Gravity is like a plumb line: Galileo and Einstein showed that all bodies fall equally — the principle of equivalence in gravity. But tests always removed charge — as a possible interference. Physicists have now checked whether charge itself changes the "plumb line". Even with a huge charge — a coulomb per kilogram — the acceleration of falling changes by only 0.02%. This is like the plumb line's thread deviating by a hair's breadth over a meter. Instruments don't notice.

Why is this important? Many theories, from dark matter to strings (Witten) and quantum field theory, predict a tiny connection. Even in curved spacetime, κ might show up. Now charge becomes a probe: in atomic interferometers, deviations will be sought, testing dark energy, dark photons, and the fall of antimatter.