Seminar of Theory of Relativity and Gravitation

In a spacetime where the Misner string is present - due to the NUT parameter or a time-shift - the axis of rotation is singular and cannot be a part of the spacetime manifold. I will discuss a geometric definition of the axis, the corresponding axial symmetry and the associated measure of the conical singularity - namely the conicity. It turns out that in the generic case, the conicity is necessarily observer-dependent and in particular in the spacetime with a NUT parameter there exists an observer measuring no difference of conicities for both parts of the axis. Consequently, any calculation concerning the Misner string - e.g. black holes thermodynamics or acceleration - should take the observer-dependency into account. The definition of conicity correctly reproduces the previously known results for Plebański-Demiański spacetimes.Based on a joint work with Ivan Kolář and Pavel Krtouš.