I heard that someone did an experiment with two atomic clocks, one stayed in London and one went around the world in the opposite direction to the spin of the earth at mach1. When it got back to London it had lost more time than it normally would in a millennia(fraction of a second). I can't remember where I heard this but it has been playing on mind if it was true, surely some military jets can do about mach4 then time travel would be possible(if impractical)?