填空题

Can time travel really be done Physicists think that travel into future is possible. Einstein’s special theory of (26) , published in 1905, predicted that time should be elastic (弹性的), (27) as an observer moves. To get a really big time leap it is necessary to travel at near the speed of light—300,000 km per second. At 99% of this speed, (28) a distant star and back would take 15 months, but travelers would return home to find that nearly nine years had passed on Earth. (29) , you would have leapt several years into Earth’s future.
Gravity offers another way to slow time. On the Earth’s (30) , clocks tick a little slower than on the moon, for example. Near a black hole, gravity is so (31) that time is slowed to a crawl relative to us. These facts are accepted by almost all scientists. Traveling forwards in time has been (32) convincingly in experiments, but the possibility of traveling backwards in time is far more (33) .
Worm holes are similar to black holes, but there exists a key difference. Whereas black holes offer a one-way journey to nowhere—fall in and you never get out—worm holes have (34) . Scientists at California Institute of Technology found that if you tried to make a worm hole out of any normal form of matter, it would (35) under its own gravity and turn into a black hole. For a worm hole to remain stable, it would have to be made of exotic material that would create an anti-gravity force.