The general theory of relativity, which Albert Einstein formulated 100 years ago, is one of the great pillars of modern physics. It is based on profound and elegant principles that connect the physics of motion and mass to the geometry of space and time. With Einstein’s equations, even the universe itself became an object of study. Only now, after a century of calculations and observations, the full power of this theory has become visible, from black holes and gravitational lenses to the practical use of GPS devices. The quest to find the ultimate nature of reality continues, however. The laws of quantum theory suggest that in the end space and time might all just be an illusion.