Most of us think of time as a way to measure things like the length of our days and the span of our lives. But if you had access to a pair of extremely high-precision clocks, you could use time in a different way -- to measure the height of mountains.
This week, scientists described a major step forward in using time to determine height above sea level. For the first time, they took an optical atomic clock out of the lab. Their liberated device was brought into the French Alps.
By comparing the tick rate of the portable atomic clock on a mountain with a similar clock in a lab in Torino, Italy, the researchers were able to show that the altitude difference between the two locations was roughly 1,000 meters, or 3,280 feet.
The work was published in Nature Physics.
"The idea of using portable clocks this way has been in the geophysical literature for a long time," said Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography in San Diego who was not involved in the work. "What these guys managed is to actually do it."
According to Einstein's theory of relativity, time moves differently depending on where you are in a gravity field.
For example, a clock on top of a tall mountain -- far from the center of the Earth -- will move a tiny bit faster than a clock at the base of that mountain, where the gravity is stronger.
It's not a mechanical error. Time itself actually passes faster at the top of the mountain.
That means your friend who lives in the Rockies is aging just a tiny bit faster than your friend who lives on the beach in Malibu.
"Your body and your biological experience exist in the real time of whatever place you are in," said Christian Lisdat, a physicist at Germany's National Metrology Institute who worked on the study. "And that is no different than clocks."