The decay of an unstable particle is the simplest example illustrating both annihilation and creation of mass. In the beta decay of the neutron, for example, written
n → p + e– + νe ,
a neutron is annihilated, and a proton, an electron, and an antineutrino are created. This is the only way in which a neutron may decay, except for the rare case in which it also emits a photon. A kaon, on the other hand, has a variety of decay modes. Positively charged kaons can vanish in more than half a dozen different ways, producing electrons, muons, neutrinos, charged and neutral pions, and gamma rays. Each mode of decay is constrained by conservation laws and controlled by probability.
Spontaneous decays and particle-antiparticle annihilation are “downhill” events. The combined mass of the product particles is always less than the mass of the initial particle or particles, and the difference is converted into energy of motion imparted to the product particles. “Uphill” events, in which new mass is created, can be stimulated by the use of high-speed projectile particles, either furnished free in the cosmic radiation, or furnished at great expense by man-made accelerators.
When relativity swept away the law of mass conservation, it swept away the idea of a solid and reliable material basis of the universe. The current view, based on the mass-energy equivalence predicted by relativity and the routine annihilation and creation of matter predicted by quantum mechanics, is much more tenuous. Most of the material particles do not live long enough to be of any use for building the world. Even those that do can be annihilated if struck by other energetic particles or if brought into contact with their antiparticles. The current view might be described as follows: Because of certain conservation laws, a very few of nature’s particles happen by chance to be stable. Even these are not indestructible, but because where we live the flux of projectile particles is very low, and because our corner of the universe happens to contain a great deal of matter and very little antimatter, the stable particles have time enough to build a durable material world.