Remarkably, Wells's story was written about ten years before the publication of Einstein's special theory of relativity was published. Special relativity showed that time is elastic, flexible. It isn't simply there — the same for everybody, as Newton supposed. There's your time and my time, and they can differ depending on how we move. If I jump in a rocket ship and head off at nearly the speed of light to a nearby star and come back again ten earth years later, I may have aged only, say, one year. This is called the twins effect, because if I left my twin brother at home, when I returned we would no longer be the same age. He would be ten years older, and I only one year older. In effect, I will have time-travelled nine years into his future. Bizarre though this time-stretching effect seems, we know it's true. In fact, you can even measure it using the motion of aircraft.

If I fly to London from New York, for example, then I will lose a few billionths of a second relative to you, staying here on the ground. That's a measurable effect, using atomic clocks. It has been tested. So we know that time travel is possible, but I'm talking here about travel into the future. It's easy; it's been done. You just have to move fast enough to get a significant effect. Since in daily life our speeds are much less than that of light, we don't notice anything weird going on with time. But the effect is definitely real.

Travel into the past is much more problematic, though. The significant thing is, our best understanding of the nature of time, which comes from Einstein's general theory of relativity, leaves open the possibility of travel into the past. It doesn't say you can't do it, there's no known law within the theory of relativity to forbid it. But finding a plausible scenario to actually travel into the past is not an easy thing.

The first person to come up with a proposal was Kurt Gödel, the Austrian-born logician and mathematician, who worked at Princeton's Institute for Advanced Study alongside Einstein in the 1940s. Gödel discovered that if the universe were rotating it would then be possible for an object to travel in a certain closed loop in space and come back to its starting point before it left! In other words a person could travel around a loop in space — and discover that it is also a loop in time. It has to be said that Gödel's scenario is highly unrealistic; there is good evidence that the universe as a whole is not rotating, but the very fact that the general theory of relativity does not forbid travel into the past is deeply unsettling. It certainly unsettled Einstein. The main reason concerns the causal paradoxes it unleashes. For example, imagine visiting the past by going on a journey through space and returning yesterday, and then, assuming you still had freewill, doing something yesterday that would prevent you from leaving in the first place (for example, blowing up the time machine). If you never left, then you wouldn't have travelled back in time to make the change. But if you didn't make the change, nothing would prevent you from embarking on the journey. Either way, you get contradictory nonsense. Because science is rational, it must always yield a consistent picture of reality, so these sort of causal paradoxes strike at the very heart of the scientific understanding of nature.