Unit 95 Scientists Look Forward to the Past Can time travel really be done? Physicists think that travel into future is possible. Einstein's special theory of relativity, published in 1905, predicted that time should be elastic, stretching or shrinking 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, a rocket trip to a distant star and back would take 15 months, but travelers would return home to find that nearly nine years had elapsed on Earth. In effect, you would have leapt several years into Earth's future. Gravity offers another way to slow time. On the earth's surface, clocks tick a little slower than on the moon, for example. Near a neutron star or black hole, gravity is so intense that time is slowed to a crawl relative to us. These facts are accepted by almost all scientists. Traveling forwards in time has been demonstrated convincingly in experiments. But the possibility f traveling backwards in time is far more controversial. The first hint that it might be possible came in 1932, when a physicist named Stockhum investigated what might happen to an observer who orbits a rapidly spinning cylinder. He showed it was possible to travel in a closed loop and return to your starting point before you left. Worm holes are like black holes, with a key difference. Whereas black holes offer a one-way journey to nowhere -- fall in and you never get out -- worm holes have an exit as well as an entrance. To find whether such an idea can be taken seriously, scientists at California Institute of Technology investigated what it would take for such a short cut though space to exist. They found that if you tried to make a worm hole out of any normal form of matter, it would collapse under its own gravity and turn into a black hole. For a worm hole remain stable, it would have to be made of exotic material that would create an anti-gravity force. Physicists know of peculiar states of matter that generate anti-gravity, and a worm hole is not physically impossible. It dawned on them that such a structure could be adapted to make a time machine that would allow an astronaut to leap instantaneously into both the past and the future. Go through the worm hole one way, and you reach the future. Go through the other way and you come out in the past. Making a worm hole presents formidable engineering challenges, but suppose it could be done, and time travel became a reality? Thorny paradoxes loom. What happens to the time traveler who goes back and murders his mother as a girl? If so, who murdered the mother? Does that mean he was never born? Because the present is linked to the past, you cannot change the past without unleashing causal disorder. Since the purpose of science is to give a rational account of reality, any theory that permits paradoxical consequences is suspect. Does this mean Einstein's theory of relativity is wrong, or that worm holes could never form? Although theoretical investigations of time travel are a popular topic among physicists, there is no consensus on how to handle the ensuing paradoxes. But one thing is agreed. Once a time machine is made, you could visit the year 2100, check out the stock prices, and then pop back and make the right investments to repay the loan.