We have investigated the basic concepts necessary to understand geometry in general relativity. The metric tensors of Black Holes - non-rotating and rotating - have been used to discuss curvature, gravitational redshift and celestial mechanics in the geometry of Black Holes. A computer code has been used to calculate trajectories in both Schwarzschild and Kerr geometry, and some differences between the two cases are shown. There are still many open questions to be answered, but this is enough for this time.
The applications of Black Holes are increasing these years as better observations gives insight to eg. accretion discs, which are naturally formed around physical Black Holes. This interesting phenomena includes charged particles, electromagnetic fields and gas (accreted from a nearby binary star). The gas is described by relativistic hydrodynamics, which is ever more complicated.
Michael Cramer Andersen
June 1996