Nib asked for
Gravity that traps light
Every object has an escape speed, the minimum speed needed to leave its gravity for good, and in a black hole that speed is greater than the speed of light, so nothing can get out.
5 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the throw
Throw a ball upward and gravity slows it, stops it, and pulls it back. Throw it harder and it climbs higher before returning. Gravity never switches off, but its pull weakens with distance, so a hard enough throw can outrun it.
Gravity always pulls back, but a fast enough launch can outrun it.
a common mix-upThings fall back only because gravity eventually 'runs out' of reach, or fly off because gravity switches off.
2escape speed
There is a threshold speed beyond which the ball never turns around, because gravity weakens faster than the ball slows. That minimum speed is the escape speed. On Earth it is about 11 kilometers per second, roughly 40,000 kilometers per hour, which is why rockets need so much power.
Escape speed is the minimum launch speed to leave a world and never fall back.
If you launch a rocket slightly below Earth's escape speed, does gravity eventually win?
Show the answer
Yes. It slows, stops, and falls back, even if it travels very far first.
3bigger, harder
Escape speed depends on how much mass is packed in and how close to its center you start. A heavier object pulls harder; starting nearer the center also means a stronger pull. The Moon is lighter than Earth, so its escape speed is only about 2.4 kilometers per second; Jupiter's is about 60.
More mass, or starting closer to the center, means a higher escape speed.
4squeeze it
Now squeeze Earth, in imagination, into a smaller ball without losing any mass. You would now stand closer to the same mass, so the surface pull and escape speed climb. Shrink it enough and the escape speed approaches the speed of light, about 300,000 kilometers per second.
Squeezing the same mass smaller raises the escape speed at its surface.
a common mix-upA black hole must be far heavier than anything else; really it is about how tightly the mass is compressed.
5black hole
Light is the fastest thing there is, and nothing can outrun it. So if the escape speed at some boundary exceeds light speed, even light launched outward cannot get away. Nothing at all can leave, and light cannot reach you from inside. That is what makes it black.
When escape speed beats light speed, not even light can leave.
Why can't a rocket with a stronger engine escape a black hole?
Show the answer
Escape requires exceeding light speed, which nothing can do, so no engine is strong enough.
A black hole isn't magic: it's escape speed pushed past the speed of light, so even light falls back.