Black holes are one of the most important predictions of Einstein's theory of general relativity (ART). Today, a large number of astronomical observations provide us with convincing arguments in favour of the existence of black holes, both supermassive black holes and stellar black holes. Supermassive black holes are located in the centres of galaxies and have a mass in the range of several million to billions of suns. For example, observations of highly elliptical Kepler orbits around the radio source Sagittarius A* in the centre of our Milky Way indicate that a supermassive black hole with a mass of around 3.7 million suns is located there. Stellar black holes are the final stage in the evolution of massive stars. These stars explode as supernovae when their nuclear fuel is exhausted. Their central region collapses into a black hole when its mass exceeds an upper limit. A good candidate for a stellar black hole with a mass of approx. 10 suns is, for example, the dark partner in the X-ray binary star system Cygnus X-1, which is located approx. 8000 light years away in the constellation Swan. However, black holes are not only of great interest for astronomy and astrophysics, they also offer us a unique laboratory: a laboratory for thought experiments. The detailed article on this topic can be found in the Physik Journal 2/2008

The authors are: Burkhard Kleihaus and Jutta Kunz, Institute of Physics, University of Oldenburg, and Francisco Navarro-Lérida, Dept. de Física, Atómica, Molecular y Nuclear, Universidad Complutense de Madrid.