system
Given the following post, respond as if you're commenting on Reddit:
human
Does a black hole experience friction drag from the matter it is travelling through?
gpt
There's something we call "friction", but it's not really friction in the same sense.

When a ball flies through the air, the atoms on its surface and constantly bumping into and interacting with the atoms in the air. These interactions transfer kinetic energy from the ball to the air, and the ball slows down.

For a black hole, it doesn't really *touch* things very much though, because space is very empty. Anything that hits it dead on will be accreted, and add its mass and momentum to the black hole. But it's pretty rare to hit a black hole head on like that. What generally happens is you form an accretion disc around a black hole that gradually drips matter onto it.

The bigger effect is what's called *dynamical friction*. This particularly affects supermassive black holes. When a supermassive black hole moves through a galaxy, it attracts the stars around it. The stars don't generally get sucked into the black hole. Instead, they tend to accumulate *behind* the black hole as it moves. There ends up being a higher density of stars behind the black hole than in front of it. These stars are all pulling the black hole with their gravity, and that means the black hole is being pulled backwards a little bit. This causes the supermassive black hole to slow down, and to start to spiral "down" to the centre of a galaxy. This is very different to how friction works on a ball thrown in the air, but we call it "dynamical friction" anyway.

For smaller black holes, scattering also acts like friction. If you have a black hole in a star cluster, it will have nearby encounters with other stars, most of which are much less massive. These random encounters tend to end up with the smaller star being thrown further out, and the black hole losing some kinetic energy and falling further towards the centre of the star cluster. Once a supermassive black hole gets somewhat close to the centre of the galaxy, the same effect also happens and this helps the supermassive black hole to fall into the very centre of the galaxy. This is another source of something like friction, but again it's not really friction in the ball-in-air sense.