There is this kind of sci-fi notion that

black holes just suck in everything

around them. Not true. You are perfectly

able to orbit around a black hole just

like you would orbit around any central

mass, but orbiting stops helping when

you get too close to the black hole. We

said that the event horizon is is 2 GM

over over c squared. In fact, already

once you're within 3 GM over c squared,

orbiting is counterproductive if you're

trying to stay away from the black hole.

And that's because there are two effects

of orbiting. One effect of orbiting

helps you stay away from the black hole.

That's the centrifugal effect. Orbital

angular momentum pushes you away from

the black hole due to the centrifugal

effect, but there's another effect which

drags you towards the black hole, and

that is the fact that in general

relativity all energy gravitates. And so

the effect of orbiting is that you have

a additional pull down towards the black

hole from the coupling between the

gravitational attraction between the

mass of the black hole and your orbital

angular energy. And when you're far away

from the black hole, the centrifugal

force is the more more important term.

When you're close to the black hole,

[music] that coupling is the more

important term. And in fact, once you

get within 3 GM, orbital angular

momentum stops helping and starts

hurting. There are no ballistic orbits

that go within 3 GM and that manage to

escape again.