David Wallace was still in high school when he first encountered the notion of parallel universes. To a British teenager hooked on science fiction and physics, it sounded “weird but cool”, he recalls. Twenty years later, the topic still absorbs him. A professor specialising in the philosophy of physics at the University of Southern California, he recently penned a book titled The Emergent Multiverse. For some people, the idea there is more than one universe out there sounds absurd. Sure, it’s fair game for the writers of Star Trek, and perhaps even for philosophers, but how can it be a serious scientific idea? In fact, it was first put forward some 60 years by a very serious physicist, Hugh Everett