To mark the passage of time, you could track the height of the sun in the sky or the passing of the seasons. Or, you could measure the vibrations of an atom.

That’s what scientists at the National Institute of Standards and Technology (NIST) in Maryland have done, creating two clocks that each trap 1,000 atoms of the element ytterbium in grids of lasers. These lasers are able to measure the atoms’ vibrations with near-perfect accuracy (there’s roughly one billionth of a billionth chance of error.)

The scientists have measured time in atom vibrations not simply to show off, but because the scientific definition of a second is determined by the frequency of these vibrations. As Katherine Foley wrote in Quartz, scientists in 1967 defined a second as “9,192,631,770 periods of the radiation” of an atom of the isotope cesium-133 atom at temperatures of absolute zero.

The research behind the atomic clocks, published in Nature on Nov. 28, show that not only are the devices highly accurate, they also excel in other measures of clock evaluation: Stability (how much a clock’s frequency changes over time), and reproducibility (how closely the two clocks tick at the same frequency). The scientists behind these clocks have created atom clocks before, but their latest version is even more accurate, thanks in part to thermal and electric shielding, which protects the atoms from external electric fields.

Indeed, they’re so accurate that they show the effect of gravity, as predicted by Einstein’s theory of relativity: The stronger the pull of gravity, the slower the vibrations of atoms and the passing of time. This effectively means that the clocks are showing not simply time, but also their distance from a center of gravity. They’re effectively measuring the space-time continuum.

That means, for example, they could potentially be used to perfectly measure the Earth’s shape, since gravity gets stronger as you get closer to the Earth’s core. The clocks’ sensitivity to gravity also means in theory that they could be used to detect the presence of dark matter. We don’t know what dark matter is, but we know it distorts gravity, and so these clocks could potentially pick up on it.

Currently, these clocks are far too big to transport. But the NIST researchers are building a portable version, which could be used to measure time around the world. Effectively, they’ll be able to show exactly where time passes more slowly, down to the tiniest vibration of atomic time.