The chronology protection conjecture is a conjecture by the physicist Professor Stephen Hawking that the laws of physics are such as to prevent time travel ("closed timelike curves") on all but sub-microscopic scales. A physicist is a scientist trained in physics. ... Stephen Hawking in 2005 Professor Stephen William Hawking, D.Phil. ... Since antiquity, people have tried to understand the behavior of matter: why unsupported objects drop to the ground, why different materials have different properties, and so forth. ... Time travel is a concept that has long fascinated humanity—whether it is Merlin experiencing time backwards, or religious traditions like Mohammeds trip to Jerusalem and ascent to heaven, returning before a glass knocked over had spilt its contents. ... From the point of view of general relativity, a closed timelike curve (CTC) is a worldline of a material particle in spacetime that is closed. ...

In a 1992 paper, Hawking uses the metaphorical device of a "Chronology Protection Agency" as a personification of the aspects of physics which make time travel impossible at macroscopic scales, thus apparently preventing time paradoxes. He says: It has been suggested that prosopopoeia be merged into this article or section. ...

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.

The idea of the Chronology Protection Agency appears to be drawn playfully from the Time Patrol or Time Police concept present in such works of science fiction as Isaac Asimov's novel The End of Eternity, Charles Stross' novel Singularity Sky and the television series Doctor Who and Star Trek. Science fiction is a form of speculative fiction principally dealing with the impact of imagined science and technology, or both, upon society and persons as individuals. ... Isaac Asimov (courtesy of Jay Kay Klein) Isaac Asimov (c. ... The End of Eternity by Isaac Asimov is a science fiction novel, with mystery and thriller elements, on the subjects of time travel and social engineering. ... Singularity Sky (ISBN 0441010725) is a hard science fiction novel by author Charles Stross. ... Main article: History of Doctor Who Doctor Who first appeared on BBC television at 5:15 p. ... http://www. ...

However, the ideas of the chronology protection conjecture are completely serious. Many attempts to generate plausible scenarios for closed timelike curves have been suggested, and all seem either implausible, contradict other principles of physical law, or appear to be contradicted by experiment. The question then arises: is this apparent prohibition a global constraint of physics, in the same way as a conservation law, or is it a series of accidental coincidences? In physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves. ...

Experimental observation of closed timelike curves would of course demonstrate this conjecture to be false. Falsifiability is an important concept in the philosophy of science that amounts to the apparently paradoxical idea that a proposition or theory cannot be scientific if it does not admit the possibility of being shown false. ...

In 1996 Li-Xin Li published a paper in which he postulates the anti-chronology protection conjecture: 1996 (MCMXCVI) is a leap year starting on Monday of the Gregorian calendar, and was designated the International Year for the Eradication of Poverty. ...

In the appearance of absorption material, the quantum vacuum fluctuations of all kinds of fields may be smoothed out and the spacetime with time machine may be stable against vacuum fluctuations. The chronology protection conjecture might break down, and the anti-chronology protection conjecture might hold: There is no law of physics preventing the appearance of closed timelike curves.

References

• Hawking, S.W., (1992) The chronology protection conjecture. Phys. Rev. D46, 603-611.
• Matt Visser, "The quantum physics of chronology protection" in The Future of Theoretical Physics and Cosmology: Celebrating Stephen Hawking's 60th Birthday by G. W. Gibbons (Editor), E. P. S. Shellard (Editor), S. J. Rankin (Editor)
• Li-Xin Li, "Must Time Machine Be Unstable against Vacuum Fluctuations?", Class.Quant.Grav. 13 (1996) 2563-2568.

See also

• Causality
• Cosmic censorship hypothesis
• Time travel
• Wormhole

The philosophical concept of causality or causation refers to the set of all particular causal or cause-and-effect relations. ... This article is in need of attention from an expert on the subject. ... Time travel is a concept that has long fascinated humanity—whether it is Merlin experiencing time backwards, or religious traditions like Mohammeds trip to Jerusalem and ascent to heaven, returning before a glass knocked over had spilt its contents. ... 2D analogy to a wormhole. ...

External links

• http://www.hawking.org.uk/lectures/warps3.html