What's going on here? (Twin Paradox)

Ever feel like time moves really quickly or really slowly? Like how the hours fly by when you're with special friends or how seconds drag on endlessly when you're standing in line for a sporting or concert event. You can't really slow time down or speed it up; it always flows at the same rate.

Einstein didn't think so. His idea was that the closer we came to traveling at the speed of light, the more time would slow down for us relative to someone not moving. He called the slowing of time, due to motion, time dilation.

Einstein came up with an example to show the effects of time dilation, which he called "twin paradox." It's a lot like a Time Traveler game. Here is an example, with a pair of pretend twins, Lea and Chantel, both of whom are 20 years old, in their imaginary universe.

Chantel decides that she has had enough of earth and needs a vacation. She has heard great things about rock resorts in the Alpha Quadrant star system, which is 25 light-years away. (A light year is the distance light travels in a year.) Lea, who has a math test next week, must stay at home to study. So Chantel sets out on her own. Wanting to get there as quickly as possible, Chantel decides to travel at 99.99 percent of the speed of light. The trip to the star and back takes just over 50 years. What happens when Chantel returns? Lea is now 70 years old, but Chantel is only 21! How can this be? Chantel was away for 50 years, but she has aged only one year!

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 into a rocket and travel at a speed near the speed of light. Yet, so far, everything we know about physics says that we can't do that.

Did you notice in the story that the rocket could travel very fast, but it never did travel at exactly 100 percent of the speed of light? Well, there is a reason for this. -According to Einstein's special theory of relativity, objects gain mass as they accelerate to greater and greater speeds. 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 does an object of lesser mass. If an object reached the speed of light, it would have an infinite amount of mass and would, therefore, need an infinite amount of push or acceleration, to keep moving. No rocket engine could do this. As a matter of fact, as far as we know, nothing can exceed the speed of light.