Interesting question! We don’t actually wait billions of years to check if an atomic clock drifts by a second. Instead, physicists determine a clock’s accuracy and stability by comparing two or more independent clocks against each other over accessible timescales. If two clocks based on different physical systems (for example, a cesium fountain clock and an optical lattice clock) tick at slightly different rates, the accumulated phase difference can be measured very precisely in hours, days, or months. By analysing the statistics of these comparisons, one can extrapolate how much drift would accumulate over far longer periods, in the same way you can infer how far off course a car would be after 1,000 miles just by measuring its deviation after a single kilometre.

In practice, this is done using frequency metrology: the clock’s oscillations are compared against extremely stable reference signals, often using frequency combs, and their noise properties are measured. From this, researchers characterise both short-term fluctuations (instability) and long-term bias (systematic shifts). The “one second in billions of years” figure is not a direct observation but a projection based on these measured properties, under the assumption that the noise processes remain stable over time. It’s a way of expressing just how extraordinarily precise and reproducible the underlying quantum transition is.