I came across the term "Misner string" in this paper: http://arxiv.org/abs/1509.07854 My understanding is that this originated in the study of spacetimes such as Taub-NUT. If I'm understanding correctly, this involves CTCs encircling the string. Misner's original interpretation of the string was that time was forced to be periodic, which caused energy to be quantized; this makes the string unobservable. Bonnor proposed instead to treat Misner string as a topological defect, with matter as a source: Bonnor, Proc Camb Phil Soc 66 (1969) 145 http://journals.cambridge.org/actio...587332E4.journals?fromPage=online&aid=2067080 . Unfortunately the old papers seem to be paywalled, so I can't access them. (As usual, the journals are doing their best imitations of Mordac, Preventer of Information Services.) Does anyone know of any freely available description of this topic?