Supplementary Radiation-Reaction Force Between Two Binaries

Radiation-reaction forces originating from the emission of gravitational waves (GWs) bring binaries to close proximity and are, thus, responsible for virtually all the mergers that we can observe in GW interferometers. We show that there exists a supplementary radiation-reaction force between two binaries interacting gravitationally, changing, in particular, the decay rate of the semimajor axis under the emission of GWs. This new binary-binary force is in some settings of the same order of magnitude as the usual 2.5PN force for an isolated binary and presents some striking features such as a dependence on retarded time even in the post-Newtonian regime where all velocities are arbitrarily small. Using effective field theory tools, we provide the expression of the force in generic configurations and show that it interpolates between several intuitive results in different limits. In particular, our formula generalizes the standard post-Newtonian estimates for radiation-reaction forces in N-body systems which are valid only in the limit where the GW wavelength goes to infinity.