The concept of the gedanken-experiment
is one of the most fruitful in theoretical physics. In situations where actual physical experiments are too difficult to carry out, a lot of information can be gleaned by taking the theory and pushing it to its limits to see what if anything breaks. If something does break, hopefully it does so in an interesting manner that yields information about possible extensions to the theory.
It is in this spirit that discussions of "traversable wormholes" and "chronology protection" should be viewed, we are pushing classical, and even semi-classical, general relativity to its ultimate limits.
When in 1988 Mike Morris and Kip Thorne (CalTech
) carefully delineated the difference between non-traversable and traversable wormholes, it became clear that traversable wormholes required violations of the classical energy conditions of general relativity — roughly speaking you need to put negative
energy on the throat of the wormhole to keep the throat open without forming a horizon. (More precisely, the null energy condition, the NEC, must be violated in the immediate vicinity of any traversable wormhole throat.)
Since violating the NEC through classical effects is at best difficult — and was thought for a long time to be impossible — attention quickly focused on using
semi-classical quantum effects to hold open the wormhole throat. While many physicists seem to have felt that it would quickly become possible to prove a "no wormhole" theorem, the actual situation has turned out to be much more complex.
- Despite a large number of constraints that can be placed on wormhole behaviour there has as yet been no knockout blow — after almost two decades of work traversable wormholes still seem to be compatible with what we know concerning Einstein gravity.
- Traversable wormholes seem, almost inevitably, to lead to closed timelike curves (in plain language, "time machines").
Time travel and its associated causality violations is deeply disturbing to most physicists, and has led Stephen Hawking to formulate his "chronology protection conjecture" whereby the universe abhors a time machine. (More technically, the onset of time travel is associated with uncontrolled fluctuations in the geometry of spacetime, so that the "chronology horizon" cannot be treated semi-classically — this pushes the onset of time travel deep into the Planck regime where full "quantum gravity" holds sway.)
In short, it seems that a traversable wormhole considered in isolation can adequately be described using semi-classical quantum gravity, but that the vicinity of any chronology horizon requires full "quantum gravity".
For background on these issues see the articles by Mike Morris and Kip Thorne, plus the "chronology protection" articles by Stephen Hawking.
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Wormholes at Victoria University of Wellington
Key items related to the Victoria University gravity Group are:
- Matt Visser, Lorentzian wormholes: from Einstein to Hawking. AIP Press, 1995.
- Matt Visser, Sayan Kar and Naresh Dadhich, Traversable wormholes with arbitrarily small energy condition violations. Phys. Rev. Lett. 90 (2003) 201102 arXiv: gr-qc/0301003.
- Matt Visser, Traversable Wormholes From Surgically Modified Schwarzschild Space-Times. Nucl. Phys. B 328 (1989) 203.
- Matt Visser, Traversable Wormholes: Some Simple Examples. Phys. Rev. D 39 (1989) 3182.
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