Our starting point for constructing perturbative quantum gravity
is the Kawai, Lewellen, and Tye (KLT) relations [85] between closed and open string tree-level amplitudes. Since
closed string theories are theories of gravity and open string
theories include gauge bosons, in the low energy limit, where
string theory reduces to field theory, these relations then
necessarily imply relations between gravity and gauge theories.
The realization that ordinary gauge and gravity field theories
emerge from the low energy limit of string theories has been
appreciated for nearly three decades. (See, for example,
Refs. [144,
118,
117,
69,
70,
110,
111].)