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# Physics/speed of gravity

Question
Hi,

I read that speed of gravity is same as that of light. Is that experimentally proved? If not, why the speed of gravity cannot be measured while the speed of light can be. Please clarify.

regards,

These guys http://www.nrao.edu/pr/2003/gravity/ have observations that put limits on the speed of the gravitational interaction at the speed of light, +/-20%.  It's an observational measurement, which is largely the only way you're going to measure that.  The reason is that there really is no good way to get significant mass to move at close to the speed of light, and gravitation depends on mass.  If you have a postage stamp (only about 0.05 grams!) traveling at about 10% of the speed of light, you have roughly the same energy as a car going 10,000 miles per hour.  Gravitation is also incredibly weak, it's a trillion*trillion*trillion times weaker (yes, multiplied) than the electromagnetic force.  If you can come up with an actual good experiment to measure the speed of such a ridiculously weak force which travels so fast in the laboratory, you'd make quite a name for yourself in physics!  :)

Physics

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#### Dr. Stephen O. Nelson

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