Hawking Radiation?

Question

Ok, from what i barely understand this is when two particles are OUTSIDE the event horizon, then one goes in and shoots the other one away. I know there is a little more to it than that. But wouldnt that just be The black hole still gaining mass? I dont understand why this would mean a black hole would eventually radiate away. Unless of course i dont understand the originall theory.

Answer 1

general relativity says that all mass creates "warps" or "gravity wells" in spacetime,and the slope of the gravity wells created by black holes is more extreme than the slope of any other. it is so extreme that ther is a huge amount of energy in the spacetime itself. this energy is sufficient for the creation of particle/antiparticle pairs in the form of electrons/positrons. these have mass and they are formed from the energy created by the warping of spacetime by the black hole. if one particle is lost in the black hole the other is free to escape, and the escaped particle represents loss mass for the black hole. less massive a black holes loose more mass than more massive ones because the slope of the gravity well is more severe for less massive black holes so they radiate faster. finally, the radiation becomes so fast that look like they end their "lives" with an explosion.

http://en.wikipedia.org/wiki/Hawking_radiation


also:
it is true, only non-spherical objects create gravitational radiation, but the mass of the object has to be oscillating. an object has to change shape, or objects have to orbit each other. if an object has to oscillate between being flattened at the poles and elongated at the poles then it is not a sphere.

http://en.wikipedia.org/wiki/Gravitational_radiation

Answer 2

Hi Stonewall Kid

Hawking radiation is not really about these particle pairs being separated near the horizon. This is an explanation invented after the fact to try and describe what's going on near the horizon in quantum terms. It's strange that it has become the common "explanation" for the phenomenon because (imo) it's not very easy to understand properly, particularly for non physicists.

Let me try to explain it another way. First let's separate our theories:
* General relativity (GR) is a theory of gravity. It's classical (ie not quantum scale) and it is the theory which describes black holes
* Thermodynamics (TD) is a theory of heat, energy and entropy. It is also a classical theory.
* Quantum Field Theories (QFTs) are theories which describe how the interactions of particles and field happen at the quantum scale, usually by pretending the field behaves like a bunch of little particles (field quanta). QFTs ignore gravity because we don't know how to treat gravity at a quantum scale.

Now - Hawking's key result was a merging of GR and TD. What he and Bekenstein found was that black holes have a non-zero temperature! That's an interesting result because one of the central tenets of TD is that heat flows from hotter things to colder things, and if we accept that the event horizons of black holes are effective one-way membranes (nothing can get out) then we'd expect the black hole to be perfectly black. If the hole has a non-zero temperature then it must radiate.

Now for the hard bit - Hawking's result is what we call "semi-classical", that is he stitched quantum fields into his result without doing full quantum gravity. He extended a generalised quantum field into the curved space-time surrounding a simple (schwarzschild) black hole and counted the particle content of the field at infinity (ie a long way from the hole where space-time is flat again). And there he found a particle distribution corresponding to a blackbody emission spectrum. The black hole at the centre of the space could be the only source for the radiation.

Notice at this point that we haven't considered what's going on near the event horizon at all. What we've found is that the BH has a non zero temperature, and that there's radiation a long way from the hole which can only have come from the hole. The next question is "how did that radiation get out of the hole?", and that is where the (clumsy) split pairs explanation comes in. the explanation is physically complex, but roughly put the curvature near the horizon attenuates the emerging pairs so that the hole loses energy. The smaller the hole the stronger the horizon curvature and the stronger the attenuation, hence the greater radiation rate.

For people with stronger physics background there's a more technical thread on the subject (with my posts) here: http://www.sciforums.com/showthread.php?t=55105


Hope this helps!
The Chicken.

Answer 3

Okay I do not know much about black holes but I did study it independently for a while a bit ago.

Well if you have two particles that the universe creates that is close to but yes, outside the event horizon. You can safely assume that they have oppsote charges. For example Positron and Electron. Which will have a very large mass to be created but anyways. So lets say the electron goes into the black hole but the positron goes outside of the back hole. The electron will react with a positron inside the black hole and react with a particle, turning it into pure energy.

Hm.... but conservation of mass/energy sais that even though they annhialate each other, the energy created will equal the same mass and you havent done anythign but convert mass into its equivolent energy. Unless the energy is in heat and because it is in a vacuum that heat is radiated?! Or disapated or absorbed by the universe?

Good question!

Answer 4

The particle pair that forms near the event horizon of a black hole can separate, with one getting sucked into the black hole and the other zinging off into space. By virtue of their very natures, one must have a positive energy and the other a negative energy signature, whether you consider charge, spin, or whatever. By definition, the one that we designate as "negative" will fall into the gravity well of the black hole because it has a lower energy, while the other one with "positive" energy escapes because it has a higher energy. We call the one that escapes matter and the one that get's captured antimatter. The black hole therefore does not gain mass, because once the antimatter particle reaches the singularity, it will annihilate a particle of matter; the singularity will get ever so slightly smaller as a result. Assuming it doesn't suck any more matter into its gravity well, a black hole will "evaporate" over a long period of time.

Answer 5

Thats particle pairing if I remember right. If the pairs originate at the event horizon , one will eventualy collide with its corisponding anti-particle and be destroyed. That wuold be a net loss of mass for the blackhole.