Surprisingly, it would leave the Moon unharmed, only causing its orbit to become more elliptical.

You wouldn’t be so lucky. The black hole would consume you before you even realized what was happening.

Scenario 2. The black hole with a mass of a coin.

If about five grams of a coin suddenly collapsed into a black hole, that black hole would be terribly tiny. Compared to an atom, it would be as small as an atom compared to the Sun. And still, it would be terrifying.

See, the smaller a black hole is, the more Hawking radiation it releases. Simply put, black holes evaporate, spewing particles back into space.

In our case, the black hole would evaporate way too fast – just in a fraction of a second. Its insignificant mass of five grams would be converted into significant 450 terajoules of energy, and cause a massive explosion.

That would be like detonating 100,000 tonnes of TNT. The explosion wouldn’t tear the whole Earth apart, but would affect anything that happened to be near it.

It would be best if this black hole-causing coin wasn’t in your pocket. Despite all the technology we’ve invented, humans aren’t able to compress matter into a black hole even that small. Maybe one day, when space travel is more widely available, we’ll be able to capture a black hole from amongst the stars, and learn from it.

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