Like a fairy-tale princess, trajectories around black holes can be sensitive to small disturbances. We describe how a small disturbance can lead to erratic orbits and an increased production of gravitational waves. 04.30.Db, 05.45.+b, 97.60.Lf
UM-P-96/45The high degree of symmetry found in the spacetime of an isolated black hole leads to regular geodesic motion for orbiting bodies. Working on the assumption that small disturbances generally have small effects, it is often tacitly assumed that this idealised, textbook picture carries over to real astrophysical situations. Here we want to emphasise that the idealised picture is not stable against small perturbations, as the non-linearity of Einstein's equations tends to amplify small disturbances.The observation that small perturbations of an idealised black hole spacetime can lead to qualitative changes in the dynamics is not new. It has previously been noted that a range of perturbations lead to chaotic dynamics. The perturbations considered include additional mass concentrations [1,2,3,4,5], magnetic fields [6], gravitational waves [7] and spin-orbit coupling [8]. Unlike the Kepler problem of Newtonian mechanics, essentially any perturbation of an isolated black hole spacetime will lead to chaotic orbits. This is because even the most pristine black hole spacetime harbours the seeds of chaos in the form of isolated unstable orbits. A small perturbation causes these unstable orbits to break out and infest large regions of phase space. Note that experience with Newtonian systems is very misleading. For example, the Kepler problem has more integrals of motion than are needed for integrability. Keplerian systems are thus impervious to small perturbations. In contrast, black hole spacetimes are at the edge of chaos, just waiting for the proverbial butterfly to flap its wings.Once it is realised that typical black hole -satellite systems are chaotic in the strong field regime, we are forced to consider the consequences. One of the most immediate consequences is that there will be no such thing as the "last stable orbit" [9]. The boundary between stable and unstable orbits will be fractal, and there may be large fractal tendrils of unstable orbits invading what would have been stable territory in an ideal black hole spacetime. This feature will be important in determining when a binary system switches from inspiral to cataclysmic collapse [9]. Another important consequence will be the increased production of gravitational waves due to the erratic motion of chaotic orbits.In what follows we will illustrate both of these effects in a simple model system. While our model does not describe a real astrophysical situation, it does capture many of the salient features we expect to find in a relativistic binary system.For the purpose of illustration we will study the orbits of non-rotating satellites around an extreme RiessnerNordstrom black hole. Almost identical results hold for motion around a Schwarzschild black hole perturbed by an orbiting third body [3]. Extreme black ...