M. Calvani

Time Machine and Geodesic Motion in Kerr Metric

The existence of nontrivial causality violation is a peculiar property of the space-time around a naked singularity. In the case of Kerr metric witha > m we have found that for a particular class of geodesies that could in principle violate causality, the conditions for causality violation are never satisfied.

Causality violation in the Kerr metric

The conjecture that geodesics in the Kerr matric witha > m cannot violate causality in the sense of meeting a turning point before making up for lost time, is found to be untrue for null geodesics.

Vortical null orbits, repulsive barriers, energy confinement in Kerr metric

We give the complete analytical description of the null trajectories in the field of a Kerr naked singularity. Two peculiar phenomena are described: the existence of repulsive barbiers in ther < 0 world and the existence of null circular bound orbits which surround the singularity in “shells.” They distribute around the surface atr = m, which is the position of the horizon in the extreme black-hole case; this suggests that a naked singularity “remembers” the position of the last horizon.

Complete description of photon trajectories in the Kerr-Newman space-time

Gives the complete description of null trajectories in the Kerr-Newman space-time in terms of the parameters of the source and of the constants of motion. The conditions for orbital and vortical motion are studied in detail and the authors are able to give the locus of turning points for any choice of the parameters.

Photon trajectories in the Kerr-Newman metric

Gives a detailed description of the null trajectories in the Kerr-Newman metric. Interesting results which were not expected from what was known in the uncharged case are a considerable enhancement of the energy storage in a restricted region around the source in the naked singularity case and the singularity is completely disconnected from the asymptotic region in the sense that no photon can reach it even in the equatorial plane.

Charged singularities: repulsive effects

The repulsive phenomena which a particle experiences in the vicinity of a naked singularity are investigated in the Kerr-Newmann space-time. The aim is to extend the knowledge of this fact to charged solutions and to have a direct indication of how, in these situations, the gravitational and electrostatic interactions are competing.

Irreducible mass for the Tomimatsu-Sato space-times

A global definition of irreducible mass for the odd-δ T-S metrics is investigated. We find that its expression in terms of the source parameters is the same for all the members of the family and reduces to the formula that holds in the Kerr case (δ=1). As a consequence, we show that processes withmir = const no longer imply zero variations of the horizons area for δ>1.