  Scattering amplitudes have their origin in quantum field theory, but have
wide-ranging applications extending to classical physics. We review a formalism
to connect certain classical observables to scattering amplitudes. An advantage
of this formalism is that it enables us to study implications of the double
copy in classical gravity. We discuss examples of observables including the
total change of a particle's momentum, and the gravitational waveform, during a
scattering encounter. The double copy also allows direct access to classical
solutions in gravity. We review this classical double copy starting from its
linearised level, where it originates in the double copy of three-point
amplitudes. The classical double copy extends elegantly to exact solutions,
making a connection between scattering amplitudes and the geometric formulation
of General Relativity.
