Exactly one hundred years ago, in 1895, H. G. Wells classic story The Time Machine was first published in book form. As befits the subject matter, that was the minus tenth anniversary of the first publication, in 1905, of Albert Einstein's special theory of relativity. It was Einstein, as every schoolchild knows, who first described time as "the fourth dimension" -- and every schoolchild is wrong. It was actually Wells who wrote, in The Time Machine, that "there is no difference between Time and any of the three dimensions of Space, except that our consciousness moves along it".
Since the time of Wells and Einstein, there has been a continuing literary fascination with time travel, and especially with the paradoxes that seem to confront any genuine time traveller (something that Wells neglected to investigate). The classic example is the so- called "granny paradox", where a time traveller inadvertantly causes the death of his granny when she was a small girl, so that the traveller's mother, and therefore the traveller himself, were never born. In which case, he did not go back in time to kill granny . . . and so on.
A less gruesome example was entertainingly provided by the science fiction writer Robert Heinlein in his story By his bootstraps (available in several Heinlein anthologies). The protagonist in the story stumbles on a time travel device brought back to the present by a visitor from the far future. He steals it and sets up home in a deserted stretch of time, constantly worrying about being found by the old man he stole the time machine from -- until one day, many years later, he realises that he is now the old man, and carefully arranges for his younger self to "find" and "steal" the time machine. Such a narcissistic view of time travel is taken to its logical extreme in David Gerrold's The Man Who Folded Himself (Random House, 1973).
Few of the writers of Dr Who have had the imagination actually to use his time machine in this kind of way. It would, after all, make for rather dull viewing if every time the Doctor had been confronted by a disaster he popped into the TARDIS, went back in time and warned his earlier self to steer clear of the looming trouble. But the implications were thoroughly explored for a wide audience in the Back to the Future trilogy, ramming home the point that time travel runs completely counter to common sense. Obviously, time travel must be impossible. Only, common sense is about as reliable a guide to science as the well known "fact" that Einstein came up with the idea of time as the fourth dimension is to history. Sticking with Einstein's own theories, it is hardly common sense that objects get both heavier and shorter the faster they move, or that moving clocks run slow. Yet all of these predictions of relativity theory have been born out many times in experiments, to an impressive number of decimal places. And when you look closely at the general theory of relativity, the best theory of time and space we have, it turns out that there is nothing in it to forbid time travel. The theory implies that time travel may be very difficult, to be sure; but not impossible.
Perhaps inevitably, it was through science fiction that serious scientists finally convinced themselves that time travel could be made to work, by a sufficiently advanced civilization. It happened like this. Carl Sagan, a well known astronomer, had written a novel in which he used the device of travel through a black hole to allow his characters to travel from a point near the Earth to a point near the star Vega. Although he was aware that he was bending the accepted rules of physics, this was, after all, a novel. Nevertheless, as a scientist himself Sagan wanted the science in his story to be as accurate as possible, so he asked Kip Thorne, an established expert in gravitational theory, to check it out and advise on how it might be tweaked up. After looking closely at the non-commonsensical equations, Thorne realised that such a wormhole through spacetime actually could exist as a stable entity within the framework of Einstein's theory.
Sagan gratefully accepted Thorne's modification to his fictional "star gate", and the wormhole duly featured in the novel, Contact, published in 1985. But this was still only presented as a shortcut through space. Neither Sagan nor Thorne realised at first that what they had described would also work as a shortcut through time. Thorne seems never to have given any thought to the time travel possibilities opened up by wormholes until, in December 1986, he went with his student, Mike Morris, to a symposium in Chicago, where one of the other participants casually pointed out to Morris that a wormhole could also be used to travel backwards in time. Thorne tells the story of what happened then in his own book Black Holes and Time Warps (Picador). The key point is that space and time are treated on an essentially equal footing by Einstein's equations -- just as Wells anticipated. So a wormhole that takes a shortcut through spacetime can just as well link two different times as two different places. Indeed, any naturally occurring wormhole would most probably link two different times.
As word spread, other physicists who were interested in the exotic implications of pushing Einstein's equations to extremes were encouraged to go public with their own ideas once Thorne was seen to endorse the investigation of time travel, and the work led to the growth of a cottage industry of time travel investigations at the end of the 1980s and in to the 1990s. The bottom line of all this work is that while it is hard to see how any civilization could build a wormhole time machine from scratch, it is much easier to envisage that a naturally occurring wormhole might be adapted to suit the time travelling needs of a sufficiently advanced civilization. "Sufficiently advanced", that is, to be able to travel through space by conventional means, locate black holes, and manipulate them with as much ease as we manipulate the fabric of the Earth itself in projects like the Channel Tunnel.
Even then, there's one snag. It seems you can't use a time machine to go back in time to before the time machine was built. You can go anywhere in the future, and come back to where you started, but no further. Which rather neatly explains why no time travellers from our future have yet visited us -- because the time machine still hasn't been invented!
So where does that leave the paradoxes, and common sense? There is a way out of all the difficulties, but you may not like it. It involves the other great theory of physics in the twentieth century, quantum mechanics, and another favourite idea from science fiction, parallel worlds. These are the "alternative histories", in which, for example, the South won the American Civil War (as in Ward Moore's classic novel Bring the Jubilee), which are envisaged as in some sense lying "alongside" our version of reality.
According to one interpretation of quantum theory (and it has to be said that there are other interpretations), each of these parallel worlds is just as real as our own, and there is an alternative history for every possible outcome of every decision ever made. Alternative histories branch out from decision points, bifurcating endlessly like the branches and twigs of an infinite tree. Bizarre though it sounds, this idea is taken seriously by a handful of scientists (including David Deutsch, of the University of Oxford). And it certainly fixes all the time travel paradoxes.
On this picture, if you go back in time and prevent your own birth it doesn't matter, because by that decision you create a new branch of reality, in which you were never born. When you go forward in time, you move up the new branch and find that you never did exist, in that reality; but since you were still born and built your time machine in the reality next door, there is no paradox.
Hard to believe? Certainly. Counter to common sense? Of course. But the bottom line is that all of this bizarre behaviour is at the very least permitted by the laws of physics, and in some cases is required by those laws. I wonder what Wells would have made of it all.