What happens when one black hole hits another black hole? Does it get more black? Seriously, though, such questions are being tackled by scientists seeking to detect gravity waves. These invisible waves are ripples in the fabric of space-time, predicted by Einstein's theory of relativity, but that remain undetected. They are thought to be emitted in copious amounts by colliding black holes, and a new simulation by researchers in Germany (J. Baker et al. Phys. Rev. Lett. 87, 121103; 2001) gives an advance preview.

Collisions between other astronomical giants, such as galaxies, produce light and other radiation, but black-hole collisions generate only gravity waves. Black hole binaries are thought to emit gravity waves all the time, but only when they collide are the waves strong enough to be detected on Earth. Three detectors are expected to start collecting data soon: the US LIGO and German–British GEO600 projects in 2002, and the Italian–French VIRGO detector in 2003.

Credit: W. BENGER, AEI/ZIB

To fully simulate the merger of two black holes, the German team merged — appropriately enough — two different approaches for calculating what might happen before and after the collision. In the computer-generated image shown here, spherical shells of intense gravity waves move outwards from the centre of the collision. The authors estimate that 3% of the total mass of the black holes is released as energy by the collision — higher than expected.

These calculations should provide experimenters with a rough estimate of what to look out for, and guide more advanced simulations that take into account, for example, the likelihood that the black holes are also spinning. Prepare for a glimpse of the darkest corners of the Universe.