Contrary to the old warning that time waits for no one, time slows down when you are on the move. It also slows down more as you move faster, which meansastronauts(宇航员) someday may 1 so long in space that they would return to an Earth of the 2 future. If you could move at the speed of light, your time would stand still. If you could move faster than light, your time would 3 . Although no form of matter yet 4 moves as fast as or faster than light, 5 experiments have already confirmed that accelerated 6 causes a traveler"s time to be stretched. Albert Einstein predicted this in 1905, when he 7 the concept of relative time as part of his Special Theory of Relativity. A search is now under way to confirm the suspected existence of particles of matter that move at a speed greater than light, and therefore, might 8 our passports to the past.Anobsession(沉迷) with time—saving, gaining, wasting, losing, and 9 it—seems to have been a part of humanity for as long as humans have existed. Humanity also has been obsessed with trying to capture the meaning of time. Einstein used a definition of time for 10 , as that which is measured by a clock.Thus, time and time"s relativity are measurable by any hourglass, alarm clock, or an atomic clock that can measure a billionth of a second.