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A whisper in a roaring waterfall: that's roughly how physicists are hunting for the gravitomagnetic effect, the tiny influence of gravity on the rotation of particles. In highly charged ions, where electrons are stripped away almost completely, this effect should be more noticeable. But the atom itself creates an electric noise that drowns out the signal.

The authors broke this noise down into its components. The first nuisance is the selection rule: to feel gravity, you need an electron with a spin of at least 3/2 (like a top that spins fast enough). But such states are precisely the ones sensitive to the electric field inside the atom, which creates a background in spectroscopy — a method of studying matter through its light. The second nuisance is the interaction of the nucleus with electrons, producing a background 10¹⁸ times stronger than the signal. The third is residual noise after clever tricks. The fourth is nuclear polarization, which depends on the nucleus's shape, not its charge.

To separate the wheat from the chaff, the scientists propose a method called the 'King plot'. They take several isotopes of one element, atoms with the same charge but different weights, and compare shifts in three different transitions. This is like listening for a whisper on three frequencies: the waterfall noise changes predictably, so it can be subtracted. For a chain of molybdenum isotopes, this approach allows, for the first time, setting an upper bound on deviations from general relativity. The parameter χ (chi)—the ratio of gravitational charge to mass — should not differ from unity by more than 10⁸–10⁹ times. This opens the way to laboratory tests of gravity at the quantum level.