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
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).