Acceleration of particles in Schwarzschild and Kerr geometries

TL;DR

This paper analyzes particle acceleration in Schwarzschild and Kerr spacetimes using a (1+3)-split, revealing no repulsive acceleration, calculating escape and terminal speeds, and showing differences in escape energy depending on geometry and position.

Contribution

The study applies the Landau-Lifshitz (1+3)-split to analyze velocities and accelerations in Schwarzschild and Kerr spacetimes, providing new insights into escape speeds and particle trajectories.

Findings

No positive acceleration (repulsion) in either spacetime.

Escape speed in Schwarzschild matches Newtonian predictions.

Escape speed in Kerr depends on polar angle, lower near the axis.

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.