NUT wormholes
Abstract
We show that supercritically charged black holes with a Newman--Unti--Tamburino (NUT) parameter provide a new setting for traversable wormholes. This does not require exotic matter, but there is a price---the Misner string singularities. Without assuming time periodicity to make Misner strings unobservable, we show that, contrary to expectations, geodesics do not stop there. Moreover, since there is no central singularity, the spacetime turns out to be geodesically complete. Another unpleasant feature of spacetimes with NUTs is the presence of regions where the azimuthal angle $\ensuremath{\varphi}$ becomes timelike, signalling the appearance of closed timelike curves (CTCs). We show that among them there are no closed timelike or null geodesics, so the freely falling observers should not encounter causality violations. Considering worldlines of charged particles, we find that, although these can become closed in the vicinity of the wormhole throat for large enough charge-to-mass ratio, the noncausal orbits are still disconnected from the distant zones. Integrating the geodesic equations completely, we demonstrate the existence of timelike and null geodesics connecting two asymptotic regions of the wormhole, such that the tidal forces in the throat are reasonably small. We also discuss bounds on the NUT charge which follow from the Schwinger pair creation and ionization thresholds.