If you try to break a proton apart and pull out a quark, you're in for a surprise. Ordinary forces, like the electric force, weaken as particles fly apart. Here, it's the opposite. The farther a quark gets from its neighbors, the harder it's pulled back. It's as if it's attached to a rubber band: the band stretches easily at first, but then the tension becomes constant, and the only way to break it is by creating new particles.

This property (confinement, or the inability to free quarks) has remained a mystery. The theory of the strong interaction, quantum chromodynamics, does a great job describing how quarks and gluons exchange color charges. But no one could mathematically prove why they are always locked up. The authors of the new work found a workaround: instead of solving the equations head-on, they learned to extract the force directly from experimental data obtained at accelerators.

The measurements showed that at distances comparable to the proton's size, the attraction does not drop off but reaches a plateau. Such behavior is the hallmark of confinement. If quarks could fly apart, the force would tend to zero. Instead, it remains unchanged, as if the proton were girded by an invisible string that keeps its ends from running away.

This result is not only direct evidence of confinement but also a compass for future experiments. The new electron-ion colliders currently under construction will be able to probe the force at different distances with unprecedented precision. In this way, we may finally understand how the massive world around us emerges from light quarks and massless gluons.