Here is an interesting article about Einstein's theory of relativity:

http://www.gravitywarpdrive.com/Rethinking_Relativity.htm

I've always been bothered about the conundrum of the slowing down of clocks moving in a staight line in opposite directions because there is no way to do an experiment to prove the theory. The whole theory of relativity is like that; it explains some things, but it requires that others just be accepted on faith, like that the speed of light in a vacuum is constant. The GPS discussion is interesting.

The speed of gravity has always intrigued me. I always wondered how anything could be instantaneous. It would be neat if we could send signals with gravity waves. That would make it possible for us to communicate with planets many thousands of light years away.