From Scattering Amplitudes to Classical Physics: Universality, Double Copy and Soft Theorems

Understand

We introduce a covariant Multipole Expansion for the scattering of a massive particle emitting photons or gravitons in $D$ dimensions.

• We find that these amplitudes exhibit very powerful features such as universality, soft exponentiation, orbit and spin multipoles, etc.
• Using ${\rm{SO}}(D)$ representation theory we show that the photon and graviton amplitudes are related via a new double copy procedure for massive spinning states.
• All these features are then promoted to properties of the observables arising in the classical version of such theories.