The Weak Interaction

In addition to gravity, the electric interaction and the nuclear force, there is one more basic force or interaction in nature given the rather bland name the weak interaction. While considerably weaker than electric or nuclear forces, it is far far stronger than gravity on a nuclear scale.
A distinctive feature of the weak interaction is its very short range. A range so short that only with the construction of the large accelerators since 1970 has one been able to see the weak interaction behave more like the other forces. Until then, the weak interaction was known only by reactions it could cause, like allowing a proton to turn into a neutron or vice versa. Because of the weak interaction, an individual neutron is not stable. Within an average time of about 10 minutes it decays into a proton and an electron. Sometimes neutrons within an unstable nucleus also decay into a proton and electron. This kind of nuclear decay was observed toward the end of the nineteenth century when knowledge of elementary particles was very limited, and the electrons that came out in these nuclear decays were identified as some kind of a ray called a beta ray. (There were alpha rays which turned out to be helium nuclei, beta rays which were electrons, and gamma rays which were photons.) Because the electrons emitted during a neutron decay were called beta rays, the process is still known as the beta decay process.
The electron is emitted when a neutron decays in order to conerve electric charge. When the neutral neutron decays into a positive proton, a negatively charged particle must also be emitted so that the total charge does not change. The lightest particle available to carry out the negative charge is the electron.
Early studies of the beta decay process indicated that while electric charge was conserved, energy was not. For example, the rest mass of a neutron is nearly 0.14 percent greater than the rest mass of a proton. This mass difference is about four times larger than the rest mass of the electron, thus there is more than enough mass energy available to create the electron when the neutron decays. If energy is conserved, you would expect that the energy left over after the electron is would appear as kinetic energy of the electron. Careful studies of the beta decay process showed that sometimes the electron carried out the expected amount of energy and sometimes it did not. These studies were carried out in the 1920s, when not too much was known about nuclear reactions. There was a serious debate about whether energy was actually conserved on the small scale of the nucleus.
In 1929, Wolfgang Pauli proposed that energy was conserved, and that the apparenty missing energy was carried out by an elusive particle that had not yet been seen. This elusive particle, which became known as the neutrino or “little neutral one”, had to have some rather peculiar properties. Aside from being electrically neutral, it had to have essentially no rest mass because in some reactions the electron was seen to carry out all the energy, leaving none to create a neutrino rest mass.
The most bizarre property f the neutrino was its undetectability. It had to pass through matter leaving no trace. It was hard to believe such a particle could exist, yet on the other hand, it was hard to believe energy was not conserved. The neutrino was finally detected thirty years later and we are now quite confident that energy is conserved on the nuclear scale.
The neutrino is elusive because it interacts with matter only through the weak interaction (and gravity). Photons interact via the strong electric interaction and are quickly stopped when they encounter the electric charges in matter. Neutrinos can pass through light years of lead before there is a good chance that they will be stopped. Only in the collapsing core of an exploding star or in the very early universe is matter dense enough to significantly absorb neutrinos. Because neutrinos have no rest mass, they, like photons, travel at the speed of light.