Scientists have made the most accurate comparison of atomic clocks to date, finding agreement to the 18th decimal place on three atomic clocks in Colorado (Nature 2021, DOI: 10.1038/s41586-021-03253-4). The demonstration could lead to a new definition of the second, and the researchers say it will enable new tests of relativity theory, dark matter, and even topography. Atomic clocks rely on measurements of excitation energy. In the International System of Units, the second is based on cesium-133 clocks, but newer clocks, like the ones used in this research, can be 100 times as accurate. Creating a new standard of time or probing fundamental physics depends on verifying agreement between multiple clocks, however. Using fiber optics and a new, through-the-air laser system, researchers at the National Institute of Standards and Technology and the University of Colorado Boulder and colleagues compared three alternative clocks based on aluminum, ytterbium, and strontium and found that they agreed to within 8 parts per quintillion (8 × 10–18) or better. They’ll need to improve to 5 × 10–18 to redefine the second, but the researchers say they’re laying the groundwork for comparisons across the globe or to space that could better constrain fundamental physical constants, more accurately measure altitudes, and more.