An atomic gravimeter works like a very precise scale for gravity: it tosses up a cloud of atoms and watches how fast they fall. But these instruments have a hidden flaw — the shape of the signal they produce is slightly skewed. This skew, like a ruler with a chipped edge, subtly shifts every measurement.

In the new work, this shift was observed live for the first time on a short-baseline atomic interferometer gravimeter. It turns out that it doesn't grow smoothly but very sharply: if you halve the free-fall time of the atoms, the shift grows eightfold, like a cube whose side is halved but whose volume drops to an eighth. At times of just a few milliseconds, the error in measuring the gravitational acceleration g reaches 0.1–1 mGal.

The result matched exactly the calculations the authors had done earlier. For compact atomic gravimeters, where the flight time of atoms is inevitably short, this shift can no longer be ignored — otherwise, the map of the underground world will have a subtle but systematic curvature.