The world’s most precise clock sits on a table in Jun Ye’s lab in Boulder, Colorado. A tangle of electronics, fiber-optic cables, and laser beams, the clock is still a prototype, so no one actually uses it to tell time. Ye, a physicist at the research institute JILA, and his team have demonstrated that the clock can produce a second with precision in the parts per quintillion—that’s 10 -19, some hundred billion times more precise than a quartz wristwatch. Put another way, if the clock had started ticking at the Big Bang, by today it would have lost or gained no more than a second. It’s not just the most precise clock in the world—it’s the most precise device in the world.

The heart of the clock is a chamber of around 100,000 strontium atoms that Ye has trapped using lasers. These atoms, when hit with a certain laser, emit red light with a wavelength of exactly 698 nanometers, which corresponds to some 430 trillion cycles of an electromagnetic wave per second. The rate of the oscillation depends on the fundamental structure of the atom, which means Ye’s sheltered strontium atoms tick with exceptional consistency. Compare that to the pendulum of a grandfather clock, which expands and contracts with changes in temperature and humidity to speed up or slow down.

Jun Ye makes the world's most precise clocks using lasers and oscillating strontium atoms. J. Burrus/NIST

In the future, the US government will likely use some iteration of Ye’s clock to set the time across the country, so you can get to your social engagements on time. But that’s probably the least interesting use for this clock. Astrophysicists have their eye on these tools too. They think that this clock’s near-perfectly spaced ticks can help them venture deeper into space.

That’s right: By studying time, they can study space. The concept relies on a postulate in Einstein’s theory of special relativity, which says that light travels at a fixed speed of 299,792,458 meters per second in the vacuum of empty space. If you can precisely measure how long it takes light to travel from point A to point B, you can figure out the distance between A and B. This is actually how GPS works. Satellites pinpoint your location on Earth by precisely measuring how long it takes a radio signal to bounce from your phone back to space. Hence, the word “spacetime”—measuring time is equivalent to measuring spatial distances, and vice versa. A clock doesn’t just count seconds; because the speed of light is predictable, a clock is also a cosmological tape measure.