From 'The Time Machine' by HG Wells
Of all the classic subjects of science fiction, perhaps the most out of reach - and therefore the most stimulating for authors, readers and scientists alike - is time travel.
The most prominent opponent of time travel ideas has been Stephen Hawking. But now Professor Hawking has started to concede the possibility, in his foreword to a new book on the science of Star Trek.
Hawking makes the point that there is a "two-way trade between science fiction and science". This has worked in the past: Verne and Wells inspired early researchers into space flight, such as Tsiolkovsky, Oberth and Ley, which led directly to Wernher von Braun's Apollo triumph in the 1960s.
But could this be true of time travel? A century after publication of The Time Machine, what does modern science fiction - and science - tell us of the possibilities of time travel? And what get-out clause in the laws of physics has made Hawking change his mind?
After Wells, sci-fi writers fell on time travel ideas, fans explored past and future, and developed new speculations about time. Ray Bradbury's A Sound of Thunder (1952), with its dinosaur hunters inadvertently killing a butterfly, memorably dramatised history-changing. In Behold the Man (1969), Michael Moorcock's time tourist finds himself taking Jesus's place on the cross.
Perhaps the most famous time traveller of all is our own, beloved, Dr Who. The best of the Who tales dealt with the theoretical and moral implications of time travel. But is there any way to build a Tardis?
Modern physicists have dreamt up several ways for time machines to work. Following the work of the American physicists Frank Tipler, Kip Thorne and others, most of these paper travellers journey in spacecraft around great loops through space and time.
The loops are closed - the travellers finish up at their starting point in space and in time - by exploiting the space-bending possibilities of Einstein's relativity theory.
Thorne will have us travel through fixed wormhole time tunnels, for example, while Tipler urges us to fly around massive, rapidly rotating cylinders. A Kip Thorne time tunnelrequiring huge outer-space construction projects, doesn't match up to our archetypal dream of time cars.
In 1949 Kurt Godel - who had already, at 25, proven the incompleteness of mathematics - described a rotating universe so distorted by its own spin that it contained paths looping into the past. In such a universe a Tardis could be built.
And, oddly, Wells himself, writing 50 years earlier, described "spinning" as an aspect of time travel: "I seemed to see a ghostly, indistinct figure sitting in a whirling mass of black and brass" (The Time Machine). Unfortunately, there is no evidence that our own universe is rotating.
The idea of time-paradoxes developed in SF soon after Wells: if I go back in time and shoot my grandmother, I will not be born, and therefore could not go back and ...
The acme of time-paradox stories is Robert Heinlein's All You Zombies (1959), in which, thanks to a time machine and a sex change, a person acts as his/her own father and mother. In Isaac Asimov's The End of Eternity (1955), time police protect a "pure" version of history.
And now, as the physicists devise almost-plausible time machines, they, too, must wrestle with the possibility of granny-strangling causality loops. This has been Hawking's main sticking point, in fact.
But the small print of the laws of physics does contain loopholes. Quantum physics allows the possibility of multiple universes: the idea is that, rather than creating a paradox within an individual history, a new world- line is created each time history is adjusted. Thorne and his co-workers have documented a remarkable series of thought experiments involving billiard balls colliding with themselves after passing through time-spanning wormholes.
We are a long way away from developing a viable prototype time machine. Still, any good SF author knows that anything not outlawed by the laws of physics is only a matter of engineering. And in the parallel development of fictional and scientific time travel ideas, we can see that the feedback loops between science and SF are still working. It would be wonderful to suppose that, among the readers of The Time Machine, or Gribbin or Hawking - or even my own books - there will be somewhere the Tsiolkovsky of time travel ... Or even the von Braun.
The writer is author of 'The Time Ships', HarperCollins, pounds 4.99.
Miles Kington is on holiday.