In physics, the twentieth century started with the twin revolutions of relativity and quantum mechanics. Much of the second half of the century was devoted to the construction of a theoretical structure unifying these radical ideas, confirmed experimentally to exquisite precision over the past three decades. Yet questions remain. The union of quantum mechanics and gravity strongly suggests that space-time is doomed—but what replaces it? The unification of relativity and quantum mechanics predicts violent short-distance quantum fluctuations that make the existence of a macroscopic world wildly implausible, and yet we comfortably live in a huge universe. What tames these violent fluctuations, and why is there a macroscopic universe? A spectacular new experiment—the Large Hadron Collider—is now poised to shed significant light on at least some of these mysteries. In this talk, Nima Arkani-Hamed, Professor in the School of Natural Sciences, describes these ideas and discuss what we can expect to know by 2020.