Free Many Worlds in One: The Search for Other Universes by Alex Vilenkin

infinite, so each of them contains an infinity of O-regions. And it follows from quantum mechanics that there is only a finite number of histories that can unfold in any O-region. Putting these two statements together, we arrive at the inevitable conclusion that every single history should be repeated an infinite number of times. According
to quantum mechanics, anything that is not strictly forbidden by conservation laws has a nonzero probability of happening. And any history that has a nonzero probability will happen—or rather has happened—in an infinite number of O-regions!
    Included among these infinitely replayed scripts are some very bizarre histories. For example, a planet similar to our Earth can suddenly collapse to form a black hole. Or it can emit a huge pulse of radiation and switch to another orbit, much closer to the central star. Such occurrences are extremely unlikely, but this only means that one will have to survey a very large number of O-regions before encountering one of them.
    A striking consequence of the new picture of the world is that there should be an infinity of regions with histories absolutely identical to ours. Yes, dear reader, scores of your duplicates are now holding copies of this book. They live on planets exactly like our Earth, with all its mountains, cities, trees, and butterflies. The earths revolve around perfect copies of our Sun, and each sun belongs to a grand spiral galaxy—an exact replica of our Milky Way.
    How far away are these earths populated by our duplicates? We know that matter contained in our O-region can be in about 10 to the 10 90 different states. A box containing, say, a googleplex (10 to the 10 100 ) O-regions should exhaust all these possibilities, with a large margin. Such a box should be, roughly, a googolplex light-years across. At larger distances, O-regions, including ours, should recur.
    There should also be regions where histories are somewhat different from ours, with all possible variations. When Julius Caesar stood with his legions on the bank of the river Rubicon, he knew he was about to make a momentous decision. Crossing the river would amount to high treason, and there would be no way back. With the words “Iacta alea est!” —“The die is cast!”—he ordered the troops to advance. And the die was cast indeed: on some earths Caesar went on to become the dictator of Rome, while on others he was defeated, tried, and executed as an enemy of the state. Of course, on most earths there has never been a person by the name Caesar, and most places in the universe are nothing like our Earth—since there are many more ways for things to be different than for them to be the same.
    It may be fitting that this surreal picture of the world originated in the
town haunted by the spirit of Salvador Dalí. Like Dalí’s paintings, it blends weird, nightmarish features with recognizable reality. It is, however, a direct consequence of the inflationary cosmology. Jaume and I wrote a paper describing the new worldview and submitted it to the Physical Review , the leading physics journal. We ran the risk that the paper could be rejected for being “too philosophical,” but it was accepted without a glitch. In the discussion section at the end of the paper we wrote:

    The existence of O-regions with all possible histories, some of them identical or nearly identical to ours, has some potentially troubling implications. Whenever a thought crosses your mind that some terrible calamity might have happened, you can be assured that it has happened in some of the O-regions. If you narrowly escaped an accident, then you were not so lucky in some of the regions with the same prior history … . On the positive side, … some readers will be pleased to know that there are infinitely many O-regions where Al Gore is President ad and—yes!—Elvis is still alive. 1

    The press responded instantly—as Jaume had anticipated. The next month’s issue of the British