E = mc² is the most famous formula in the world. With it, Einstein made it clear: energy and mass are two sides of the same coin, and the speed of light c is their exchange rate. But why does this seemingly simple relationship exist? How did Albert Einstein come to this conclusion? And what implications does it have for our understanding of the universe? Brian Cox, a professor of physics who is well-known in England through his shows on the BBC, along with his colleague Jeff Forshaw, a professor of theoretical physics, has taken on the seemingly simple Einstein equation to explain it in detail and in an engaging way.