X-Nico

Cosmic string

In 1995, Visser et al. proposed that cosmic string could theoretically also exist with an angle excess, and thus negative tension and hence negative mass.

Wormhole

The debate on this matter is described by Kip S. Thorne in the book Black Holes and Time Warps, and a more technical discussion can be found in The quantum physics of chronology protection by Matt Visser.

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Later, other types of traversable wormholes were discovered as allowable solutions to the equations of general relativity, including a variety analyzed in a 1989 paper by Matt Visser, in which a path through the wormhole can be made where the traversing path does not pass through a region of exotic matter.

see also