[NAHTMMM]

[color=#000000ost_uid0][quoteost_uid0="PointyHairedJedi"]There also the fact that it would (and this has just occured to me) unbalance the weight of matter in the universe. 'Cos, you see, if you jump back then you'd be removing matter from your time and adding it to the universe of the prevoius time that you were visiting.[/quoteost_uid0]
It wouldn't be any worse than if you had set a bowling ball at the origin of a Cartesian grid painted on the floor, then returned and put it somewhere else on the y-axis. Time is just another axis (or set of axes), like the x-axis that you just deprived of mass, the ordinates of which when taken with all other coordinates define a particular place, time, and whatever else needs defining, in a spacetime continuum.

Now, if you were to remove a nontrivial amount of mass from one universe and scatter it around another universe so that it couldn't be measured or returned, I agree, you'd have messed up a universe or two pretty badly.[/colorost_uid0]