Quiz
Earth's Invisible Twin
John Wheeler gave us the term ‘black hole’ for what happens when enough mass gets crushed into a small enough space that not even light can escape.
If you hypothetically crushed the entire Earth down to a black hole without losing a single kilogram of mass, how big would the resulting event horizon (the Schwarzschild radius) actually be?
The Schwarzschild radius scales directly with mass: Rs = 2GM/c². Earth is nowhere near massive enough to naturally form a black hole, but the math doesn’t care about ’naturally’ — it only cares about how tightly the mass is packed.
The correct answer is 2: about the size of a marble, roughly 9mm across. Plugging Earth’s mass (5.97 x 10^24 kg) into the Schwarzschild radius formula gives a horizon just under 9 millimeters wide. To turn Earth into a black hole, you would have to compress the entire planet, oceans, core, and all, into a sphere smaller than a marble. It’s a good reminder that a black hole isn’t some exotic new kind of matter; it’s just a statement about how much mass is packed into how little space. Every object, including you, technically has a Schwarzschild radius. Yours is about 10^-25 meters, roughly a hundred-billionth the width of a proton, which is exactly why you don’t need to worry about spontaneously collapsing on your way to work.