A lot of the physics based around general relativity relies on physical laws remaining the same as rotation and speed increase (Lorentz transformations), and under the combination of Charge conjunction, Parity inversion, and Time reversal (CPT Invariance). Robert Bluhm, Alan Kostelecky, Charles Lane, and Neil Russell are among the people trying to find instances where these symmetries are violated. Their recent paper in Physical Review Letters describes experiments using clocks located in space, like the one scheduled to be launched to the ISS in 2005, to detect symmetry violations.

See also:

• Relativity: The Special and General Theory
• Sean Carroll's Lecture Notes on General Relativity