In our general relativity research projects the Cactus computer code is used to numerically evolve space-times, which cannot be treated by analytic means, e.g. the collision of black holes or neutron stars. Such simulations require immense computational resources and - for large-scale simulations - run on supercomputers. In such a numerical space-time we may place `seeds' of particles, being at rest initially. These particles will then be influenced by the simulated gravitational field.

While CACTUS runs on a supercomputer located in a high-performance computing center somewhere in the world, the particle positions are sent to a local graphics workstations running Amira for visualization. The `Illuminated Stream Line' technique, special to Amira (developed at ZIB), is used to display the path of the virtual particles as they are dragged around in the numerically evolved space-time.

The examples presented here show particles in the Kerr space-time. This is the analytically known solution of a rotating black hole. However, while the space-time is known in this demonstration example, the particle's world lines cannot be determined analytically and have to be computed numerically.

Note that the influence of the rotation of the black hole on the particle paths is a non-newtonian effect. This effect, also called the `gravitomagnetism', is one of the yet unverified expectations of Einstein's theory of general relativity.