Every atom has its own 'voice'—the frequency of light it emits. For isotopes, atoms with different numbers of neutrons, the voices differ slightly. It's like a choir: if you know the score precisely, the slightest false note reveals external interference. The King plot is a way to compare how harmoniously two notes from different isotopes sing together. This analysis is a powerful spectroscopic test beyond the Standard Model, capable of detecting even dark matter particles.
Working with ytterbium, physicists found a 'magic' wavelength that works like an ideal microphone: it adds no noise and lets you hear the finest details. After measuring four isotope pairs, they constructed a three-dimensional King plot. The curvature, previously taken as a hint of new physics, turned out to be due to inaccurate reference data on nuclei. The measurement precision was so high that if GPS worked on this principle, it would pinpoint coordinates with an error of a fraction of a millimeter. No new force has been found, but the method has been purified — now it's an ultra-reliable detector of the unknown.
The frequency of atomic transitions in ytterbium has been measured to 18 decimal places. If GPS used such clocks, it would determine your position with millimeter accuracy.