# Science Questions

## Does Hawking radiation selectively emit matter, rather than antimatter?

Tue, 25th Jan 2011

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### Question

So here lies my first problem; I could see that if the anti particle got sucked in it would annihilate releasing energy (although I'm not sure where this energy would go?) and the black hole would get smaller, and the mass of the radiation would be equal to the reduction in mass of the B hole. So that makes reasonable sense but it is equally likely that the matter particle gets sucked in and the anti matter emitted, surely the net effect would be no effect, on changing the mass of the B hole?

My lack of understanding of that may also account for my second problem; B holes emit more radiation when they get small (less mass). This is obviously counter-intuitive. I've heard it has something to do with thermodynamics, bigger B holes are cooler, while smaller ones are hotter? something to do with entropy?

Could you please clear up these issues for me, it would be greatly appreciated-don't be afraid to include equations :p they can make things easier to understand sometimes.

btw keep up the good work, I'll come and visit you if I get into Cambridge in acouple of years :p well maybe

David

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David Walker asked the Naked Scientists: Hi, I have a question about hawking radiation. Let me first outline what I understand by hawking radiation, due to particles (matter and anti matter pairs) constantly popping into existence then annihilating. The immense gravity of a black hole stops these from colliding, one falls in while the other shoots off into space - the radiation. This allows black holes to evaporate - lose mass. So here lies my first problem; I could see that if the anti particle got sucked in it would annihilate releasing energy (although I'm not sure where this energy would go?) and the black hole would get smaller, and the mass of the radiation would be equal to the reduction in mass of the B hole. So that makes reasonable sense but it is equally likely that the matter particle gets sucked in and the anti matter emitted, surely the net effect would be no effect, on changing the mass of the B hole? My lack of understanding of that may also account for my second problem; B holes emit more radiation when they get small (less mass). This is obviously counter-intuitive. I've heard it has something to do with thermodynamics, bigger B holes are cooler, while smaller ones are hotter? something to do with entropy? Could you please clear up these issues for me, it would be greatly appreciated-don't be afraid to include equations :p they can make things easier to understand sometimes. btw keep up the good work, I'll come and visit you if I get into Cambridge in a couple of years :p well maybe David What do you think? David Walker , Wed, 12th Jan 2011

No this is a very good question

My gut feeling is that there shouldnt be a bias in whether an anti matter particle or matter particle is ejected from the event horizon - ie 50/50 split

But there may be other processess at work here that favour one or the other

We know that anti-matter particles have opposite spin and charge to normal matter particles. Foolosophy, Wed, 12th Jan 2011

Oh there is a reason... What that reason is escapes me now, and I will try and find out, but contrary to what foolosophy states, physics is not truely a matter of biasm. There are fundamental and sturdy reasons for us to assume or postulate certain physics phenomena. QuantumClue, Wed, 12th Jan 2011

Just trying to get the conversation going to a very good question that is posed in this thread.

I don't know if there is a bias at the event horizon for anti particles or particles.

There may be a theoretical mechanism for such a bias (certainly not an experimental or observational basis for it at this time) Foolosophy, Wed, 12th Jan 2011

David - Hawking's initial and fairly intuitive explanation of virtual pair production is a tiny fraction of the story; its a very useful vehicle to explain the idea but is nowhere near well-founded enough to make further predictions on. Hawking describes it as follows in his original paper:

The actual mechanism is, apparently, pretty fiendish and I would guess well beyond my maths and physics.

But back to the heuristic mechanism - you have it slightly mixed up.  The idea isn't that an anti-matter particle falls into the BH, is annihilated, and radiates away energy - the radiation wouldn't get out, its a black hole radiation cannot get out.  It is that of the virtual particle pair the particle which falls into the black hole would from a distant observers viewpoint would have negative energy - and within the special conditions of black hole this virtual particle can be treated as a real entity.  thus the black hole has an increase in negative energy - ie loses energy and the other particle of the pair is emitted ie the rest of space gains energy. imatfaal, Wed, 12th Jan 2011

You're the lucky one, you are :)
This is one of the things we constantly debate here.