In a basement lab at the University of Colorado Boulder, researchers have built the world's most accurate atomic clock — it can keep perfect time for 5 billion years. But the clock is so precise that it's run into a problem. Time doesn't move at the same rate everywhere in the cosmos, or even on the surface of Earth.

The clock is a mess to look at, spread out on a large table amid a tangle of cables wrapped in tinfoil and held together with binder clips. At the heart of the device are strontium atoms, suspended in laser beams, which vibrate at a superfast frequency.

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As NPR reports, though, the clock is so sensitive that it presents researchers with an unprecedented problem:

Right now, on the top of Mount Everest, time is passing just a little bit faster than it is in Death Valley. That's because speed at which time passes depends on the strength of gravity. Einstein himself discovered this dependence as part of his theory of relativity, and it is a very real effect.

The relative nature of time isn't just something seen in the extreme. If you take a clock off the floor, and hang it on the wall, says scientists Jun Ye, "the time will speed up by about one part in 10^16."

That is a sliver of a second. But this isn't some effect of gravity on the clock's machinery. Time itself is flowing more quickly on the wall than on the floor. These differences didn't really matter until now. But this new clock is so sensitive, little changes in height throw it way off. Lift it just a couple of centimeters, Ye says, "and you will start to see that difference."

That's a problem, because to actually use time, you need different clocks to agree on the time. Think about it: If I say, "let's meet at 3:30," we use our watches. But imagine a world in which your watch starts to tick faster, because you're working on the floor above me. Your 3:30 happens earlier than mine, and we miss our appointment.

This clock works like that. Tiny shifts in the earth's crust can throw it off, even when it's sitting still. Even if two of them are synchronized, their different rates of ticking mean they will soon be out of synch. They will never agree.

The world's current time is coordinated between atomic clocks all over the planet. But that can't happen with the new one.

"At this level, maintaining absolute time scale on earth is in fact turning into nightmare," Ye says.

But Tom O'Brien, who oversees the U.S. master clock at the National Institute of Standards and Technology, believes this newest generation of timepieces could be "made into exquisite devices for sensing a whole bunch of different things." Their sensitivity to gravity, he says, could help scientists map the interior of our planet or detect gravitational waves from black holes.