Is time travel possible? Paul Davies, author of "How to Build a Time Machine" thinks so. "The short answer is that time travel into the future is not only possible, it's been done, and we've known about it for over a century," he says. He thinks that the closer you travel to the speed of light, the slower time moves for you. For example, if you took off in a fighter jet, flew around for a few hours, and came back, your clock would be billionths of seconds off from the clocks on the ground.