Before Albert Einstein (1879-1955) developed general relativity in 1917, Isaac Newton's theory of gravity required that gravity and gravitational forces traveled instantaneously from place to place. The most elementary assumption behind Einstein's theory is that gravity must travel at exactly the speed of light. In 1975, astronomers Joseph Taylor and Hendrik van der Hulst discovered two pulsars (see Glossary) orbiting each other at very close range. From a decade of careful study, they discovered this system collapsing, and the speed of this collapse is within one percent of what you would predict if gravity waves were carrying off energy at the speed of light. What has been claimed as the first direct measurement of the speed of gravity was attempted by Ed Fomalont of the National Radio Astronomy Observatory and Sergei Kopeikin from the University of Missouri in Columbia. On September 8, 2002 the planet Jupiter passed in front of a powerful quasar J0842+1835 and from the gravitational distortions that Jupiter's field produced on the passing radio waves, they found that the speed of gravity was about equal to the speed of light. Several world-renowned experts on general relativity disputed this result, and suggested that the astronomers had merely measured the speed of light, not gravity. We will likely not hear the end of this until the calculations are actually published and the rest of the community can go over them too. Meanwhile another astronomer, Tom Van Flandern in Washington D.C. has argued for many years that the speed of gravity is probably infinite. Van Flandern claims that if gravity traveled at the speed of light, the force upon Earth would be caused by the gravitational forces of the other planets based on where these planets were in the past, not where they are now. By including these delays, all the predictions seemed to be significantly off, and worse still, planetary orbits become unstable. Physicists quickly discovered that his calculations did not include a relativistic effect that exactly canceled the problems that Van Flandern had found. In the next ten years, more tests will be attempted, but for now 'Einstein Rules'.
