向过去旅行似无可能，但向未来旅行则有可能（Travel into the past is probably impossible. Even if it were possible, Hawking and others have argued that you could never travel back before the moment your time machine was built. But travel to the future? That’s a different story.）
Time travel is a familiar subject of science fiction and fantasy stories, from H.G. Wells to Rod Serling to Hot Tub Time Machine. Some scientists say time travel is theoretically possible, at least if you want to jump ahead to the future, but don't start saving up for a time machine just yet. "Fly to the star Vega, 25 light years away, and back again at 99% of the speed of light, and when you return to Earth in 2062, you will have experienced only seven years' travel time in the spacecraft.
