Why one of NASA's twin astronauts is younger than the other

NASA will conduct an experiment using its twin astronauts to assess the effects of space travel on humans. Here's why one of the twins is older than the other.

By Elizabeth Barber,?Contributor / August 7, 2013

US astronaut Scott Kelly prepares for launch to the ISS in October 2010. He will return to the station in March 2015, where he will be the subject of a NASA experiment assessing the effects of space travel on humans.

Shamil Zhumatov/Reuters

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Beginning in March 2015, NASA astronaut Scott Kelly will spend one year at the International Space Station. His twin brother, a former astronaut, will spend that time at home in Arizona.

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So, at the twin?s request, NASA plans to use the opportunity to measure the effects of space flight on Scott?s body, using his twin?s earthbound body as a baseline for assessments. Last month, the organization opened a public call for research proposals under the topic?"Differential Effects on Homozygous Twin Astronauts Associated with Differences in Exposure to Spaceflight Factors."

NASA?s experiment ? more public relations than ground-breaking science, as Scott has already logged significantly more time in space than Mark and, technically, the two could already be test subjects without one of them going back to space ? will likely show that Scott is biologically "older" than Mark, given the toll that spaceflight is believed to take on astronaut?s bodies.

But even if space travel has made Scott biologically older than Mark, it has also make him in a different sense younger ? thanks to special relativity.?

In 1911, the French physicist Paul Langevin put forward a thought experiment: What if one twin flies away from Earth at 99.99 percent of the speed of light? When the twin returns two years later, he expects that his twin, like himself, is two years older. But his twin isn?t there anymore ? in the traveler?s absence, 200 years have passed on Earth, and his Earth-bound twin is long dead.

Langevin called it a paradox, but in fact it?s not so paradoxical.

In 1905, Albert Einstein upended the notion that time is fixed and absolute. According to his Special Theory of Relatively, time is relative. It?s the speed of light that?s fixed.

To unpack that, suppose you are on a train going 100 miles per hour, and then another train passes on the adjacent track at 101 miles per hour. To you, the adjacent train looks as if it?s going at just one mile per hour, and its possible to lean out your window and hand a cup of tea to a passenger on another train. That?s classical relatively, as Galileo first described it (minus the train and the tea) in 1632.

But with light, it?s different ? now we?re dealing with special relativity. No matter how fast you go, or don?t go, the speed of light does not change: Experiments have repeatedly demonstrated that light always travels at 299,792,458 meters per second in a vacuum, no matter what. Even if the light is emanating from a flashlight that's moving a million miles an hour (relative to you) it appears to you to be moving at the same speed as the light from a stationary flashlight.

Source: http://www.csmonitor.com/Science/2013/0807/Why-one-of-NASA-s-twin-astronauts-is-younger-than-the-other

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