What are jets in black holes?

19 April 2016


We often hear that blackholes are so massive that even light cannot escape its grasp, therefore light bends when close a blackhole. Come to close and light enters the event horizon and its lost.

Then, why do we hear that black holes spew out particles? Do the particles travel faster than light then?


Francois Nel


Kat Arney put this question to Cambridge astronomer Matt Middleton... Matt - Jets are the most powerful events in the universe - jets from black holes. They carry away huge amounts of material and very often they're moving extremely close to the speed of light - say 99.9% of the speed of light. I'm sure that we all roughly remember around our GCSEs - something like this, where you could work out the kinetic energy of a moving body from 0.5 x mv2. Take that mass - oh Kat's wondering about it, alright.

Kat - I'm a biologist.

Matt - You're a biologist - you don't do equations. So if you take the mass that's coming out of it, take the velocity, clearly there's a huge amount of energy. And, in fact, there's a very nearby jet coming from a supermassive black hole called CenA, and the amount of power that is coming from that is 10 to the 12 times the power that is coming from the sun.

Chris - How do we know the jets there?

Matt - You can see it. So these jets in particular emit everything from optical all the way through to x-rays and probably beyond.

Chris - Ah, so the stuff coming out which is radiating this.

Matt - Absolutely. It radiates across.

Chris - So is it just radiation or is it particles. What is in the jet?

Matt - So it is particles and those particles are radiating. So essentially for those who are on a geek out for some science you have magnetic fields and you have electrons that spiral around those magnetic fields and, because they're constantly changing direction, they have acceleration.

Chris - Why are they firing out of the black hole and where from the black hole are they coming from?

Matt - Okay. So, the point is these are not actually from inside the black hole. The old adage of, you genuinely cannot get out of a black hole, is true. You cannot escape from it. And in fact, we should point people to the podcast. But they're coming from close to the black hole and, in fact, in another nearby supermassive black hole, people have been able to use radio interferometers. That's when you have multiple radio dishes, and they provide a very high angular resolution view of these structures and they've been able to work out that it's coming from about 5 times the size of the black hole, above the black hole. So that's incredibly close.

Chris - Do we know what concentrates the material into a jet? Why isn't it just sort of spinning round, getting excited, and then just radiating in all directions like our sun radiates radiation at us in all directions.

Matt - Sure. So if you ever ask an astronomer a question they can't answer, they'll always say magnetic fields or dust. It turns out it is magnetic fields and it genuinely is the answer. We know there are magnetic fields there because we see what we call synchrotron emissions, so these are the electrons spiralling around magnetic field lines. And that's why you basically collimate all this emission that's coming away from the black hole through these magnetic fields, so it actually does look like a jet. It's perhaps worth mentioning that there's aren't the only particles that we see from black holes, there's also Hawking radiation and that's when the black hole itself decays. So you have a particle that's created on the event horizon. It's actually is a particle-antiparticle pair. One of them goes into the black hole, the other one gets kicked out and that's how black holes decay.

Chris - The black hole loses a little bit of mass and that makes it shrink?

Matt - Absolutely!

Chris - So black holes should evaporate over time?

Matt - And that's why we're not going to be destroyed by the Large Hadron Collider.

Kat - Hooray!

Chris - That's reassuring to know isn't it Kat?

Kat - It is reassuring.

Chris - Thanks Matt.

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