One of Albert Einstein's greatest insights was realizing that time is relative. It speeds up or slows down depending on how fast one thing is moving relative to something else. How much does it change? In this feature originally designed for students in 1996, "Captain Ein" and "Major Stein" have volunteered to help you find out. Send Captain Ein on a round-trip journey to a star and then compare her age with Major Stein's on Earth.—Jenny Lisle Instructions Click on Captain Ein or Major Stein to set their starting ages. Click on the spaceship to set its speed. Click one of the twinkling stars to choose a destination and start the journey. When the spaceship lands, compare Captain Ein's and Major Stein's ages. Suggested experiments What happens if: you change the speed of the spaceship?

you send the spaceship at the same speed to two stars—one that is close by, and one that is far away? Also, can you find ways to set the ages, speed, and destination so that Captain Ein and Major Stein are almost the same age when Captain Ein returns from her journey? Why does time change? Ever feel like time moves very quickly and sometimes very slowly? Like how the hours fly by when you're hanging out with a close friend, or how seconds drag on endlessly when you're stuck in traffic on a hot day? But you can't actually speed time up or slow it down—it always flows at the same rate, right? Albert Einstein didn't think so. His idea was that, theoretically, the closer we come to traveling at the speed of light (186,000 miles per second), the more time would appear to slow down for us from the perspective of someone who, in relation to us, was not moving. He called the slowing of time due to motion time dilation. Imagine you're standing on Earth holding a clock. Your friend is in a rocket zooming past you at nearly 186,000 miles per second. Your friend is also holding a clock. If you could see your friend's clock, you'd notice that it seems to be moving a lot more slowly than yours. Your friend, on the other hand, thinks the clock in the rocket is moving just fine, while your clock on the ground seems to be moving very fast. Sound confusing? Well, remember, it took Einstein years to figure this out, and he was pretty smart (see Genius Among Geniuses). The twin paradox Einstein came up with an example to show the effects of time dilation that he called the "twin paradox." It's a lot like the Time Traveler game you just played. Let's try it out with a pair of pretend twins, Al and Bert, both of whom are 10 years old in their highly futuristic universe. Al's parents decide to send him to summer camp in the Alpha-3 star system, which is 25 light-years away (a light-year is the distance light travels in a year). Bert doesn't want to go and stays home on Earth. So Al sets out on his own. Wanting him to get there as quickly as possible, his parents pay extra and send him at 99.99 percent the speed of light. The trip to the star and back takes 50 years. What happens when Al returns? His twin brother is now 60 years old, but Al is only 10 and a half. How can this be? Al was away for 50 years but only aged by half a year. Has Al just discovered the fountain of youth? Not at all. Al's trip into space lasted only a half year for him, but on Earth 50 years passed. Does this mean that Al can live forever? Nope. He may have aged by only half a year in the time it took 50 years to pass on Earth, but he also only lived half a year. And since time can slow down but never goes backwards, there's no way he could grow younger. The ultimate speed limit Einstein's idea about time slowing down sounds fine in theory, but how can you be sure he's right? One way would be to hop in a rocket and travel near the speed of light. Yet everything we know about physics says we can't do that. Why? Did you notice in the Time Traveler game that the rocket could travel very fast, but could never reach 100 percent the speed of light? Well, there's a good reason for this.... According to Einstein's special theory of relativity, objects gain mass as they accelerate to greater and greater speeds. Now, to get an object to move faster, you need to give it some sort of push. An object that has more mass needs a bigger push than an object with less mass. If an object reached the speed of light, it would have an infinite amount of mass and need an infinite amount of push, or acceleration, to keep it moving. No rocket engine, no matter how powerful, could do this. In fact, as far as we know, nothing can exceed the speed of light. An historic flight There are other ways, however, to put his ideas to the test. How do we know Einstein had it right? One experiment in the 1970s provided some pretty strong evidence: Atomic clocks are extremely accurate clocks that can measure tiny amounts of time—billionths of a second. In 1971, scientists used these clocks to test Einstein's ideas. One atomic clock was set up on the ground, while another was sent around the world on a jet traveling at 600 mph. At the start, both clocks showed exactly the same time. What happened when the clock flown around the world returned to the spot where the other clock was? As Einstein had predicted in a general way, the clocks no longer showed the same time—the clock on the jet was behind by a few billionths of a second. Why such a small difference? Well, 600 mph is fast but still just the tiniest fraction of the speed of light. To see any significant differences in time, you'd have to be traveling many millions of miles an hour faster.

