Can someone explain how to derive the formula for geodesic deviation? i didn't quite understand it. I'm using schtuz's book.
You can try, for instance, http://www.mth.uct.ac.za/omei/gr/chap6/node11.html This seems like a rather direct and straightforwards approach to me, even more direct than MTW's. Of course it is a bit mathematically impure, in that it assumes a convenient coordinate system, but I think assuming an inertial coordinate system for one of the observers makes the physics more apparent.
The derivation I placed on my web page at http://www.geocities.com/physics_world/gr/geodesic_deviation.htm started out being a translation from Ohanian's book to my web page. However it seemed to end up being Ohanian's book and part of what I had to say since Ohanian made somethings hard to understand there. Enjoy. Any problems then e-mail me so we can set straight if there is something wrong. Pete