By definition, the universe seems like it should be the totality of everything that exists. Yet a variety of arguments emerging from cosmology, particle physics and quantum mechanics hint that there could also be unobservable universes beyond our own that follow different laws of nature. While the existence of a multiverse is speculative, for many physicists it represents a plausible explanation for some of the biggest mysteries in science. In this episode, Steven Strogatz explores the idea of a multiverse with the theoretical physicist David Kaplan and learns what it might mean about our own existence.