Multiversal concepts pop up in several branches of modern physics. In quantum mechanics, for instance, a particle exists in a superposition of all possible states at once—until, that is, someone tries to make a measurement of it. At that point, the possibilities collapse, and one physical state becomes apparent to the observer. The “many worlds” interpretation of quantum mechanics, though, posits that all the possible states the measurement might have shown play out in different universes, each with a different version of the observer.