Black hole danger

Black holes are famous as some of the most dangerous objects in the universe and with good reason. Even still most people don’t really understand the actual dangers posed. In every book or movie with a black hole, it is portrayed as a giant vortex that sucks in everything. While this description is technically correct, the range of the destruction is almost always greatly exaggerated.

I think an illustration would best explain the actual limitations. Let’s look at what would happen if the moon were a black hole; a black hole is formed on the moon which then consumes it. To be clear, this scenario does not add any mass to the moon, it only compresses the existing matter to a singularity. The Schwarzschild radius formula (r = 2 * G * M / c²) says the resulting black hole would have an event horizon 0.22 mm in diameter. The event horizon is the point from which it is physically impossible to escape, but, of course, anything not moving close to the speed of light is going to be in trouble from a lot farther away.

So how dangerous would this black hole moon be to the earth? The answer depends on the gravitational pull, which we can determine using Newton’s law of gravity (F = G * m₁ * m₂ / r²) for anything reasonably far from the event horizon. Since the only variables in this formula are the distance to the moon’s center of mass, the mass of the moon and the mass of the object being pulled on by the moon, none of which have changed in this scenario, the effect of the moon’s gravity would be the same as it is now for anything on earth. In fact, it would be the same for anything more than 1,080 miles (current the radius of the moon) from the black hole.

All this means that the moon being a black hole would not pose any danger to anyone on earth. There would be differences, like the lack of moonlight and a tiny black spot in the sky that occasionally distorts a star, but the only ones who would actually be in danger are astronauts that try to leave earth’s orbit.