Physicists have always hoped that once we understood the fundamental laws of physics, they would make unambiguous predictions for physical quantities. We imagined that the underlying physical laws would explain why the mass of the Higgs particle must be 125 gigaelectron-volts, as was recently discovered, and not any other value, and also make predictions for new particles that are yet to be discovered. For example, we would like to predict what kind of particles make up the dark matter. These hopes now appear to have been hopelessly naïve. Our most promising fundamental theory, string theory, does not make unique predictions. It seems to contain a vast landscape of solutions, or “vacua,” each with its own values of the observable physical constants.