Clocks Run Faster At Altitude

It may sound like science fiction, but the rate at which time passes is not universal. Albert Einstein’s theory of general relativity revealed that gravity can warp the very fabric of reality, or spacetime. The stronger the gravitational pull, the more it slows down the passage of time. Consequently, in a location with weaker gravity, such as on a mountaintop or even on the top floor of a skyscraper, time literally moves a fraction of a second faster than it does at sea level. This effect is known as gravitational time dilation.

While this concept was once purely theoretical, the development of ultra-precise atomic clocks allowed scientists to prove it. In a famous 1971 experiment, scientists flew atomic clocks on commercial jets around the world and found their clocks disagreed with a reference clock on the ground, just as Einstein's equations predicted. The effect is imperceptible to humans, but it has a profound impact on modern technology.

This principle is essential for the function of the Global Positioning System (GPS). Satellites orbit far above the Earth where gravity is weaker, causing their onboard atomic clocks to run faster than clocks on the ground by about 38 microseconds every day. If engineers didn't constantly correct for this relativistic effect, the entire GPS network (Review) would fail, accumulating navigational errors of several kilometers within a single day.