This article appeared in: IAU Colloquium 109 (1988/1989) ...

A Laboratory for Gravitational Scattering Experiments

Piet Hut

The Institute for Advanced Study, Princeton, NJ 08540


An accurate macroscopic description of the evolution of star clusters, galaxies, and clusters of galaxies should be based on a detailed knowledge of the underlying microscopic processes. Examples of such processes are the interactions between single stars and binaries, in the context of a star cluster, or between whole galaxies, in the context of a cluster of galaxies. A statistical description of these processes is most conveniently given in terms of cross sections and reaction rates, just as in atomic or nuclear physics. The main difference is that in gravitational physics our particle accelerator is replaced by a computer program, which plays the role of star accelerator.

Work in progress towards realization of a laboratory for gravitational scattering is summarized, by concentrating on the three levels of software needed to build such a laboratory. First, one needs a program which computes the actual particle orbits (the accelerator). Second, one needs a program which monitor the progress of individual scattering experiments, and decides automatically when to halt computation (the servo mechanism which guide and control the accelerator). Third, one would like to have an intelligent program which takes the physics questions, such as "what is the reaction rate for the occurrence of hierarchical scattering for a binary with a binding energy of 10 kT in a heat bath with a Maxwellian distribution of single star velocities?", and translate that question in an agenda of scattering experiments to be carried out (the laboratory assistant).