Abstract
One of the fundamental aspects of statistical behaviour in many-body systems is exponential divergence of neighbouring orbits, which is often discussed in terms of Liapounov exponents. Here we study this topic for the classical gravitational N-body problem. The application we have in mind is to old stellar systems such as globular star clusters, where N∼106, and so we concentrate on spherical, centrally concentrated systems with total energy E<0. Hitherto no connection has been made between the time scale for divergence (denoted here by t e ) and the time scale on which the energies of the particles evolve because of two-body encounters (i.e., the two-body relaxation time scale, t r ), even though both may be calculated by similar considerations. In this paper we give a simplified model showing that divergence in phase space is initially roughly exponential, on a timescale proportional to the crossing time (defined as a mean time for a star to cross from one side of the system to another). In this phase t e <<t r , if N is not too small (i.e., N≫30). After several e-folding times, the model shows that the divergence slows down. Thereafter the divergence (measured by the energies of the bodies) varies with time as t 1/2, on a timescale nearly proportional to the familiar two-body relaxation timescale, i.e., t e ∼t r in this phase. These conclusions are illustrated by numerical results.
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Hut, P., Heggie, D.C. Orbital Divergence and Relaxation in the Gravitational N-Body Problem. Journal of Statistical Physics 109, 1017–1025 (2002). https://doi.org/10.1023/A:1020472526203
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DOI: https://doi.org/10.1023/A:1020472526203