The Weak Interaction

The fact that some nuclei are unstable. These are called radioactive nuclei, and their state and even identity can change spontaneously. These changes occur by the absorption and emission of energy in the form of alpha particles, electrons, positrons, and gamma rays. Radioactive nuclei that have too many neutrons compared to protons emit electrons in order to regain their stability. This process is called “beta decay.” It occurs when one of the excess neutrons changes into a proton with the creation of an electron and a neutral particle called an antineutrino.

Similarly, a proton in a nucleus can decay into a neutron with the emission of a positron and a neutrino. This latter particle was so difficult to detect that it was not observed until 1956, even though beta decay had been observed since 1898. Note that the emitted electron is not one of the orbital atomic electrons, but a brand spanking new one created in the decay process.

Although there was awareness of the gravitational, electromagnetic, and strong interactions by the early 1930s, none of these could help explain the process of beta decay. This naturally implied that there had to be a fourth fundamental interaction. In 1932, Italian physicist Enrico Fermi proposed the weak interaction to fill this void.

As you might think based on our discussion of beta decay, electrons are influenced by the weak interaction. Quarks are too, and it is actually the conversion of one type of quark to another that leads to the change of identity from proton to neutron or vice-versa. All fermions (e.g., electrons, quarks) are influenced by the weak interaction.

This weak force doesn’t play a major role in the binding of nuclei since its strength is significantly weaker (nearly a million times) than the strong interaction. Furthermore, the range of the weak force is about a hundredth of a femtometer. Beyond this, the weak interaction dies away quickly. For this reason, there is no weak interaction between an atom’s electron cloud and its quark-based nucleus.

Despite its diminutive label, the weak interaction is important because it is also responsible for other decay processes involving fundamental particles besides simple beta decay. It plays a major role in determining the rates of certain nuclear fusion processes, which are important both to the lifetimes of stars and the evolution of the very young universe (shortly after the Big Bang). For now, however, we must leave the weak interaction and go back to the larger picture.