# Colloquium: Clifford Cheung, Caltech

Scattering amplitudes are fundamental observables encoding the dynamics of interacting particles. In this talk I describe how to systematically construct these objects without reference to a Lagrangian. The physics of real-world particles like gravitons, gluons, and pions are thus derived from the properties of amplitudes rather than vice versa. Remarkably, the expressions gleaned from this line of attack are marvelously simple, revealing new structures long hidden in plain sight. In particular, I describe how gravity serves as the "mother of all theories" whose amplitudes secretly unify, among others, all gluon and pion amplitudes. This fact has far-reaching theoretical and phenomenological connections, e.g. to fluid mechanics and to new approaches to the black hole binary inspiral problem.