Editing Chronology protection conjecture (section)

==Etymology==
In a 1992 paper, Hawking uses the metaphorical device of a "Chronology Protection Agency" as a [[personification]] of the aspects of physics that make time travel impossible at macroscopic scales, thus apparently preventing [[temporal paradox]]es. He says:

{{quote|It seems that there is a Chronology Protection Agency which prevents the appearance of closed timelike curves and so makes the universe safe for historians.<ref>{{cite journal | last1 = Hawking | first1 = S. W. | year = 1992 | title = Chronology protection conjecture | journal = Phys. Rev. D | volume = 46 | issue = 2| pages = 603–611 | doi=10.1103/physrevd.46.603| bibcode = 1992PhRvD..46..603H | pmid = 10014972}}</ref>}}

The idea of the Chronology Protection Agency appears to be drawn playfully from the Time Patrol or Time Police concept, which has been used in many works of [[science fiction]]<ref>{{cite web|url=http://www.sf-encyclopedia.com/entry/time_police |title=Time Police : SFE : Science Fiction Encyclopedia |publisher=Sf-encyclopedia.com |date=December 21, 2011 |access-date=2014-08-25}}</ref> such as [[Poul Anderson]]'s series of ''[[Time Patrol]]'' stories or [[Isaac Asimov]]'s novel ''[[The End of Eternity]]'', or in the television series ''[[Doctor Who]]''.  [[Phil D'Amato#"The Chronology Protection Case"|"The Chronology Protection Case"]] by [[Paul Levinson]], published after Hawking's paper, posits a universe that goes so far as to murder any scientists who are close to inventing any means of time travel. [[Larry Niven]], in his short story ‘Rotating Cylinders and the possibility of Global Causality Violation’ expands this concept so that the universe causes environmental catastrophe, or global civil war, or the local sun going nova, to any civilisation which shows any sign of successful construction.