In his new book How to Build a Time Machine, Brian Clegg takes a "pop science" look at time travel, explaining quantum entanglement and superluminal speeds in terms that even a technophobe could understand. We asked Clegg about his book, some of his favorite time travel stories, and the most important scientific discovery of his lifetime.

You have a quote from H.G. Wells’s The Time Machine at the beginning of your book. What are some of your favorite time-traveling books?
Oddly enough, not particularly The Time Machine itself. Although the way Wells uses the idea of time being a fourth dimension is very effective, the actual story doesn’t compare with some of his other novels like First Men in the Moon and War of the Worlds. One of my all time favorites is a short story by Robert Heinlein called “All You Zombies.” Heinlein sets up a wonderful time paradox, where the main character, who has had a sex change, goes back in time to impregnate his younger, female self. The resultant child is then moved back through time to become the mother. The character has literally come from nowhere. Perhaps my favorite novel with a time travel theme is Joe Haldeman’s Forever War, originally envisaged as a counter to Heinlein’s gung-ho Starship Troopers. Because the protagonists in Forever War are always taking long journeys at near the speed of light, they travel far into the future. By the time they return home everyone they once knew is dead, the world is not the one they remember – so there is nothing for it but to sign up for another tour.

Although not a book, I’m also very fond of the time-traveling Star Trek IV movie, bringing the original Star Trek cast to the 1980s. And on TV, although rarely making much use of time travel paradoxes, I still enjoy Dr. Who, which is a show that has been around since I was very young.

Can you explain, in the most basic terms possible, how time travel works? Or is that impossible to describe?
It’s not impossible, though it’s easier to do in a book than a couple of paragraphs! Real time travel makes use of Einstein’s relativity. There are two parts to this. Special relativity tells us that time slows down for anything moving. If, for example, you fly off at high speed in a spaceship, your time goes slower than it does on Earth. So you will return to Earth to find that you have traveled into the future. This happens at any speed. A frequent flyer crossing the Atlantic once a week for 40 years will shift 1/1000th of a second into the future. But to make a significant trip you need to get near to the speed of light. The faster you go, and the longer you travel at that speed, the further you move into the future.

Unfortunately, special relativity doesn’t have a way to get back. You are stuck in the future. But the other part of Einstein’s theory, general relativity, can help out. This describes how gravity works, saying that anything with mass will warp space and time. If the warp in time can be made sufficiently large, it’s possible to travel backwards or forwards in time using general relativity. But doing this is vastly more complex than using special relativity. One approach would be to get hold of around 10 neutron stars—stars that have collapsed so all the space inside their atoms has disappeared—and form them into a cylinder and spin this at a high speed. Oh, and you’d need antigravity to stop gravity pulling the cylinder into a sphere and it collapsing as a black hole. Not exactly everyday engineering. There is a little hope, though. An American physicist, Ronald Mallett, believes this approach can be used on the desktop with a spiraling laser light instead of a neutron star cylinder, but the experiment is yet to be tested.

What, to you, has been the most significant scientific discovery in your lifetime? Do you think it'll be surpassed in your lifetime?
If you were to say the last 100 years it would be easy—relativity and quantum theory. In my lifetime, I think the really amazing discoveries have been in cosmology—the evidence for the Big Bang, for example, plus the remarkable discovery that the acceleration of the universe is expanding. The great thing about science, though, is that there is so much more to discover. I’m absolutely sure there will many more amazing breakthroughs this century, and I hope that I’m around to see many of them.

Do you think we're getting closer to making time travel a reality?
On a small scale it’s real today. If we didn’t make corrections for the fact that the GPS satellites are shifting slightly in time with respect to the Earth, the locations they give to your satellite navigation would be out by a couple of miles in just a day. We can certainly go significantly further using special relativity if we can get fast enough spaceships. That’s quite a challenge though. To get a really good journey through time you need to get up to around 99 percent of the speed of light. The fastest humans to date were on Apollo 10, travelling at just 0.0037 of light speed. Most of the ideas for using general relativity are way beyond our current technology. We’re talking thousands of years away. The only hope in our lifetime is probably Ronald Mallett’s idea. But for me the amazing thing is that it’s only a matter of getting the technology right. There’s nothing in physics that prevents time travel.