  Three of the four fundamental forces in nature are described by so-called
gauge theories, which include the effects of both relativity and quantum
mechanics. Gravity, on the other hand, is described by General Relativity, and
the lack of a well-behaved quantum theory - believed to be relevant at the
centre of black holes, and at the Big Bang itself - remains a notorious
unsolved problem. Recently a new correspondence, the double copy, has been
discovered between scattering amplitudes (quantities related to the probability
for particles to interact) in gravity, and their gauge theory counterparts.
This has subsequently been extended to other quantities, providing gauge theory
analogues of e.g. black holes. We here review current research on the double
copy, and describe some possible applications.
