Diamond crystals sometimes harbor 'guests'—single atoms of other elements that take the place of carbon. One such guest, a tin atom, emits light of a strictly defined color when irradiated by a laser. This color changes if the diamond is squeezed. Physicists calculated exactly how much.

It turns out that the color shift is the same for two states of the defect — with an extra electron and without it. Both shift by about 1.4 nanometers for each gigapascal of pressure increase. It's as if a clock's hand always moved at the same speed, regardless of whether the clock is wound or not.

The accuracy of the calculations was verified by comparing them with real measurements. The agreement is good, meaning such a defect could be used as a miniature pressure sensor — for instance, inside living cells or deep within the Earth.