In June 1925, 23-year-old Werner Heisenberg, suffering from hay fever, retreated to a small, treeless island in the North Sea called Helgoland. It was there that he came up with one of the most transformative scientific concepts: quantum theory. Almost a century later, quantum physics has given us many startling ideas: ghost waves, distant objects that seem magically connected to each other, cats that are both dead and alive. Countless experiments have led to practical applications that shape our daily lives. Today our understanding of the world around us is based on this theory. And yet it is still profoundly mysterious. Here, Carlo Rovelli tells the extraordinary story of quantum physics and reveals its deep meaning: a world made of substances is replaced by a world made of relations, each particle responding to another in a never ending game of mirrors.