A binary black hole is two black holes in close orbit around each other. Although only a theoretical idea, they are important in astrophysics in that they would be the strongest known gravitational wave source in the universe. As the orbiting black holes give off these waves, the orbit decays, and the orbital period decreases. This stage is called binary black hole inspiral. The black holes will merge once they are close enough. Once merged the single hole goes though a stage called ring down where any distortion in the shape is dissapated as more gravitational waves.^[1]

An unexpected result can occur with binary spinning black holes in that the gravitational waves carry momentum and the black hole pair accelerates seemingly violating Newton's third law.

• Binary Black Holes Orbit and Collide by djxatlanta