Some time-travel phenomena are internally consistent and yet utterly defy common sense. Consider objects (called jinn) that exist in closed time loops and have no point of origin, materializing from nowhere. Gott cites the gold watch in the movie Somewhere in Timethat passes from old Jane Seymour to young Christopher Reeve in 1972 and (after he wills himself to 1912) back to young Seymour, who keeps it with her for the next 60 years before relinquishing it to Reeve. Jinn can also take the form of information transmission. A journalist on deadline could save himself a lot of work by leaping into the future, reading his published article (this one?), coming back, and writing it upthe content of the piece has essentially been pulled from thin air. Nahin stresses that loops like these, anti-rational though they may be, are not satisfactory rebuttals. On the contrary, "if time travel is possible," he writes, "then it would seem that we will have to accept causal loops, too."
In a 1998 paper, Gott and his Princeton colleague Li-Xin Li extended this circularity to the whole megillah, proposingin what amounts to a cosmic extrapolation of the aforementioned Heinlein storythat "the Universe could be its own mother." At the beginningand the endis a time machine; there is no earliest or latest event. A provocative answer to the conundrum of first cause, this theory relates to a controversial question Stephen Hawking posed in A Brief History of Time; describing the notion of "completely self-contained" space-time, he muses, "What place, then, for a creator?" Gott, a Presbyterian, writes in his book, "I would not pretend that a self-creating Universe is not a troubling notionbut perhaps we should find the Universe troubling."
As in all matters temporal, things unavoidably turn fuzzy. The metaphysics here are inextricably linked to the philosophy of timemysteries that have gripped thinkers from Aristotle (time is eternal in both directions) to Kant (the past cannot be infinite). Time travel also adds new twists to the age-old free will vs. omniscience debate, though Davies, who's "not religious in any conventional sense," points out it's not worth getting too worked up just yet: "The possibility of time travel would be such a transformation of our understanding of reality it would transform our entire value system."
So, in a best-case scenario, how far are we from achieving time travel? Davies says, "It seems to me the best hope for building a wormhole time machine is if our understanding of gravity is not yet the last word. Some people believe the next generation of particle accelerators could bring about dramatic gravitational effects at modest energies, and we would be able to sculpt space-time with those machines. I could well imagine a situation in which some sort of time travel is possibleyou could send a particle back through time, saybut for one reason or another, humans never could do it."
Mallett's optimism is less guarded. "Once it can be done, even in the simplest situation, in the most primitive way, the engineering obstacles will be overcome," he says. "Just think of the Wright brothers: All they did was get their plane to fly a hundred yards or so, and we had jet travel by the middle of the century. It's only a matter of time before we find the right combination of things that permits it. I honestly believe this will be the century for time travel."