The double copy: from BRST ghosts to classical fields

We present a procedure for perturbatively constructing the field content of gravitational theories from a convolutive product of two Yang-Mills theories. A dictionary “gravity=YM X YM” is developed, reproducing the symmetries and dynamics of the gravity theory from those of the YM theories. We explain the unexpected, yet crucial role played by the BRST ghosts of the YM system in the construction of gravitational fields. The dictionary is expected to develop into a solution-generating technique for gravity.