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.