Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: DoctorBeaver on 24/06/2008 21:38:28
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Can someone explain this to me please?
Let's start with the assumption that Quantum Field Theory is correct insofar as gravity is mediated by gravitons. Gravitons interact with all other particles including other gravitons. That means that gravitons are affected by gravity. So far so good.
Black holes. Nothing can escape from them.
Hang on... rewind...
Nothing can escape from them.
The gravity of a black hole is so extreme that even photons cannot escape. But wait a minute - the gravitational field of a BH extends far beyond its event horizon. Can you see where this is going?
A BH generates a gravitational field that extends beyond its EH. Gravity is mediated by gravitons. Gravitons are subject to gravity.
Is it just me, or can anyone else see a problem here? If gravitons are affected by gravity, how do they escape from the BH?
Since this occured to me a while ago, I've been checking in my text books and on internet sites, and all of my statements are verified. But they are incompatible. Either gravitons must be able to escape from a BH, or the BH will have no gravitational field.
So come on boffins. Explanation please. [???]
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The first thing that strikes me is that Gravitons in as much as they can be regarded as particles are very light, I have seen a figure of 10^-11 that of the Neutrino quoted and they are incredibly light particles.
Is it possible that gravity does not operate below a certain mass, a sort of Planck limit ?.
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syhprum - I was under the impression that gravitons were thought to have the Planck scale mass. There is now, however, a theory that they could be as light as 1TeV. I have never seen them quoted at the figure you gave.
I'm not sure about there being a lower limit. What about photons? They are affected by gravity.
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You have some serious wrong thinking here. The existence of a gravitiational field does NOT create gravitons in the same way that the existence of an electrical field does not create photons.
The graviton and the photon only come into the process when another particle comes into the gravitational or electrical field and is affected by it and then the total interaction is then the the sum total of all the interaction alternatives that are possible if the interaction is to be viewed in a quantum way.
There is also no good reason to believe that there is any lower limit on the energy of an individual graviton as there is also no lower limit on the energy of a photon. In fact I expect that assuming the energy is related to plancks constant times the frequency like photons that most gravitons are extremely low energy indeed.
On this basis the event horizon does not have to emit anything it is just that the quantum effect of a body moving through the gravitational field and being deflected by it invoves the exchange of gravitions betweeen the body and the field around the black hole this of cours will apply all the time the body is outside the event horizon.
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Are you having a bit of a memory lapse DrB?
After you suggested this solution:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Ftiger100.co.uk%2Fjpg%2520folder%2Fthejump.jpg&hash=826a2037bd14abcf8b76dc905cec2ee3)
you then put me on to Randall-Sundrum space, after I asked the same question about a third of the way through this thread:
http://www.thenakedscientists.com/forum/index.php?topic=14372.0
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The figure I quoted for the Graviton mass came from an article in the Scientific American on the possibility of building a Graviton detector similar to a Neutrino detector.
The conclusion was that this was quite impossible.
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Ian - I was not implying that a gravitational field creates gravitons. Maybe my terminology was wrong. What I meant to say is that as it is gravitons that mediate gravity, then for an object to feel gravitational attraction there must be a gravitonic interaction. Where do the gravitons originate if not from inside the BH?
There is also no good reason to believe that there is any lower limit on the energy of an individual graviton as there is also no lower limit on the energy of a photon. In fact I expect that assuming the energy is related to plancks constant times the frequency like photons that most gravitons are extremely low energy indeed.
There is a very good reason for believing there is a lower limit - none have been detected. In fact, if their energy were arbitrarily low then even chemical reactions would produce them. That is clearly not the case.
LeeE - I haven't forgotten. But what I have read recently indicates that is only 1 possibility. I have been pondering on other theories that have been put forward. For what it's worth, I think my reply in the other thread is more likely. However, that would imply that there is no direct interaction between gravitons and other particles and what I've just read in various books says that there is.
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Daylight on a cloudy day is a mix of photons of a wide range of frequencies going in all sorts of directions. Under these conditions it is very difficult to detect an individual photon unless you can create a very specific filter. Electrical circuits can emit electrical fields down to several seconds per cycle and particles can be deflected by them the very low energy photons produced by such circuits are undetectable individually because they are such low energies and there are so many of them. The gravitational interactions of bodies are just like that, and involve even lower frequencies there is a background noise of gravity that makes it difficult to detect individual gravitons even if there was a way of designing a suitable filter. The current gravity wave detectors are aimed at detecting large numbers of gravitons produced in very violent cataclysmic events.
There may be some very high energy gravitons around from violent gravitational interactions during the early stages of the big bang and they may constitute a part of the dark matter/energy of the universe. current attempts to detect dark matter from unexpected recoils of particles under very quiet conditions may detect them but it would be very difficult to identify such a particle from other types of dark matter.
It would be very interesting if there was a lower limit to a gravitational interection and it might be measurable using very low mass small gravitational experiments (people are doing experiments like this to look for a fifth force) it could also be a contributor to modified Newtonian dynamics.
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PS the gravitons do not come from inside the black hole they come from an interaction of a gravitiating particle with the gravitational field outside of the black hole.
Another idea for detecting a lower limit on gravitiational interactions it is quite possible now to create very slowly moving beams of light atomic particles that can fall under gravity if there was a lower limit these would not fall smoothly but would fall in a series of steps that could be detected. As far as I know no such steps have ever been detected.
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Where do the gravitons originate if not from inside the BH?
Why?
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Here's a bit of wild speculation about something that ocurred to me concerning photons and BHs that might have some relevance to gravitons and BHs.
If a photon is heading directly towards a BH it will not accelerate, but it will be blue-shifted i.e. it's frequency, and therefore it's apparent energy, will increase. However, as the rate of time depends upon the gravitational field strength i.e. time slows in a stronger gravitational field, the increase in frequency can be seen as a conservation of energy. That is, if the frequency of a photon is measured in terms of a number of oscillations per unit of time then increasing the period of time that it takes that unit of time to pass will in effect reduce it's effective frequency and energy. If this is so, then a BH has no direct gravitational effect upon the photon and only influences it by virtue of it's local change to the time rate.
It is not as simple though, for a photon heading obliquely towards a BH, because in addition to the stuff above, it's course will also be changed. The implication of this would seem to be that a photon is affected differently by gravity according to the direction it comes from relative to the orientation of the photon. Now if that's so, we could hypothesise that a graviton is like a photon except that it's not affacted by gravity from any direction, and as directionality is related to shape, a graviton could be similar to a photon except that it lacks some aspect of the photon's shape.
Also, if gravitons were to have no frequency (which might be linked to their structure or shape), would they be affected by changes in the rate of time, would energy conservation be irrelevent, and might this then allow them to escape?
Like I said, just a bit of wild speculation.
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[???]
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Erm... which bit aren't you clear on?
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The bit between
Here's a bit of wild speculation about something that ocurred to me concerning photons and BHs that might have some relevance to gravitons and BHs.
and
Like I said, just a bit of wild speculation.
Come on... it's 6:38am and I didn't get to bed until almost 2am. Let me read it when I'm a bit more awake.
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[:D]