The concept of nucleons (protons and neutrons) binding each other through the exchange of pions is an approximation that helps us understand the strong nuclear force, which is the force responsible for binding nucleons within atomic nuclei. The strong nuclear force is ultimately mediated by the exchange of particles called gluons between quarks, but the pion exchange approximation provides a simpler and more intuitive explanation.
The strong nuclear force is one of the fundamental forces in nature and is responsible for holding quarks together within nucleons and binding nucleons within atomic nuclei. It is transmitted through the exchange of particles called gluons, which are the force carriers of the strong interaction. The interactions between quarks are described by the theory of quantum chromodynamics (QCD), which is a part of the Standard Model of particle physics.
However, the calculations involving the interactions between quarks can be extremely complex, and it is difficult to directly calculate the behavior of the strong force within nuclei from first principles. As a result, physicists have developed approximations and effective models to simplify the understanding of nuclear interactions.
One such approximation is the concept of meson exchange, specifically pion exchange, as an effective description of the strong nuclear force between nucleons. Pions are the lightest mesons and are produced through the interactions of quarks and gluons. They can be exchanged between nucleons, resulting in an attractive force that helps bind the nucleons together.
The pion exchange approximation considers the residual effect of the strong force between quarks within nucleons. While the strong force between quarks is primarily mediated by gluons, the interactions between nucleons can be described approximately by the exchange of pions. Pions are pseudo-Goldstone bosons, which means their properties are connected to the spontaneous breaking of chiral symmetry in QCD.
By using pions as an effective exchange particle, physicists can simplify calculations and gain qualitative insights into the behavior of nucleons within atomic nuclei. While the underlying force between quarks is ultimately described by QCD and mediated by gluons, the pion exchange approximation provides a useful framework for understanding the strong nuclear force and nuclear structure.