The narrator explains that the multiverse theory is one possible solution to the grandfather paradox. The theory states that every version of someone's past and future does exist in an alternate universe. Essentially, whenever someone makes a choice, millions of universes spring up in which they made different choices. The narrator says that per this theory, there's a universe where Samuel doesn't ruin Natasha's life, one where he does but Natasha can fix it, and one where Natasha can't fix it. Natasha isn't sure which universe she's living in.
The multiverse theory proposes that there are millions of interconnected worlds—and further, that they connect through science. This shows that emotional pursuits and rationality aren't necessarily at odds. According to the theory, Natasha could have a million different emotional (or unemotional) relationships, all with different outcomes.