Parallel universes are a staple of science fiction, and it's no wonder. They allow us to explore the question, "what if?" in a way that lets us step completely outside of the world we know, rather than question how that world might have turned out differently. For cosmologists, the question isn't "what if the South won the Civil War?" but "what if the constants that make up the fundamental building blocks of physics were different?" Physicists argue that any slight change to the laws of physics would mean a disruption in the evolution of the universe, and thus our existence. Take gravity, for example: too strong and stars would burn through their fuel far more quickly. If the universe expanded too fast, matter would spread out too thin for galaxies to form. The list of examples goes on – to the point where the laws of physics might seem finely tuned to make our existence possible. Short of a supernatural or divine explanation, one possibility is that our universe isn’t the only one. That's the idea explored in this eBook, Possibilities in Parallel: Seeking the Multiverse. In Section 1, "Undiscovered Worlds," we explore why scientists think other universes could exist. After that, we get a look at the implications. Is it possible to have life in a universe with different physical laws? It would seem so. In “Cracking Open a Window,” George Musser discusses the possibility that our universe has more than three spatial dimensions – the others happen to be very small. Other articles, including “The Universe’s Unseen Dimensions,” analyze the idea that our universe is one of many "branes" – three-dimensional structures stretched out over a higher-dimensional space. The concept of a parallel universe also touches time travel, and then there's the question of what the term "parallel universe" actually means. It's a triumph of the sciences that the very question of why the universe looks as it does can be asked at all. There are currently several possibilities for a multiverse, if it exists. Time and a lot of scientific spadework will reveal which one is right – and get us closer to answering those metaphysical questions: what if, why us, why now?