Hidden inside the scandium-45 nucleus is a nearly perfect pendulum. Its oscillations — the emission of X-rays — are so steady that it ticks $10^{19}$ times in half a second without ever missing a beat. That’s billions of times more stable than the best atomic clocks.

Scientists 'listened' to this pendulum by exciting the nuclei with an X-ray laser and using spectroscopy and light measurement. However, in a solid crystal, neighboring atoms jiggle like a wobbly table under a pendulum. This disorder from vibrations blurs the signal at least 500-fold, muddying the clock’s ideal rhythm. Even under these conditions, scandium-45 remains a record-breaker.

A nuclear clock based on it would notice a shift in time if raised by just one centimeter — gravity’s hand, just as Einstein predicted. This means we could not only improve navigation but also test whether the laws of physics have changed over billions of years.