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Possible Duplicate:
The speed of gravity
Does gravity travel at the speed of light?

Imagine there is a large mass $m_1$ (e.g. a star) 1 light-year away from us. It is stable, stationary relative to us and has been in place for a long time, much more than a year. A small mass $m_2$ (e.g. a proton) has just been created locally, 1 light-year away from $m_1$.

How much time does it take for $m_2$ to feel the gravitational pull of $m_1$, and how can this be explained with the virtual-graviton theory of gravity?

Some possible answers I can imagine:

a) Immediately $m_2$ interacts with virtual-gravitons sent by $m_1$, a year ago.

b) 1 year. It takes this long for freshly launched virtual-gravitons from $m_1$ to reach $m_2$ and vice-versa before any effect is felt on either mass

c) 2 years. There needs to be an exchange of information / virtual-gravitons between $m_1$ and $m_2$ and this is the minimum time it could take.

d) None of the above

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roblev
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    Possible duplicates: http://physics.stackexchange.com/questions/26742/does-gravity-travel-at-the-speed-of-light http://physics.stackexchange.com/questions/38525/faster-than-light-communication-using-gravity/38527#38527 –  Jan 19 '13 at 19:33
  • @Chris White I do not think either if these questions are duplicates, one is about FTL information exchange (which I am not covering), one is about speed of gravity transmission (which is clearly speed-of-light). I am asking a different question about the nature of gravity. – roblev Jan 19 '13 at 19:46
  • Okay, Roblev. Removed the tag. Atleast, you've mentioned it. BTW, Keep in mind that we use TeX markups for equations, parameters, etc. :-) – Waffle's Crazy Peanut Jan 19 '13 at 19:53
  • @CrazyBuddy thanks, will try to master the markup next time – roblev Jan 19 '13 at 20:03
  • Possible duplicate: http://physics.stackexchange.com/q/5456/2451 – Qmechanic Jan 20 '13 at 06:38
  • Maybe I'm missing something but not one of the linked "duplicate" questions asks anything about gravitons, which is the whole point of my question. – roblev Jan 20 '13 at 14:40

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This isn't really an answer, since Chris White's comment links a couple of questions that cover the same ground as your question. However you specifically ask about virtual gravitons, so I thought it might be worth a note on this.

Even if virtual gravitons are a good way to describe quantum gravity (and to be fair at such low energies they probably are) the energy of a graviton is so low that the interaction of even something a small as a proton is well described by classical gravity and there is no need to involve gravitons. The proton will feel the spacetime curvature created by the distant star immediately i.e. the answer is (a). However the star won't feel the (very small!) change in the spacetime curvature created by the proton for a year.

John Rennie
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  • Thanks John, I agree that the answer (a) is obvious considering relativity / curvature of space arguments, but I am struggling to visualise how it could possibly work with gravitons. It's not so much that there is no need to involve gravitons - I agree that different models handle the situation well - it is just that I struggle to understand the graviton model! – roblev Jan 19 '13 at 20:01
  • For example, it implies that the star launched enough virtual gravitons 1 year ago to be able to interact with any particle on a shell 1 light year radius! that is a lot of virtual gravitons, and the density of them a year ago would be immense. It just seems weird! – roblev Jan 19 '13 at 20:02
  • @roblev, exactly as weird as the density of photons launched, and yet you see the star with your naked eye... And what about light from a quasar at the other end of the Universe, to say so... – Eduardo Guerras Valera Jan 19 '13 at 20:09
  • Be careful about interpreting the term "virtual particle" too literally. Matt Strassler has written a good article on the pitfalls of this at http://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/. – John Rennie Jan 19 '13 at 20:16
  • "However the star won't feel the (very small!) change in the spacetime curvature created by the proton for a year." Er...that proton didn't come from nowhere, the energy was already presents and the star was already feeling the (retarded) gravitation effects of that energy in it's earlier configuration. – dmckee --- ex-moderator kitten Jan 19 '13 at 20:17
  • @EduardoGuerras I'm not sure these are the same, the photons represent real energy emitted from the star, the virtual-gravitons do not. At a certain distance from the star I will stop seeing its photons, but I should still "see" it's virtual gravitons – roblev Jan 19 '13 at 20:38
  • @JohnRennie thanks for the link, looks interesting. – roblev Jan 19 '13 at 20:51
  • @roblev, you never stop receiving photons, no matter how far you are. You may be so far that you receive, in average, one photon every five years, but still you can focus them and have an image if you wait long enough. In practice, the problem (with the attempts to detect gravitons too) is the S/N ratio, so that your signal gets undetectable, buried under the noise of nearer sources, so to say. But you never stop receiving photons, although they are (would be) of course, completely different particles as gravitons. – Eduardo Guerras Valera Jan 19 '13 at 21:07
  • @roblev, gravitons should carry energy too, able to shake an object in a tiny amount. This is supposed to be the way they are trying to detect gravitational waves. Is that wrong? I am not much keen on QFT yet. – Eduardo Guerras Valera Jan 19 '13 at 21:18
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    @EduardoGuerras yes you are right about the photon behaviour at a distance, but that is different to the virtual graviton behaviour; receiving one photon every 5 years is fine but receiving one graviton every 5 years would not be fine, as it would then take 5 years for the star to influence the particle via gravity. I think the key thing here is the difference between real and virtual particles, and the link from JohnRennie looks good on this topic. – roblev Jan 19 '13 at 22:55
  • @roblev, thanks! (+1). Additionally, I am afraid that mixing classical gravitational waves with gravitons is a conceptual mess, so I better not follow in that path... – Eduardo Guerras Valera Jan 19 '13 at 23:03
  • @roblev, Mmm... That interaction supposed to happen every 5 years seems a conceptual mess for the photon too. Electrostatic interaction would happen only every 5 years too... That seems fishy too... But I rather not discuss the topic anymore, at least not until I have finished with Zee's nutshell... – Eduardo Guerras Valera Jan 20 '13 at 00:37