A place light can't leave

Throw a ball straight up and it comes back down. Throw it harder and it climbs higher before it falls. Somewhere there is a speed where it never comes back. Earth's gravity keeps slowing it, but never enough to turn it around. That speed is called escape velocity, and for Earth it's roughly 11 kilometers per second, about 40,000 km/h. That's why rockets are mostly fuel.

Squeeze it and the speed goes up

Escape velocity depends on two things. One is how much mass is pulling on you, and the other is how close you are to its center. From the surface of the Sun, which is far heavier than Earth, you'd need roughly 600 kilometers per second.

Now keep the mass the same and shrink the size. You're standing closer to all that mass, so escape gets harder. Squeeze far enough and escape velocity passes the speed of light, about 300,000 kilometers per second. Nothing can go faster than light, so nothing gets out. Not even light.

That's a black hole. It isn't a hole and it isn't a thing made of black stuff. It's a region of space where gravity has won completely.

Choose outside, at the horizon, then inside. The blue wedge shows possible radial futures, with light on its edges; it does not show light slowing down locally.

An old idea that turned out real

In 1783 an English clergyman named John Michell made this exact argument with Newton's physics. He imagined "dark stars" so heavy that their own light would fall back. It was a clever guess, and nobody could check it.

The real version came from Einstein, who described gravity as the bending of space and time. Within weeks of his 1915 theory, a German astronomer named Karl Schwarzschild, serving on the Russian front in World War I, sent him an exact solution for the space around a ball of mass. Hidden in it was a boundary that nothing could come back from. Schwarzschild died a few months later. The name "black hole" came decades after that, made popular by the physicist John Wheeler in 1967.

How small is small enough?

The size of that boundary depends only on mass. Double the mass and it doubles in size. Pick an object below and squeeze it.

Pick an object and tap Squeeze, then Reset. The picture is not to scale; the text gives the size needed for that mass.

Earth would need to fit in a marble. The Sun would need to shrink to a ball you could walk across in a little over an hour. Nothing in your daily life comes close to that kind of squeezing, which is why black holes sound so strange. Nature can do it, though, and the next lesson shows how.

They don't suck

One myth to drop now. A black hole doesn't vacuum up everything around it. From far away, it pulls exactly like any other object with the same mass. If the Sun were swapped for a black hole of equal mass, Earth would keep orbiting on the same path. It would get very cold and very dark, but it wouldn't fall in.