Particles and Fields

The number of the particles of each type in the present universe is the result of a complicated history. Most of the particle types that were abundant in the early universe have long ago disappeared. We only observe them when they are produced briefly in laboratories, and then annihilate or decay. Because of this we are uncertain of how many particle types may exist.

In the present universe, quarks and electrons have properties that allow them to form the tightly bound clusters that we call nuclei and atoms. Photons and neutrinos cannot do this, and so exist much more diffusely throughout the universe.

Nevertheless, most of the universe we know is made of quarks and electrons, and the present picture we have of the world is largely an expression of the properties of these particles. Of the two, quarks have a greater tendency to cluster together. Indeed, this tendency is so pronounced that quarks are believed to be never found in isolation, but only in combinations containing either three quarks or one quark and one antiquark. These are the combinations that make up most of the subatomic particles that are observed, such as protons and neutrons, the particles found in the nuclei of atoms.

The reasons why quarks insist on clustering in this way are not completely understood. There is a general theory, known as quantum chromodynamics (QCD) that attempts to describe how quarks behave. QCD involves the interactions of fields associated with quarks and fields associated with another type of particle called gluons (so named because they bind the quarks together). Most physicists believe that when the predictions of this theory are better understood, we will know why quarks cluster as they do.

Ever since the first microsecond after the origin of the universe, quarks have been bound together, in groups of three, into neutrons or protons. All of the other combinations of quarks or the other quark types, which also can bind together, are unstable under present conditions. That is, if they are produced, they change spontaneously into less massive particles, and eventually into some combination of the stable ones. Even neutrons are unstable when they are found in isolation– as when they arc produced in nuclear reactors– and decay into protons in a few minutes. The reason that neutrons exist at all in the present universe is that when given the chance they bind together into more complex and lasting objects. Neutrons can bind with protons into atomic nuclei, and with one another in immense numbers into neutron stars.