Relaxation of spherical stellar systems
  10,000 simulations of 1000-particle realisations of the same cluster are
computed by direct force summation. Over three crossing times the original
Poisson noise is amplified more than tenfold by self-gravity. The cluster's
fundamental dipole mode is strongly excited by Poisson noise, and this mode
makes a major contribution to driving diffusion of stars in energy. The
diffusive flow through action space is computed for the simulations and
compared with the predictions of both local-scattering theory and the
Balescu-Lenard (BL) equation. The predictions of local-scattering theory are
qualitatively wrong because the latter neglects self-gravity. These results
imply that local-scattering theory is of little value. Future work on cluster
evolution should employ either N-body simulation or the BL equation. However,
significant code development will be required to make use of the BL equation
practicable.
