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Dark Stars ??

beamish

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Paxo

Paxo

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Bekenstein-Hawking radiation is a thermal radiation with a black body spectrum which is emitted by black holes and is caused by quantum gravity effects. It was shown in the 70's (theoretically by Zeldovich and Starobinsky (I think)) that rotating black holes should create and emit particles.

If this radiation truely exists, which is still under debate as no observations have been made and currently no tests exist for it, it means that black holes can lose mass and ultimately shrink and dissapear. This is one of the quantum gravity effects that the new Hadron collider at Cern will set out to determine/observe, by creating and subsequently evaporating micro-black holes.

Steve..

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InvaderXan

InvaderXan

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I think 'Hawking Radiation' has something to do with Hawking's new thoughts that things can actually escape from a black hole. :shocked:

Hawking radiation... :lol: Ok, well, here goes. :D

I hope this is a good explanation. Believe it or not, this is the simplified picture. I haven't even read the technical stuff -- I'm still reeling from the string theory course I took last semester! :)

Seriously though, there are a lot of genuinely difficult concepts here. I apologise if this is overly technical.

So one way of explaining hawking radiation is with a concept called "vacuum fluctuations". This requires a little rearrangement of how you think about the fabric of spacetime!

Firstly, as everyone probably knows, relativity tells us that E=mc^2, or in other words -- mass and energy are effectively interchangeable. Secondly, is the concept of "vacuum energy". What we consider to be vacuum actually contains energy, even if seemingly devoid of anything. For simplicity, you can consider it to be the energy of spacetime itself. :lol:

So, vacuum contains energy and energy may interconvert into mass. Quantum field theory expands on this with the concept of "virtual particles". Particles which exist for only a brief time. A pair - a particle and an antiparticle - spontaneously come into existence by stealing a small amount of this energy. They then immediately canel each other out, returning the exact same amount of energy.

Put simply, what we consider to be a vacuum isn't really a "vacuum" at all. It could be better described as a seething mass of particles bursting in and out of existence. Pairs of particles constantly appearing and disappearing. Energy to matter and back to energy again.

Does that all make sense...? :scratch:

So, if you can visualise that, consider this -- if this happens everywhere in the universe, then logically it must also happen at the event horizon of a black hole. A particle-antiparticle pair then could be created at a black hole's event horizon, with one particle travelling away from the black hole, and the other falling back into it.

Thus, the black hole loses mass and appears to emit a particle.

If all of that makes sense to you, then you should seriously consider a career in cosmology! :lol:

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dogfish

dogfish

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Yup, that was exactly my understanding. As the opposite side of the pair is lost into the black hole, then there is nothing to cancel the other out and so the black hole appears to emit radiation, lose mass and ultimately shrink. I thought there had been some observations though, but of course I could easily be wrong.

Now if someone could explain string theory to me... I tried reading the Elegant Universe, but got stuck at some point. Maybe I just need to concentrate harder?

Cheers, Martin

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beamish

beamish

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Thanks IX, - there you see simple really ! That refreshed my memory- perhaps you could pass on your knowledge re String Theory, I feel it's fair to say you'll have a better grasp (having studied it to some degree(no pun intended) )than someone like myself. There was a request earlier I think.

Must say I'm really enjoying this thread ! :)

Martin you still here-thought you had an early flight !! :shocked::lol:

Karlo

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JTankers

JTankers

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Yes, assuming that Hawking Radiation theory is valid, recent papers report that the theory is flawed. If dark energy exists, and then reverse Hawking Radiation might be more likely to feed black holes and cause growth. Do you know what you get when you mix high energy colliders with Professor Otto Rossler's charged micro black hole theory? :shocked: Answer: A golf ball :lol:

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beamish

beamish

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I wish I had a Brane.

Kaptain Klevtsov

" you could while away the hours

Conferring with the flowers,

Consulting with the rain.

With the thoughts you'd be thinkin'

you could be another Lincoln

If you only had a Brane..... and so on..... :shocked::lol::)

If anyone here understands it (Branes) it's likely to be you KK :lol:

Karlo

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InvaderXan

InvaderXan

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If I understand this correctly, the gravitational pull of the event horizon separates the particle-antiparticle's creation/cancellation process and allows one of them to escape and appear as radiation?

:scratch:

Exactly! :shocked:

If dark energy exists, and then reverse Hawking Radiation might be more likely to feed black holes and cause growth.

That is -- if dark energy exists. Personally, I'm not convinced. :lol:

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ngc2403

ngc2403

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InvaderXan,

you got hawking radiation down to a tee. as for observations i spoke to a lecture about this and he said that they have good strong evidence or to say what he meant was that they see this radiation signiture on almost all 'blackholes' because it is not seen as the done thing call them blackholes because the theory has not been accepted by 100% of the science population! (1 in 10 medical students don't beleive in evolution yet they have no problems working with it, so why can't we ?????)

during my current astronomy course we where give the equation for the mass lose of a blackhole. it is mass dependent i.e. as the mass gets smaller the rate of mass lose inceases. the LHC hopes to see the radiation signitures of tiny blackholes quicky evaporating, but they are hoping for a lot of other things too.

ally

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ngc2403

ngc2403

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That is -- if dark energy exists. Personally, I'm not convinced. :)

have you seen/read the stuff from the Italians that showed the wave like graph which they say is DARK MATTER :lol: if it is then they must just be messy at reporting however i think it might just prove to be inconclusive :shocked:

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Talitha

Talitha

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If I understand this correctly, the gravitational pull of the event horizon separates the particle-antiparticle's creation/cancellation process and allows one of them to escape and appear as radiation?

:scratch:

Exactly! :shocked:

See? Your explanation made perfect sense!! :thumbright:

One question, though.. what exactly is it that allows one of them to escape and appear as radiation? Does the creation/cancellation process allow enough time, or does time stretch a bit on the event horizon and 'make' a bit more time, so to speak?

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InvaderXan

InvaderXan

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Actually, time doesn't have much effect, from the particle's perspective. Relativity uses the idea of "proper time" -- imagine a clock at the same place as the object you're considering.

At the event horizon, nothing too special would happen to the particles in proper time. If an observer was to fall into a black hole, they wouldn't even be able to tell where the event horizon was!

The only difference between the two particles is that one would be slightly closer and therefore be slightly more affected by gravity. Gravity would act on the two instantaneously -- It's only by sheer chance (based on energy, velocty, etc) that only one of the pair gets pulled in. It only happens because so many pairs are being constantly created/annhilated! :shock:

All the same, the vast majority of these particle-antiparticle pairs either both get dragged in or both escape (leaving the black hole unchanged).

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Ajohn

Ajohn

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One thing that puzzles me about radiation and black holes. Say some one is watching something fall into a black hole. To an observer it looks like it going slower and slower as it gets nearer the event horizon. To me that means that any electromagnetic radiation is getting more and more red shifted - in this case to zero - what ever that means. So just how can any from of radiation be detected coming out of a black hole?

John

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