Chronology protection conjecture
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2008-05-27T08:48:52Z
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/* Uses In Fiction */ crime traveller uses this a lot
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]] on all but sub-microscopic scales. Mathematically, the permissibility of time travel is represented by the existence of [[closed timelike curve]]s.
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 paradox]]es. 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]]''.
However, the ideas of the chronology protection conjecture are completely serious. Many attempts to generate scenarios for closed timelike curves have been suggested, and the theory of [[general relativity]] does allow them in certain circumstances (for example, it would allow for a time machine constructed from a [[wormhole]] - see [[Wormhole#Wormholes and time travel|Wormholes and time travel]]). But attempts to incorporate [[quantum mechanics|quantum]] effects into general relativity using [[semiclassical gravity]] seem to make it plausible that [[Virtual particle#Virtual particles in the vacuum|vacuum fluctuations]] would drive the [[energy density]] on the boundary of the time machine (the [[Cauchy horizon]] of the region where closed timelike curves become possible) to infinity, destroying the time machine at the instant it was created or at least preventing anyone outside it from entering it. The question then arises: is this apparent censorship of closed timelike curves a global constraint of physics, in the same way as a [[conservation law]], or is it a series of accidental coincidences?
A definite theoretical decision on the status of the chronology protection conjecture would require a full theory of [[quantum gravity]] as opposed to the [[semiclassical gravity|semiclassical]] arguments that have been mainly used to support it (there are also some arguments from [[string theory]] which seem to support chronology protection, but string theory is not yet a complete theory of quantum gravity). Experimental observation of closed timelike curves would of course demonstrate this conjecture to be [[falsifiability|false]], but short of that, if physicists had a theory of quantum gravity whose predictions had been well-confirmed in other areas, this would give them a significant degree of confidence in the theory's predictions about the possibility or impossibility of time travel.
Other proposals which allow for backwards time travel but prevent [[Physical paradox#Causality paradoxes|time paradoxes]], such as the [[Novikov self-consistency principle]] which would ensure the timeline stays consistent, or the idea that a time traveler is taken to a [[Many-worlds interpretation|parallel universe]] while his original timeline remains intact, do not qualify as "chronology protection".
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 [[Virtual particle#Virtual particles in the vacuum|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.''
==Uses In Fiction==
In [[Newton's Wake: A Space Opera|Newton's Wake]] by Ken MacLeod, post-humans (and other [[Singularity]] driven intelligences) that have transcended physical limitations of the [[Planck limits]], also termed as quantum angels, are speculated as responsible for enforcing CPC.
The chronology protection conjecture plays a large part in the British [[science fiction]] television programme [[Crime Traveller]] (named the "chronology protection hypothesis" within the show), stated as the reason that while the main characters can travel back in time and observe a crime taking place to solve how it was done, they cannot prevent the crime or otherwise alter any events which have already taken place.
In the [[Lost (TV series)|LOST]] episode "The Constant," a blackboard belonging to character Daniel [[Faraday effect|Faraday]] contains the phrase "[[Kerr metric|Kerr]] might work! Can it evade Hawking's chronology protection conjecture?"
==References==
* Hawking, S.W., (1992) ''The chronology protection conjecture.'' Phys. Rev. D46, 603-611.
* Matt Visser, [http://arxiv.org/abs/gr-qc/0204022 "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, [http://arxiv.org/abs/gr-qc/9703024 "Must Time Machine Be Unstable against Vacuum Fluctuations?"], Class.Quant.Grav. 13 (1996) 2563-2568.
==See also==
* [[Causality]]
* [[Cosmic censorship hypothesis]]
* [[Time travel]]
* [[Wormhole]]
== External links ==
* http://www.hawking.org.uk/lectures/warps3.html
[[Category:Time]]
[[Category:Causality]]
[[Category:Time travel]]
[[Category:Theories of gravitation]]
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