I never knew what a GRR was, I must have missed that thread. Still give me a TARDIS any day, telepathic interface, time and space and relative dimensions. I might even take it for a spin into E-Space. :) Plus it can look like anything if the Chameleon circuit is not broken.
I always felt that there was a link to time travel between gravity and speed/velocity. In high school I took Physics and got an A out of it and wrote some time travel equasions based on existing equasions for gravity, speed, and other factors. Too bad I lost it and forgot what I had written down. There was a square root involved because I also used E=MC^2, so it could go forward or backward in time, but I had no idea how to control that.