What is a black hole?
Most people think of a black hole as a voracious whirlpool in space, sucking down everything around it. But that’s not really true! A black hole is a place where gravity has gotten so strong that the escape velocity is faster than light. But what does that mean, exactly?
Gravity is what keeps us on the Earth, but it can be overcome. If you toss a rock up in the air, it will only go up a little ways before the Earth’s gravity slows it and pulls it back down. If you throw it a little harder, it goes faster and higher before coming back down. If you could throw the rock hard enough, it would have enough velocity that the Earth’s gravity could not slow it down enough to stop it. The rock would have enough velocity to escape the Earth.
For the Earth, that velocity is about 11 kilometers per second (7 miles/second). But an object’s escape velocity depends on its gravity: more gravity means a higher escape velocity, because the gravity will “hold onto” things more strongly. The Sun has far more gravity than the Earth, so its escape velocity is much higher—more than 600 km/s (380 miles/s). That’s 3000 times faster than a jet plane!
If you take an object and squeeze it down in size, or take an object and pile mass onto it, its gravity (and escape velocity) will go up. At some point, if you keep doing that, you’ll have an object with so much gravity that the escape velocity is faster than light. Since that’s the ultimate speed limit of the Universe, anything too close would get trapped forever. No light can escape, and it’s like a bottomless pit: a black hole.