Hawking Radiation, again.

[Jay-qu]

I know this has been discussed here before numerous times but I saw the need to start a new thread so that I could clear up some finer details. :confused:

 

I have heard Hawking Radiation explained in two ways and am not clear on which one is the accepted theory, or whether they are both not only plausible but add up to the overall effect.

 

So the main way I have come to understand it is that on a very small scale virtual particles are constantly created and destroyed. These particles are created in particle/anti-particle pairs; the energy needed to create them is temporarily 'borrowed' from the vacuum. Normally these two particles would then come in contact and be annihilated giving back the energy but when this happens in close proximity to a black hole one of the particles can be sucked in, leaving the other particle to escape, the energy needed to create this particle has come from the black hole so it reduces in size (ever so slightly!)

 

Secondary explanation – I think it may have been in Hawking’s ‘Black holes and baby Universes’ book. Basically as the position of a particle that is in a black hole is very tightly defined, its velocity must be relatively undefined because of the uncertainty principle. Because of such a large uncertainty in the velocity, it is possible that the particle could temporarily exceed the speed of light and tunnel out of the black hole (faster than light:shrug: )

 

Also one final question (as if the first wasnt enough!) Large black holes radiate slower than smaller ones, is there a point when they are so big that they absorb more background radiation than they are able to radiate and hence will never end up evapourating into nothing?

[InfiniteNow]

My understanding is that both of the explanations you gave are valid. The pair popping into existence, but one being sucked in the other not, and also the particle picking up energy from the BH.

 

Your last paragraph, however, would seem to violate thermodynamics. However, I believe your grasp on the subject is already better than my own, so will stop here in hopes that a more informed individual will step in.

 

 

Cheers. :confused:

[Jay-qu]

Thanks InfNow :cup: I dont know how my grasp is but I definitely am starting to get a much better understanding :shrug:

[TheBigDog]

What I don't understand about hawking radiation is how the black hole loses energy. If half the particle pair is outside the event horizon and is observed as radiation out of the BH, and the other half of the pair goes into the black hole, then how is the BH losing mass and energy if is absorbing mass/energy at the same rate it is "radiating" It would seem to me more likely that the hole grows as a result of this process instead of shrink, but I will defer to those who have studied it more closely.

 

Bill

[InfiniteNow]

TBG,

 

I think the phenomena happens whereby the pair pops into existence right at some critical distance, whereby one falls in, the other doesn't, and there is a negative "hole" left over. The "hole" is filled by a tunneling effect (which I haven't even yet begun to grasp) and energy actually comes out of the BH. It's still controversial though...

 

 

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

 

Classically, the gravitation is so powerful that nothing, not even radiation, can escape from the black hole. However, by doing a calculation in the framework of quantum field theory in curved spacetimes, Hawking showed quantum effects allow black holes to emit radiation in a thermal spectrum.

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A more precise, but still much simplified view of the process is that vacuum fluctuations cause a particle antiparticle pair to appear close to the event horizon of a black hole. One of the pair falls into the black hole whilst the other escapes. In order to fill the energy 'hole' left by the pair's spontaneous creation, energy tunnels out of the black hole and across the event horizon. By this process the black hole loses mass, and to an outside observer it would appear that the black hole has just emitted a particle.

 

 

Hence, the evaporation is specific to a particular frame of reference too...

[Jay-qu]

What I don't understand about hawking radiation is how the black hole loses energy.

 

2 particles have just come into exsistance that were not there before - the particles require energy to be created, which is tunneled out of the black hole.

[Qfwfq]

It's still controversial though...

Ya and it's a thing I'm a little sceptical of.

 

One thing fundamental in quantum field theory is that the pair created can't be observed while it is virtual and the energy is "borrowed", any interaction that maks it possible to observe at least one of them would have to supply the energy so as to pay the mortgage off. I can't quite see how this would be done, by energy coming out from the black hole.

[Jay-qu]

but even if the black hole isnt the source of the energy - then it is still radiating, but this time would be growing!

[Qfwfq]

It wouldn't strictly be the black hole that's radiating, if the energy is coming from elsewhere. It would just be that some of that energy is ending up in and some isn't.

[Jay-qu]

sorry, the black hole would 'appear' to radiate, but it would still remain the cause of the radiation.

[InfiniteNow]

Have you looked into vacuum fluctuations at all?

[Jay-qu]

nope, I have only been introduced for the purposes of explaining Hawking Radiation.. I guess I should look into that :doh: