Stability of Matter

The conservation of electric charge may be related to the stability of matter. The decay of elementary particles is not an exceptional occurrence, it is the general rule. Of the hundreds of particles that have been observed, only four are stable, the proton, the electron, the photon and the neutrino. (Neutrons are also stable if buried inside a nucleus, for reasons we will discuss in the next chapter.) All the other particles eventually, and often very quickly, decay into these four.
The question we should ask is not why particles decay, but instead why these four particles do not. We know the answer in the case of two of them. Photons, and perhaps, neutrinos, have zero rest mass. As a result they travel at the speed of light, and time does not pass for them. If a photon had a half life, that half life would become infinite due to time dilation.
Why is the electron stable? It appears that the stability of the electron is due to the conservation of energy and electric charge. The electron is the least massive charged particle. There is nothing for it to decay into and still conserve charge and energy.
That leaves the proton. Why is it stable? We do not know for sure. There are a couple of possibilities which are currently under study. One is that perhaps the proton has some property beyond electric charge that is conserved, and that the proton is the least massive particle with this property. This was the firm belief back in the 1950s.
In the 1960s, with the discovery of quarks and the combining of the electric and weak interaction theories, it was no longer obvious that the proton was stable. Several theories were proposed, theories that attempted to unify the electric, weak, and nuclear force. These so– called Grand Unified Theories or GUT for short, predicted that protons should eventually decay, with a half life of about 10³¹ years. Since the universe is only 10¹⁰ years old, that is an incredibly long time.
It is not impossible to measure a half life of 10³¹ years. You do not have to wait that long. Instead you look at 10 31 or 10 32 particles, and see if a few decay in one year. Since a mole of particles is 6× 10²³ particles, you need about a billion moles of protons for such an experiment. A mole of protons (hydrogen) weighs one gram, a billion moles is a million kilograms or a thousand metric tons. You get that much mass in a cube of water 10 meters on a side, or in a large swimming pool. For this reason, experiments designed to detect the decay of the proton had to be able to distinguish a few proton decays per year in a swimming pool sized container of water.
So far none of these detectors has yet succeeded in detecting a proton decay (but they did detect the neutrinos from the 1987 supernova explosion). We now know that the proton half-life is in excess of 10³² years, and as a result the Grand Unified Theories are in trouble. We still do not know whether the proton is stable, or just very long lived.