"Maybe... the familiar notion that any given experiment has one and only one outcome is flawed. The mathematics underlying quantum mechanics - or at least, one perspective on the math - suggests that all possible outcomes happen, each inhabiting its own separate universe. If a quantum calculation predicts that a particle might be here, or it might be there, than in one universe it is here, and in another it is there."
- The Hidden Reality, Parallel Universes and the Deep Laws of the Cosmos, by Brian Greene.
So it's not really the first lines of the book. But they are the most fascinating ones in the introduction and the first chapter.