Light cones and repulsive gravity

Contrary to common belief, gravitation can also be repulsive. Examples of repulsive gravity are provided by the naked singularity solutions of the Einstein equations corresponding to the negative mass Schwarzschild, the Reissner-Nordstrom, and the Kerr spacetimes. We show that their repulsive gravity regions can be identified by a particular behavior of the light cones when use is made of symmetry-adapted coordinate systems. (C) 2008 American Association of Physics Teachers.