On day 3 of HowTheLightGetsIn at Hay, an extraordinary panel of theoretical physicists debated the reality of the multiverse. Nobel prize-winner Roger Penrose, string theory pioneer Michio Kaku, and quantum gravity researcher Sabine Hossenfelder, argued over whether the idea that our universe is one of many can be taken to be a scientific claim, backed up by observational evidence, or whether it’s merely a popular fiction.

Our universe isn’t the only one. It’s one of many, an infinite number of other universes, some of which are very similar to our own with only minute differences, others in which space has more than three dimensions, and others still in which the laws of physics themselves are completely different. Sounds like a crackpot theory, something someone came up with when high on drugs. In fact, it’s a serious proposal by theoretical physicists and goes by the name the multiverse. But does the fact that it’s a theory put forward by scientists automatically make it scientific? Is there really observational evidence for it? Could there ever be? And if not, where does that leave the theory? Is it delegated back to crackpot fiction, or do we simply have to stretch our idea of what counts as a scientific theory?