travel and time travel appear to be two sides of the same coin: solve one, and the other is solved.
Why do we not keep our universal x,y,z co-ordinates (a fallacy, but will serve to prove my point)? Simple - gravity. If time travel is non-instantaneous (eg: 1 year/sec) then gravity should keep the time machine in the same place, as to the outside observer the time machine is not moving (it has always existed there).
I think so, anyway. All of this temporal mechanics gets me confused...