The hardest thing to prove is something you think you already know. How can you be sure that you're proving it, rather than merely reasserting your belief? So it is with the latest test of Einstein's general theory of relativity--a measurement of the speed at which changes in a gravitational field propagate. If the sun suddenly shattered into a million pieces, this speed would determine how many minutes of blissful ignorance the denizens of Earth would have until our orbit went haywire. In Einstein's theory, the speed of gravity (abbreviated c_g) exactly equals the speed of light in a vacuum (c).

Lo and behold, that is what a physicist-astronomer duo announced at the American Astronomical Society meeting in January. Einstein, they concluded, was right once again. Yet most relativity researchers are skeptical. "It's a beautiful experiment that gives a very nice new confirmation of general relativity, but it's still unclear whether it's testing the speed of gravity," says Steven Carlip of the University of California at Davis.

This article was originally published with the title A Tale of Two C's.

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