Black holes take the plunge

For the first time computer simulations by the Max Planck Institute for Gravitational Physics predict what astronomers will "see" with gravitational wave telescopes during the collision of two black holes.

September 17, 2001
Visualization showing the emission of gravitational waves following the merger of two black holes with a combined mass of 35 solar masses. The gravitational wave, which propagates at the speed of light, has the structure of convoluted spherical shells. Of the cubic region of space simulated, only the lower half is shown; this way, the spiral structure of the gravitational waves in the central plane is clearly visible. The gravitational wave data was obtained in the course of the Lazarus project. In this project, the results of a three-dimensional numerical simulation of a black-hole merger were used as initial data for a calculation involving a suitable approximation method. In this way, the researchers were able to follow the evolution of the merger for a far longer time than with a three-dimensional simulation alone – such simulations tend to become unstable and break down as the strong gravitational attraction behind the black hole horizon asserts itself.

The merging of two black holes is one of the strangest occurrences expected in modern astronomy. Now physicists using the world’s biggest computers have shown astronomers what to look for and have brought the first observations of these events much closer.

In a paper that is to appear in Physical Review Letters on Sept. 17, 2001, a team of young researchers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute in Golm, near Potsdam and Berlin, Germany) has predicted the gravitational waves that should be emitted when black holes plunge towards each other and merge. The team consists of John Baker (now at NASA's Goddard Space Flight Center in the USA), Bernd Brügmann, Manuela Campanelli, Carlos Lousto, and Ryoji Takahashi. They call themselves the Lazarus Team.

The most important result of the Lazarus simulations will be to provide gravitational wave astronomers with a set of templates which they can use to recognize the signals in the noise at the output of their detectors. The Lazarus simulations make predictions that are more detailed and more reliable than any before. The Lazarus scientists expect the gravitational waves to be stronger than previously accepted estimates.

Bernard Schutz, one of the directors of the Max Planck Institute for Gravitational Physics, observes: “The success of the Lazarus Project at the AEI comes at just the right time. Black hole mergers could provide the first-ever detections, which will be a landmark for Einstein's theory of general relativity. Numerically computed gravitational wave-forms will not only help us to detect and recognize waves from these events, but will help us to deduce from the observations the masses of the holes and their distance from us. Black hole mergers emit no light, radio waves, or X-rays. We can only detect them by catching their gravitational waves.”

Previous simulations have not been able to follow the black holes through the whole merger event. Deep inside a black hole lurks a "singularity", a place where gravity gets huge. Computer simulations have had difficulty modeling the waves outside the hole at the same time as the inside.

The key advance by the Lazarus team at the AEI came when they combined two approaches, full numerical simulation for the essentially strong-field regime of the collision and an approximation method, perturbation theory, for computing the radiation from the resulting distorted single black hole. They cut off the full simulation before it went bad, and then used a different method that looked only at the gravitational waves outside the merged hole. Computers again had to calculate this radiation, but they could avoid the problems caused by looking inside the holes.

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