Weir(Looking at her older self): Very. Very, very weird. Looking at yourself, how you will be.
McKay: Actually, how you will be will be different than how she is right now. You see, the moment she went back in time, she created a separate reality, a second "you" living in a parallel world; well, according to one of many interpretations of quantum theory. I mean, simply put, this interpretation states that, uh, the universe is in fact split into an infinite number of copies of itself in which every possible outcome to every decision ever made all exist somewhere in this infinitely layered multi-universe.
Weir: Simply put.
McKay: Yeah… In a nutshell.