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Strike a diamond and it rings, but sound lives inside it even without a strike. Waves of compression and rarefaction constantly ripple through it, like the ripples a stone makes on water. Except this ripple is the thermal jitter of the atoms themselves, and it can be detected with a laser.

The authors aimed a green laser at a diamond and watched how light scattered off these invisible waves. This method, a sort of microphone for solids, let them distinguish three types of surface sound waves. One is like ocean swell that bobs a float up and down (a Rayleigh wave). Another is like a string vibrating side to side (a shear horizontal wave). The third is a quick push along the surface, like the fold in a carpet being beaten with a stick (a pseudo-longitudinal wave). Measurements were taken at temperatures from 10 to 300 kelvins, spanning from the cold of space to room warmth.

The big surprise was how indifferent sound is to temperature. The frequencies of all three waves barely changed, and their speeds matched theory. This stability is a boon for engineers: if the acoustic track in a diamond chip doesn't drift with heat, you can build ultra-precise sensors on it. For example, quantum sensors based on diamond defects that can detect magnetic fields a thousand times weaker than Earth's. Or surface acoustic wave devices — the kind that filter signals in every smartphone.

Knowing how sound travels across a diamond facet in the cold also matters for thermal calculations. When two materials touch, heat is transferred precisely by such surface waves. An accurate model of their behavior could help cool processors better or, conversely, trap heat where it's needed. So a laser 'microphone' turns a diamond from a gemstone into a blueprint for future technology.