## Question of the Month: We Hear of Humans Going to Distant Planets in the Future. But if Some Planets Are Light-years Away, How Could An Astronaut Live Long Enough To Get There?

Question of the Month Submitted by: Richard Scott, Whittier, Calif., and answered by: Teviet Creighton, Caltech doctoral student in theoretical physics.

At first, it might seem unlikely. After all, even the nearest star (after the Sun) is over four light-years away — that is, it would take more than four years for light, traveling at 300,000 kilometers per second, to make the trip. Most points of interest are much farther; the center of our galaxy, for instance, is roughly 30,000 light-years away. And as you may know, Einstein's theory of relativity tells us that nothing can accelerate past the speed of light. How could anyone hope to live through a flight lasting over 30,000 years?

Fortunately, the same theory which imposes this universal speed limit also provides a sneaky way of getting around it, at least for the people making the trip. In the theory of relativity, the time measured by a person moving very quickly (near the speed of light) is not the same as the time measured by someone standing still. In particular, if someone (call her Alice) accelerates to very near the speed of light and then decelerates to a stop, the time that she measures for the trip will be less than the time measured by her brother Bert, who was standing still. And the closer she comes to the speed of light (without, of course, ever going faster than light), the bigger the difference in times. This effect is called "time dilation."

Now let us imagine Alice taking a trip to a distant star. Her starship is designed to accelerate at "one gee," which means that the acceleration of the ship pushes her as hard as the force of gravity would on Earth. The farther she goes, the more speed she builds up along the way, and the more time she saves because of time dilation. In this way even the most distant galaxies can be reached within her lifetime! Here are her travel times to a few interesting destinations:

Nearest star (4.3 light-years away) ................... 3.6 years Center of our galaxy (30,000 light-years away) ......... 21 years Andromeda galaxy (2 million light-years away) .......... 29 years Edge of known universe (15 billion light-years away) ... 47 years

If she returns to Earth, the return trip will take an equal amount of time. However, people on Earth will have continued to age at the normal, undilated rate. So while Alice might be able to travel to Andromeda and back in only 60 years (as she sees it), the Earth and its people will have aged 4 million years!