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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.

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.

Think of it this way: Suppose an editor sitting next to you tossed a tennis ball five meters in the air and caught it one second later. You would both observe the ball moving in a straight line, at an average velocity of five meters per second.

So far, so good, right? Now, let's suppose that, instead of standing still, you're rolling past your editor in an office chair at five meters per second. From your perspective, the ball would not travel straight up and down. Instead, as you cruise by, you would see the ball describing a isosceles triangle. So, to you, the point where your editor tosses the ball is five meters away from the point where he catches it. That means that the tennis ball is traveling a greater distance in one second. (Using the Pythagorean Theorem, we can calculate that you would perceive it traveling at just over 7 meters per second, relative to you.)

But light is different from tennis balls. It moves at the same speed regardless of how fast you're moving. So now, let's say that, instead of tossing a tennis ball, your editor is pointing a laser pointer upward and bouncing the beam off of a distant mirror, where it returns exactly one second later (in reality, this mirror would be about 93,000 miles away, but whatever).

Your editor perceives the light from the laser as traveling in a straight line at the speed of light. And, unlike with the tennis ball, as you roll past your editor you also perceive the light moving at exactly the same speed.

Except that, from your perspective, the light is moving not in a straight line. It's moving in a triangle. The light is traveling a greater distance. But, unlike with the tennis ball, you don't perceive it as moving faster. Distance equals velocity times time. The distance increases. The velocity is fixed. Therefore, time must increase.

In other words, because you're moving relative to your editor, a second lasts longer for you than it does for him. Time, as Einstein demonstrated, is relative.

Which means that age is relative, too.

So when Scott comes back to Earth in March 2016, he will have spent over his career a total of 540 days in low Earth orbit. Mark will have logged 54 days there.

And if we assume – alright, improbably – that Scott and Mark were born at exactly the same time, and if we rule out all other factors (including gravity, which also slows time) that means that Scott is younger than his twin, having spent more time in a fast-moving space station in which time slows down.

How much younger? By our calculations, those 486 additional days that Scott spent moving at about 17,0000 miles per hour slowed his aging by about two one hundredths of a second compared to his twin.

“No regrets growing older. It's better than the alternative. #Thanks for all the birthday wishes (and the cupcakes!),” tweeted Scott, 49, in February. No reason to have regrets, Scott: you’re younger than your birth certificate says.

The deadline for twin-experiment research proposals is 5 p.m. EDT Sept. 17.

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*Using a tennis ball, a bike light, an office chair, and some math, Online News Editor Eoin O'Carroll contributed to this report.*