The energy scale of processes that occur naturally on the surface of the Earth is relatively low. At these energies, the forces of nature are different from one another because the particles that transmit them have very different properties. However, above 1015 degrees the electromagnetic and weak forces become indistinguishable. That is, they take on similar identities. Indeed, such modifications have been observed in the large particle accelerators. This fundamental similarity between the electromagnetic and weak forces suggests that they might be two components of a more fundamental force. The two forces are said to become unified into the electroweak force above 1015 degrees.

What causes such a unification at this particular temperature? We know that temperature is just a measure of the amount of energy present; higher temperatures correspond to higher energies. Furthermore, energy and mass are equivalent. In this sense, then, mass is closely related to temperature. For every mass there is a corresponding energy and temperature.

The masses of the W and Z particles correspond to this temperature of 1015 degrees. These particles have two types of energy associated with them: one is the energy stored in their mass, and the other is the energy determined by how fast they are moving. This latter type is called kinetic energy. As the temperature increases, so does the kinetic energy of the particles. The energy stored in the mass remains fixed, however, because the masses of the particles are constant.