Time travel is clearly a trickier proposition than space travel, though. Put simply, Einstein's idea was that every object in the universe has it's own “time”, and these vary as objects move. The faster an object moves, the slower its time is, compared to the time of a slower moving object. The extreme situation would be if an object could move as fast as the speed of light, its time would be completely halted. But whatever an object——or person's——time is, it's only evident in contrast to other objects. In other words, it's all relatives.

　　与太空旅行相比，时间旅行很显然是一个更为复杂的命题。简而言之，爱因斯坦的观点就是，宇宙中每一物体都有其自身的“时间”，并随其运动的不同而不同。物体运动越快，其时间越慢(与运动较慢的物体相比而言)。最极端的假设是，如果一个物体运动达到光速，那么其时间就会完全停止。但无论一个物体或者一个人的时间为何，它只能参照其它物体才能得以体现。换句话说，万物皆相对。