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QUESTION: Hi,

I read that speed of gravity is same as that of light? Is that true? What are the experimental evidences available for this?

Regards.

ANSWER: > I read that speed of gravity is same as that of light?
> Is that true?

If all of general relativity is true, then changes in gravity propagate through space at the speed of light.
The best way to explain this is through this thought experiment: how quickly would we humans know if our Sun completely disappeared in an instant? Since we would continue to see our Sun's light for eight minutes (the time for our Sun's light to reach us), would we detect a change in gravity more rapidly? General relativity says we would not.

> What are the experimental evidences available for this?

At this point, none. And, since we can't yet detect changes in gravity (ie, "gravity waves"), it may be a while before we have direct evidence of the speed of gravity.

However, general relativity has been repeatedly tested for almost 100 years, and it's passed every test. At the very least, it's very close approximation to reality.

---------- FOLLOW-UP ----------

QUESTION: Thanks for your reply.

You wrote that there not yet  direct experimental evidences to measure the speed of gravity. If we can measure speed of light with so many different experiments with accuracy, what is the hindrance for finding the speed of gravity?

Also the objects attract each other according to Newton's law of universal gravitation, does the speed of gravity comes into picture in this case also.  

Regards

Answer
> what is the hindrance for finding the speed of gravity?

The table in this section should help explain why.
http://en.wikipedia.org/wiki/Fundamental_interaction#Overview_of_the_fundamental

Note how much stronger is the electromagnetic force (the one that causes light) over the gravitational force. Not a trillion times stronger. Not a trillion trillion times stronger. It's a trillion trillion trillion times stronger than gravity!
Want to see some evidence of this? Take a magnet you buy in a toy store, and use it to pick up a piece of metal. Note what's happening -- the magnetic force in this hundred gram magnet is more powerful than the gravity force OF THE ENTIRE EARTH!

Because gravity is such a weak force, we don't have the means to measure fluctuations in it. We can detect changes in light intensity (ie, the EM force) due to a supernova from over ten billion light years away; however, we can't yet detect the changes in gravity due to a supernova even within our own galaxy. We're working on it, and we'll probably be able to measure fluctuations of gravity in the next twenty years or so.
http://en.wikipedia.org/wiki/Gravity_wave_detector#Detecting_gravitational_waves
But it's still going to be a problem to determine how far away is the thing we're detecting.

> objects attract each other according to Newton's law of universal gravitation,
> does the speed of gravity comes into picture in this case

No.
Newton was troubled by the fact that his view of gravity was that it was instantaneous over large distances, but couldn't devise a solution to this.  

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