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Wormholes are a marginal and very speculative idea, but from what we understand of the nature of gravity when combined with quantum physics, it looks like yes, in principle, such an entity would be possible. As a practical matter, however, I have to say that it would be a very expensive proposition. To make one, probably you would need to capture something like a black hole, and then adapt its interior to create a wormhole. We're talking about cosmic-scale engineering here; I don't think any of my professional colleagues regard this as terribly credible. But that's not the issue. The point is that if it is in principle possible for a wormhole to exist, if it could either be engineered or delivered to us ready-made by Mother Nature, then it opens up the possibility of paradoxical time loops.

By providing an insight into the nature of reality, and the nature of the physical universe, this whole area is really fascinating. I've thought a lot about it over the years, and I'm still undecided as to whether nature could never permit such a crazy thing, or whether wormholes, or some other type of gravitational system, might be possible so that in principle one could visit the past. If so, we must find some way of avoiding the paradoxes, maybe by giving up freewill. In daily life we imagine that we are free to do most of what we want, but if you find yourself in a causal loop, you might discover that you just can't do anything that is going to change the world in a manner that is inconsistent with the future you've come from.

There's a famous story, I think originating with Richard Feynman, about the time traveler who goes back in time and, in an adaptation of the grandmother-killing scenario, decides to shoot his younger self to see what would happen. He takes a rifle with him, seeks out his younger self and raises the rifle to shoot through the heart. But his aim isn't very good, it's a little bit wobbly, so he hits his younger self in the shoulder instead, merely wounding him. The reason his aim isn't so good is because he's got this shoulder wound from an earlier shooting incident! So you see, it's possible to conceive of temporal loops of that sort without encountering a paradox.

If you look at the way science fiction writers deal with this — well, most of them just fudge the whole issue. Then some of them have the time traveler go back in time, and change the past ­ stepping on a beetle perhaps, or shooting Adolf Hitler — and then when they return to their own time, they find everything has changed. Well that's simply inconsistent if there is only one world, one reality. That's no way out at all. It may make a good story but it doesn't make sense. So this is a subject that goes right to the heart of physics, and right to the heart of the nature of reality. I think it's a terrific topic.

EDGE: I am aware that the work of physicists influence science fiction writers, but is it a two-way street?

DAVIES: Oh yes, there's no doubt about that. For a start, a lot of young people get into doing science through reading science fiction. I remember a postdoc colleague of mine who reckoned he got into physics from reading "Superman" comics. 'I owe a great debt of gratitude to that guy,' he once remarked. If I think of my own scientific development, I read a lot of H.G. Wells in my teens — War of the Worlds, The Time Machine, plus a number of his books on social and political issues — so they certainly had an influence on me. I also read most of John Wyndham's books— this was in the 50s and early 60s. It's a bit hard to say whether the science fiction turned me on to the science, or whether I was already interested in the science and naturally gravitated to science fiction. I was never a great fan of Isaac Asimov, but a lot of my scientist friends have been. I prefer Arthur C. Clarke. These writers are definitely inspirational. If you think back to the 60s — for most people that was an era of rebellion, drugs, Vietnam War protests and so on. But for me the influences of the 60s were less John Lennon, more Arthur C. Clarke. Stanley Kubrick's movie 2001 A Space Odyssey came out in the late 60s when I was a PhD student in London, and I found it wonderfully confident and inspiring, a great antidote to the pessimistic dropout culture of the times.