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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Does gravity travel at the speed of light? ?

We don't know, because gravitational waves are so weak that we cannot detect them in order to time them. However, gravitational radiation is described very accurately by Einstein's theory of general relativity and in particular his field equation for gravity. The basis of this theory is that the velocity of gravity is limited to the speed of light. A very precise study of the so-called Taylor-Hulst binary pulsar system shows that the system is loosing energy at exactly the rate proscribed by Einstein's field equation for gravity. This would only be the case if this equation represented the essential physics of the phenomenon of gravity with high fidelity. The key assumption is that, like all other forces, gravity travels at exactly the speed of light. I don't know if this is compelling evidence to the non- physicist, but to me it does. In the future, we may be able to test this directly if we can figure out some way of detecting under laboratory conditions a gravitational wave.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.