Chronology protection conjecture
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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.
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 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, Marvel Comics' Time Variance Authority, and the television series Doctor Who and Star Trek.
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?
Experimental observation of closed timelike curves would of course demonstrate this conjecture to be false.
In layman's terms, anyone travelling into the past would actually travel into the past of an identical and alternate universe; thus, any change they make would not affect the universe they came from (preserving the chronology of subsequent events), but only the alternate universe, where any 'changes' would be part of the original timeline.
In 1996 Li-Xin Li published a paper in which he postulates the anti-chronology protection conjecture:
- 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.