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 ideas in science fiction, such as Isaac Asimov's novel The End of Eternity, Charles Stross' novel Singularity Sky 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, or contradict other principles of physical law, or appear to be contradicted by experimental observations. 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.

References

• Hawking, S.W., (1992) The chronology protection conjecture. Phys. Rev. D46, 603-611.
• Matt Visser, "The quantum physics of chronology protection" (http://arxiv.org/abs/gr-qc/0204022) 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)

See also

• Causality
• Cosmic censorship hypothesis

External links

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