The research activity of the group is focused towards the study of the two body problem for two black holes. The classical two body problem in physics is formulated for two point-like masses. In Newtonian physics this problem has the well-known solution in terms of

Kepler orbits, but do we really understand how two masses move in their mutual gravitational field? As surprising as it may seem at first, the answer to this question is not really known, even if we restrict ourselves to classical, non-quantum physics and to no more than two bodies.

Einstein's theory of general relativity, which by all accounts is an extremely successful description of the gravitational interaction in the classical regime. In the limit of velocities much below the

speed of light and for weak gravitational fields, Newton's theory of gravity is an excellent approximation and with post-Newtonian approximations we can obtain good approximations for about one tenth of the speed of light. However, the question has to be asked how for example two black holes, which are prime examples for extreme gravity, move around each other when they approach each other at relativistic velocities.

Is there perhaps in general relativity a solution to the equations of motion for two black holes which is as simple and simultaneously as astrophysically relevant as the Kepler orbits of Newtonian physics? The answer is, in a rather satisfying manner, no! The motion of two masses generates gravitational waves, which remove energy and momentum from the system, such that a Kepler

ellipse is no longer a stable solution for an orbit. Two black holes will rather move on an inward spiral, first slowly, then faster and faster, until they collide and merge to a single black hole.

This loss of stability in the Einstein equations is by no means tragic. Quite to the contrary, a growing international community of gravitational wave researchers hopes that

gravitational waves can be detected in order to establish an entirely new branch of astronomy, namely gravitational wave astronomy. Gravitational waves have not been directly detected yet, but detectors are in operation and further improvements are under development, which should lead to the first detection in a matter of years.