So, is there any gravity particle discovered yet? I haven’t heard of it. Is the effect of gravity assumed to be instantaneous through the universe? If so, could gravitons somehow be used for instant communication?
Just this side of Star Trek fantasyland I realize…
Gravitons and Gravitinos have not yet been discovered. Unfortunately, as is the case with many aspects of subatomic physics, they are theoretical and I don’t believe any extensive experiments have been carried out. Particle physics relies on a ‘standard model’ of the atom with its various components. Experiments are substantiating different parts of this model bit by bit.
(Last summer I was working on a software package which is to be used (or is being used) to detect the paths of muons, neutrinos, and positrons in a model of controlled muon decay.)
I don’t think that gravity could ever be used practically for communication, as there is so much movement of mass that to get a significant, identifiable signal, one would have to move an immense amount of matter.
I doubt, too, that gravity acts instantaneously, but for our purposes in the year 2000, it’s close enough.
Nope, although people have looked. It’s difficult because gravity is **really really really really
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weak compared to the electromagnetic force.
Absolutely not. ALthough the speed of gravity has not been measured experimentally, we have really good reasons to believe that it propagates at the speed of light in a vacuum. The Newtonian model requires infinite propagation speed for gravity, and is therefore not a very good esplanation for the fundamental phenomenon (although it’s still really useful for calculation). See Does Gravity Travel at the Speed of Light?, Has anyone ever measured the speed of gravity to see if it is the same as the speed of light? and [url=http://www.desy.de/user/projects/Physics/FTL.html]Faster than Light
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No.
The huge problem with gravitons is that the renormalization techniques whereby the infinities caused by the spontaneous creation of virtual particle pairs fail with gravity. QM basically has no idea what graviton look like or even what they should look like.
Sadly, the Europeans will probably find the answers to this question and many more since the Texas Supercollider was cancelled. 
I’m genuinely confused, here - why is this a problem? Are we afraid they won’t share the information with us?
Oops… The QM renormalization techniques fail when applied to gravity.
My sadness at the cancellation of the Texas SSC (Superconducting Super Collider) is a matter of, IMHO, national prestige. Obviously the congress and the voters feel differently than I do.
No, they have one idea: string theory. The infinities can be renormalized, and gravitons fall naturally out of the mix. I don’t think they have any predictions which differ from the standard model, and which are currently testable, buit they’re working on it.
It is too clear, and so it is hard to see.