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 relativeto something else. How much does it change? In this feature originally designedfor students in 1996, "Captain Ein" and "Major Stein" have volunteered to helpyou find out. Send Captain Ein on a round-trip journey to a star and thencompare her age with Major Stein's on Earth.—Jenny Lisle
Requires the Shockwave plugin
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 isclose by, and one that is far away?
Also, can you find ways to set the ages, speed, and destination so thatCaptain Ein and Major Stein are almost the same age when Captain Ein returnsfrom her journey?
Why does time change?
Ever feel like time moves very quickly and sometimes very slowly? Like how thehours fly by when you're hanging out with a close friend, or how seconds dragon endlessly when you're stuck in traffic on a hot day? But you can't actuallyspeed 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 closerwe come to traveling at the speed of light (186,000 miles per second), themore 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 motiontime dilation.
Imagine you're standing on Earth holding a clock. Your friend is in a rocketzooming past you at nearly 186,000 miles per second. Your friend is also holding aclock. If you could see your friend's clock, you'd notice that it seems to bemoving a lot more slowly than yours. Your friend, on the other hand, thinks theclock 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 Einsteinyears to figure this out, and he was pretty smart (see Genius AmongGeniuses).
The twin paradox
Einstein came up with an example to show the effects of time dilation that hecalled the "twin paradox." It's a lot like the Time Traveler game you justplayed. Let's try it out with a pair of pretend twins, Al and Bert, both ofwhom 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 ayear). Bert doesn't want to go and stays home on Earth. So Al sets out on hisown. Wanting him to get there as quickly as possible, his parents pay extra andsend him at 99.99 percent the speed of light.
The trip to the star and back takes 50 years. What happens when Al returns? Histwin brother is now 60 years old, but Al is only 10 and a half. How can thisbe? Al was away for 50 years but only aged by half a year. Has Al justdiscovered the fountain of youth?
Not at all. Al's trip into space lasted only a half year for him, but on Earth50 years passed. Does this mean that Al can live forever? Nope. He may haveaged by only half a year in the time it took 50 years to pass on Earth, but healso only lived half a year. And since time can slow down but never goesbackwards, 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 yoube sure he's right? One way would be to hop in a rocket and travel near thespeed 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 travelvery 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 theyaccelerate 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 abigger push than an object with less mass. If an object reached the speed oflight, it would have an infinite amount of mass and need an infinite amount ofpush, or acceleration, to keep it moving. No rocket engine, no matter howpowerful, could do this. In fact, as far as we know, nothing can exceed thespeed of light.
An historic flight
There are other ways, however, to put his ideas to the test. How do we knowEinstein had it right? One experiment in the 1970s provided some pretty strongevidence:
Atomic clocks are extremely accurate clocks that can measure tiny amounts oftime—billionths of a second. In 1971, scientists used these clocks totest Einstein's ideas. One atomic clock was set up on the ground, while anotherwas sent around the world on a jet traveling at 600 mph. At the start, bothclocks showed exactly the same time.
What happened when the clock flown around the world returned to the spot wherethe other clock was? As Einstein had predicted in a general way, the clocks nolonger showed the same time—the clock on the jet was behind by a fewbillionths of a second. Why such a small difference? Well, 600 mph is fast butstill just the tiniest fraction of the speed of light. To see any significantdifferences in time, you'd have to be traveling many millions of miles an hourfaster. 
Note: This feature originally appeared, in slightly different form, on NOVA's"Einstein Revealed" Web site, which has been subsumed into the"Einstein's Big Idea" Web site. The "Time Traveler" Shockwave game wasdesigned by ?! Productions (Really Wow).
