Physicists in Singapore have built the most accurate clock ever made, and it could change how the world measures time. Here is what happened.

Researchers at the National University of Singapore built an optical atomic clock from a single lutetium ion. They measured its frequency to 19 decimal places.

The team reported an uncertainty of one part in 10^19, the lowest for any optical clock, in the journal Nature on September 23. Two independently built clocks ran side by side for 200 hours and agreed to within 5.7 parts in 10^19.

The design is unusually resistant to changes in temperature and magnetic fields, which are the usual enemies of precision.

Such clocks could help redefine the second. The current definition is based on caesium atoms, and metrology institutes are comparing optical designs to choose a replacement.

Optical clocks are already used to test Einstein's relativity in laboratories.