A translator who once caught only fragments of sentences can now hear the entire conversation. In much the same way, physicists have long been unable to calculate how two particles respond to a flash of light when they don't form a stable nucleus. In the new work, they finally managed to carry the calculation through to the end and compare the response with the exact charge conservation rule.

Two pions flying apart after being struck by a photon are like two dancers moving apart after a joint leap. Previously, scientists could only see individual frames of this scene: the energies at which the pair exists, but not how it responds to an external push. Now a method in which space and time are replaced by a grid of points has allowed them to connect the frames into a continuous recording.

The result was compared with a rule that follows from the law of conservation of electric charge. If the calculation had been wrong, the answer would have diverged from this rule, like a translation in which half the lines are lost. But the agreement was perfect. Now physicists can apply the same approach to particles that live for billionths of billionths of a second and decay without leaving a trace. This will help us understand how atomic nuclei are structured and why some particles stick together.