Q&A: Galaxies, Galaxy Clusters, AGN, and Quasars

Q:
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?

A:
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.