Now we know the building blocks of matter, but we must also ask: What holds it together? All forces are due to the underlying interactions of the particles. Interactions come in four types: gravitational, electromagnetic, strong, and weak. Gravity is perhaps the most familiar force to us, but it is not included in the Standard Model because its effects are tiny in particle processes and, furthermore, physicists have not yet figured out how to include it.

Electromagnetic forces are also familiar; they are responsible for binding the electrons to the nucleus to form electrically-neutral atoms. Atoms combine to form molecules or crystals because of electromagnetic effects due to their charged substructure. Most everyday forces, such as the support of the floor or friction, are due to the electromagnetic forces in matter that resist displacement of atoms or electrons from their equilibrium positions in the material.

In particle processes the forces are described as due to the exchange of particles; for each type of force there is an associated carrier particle. The carrier particle of the electromagnetic force is the photon. For one range of energies we see photons as light; gamma rays are the photons from a nuclear transition.

For distances much larger than the size of an atomic nucleus, the remaining two forces have only tiny effects -- so we never notice them in everyday life. But they are essential for the existence of all the stuff from which the world is made, and for the decay processes that make some types of matter unstable... See the next page.