I was thinking about the possibility of time travel...various theories etc... and what never sems to be brought into the conversation is that the universe is not only expanding..it's expansion is accelerating. That being the case...there is no reliable method to know where when is. Even if there are "worm holes" and assuming even if fleeting their "location" is stable even for an instant that location must change very rapidly as the universe expands. Also one must consider gravitational flex of the earth ... Your time traveller may find him or herself imbedded in rock as he or she "re-appears" on earth.