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Date: 1991 


Question:
What is the "speed" of gravity?
I know that over the centuries we have found equations which mathematically relate most of the natural forces, but gravity is one that has resisted all such efforts. Since gravity is apparently so different perhaps it is not constrained like electricity and magnetism to a maximum speed - light speed. Are there any tests/experiments to measure the time it takes for gravity to act between two bodies? Thought experiment: If everything in the universe was divided into two enormous masses and placed at opposite sides of the universe how long would it take for them to begin moving toward each other? -- assuming we did not have two Medium Bangs :-)
a) Immediately.
b) The time it would take light to travel between them.
c) Half the time it would take light to travel between them.
d) Never, the gravitons would be part of the masses and thus could not transmit the gravitational forces.
e) None of the above
Now that I think of it a graviton would have to be FASTER than a photon or black holes would have no gravity - Right?



Replies:
Gravitons are still hypothetical since no one has managed to quantize gravity yet, but gravitational waves should exist and are expected to travel at the speed of light. We should be close to detecting G gravitational waves any year now. Your comment about black holes is not quite right: remember gravity is really a distortion of the space-time continuum caused by mass. This is part of Einstein's General Theory of Relativity.

John Hawley


Nope.
Gravitons travel at the speed of light, in the theory of general relativity (at least to the extent that the theory recognizes the existence of quantum mechanics). A prediction of the general theory is that gravitons will carry off a definite amount of energy from millisecond pulsars, making the slow down at a predictable rate. That rate seems to be observed. See the 1993 Nobel Prize in physics to Taylor and his colleague who made the observations.

Jack L. Uretsky



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