The strong nuclear force, described by the theory of quantum chromodynamics (QCD), exhibits two key features that may seem contradictory at first glance: it is both short-range and responsible for quark confinement.
The short-range nature of the strong force arises from the properties of the force-carrying particles called gluons. Gluons themselves carry color charge and can interact with quarks and other gluons. Unlike the electromagnetic force, which diminishes with distance according to the inverse square law, the strong force between quarks becomes stronger as they move apart. This behavior is due to a phenomenon known as asymptotic freedom, which was discovered by physicists David Gross, David Politzer, and Frank Wilczek in the 1970s. Asymptotic freedom means that at very high energies or short distances, the strong force weakens, allowing for the precise calculations used in perturbative QCD.
On the other hand, the strong force also leads to quark confinement, which explains why individual quarks are never observed in isolation. When quarks are pulled apart, the energy stored in the strong force field increases. At some point, the energy becomes so high that it becomes more favorable for new quark-antiquark pairs to be created out of the vacuum, forming new hadrons. This process, called hadronization, ensures that quarks are always confined within color-neutral hadronic particles. As a result, attempts to separate individual quarks require an increasing amount of energy until the creation of new quark-antiquark pairs occurs, preventing the observation of isolated quarks.
Regarding the stability of nuclei, the short-range nature of the strong force plays a crucial role. In the nucleus, protons and neutrons are held together by the residual strong force, which is an effective force that binds nucleons (protons and neutrons) despite their mutual electromagnetic repulsion. The strong force between nucleons operates at very short distances, allowing them to overcome the electromagnetic repulsion and create a stable nucleus. However, as the size of the nucleus increases, the electromagnetic repulsion between protons becomes stronger, eventually overcoming the short-range strong force. This leads to the destabilization of large, heavy nuclei, resulting in radioactive decay.
So, while the strong force is short-range, it is still responsible for quark confinement and also contributes to the stability of smaller nuclei. The interplay between the short-range nature of the force and its confinement properties creates these seemingly contradictory effects.