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Twin Paradox a Paradox in Low-Earth Orbit

Relativity enthusiasts will be excited to learn that in a few months, twin brothers will meet in space for the very first time! But who will age more, the brother spending six months in orbit, or the brother on the quick shuttle hop to the International Space Station?


In March 2011, if all goes as planned, two twin brothers will meet in space for the first time ever. On Feb. 27, astronaut Mark Kelly (the one with the mustache) will launch aboard NASA’s space shuttle Endeavour to meet up with his twin brother Scott who's currently flying aboard the International Space Station (ISS). Scott made the trip up to the orbiting laboratory on a Russian Soyuz rocket launched on Oct. 7 and will be on board for the next sixth months.

The event evokes Einstein's theoretical experiment called the "twin paradox," involving two twin siblings. In the experiment, one twin travels aboard a spaceship cruising near the speed of light to a star several light-years away, while the second twin stays on Earth. Years later, the star-voyaging twin returns to find his brother has aged many years while he is still rather young. But will we see similar behavior among the Kelly brothers come March?

To answer the question we have to consider Einstein’s two theories of relativity: General relativity and special relativity. General relativity tells us that the greater the amount of gravity the slower the movement of time. Indeed, recent research has proven this to be true, showing that a man on a mountaintop ages faster than a man at sea level. So, one could argue that a man in low-earth orbit, about 200 miles above the Earth’s surface, would age faster than a man on the ground.

However, special relativity tells us that time slows down for an object that is moving, giving us the twin paradox. Even though the ISS travels at a slow 17,000 mph – slow relative to the speed of light, or 671 million mph, where the theory of special relativity really kicks in – its speed is still be enough to cause the on-board clocks to tick a little slower than those back on Earth and the astronaut could therefore age more slowly.

It seems that in considering Einstein’s twin paradox, we find our own low-earth orbit based paradox. Which theory of relativity will have a greater effect on the twin in space, gravity-based general relativity or motion-based special relativity? Will Scott in the ISS age faster or slower than his Earth-bound twin brother Mark? Tune in tomorrow for more!

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