Q&A: Galaxies, Galaxy Clusters, AGN, and Quasars
Regarding the story on Perseus, I thought that no information
leaves a black hole. Light can't escape, so how can sound? Especially when
the sound is generated from the black hole itself?
You're correct that information can't escape a black hole, so that's
a good question. Imagine firing an object like a rocket away from the
Earth. The escape velocity is the velocity an object would require to
completely escape from the Earth's gravitational pull. The escape velocity
for the Earth is about 25,000 miles per hour. If an object had the same
radius as the Earth but a higher mass, the escape velocity would be larger.
If this object had an enormous mass, the escape velocity would be higher
than the speed of light (675 million miles per hour) and you would have
a black hole. The "event horizon" is where the escape velocity is equal
to the speed of light. At closer distances to the black hole then yes,
neither light nor matter can escape.
What's happening with the black hole in Perseus, as with many other black
holes, is that material (gas) is pulled onto a hot, rapidly spinning disk
before being pulled over the event horizon, the point of no return. But,
not all of the material passes over the event horizon. A fraction of the
material gets ejected along jets in a process that isn't very well understood,
but it must be related to the large amount of energy the material gathers
when spinning around the disk, and the strong magnetic fields generated.