If you place a kaon in an electric field, it stretches slightly, like a balloon rubbed on wool. This response, the electric polarizability, shows how rigidly the particle is built inside. For the charged kaon, it has been measured for the first time, and the number is about one ten-thousandth of the volume occupied by a proton.

The calculation was done on a supercomputer, dividing spacetime into a tiny grid, as if freezing moments to examine each one. In this grid, the kaon is not a point but a cloud of quarks, and its response to the field has two parts. The first is elastic: the particle behaves as a whole, and its size was already known from electron scattering. The second is inelastic: the field makes the internal quarks oscillate like strings, and this part was isolated by subtracting the first from the total response.

The result is still modest in precision: the uncertainty is more than half the value itself. But it proves the method works for particles with a strange quark. In the future, with more computing power and including virtual quark pairs, physicists hope to obtain a number that can be compared with experiments.