Acceleration of particles in Schwarzschild and Kerr geometries

Abstract: The Landau-Lifshitz decomposition of spacetime, or (1+3)-split, determines the three-dimensional velocity and acceleration as measured by static observers. We use these quantities to analyze the geodesic particles in Schwarzschild and Kerr spacetimes. We show that in both cases there is no room for a positive acceleration (repulsion). We also compute the escape and terminal speeds. The escape speed in the case of a static black hole coincides with the Newton result. For the Kerr spacetime, the escape speed depends on the polar angle, showing that a particle needs less energy to escape in the direction close to the polar axis. The terminal speed at the Schwarzschild horizon and at the Kerr ergosphere turns out to be equal to the speed of light. For a local, stationary observer near to a massive particle in geodesic motion on the equatorial plane of the Kerr spacetime, the analysis of the velocity reveals that such a particle never reaches the ergosphere. And, in the case of the Schwarzschild spacetime, we found that the geodesic trajectories reach the horizon perpendicularly.