Apologies for the title :P but couldn't think of a better one.This is just a shower thought I had the other day. As you will see, I know nothing about Physics, Astronomy etc, so please don't be too harsh.Here goes:Intuitively, it seems to me the observable universe can be sliced into a 3d grid with coordinates and we can track the relative position of things within it over time.Since time and space are intertwined, if we had some means to travel back to a time-position point in the past on that grid, we'd accomplish time travel. Yay.But... we don't know where we are in relation to the rest of the universe.Even if our galaxy or cluster or whatever was moving in a given direction within the context of the observable universe, within the rest of the universe, we could still be moving net the opposite direction depending on what direction our observable universe is moving within the rest of the universe.And since the rest of the universe isn't observable, we could never hope to construct any 3d grid with absolute time-position coordinates of anything.Isn't this one (of many) reasons why time travel would be impossible?