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#3
12-02-2006, 05:39 PM
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Re: How do gravitons escape a black hole?
This is a really good question and I'll tell you what I think about it. I'm not 100% confident of my understanding on the matter and would welcome any corrections or criticisms.
Unfortunately, the answer may boil down to the fact that we just don't have a good quantum gravity theory yet so we aren't yet equipped theoretically to answer this question. One way to look at it is from a general relativity perspective. In GR, "gravity" is just the result of the curvature of space-time. There are no gravitons that need to escape and black holes are just an extremely curvy bit of space-time. Gravity in GR isn't something that travels like photons, it is a property of space itself. For a theory of gravitons it is trickier. I would attribute it to quantum tunneling. Quantum mechanical particles can overcome potential barriers that classical particles can not, due to the uncertainty principle. This property, often called tunneling, is responsible for a phenomenon known as Hawking radiation, which is basically a radiation of photons every black hole gives off. I'd imagine a similar thing could happen for gravitons. 
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#4
12-02-2006, 06:05 PM
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Re: How do gravitons escape a black hole?
Virtual particles aren't limited to the speed of light. In the same way that virtual photons can escape a charged black hole to mediate the electromagnetic field, virtual gravitons can escape to mediate the gravitational field.
We don't actually have a full theory of gravitons, so we don't really know if actual gravitons (i.e. gravitational waves) travel at the speed of light, or are even affected by gravity, but both are common assumptions. Anyway, actual gravitons would only be produced *outside* the event horizon, (e.g. by two black holes circling each other before collision) and so would still be able to escape.
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#6
12-02-2006, 07:28 PM
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Re: How do gravitons escape a black hole?
[ QUOTE ]
How does any interaction cross the event horizon? [/ QUOTE ] I'm not sure if anything can actually cross the event horizon. I haven't read up on physics lately, but from what I remember the curvature of space becomes infinite at the event horizon and time stops, so I would think that things just get stuck at the event horizon. Any experts please feel free to correct me on this.
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#7
12-02-2006, 07:40 PM
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Re: How do gravitons escape a black hole?
A couple of good responses so far. Let's expand the list of questions.
1. How do gravitons escape a black hole? 2. How does gravity escape a black hole? 3. Is there anything special about the event horizon? 4. If there is a graviton version of Hawking radiation, call it graviton-Hawking-radiation, then what is its strength? 5. Is the strength of the graviton-Hawking-radiation basically the same strength as the gravity (gravitational acceleration) itself? (Set hbar, c, G to 1.) 6. If so, or if not, what role does graviton-Hawking-radiation play in gravity? 7. Since Hawking radiation comes from gravitional acceleration, does it not occur also inside the event horizon all the way down to the `singularity'? Here `singularity' means whatever there is in quantum gravity in place of the classical singularity of GR (general relativity). 8. If not, then how does all the mass get from the `singularity' to the outside during the complete Hawking evaporation of a black hole via Hawking radiation? 9. If so, how much of the mass of the black hole consists of Hawking radiation between the `singularity' and the event horizon? 10. If you are accelerating `forwards' in space, does the Unruh radiation (similar concept to Hawking radiation, relating gravity to acceleration) hit you from the front or the back, or from all around, or what? 11. What are all the similarities, and differences, between gravitons and photons? 12. (For genuine experts only) how much of the above is nonsense based on ignorance?
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#8
12-02-2006, 10:22 PM
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Re: How do gravitons escape a black hole?
[ QUOTE ]
A couple of good responses so far. Let's expand the list of questions. 1. How do gravitons escape a black hole? [/ QUOTE ] A "graviton producing machine" inside the BH will not be able to send real gravitons out past the event horizon. (provided we remain in the limit where the BH solution can be thought of as a reasonable background) [ QUOTE ] 2. How does gravity escape a black hole? [/ QUOTE ] Gravity doesn't "escape" a black hole -- a black hole *is* a solution to the gravitational field equations. [ QUOTE ] 3. Is there anything special about the event horizon? [/ QUOTE ] Not locally -- you wouldn't notice anything special if you were in a box falling across the horizon. Outside the BH, though, it does have significance -- you'll never hear again from anything that crosses the horizon. [ QUOTE ] 4. If there is a graviton version of Hawking radiation, call it graviton-Hawking-radiation, then what is its strength? [/ QUOTE ] It should exist, and it should have a thermal spectrum just like any other field (provided again that the BH solution can be thought of as a good background -- i.e. the perturbative regime -- this should be an extremely good approximation for any reasonable BH, though). [ QUOTE ] 5. Is the strength of the graviton-Hawking-radiation basically the same strength as the gravity (gravitational acceleration) itself? (Set hbar, c, G to 1.) [/ QUOTE ] The temperature defining the thermal spectrum of gravitons should be proportional to the surface gravity of the BH (which will be inversely proportional to the mass of the BH). [ QUOTE ] 6. If so, or if not, what role does graviton-Hawking-radiation play in gravity? [/ QUOTE ] Due to the weakness of Hawking radiation, the gravitons can be thought of as tiny perturbations to the BH solution -- they shouldn't be significant in changing the causal structure of the BH solution. [ QUOTE ] 7. Since Hawking radiation comes from gravitional acceleration, does it not occur also inside the event horizon all the way down to the `singularity'? [/ QUOTE ] I seem to recall that a freely falling detector dropped into a BH isn't excited by Hawking radiation, but I could be mistaken. [ QUOTE ] 8. If not, then how does all the mass get from the `singularity' to the outside during the complete Hawking evaporation of a black hole via Hawking radiation? [/ QUOTE ] Hawking radiation is a property of the vacuum outside the BH -- there are some problems with thinking of Hawking radiation as something "escaping" from the BH. That said, there are some very big problems associated with "complete evaporation" of a BH that aren't properly understood by anyone. [ QUOTE ] 9. If so, how much of the mass of the black hole consists of Hawking radiation between the `singularity' and the event horizon? [/ QUOTE ] This is a sort of ill-defined question. The BH's mass is defined with respect to a "timelike Killing vector" that doesn't exist inside the BH. Energy and momentum inside the BH are only locally defined concepts. [ QUOTE ] 10. If you are accelerating `forwards' in space, does the Unruh radiation (similar concept to Hawking radiation, relating gravity to acceleration) hit you from the front or the back, or from all around, or what? [/ QUOTE ] It's isotropic. [ QUOTE ] 11. What are all the similarities, and differences, between gravitons and photons? [/ QUOTE ] This could lead to a huge, detailed discussion in and of itself. If you want to discuss it more, I wouldn't mind doing so, but I'll sidestep it for this particular post. [ QUOTE ] 12. (For genuine experts only) how much of the above is nonsense based on ignorance? [/ QUOTE ] They seem like reasonable questions, but as always a more detailed understanding reveals some "problems" to not be problems at all, and some questions not to be defined in the original sense that you imagined.
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#9
12-02-2006, 10:28 PM
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Re: How do gravitons escape a black hole?
Gravitons exist?
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Linear Mode
