Harmony of Scattering Amplitudes: From QCD to Gravity

In these lectures we explain some aspects fo the remarkable simplicity in the scattering amplitudes of gravity and gauge theories. We start by summarizing the unitarity method and how it reveals surprising structures in loop amplitudes, and gives us a tool for carrying out state-of-the-art calculations. We review a recently discovered duality between color and kinematics in gauge theories, exposing gravity theories as double copies of gauge theories; these properties have been conjectured to hold to all loop orders. We also describe some state-of-the-art results obtained with the unitarity method in QCD, supersymmetric gauge theory and supergravity.