Yesterday I was wondering, in my non-scientific, physic's-ignorant sort of way, whether parallel worlds are even possible. Star Trek having a lot of them, you see. Today, I found another post at Discovery Magazine discussing rules for Time Travellers. Parallel worlds get a mention because, of course, they might be one way of travelling through time.
During the discussion below the article, someone raises a point that I read in an SF story, which I don't remember the title of, about where things are when you move in time. In the story, when someone is asked where they were at a certain time and date, they calculate how far our Sun has moved in the galaxy, and far the Earth has moved in its rotation, and then conclude with how many thousands of kilometres away they are now. Thus, if they travelled in time they would have to travel in space or wind up in a vacuum. This idea doesn't get used much in a lot of SF stories.
Another reply to the article suggests that if you travel in time, your mass would disappear from where you started from. That is to say, if you travel back in time, as in Back to the Future, you won't be there to be affected by your actions in the past.
Alternatively, maybe time doesn't really exist, and all we've got is motion through space.