The idea that someone who lives on a mountain ages faster than a person who lives on the beach may seem a little preposterous, but it's actually the truth. This concept was first advanced about a century ago by physicist Albert Einstein, whose theory of special relativity postulated that time is not constant, but relative. (That's why they call it relativity.) In 2010, James Chin-Wen Chou and colleagues from the National Institute of Standards and Technology (NIST) conducted an experiment to test Einstein's reasoning. They positioned two atomic clocks about 30 centimeters apart above sea level, and found that the higher of the two clocks ran slightly faster. In real terms, though, the difference wouldn't be noticeable; the mountain dweller would age about 90 billionths of a second faster over a 79-year lifetime, according to Chou [source: Connor].