One of the truly weird side-effects of the wormhole/warp-drive discussion is that it seems to lead, almost inevitably, to the possibility of time travel. (At his stage the more conservative members of the physics community typically shake their heads in befuddlement and leave the room.) There are roughly four broad classes of response suitable for handling the various types of paradox induced by the possibility of time travel:
The radical re-write conjecture: Grit your teeth and proceed to re-write all of modern physics from the ground up. Painful, very painful. (I'll live with this if necessary-but you'd better give me good experimental evidence before I spend too much time worrying about this possibility.)
The consistency conjecture: Since there seems to be only one universe, insist that it must be consistent no matter what. So if you try to change history you cannot succeed no matter how hard you try, because the past is already fixed. You know that you, the reader, are alive right now, so no-one can ever send a time traveller to five minutes ago to kill you as you pick up your copy of Phlogiston. If someone tries, something must go wrong: the gun must misfire, or the time machine malfumction, or the assasin miss the bus, or any of a potenially infinite list of increasingly contrived excuses. (Not my favourite way of dealing with things; it quickly begins to look like a consistency conspiracy.)
The chronology protection conjecture: A much more conservative point of view. Even though time travel seems to be absurdly easy once wormholes/warp-drives are allowed, there are reasons to expect things to go berserk just at the onset of time travel. We know that gravitational fields distort the quantum mechanical vacuum, and that this vacuum distortion heads off to infinity at the onset of time travel. We suspect that this effect destroys the wormhole/warp-bubble just as one is getting round to building a time machine. (This is my personal favourite.)
The boring physics conjecture: Forget all this nonsense. Take a good hard look at the experimental evidence, or rather lack thereof, and move on to greener pastures.
To wrap up then, what we are doing is laying the groundwork for Planck scale physics, and groping our way toward the as yet ill-perceived theory of quantum gravity. We are surveying the lay of the land, and even if we do not yet have definitive answers, we are trying to at least formulate the right questions. In particular, whatever the true theory of quantum gravity is, at energies below the Planck scale it must reduce to semiclassical quantum gravity. So any question you can ask about semiclassical quantum gravity must be askable and answerable in the full-fledged theory of quantum gravity. Whatever the theory of quantum gravity ultimately proves to be, it must (among many other things) be able to give solid answers concerning the existence of, and properties of, wormholes, warp-drives, time machines, and other exotica. The author:
Matt Visser grew up in Lower Hutt and studied undergraduate physics at Victoria University. He obtained his Ph.D. at the University of California at Berkeley. He is presently Research Assistant Professor of Physics at Washington University in Saint Louis.
