A new definition of the second will appear in the SI system in 2026-2030.

The current definition of the second associates it with a certain number of transitions (9.192 million) between energy levels that cesium-133 ions make inside atomic clocks when interacting with electromagnetic waves. In recent years, the quality of measuring instruments and the accuracy of atomic clocks have greatly improved. This makes scientists think about the introduction of a new concept of a second, based on other measurement methods. Nine approaches, based on different types of atomic clocks and frequency standards, are currently claiming this role.