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TRAVERSABLE WORMHOLES: SOME IMPLICATIONS

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Guyfolan
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« on: December 21, 2010, 01:07:33 am »

4. TIME TRAVEL

Wormholes are constrained by relativity to travel at sublight speeds and are time-dilated as per normal. Clocks placed at the mouths of a wormhole always remain in synchronisation with each other. If I look through one end of a wormhole and compare the near clock with the far clock they always agree. Even if one end of the wormhole is travelling at relativistic speeds many light years away. Einstein says moving clocks run slow. There would appear to be a paradox here. We observe the two clocks keeping time with each other, yet relativity says the 'distant', travelling clock is running slowly. How do we reconcile this? Only by concluding that the distant clock has been displaced in space and time. If a wormhole enables someone travel from Alpha Centauri 2000 to Sol 1993 and vice versa, then no paradox because they can't travel back to Alpha Centauri (through conventional space) and arrive before they left (to cause a paradox).

Problems begin when the distant wormhole end turns about and returns home. According to the twin paradox the traveller returns aged less than the stay-at-home twin (their clocks are no longer in step). Travelling through the wormhole from the stay-at-home end to the go- away-and-come-back end transports you forward in time. Travelling in the reverse direction transports you back in time. Wormholes allow time travel. This conclusion was realised soon after the first articles on traversable wormholes were published. Depending on your view of the plausibility of time travel this is either, if you believe time travel possible, very exciting or, if you scoff at time travel, proof that traversable wormhole can't exist. No general consensus emerged in the pages of various physics journals as the subject was batted back and forth. Elaborate and very interesting papers (by Thorne's group [7] and others) reconciled time travel with quantum theory, whilst others (like Hawking ) proposed a Chronological Protection Conjecture, CPC, which says the Universe Shalt Not Allow Time Travel.

One of the time travel sceptics was Matt Visser. Early in 1993 he showed that wormholes do not enable time travel [2], by proposing physical mechanisms that enforce CPC. Visser showed, in a peer reviewed article, the mouths of a wormhole with an induced clock difference could not be brought close enough together to enable a traveller to attempt violation of causality. Quantum field and gravitational effects build up as the two ends of a wormhole approach the critical point and either collapse the wormhole or induce a mutual repulsion. Visser's work is not complete but it seems swarms of virtual particles disrupt the region around a time machine just before it would otherwise become operational.

The virtual particles around a nearly chronologically violating region are able form closed spacelike (superluminal) loops and, via Heisenberg, to borrow energy off themselves, becoming more virulent than usual. Traversable wormholes are closed, or pinched off, by the energy of the virtual particles that flow through them as they approach being time machines which prevents the more dangerous closed timelike loops (which may cause paradoxes). For the purposes of this article I'll adopt Visser's conclusion that the CPC mechanism is generic and blocks all forms of time travel via wormholes, but permits the operation of wormholes for the purpose of FTL travel.
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