The second is one of the most precisely defined quantities in science, but metrologists are preparing to redefine it. An international comparison of seven optical atomic clocks across four European countries has shown that these next-generation timekeepers can agree with one another at the extraordinary level needed for that transition.
Today's SI second is defined using the microwave transition of cesium-133 atoms. Optical clocks work at much higher frequencies, using transitions in atoms or ions that oscillate hundreds of thousands of times faster. That higher frequency gives scientists more ticks to measure and, in principle, allows much greater precision. The best optical clocks can already outperform primary cesium standards in laboratory conditions.
The challenge is no longer simply building a good clock. A global definition requires laboratories using different atoms, designs and experimental systems to compare their frequencies reproducibly. In the European experiment, seven high-precision clocks in Italy, France, Germany and the United Kingdom were linked through an optical-fibre network. Researchers measured frequency ratios between the clocks and found agreement at uncertainties reaching the 10^-18 range.
At that level, relativity becomes part of routine timekeeping. A clock at a slightly different height experiences a different gravitational potential and therefore ticks at a measurably different rate. Comparisons must account for elevation and the local gravitational field with extreme care. That sensitivity is a complication for defining time, but it is also useful: optical clocks can function as probes of gravitational potential and may one day contribute to geodesy.
A redefinition of the second will not make everyday clocks suddenly look different. Phones, computers and navigation systems would continue to count seconds in the familiar way. The change would occur underneath the infrastructure, giving national metrology institutes a more accurate reference from which other timing systems can be calibrated.
Before an optical definition can replace cesium, the international community needs confidence that different laboratories can reproduce the same frequency and maintain reliable comparisons over long distances. Researchers must also choose how the definition will be realized: through one atomic species, several approved transitions or a framework that allows multiple optical standards to contribute.
The European network demonstrates the essential idea. Optical clocks are no longer isolated experimental champions whose precision can be admired only within a single laboratory. They can be connected, compared and made to agree across national borders. That is the infrastructure a new definition of the second will require, and each successful comparison brings the change closer to practical reality.