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==History==
Physicists have long known that some solutions to the theory of general relativity contain [[closed timelike curve]]s—for example the [[Gödel metric]]. Novikov discussed the possibility of closed timelike curves (CTCs) in books he wrote in 1975 and 1983,<ref>See note 10 on p. 42 of Friedman et al., "Cauchy problem in space-times with closed timelike curves"</ref> offering the opinion that only self-consistent trips back in time would be permitted.<ref>On p. 169 of Novikov's ''Evolution of the Universe'' (1983), which was a translation of his Russian book '' Evolyutsiya Vselennoĭ'' (1979), Novikov's comment on the issue is rendered by translator M. M. Basko as "The close of time curves does not necessarily imply a violation of causality, since the events along such a closed line may be all 'self-adjusted'—they all affect one another through the closed cycle and follow one another in a self-consistent way."</ref> In a 1990 paper by Novikov and several others, "[[Cauchy problem]] in spacetimes with closed timelike curves",<ref name="friedman">{{cite journal | first=John | last=Friedman |author2=Michael Morris |author3=Igor Novikov |author4=Fernando Echeverria |author5=Gunnar Klinkhammer |author6=Kip Thorne |author7=Ulvi Yurtsever | url=http://authors.library.caltech.edu/3737/ | title=Cauchy problem in spacetimes with closed timelike curves | journal = Physical Review D | volume = 42 | year=1990 | issue=6 | doi=10.1103/PhysRevD.42.1915 | pages=1915 | bibcode=1990PhRvD..42.1915F | pmid=10013039}}</ref> the authors state:
{{quote|The only type of causality violation that the authors would find unacceptable is that embodied in the science-fiction concept of going backward in time and killing one's younger self ("changing the past"). Some years ago one of us (Novikov) briefly considered the possibility that CTCs might exist and argued that they cannot entail this type of causality violation: events on a CTC are already guaranteed to be self-consistent, Novikov argued; they influence each other around a closed curve in a self-adjusted, cyclical, self-consistent way. The other authors recently have arrived at the same viewpoint.
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