Look out if black holes collide.

• Position: Astrophysics

When black holes collide, look out! An enormous burst of gravitational radiation results as they violently merge into one massive black hole. The "kick" that occurs during the collision could knock the black hole clear out of its galaxy. The consequences of such an intergalactic collision are explained by David Merritt, professor of physics, Rochester (N.Y.) Institute of Technology, in the study "Consequences of Gravitational Radiation Recoil."

Virtually all galaxies are believed to contain supermassive black holes at their centers. According to current theory, galaxies grow through mergers with other galaxies. When this occurs, the central black holes form a binary system and revolve around each other, eventually coalescing into one. The coalescence is driven by the emission of gravitational radiation, as predicted by physicist Albert Einstein's Theory of Relativity.

Merritt and his colleagues determined how fast a black hole has to move to escape a galaxy's gravitational field completely. They found that larger and brighter galaxies have stronger gravitational fields and would require a bigger kick to eject a black hole than the smaller...