The Standard Model
At the present time, the most widely accepted theory of particle physics is the Standard Model. The discussions on the fundamental particles classification and the basic forces are based on this theory. The Standard Model is a quantum field theory that incorporate fields representing the
twelve fundamental particles and the three exchange force carriers. Many experimental works have been carried out and found no conflict with the theory.
However, the theory that withstands a variety of tests does not mean it represents the whole picture of particle physics and the universe. For example, we know that gravitational force exists but it does not include in the Standard Model.
This is because the force is so weak at the sub-atomic scale that it can be regarded as non-existance. It is believed that all forces could be combined and essentially behave as a single force.
For example, the 19th century Maxwell's theory of electromagnetism shows that electric and magnetic forces are aspects of a single interaction - the electromagnetic interaction. In other words, electrical effects could produce magnetic fields and vice versa. This link was later known to be a fundamental
part of the special theory of relativity whereby the electic fields and magnetic fields can be transformed into each other due to the relative motion of the observer.
The Electroweak force
In the 20th century this analogy was taken a step further by attempting to link the basic forces. In 1967, it was independently discovered by S. Weinberg, A. Salam and S. Glasgow that the electromagnetic and the weak forces are essentially two aspects of the same force. These two forces behave similar at high enough energy (~ 100 GeV). This unified force is called the electroweak force and the corresponding theory predicts that there should be heavy force carriers that are responsible for the weak force, just like the photon is
the carrier of the electromagnetic force. However, these heavy carriers should behave similarly to a high-energy photon at around 100 GeV. The theory links the phenomena of radioactivity and thermonuclear fusion in stars with the effects of electricity and magnetism.
This theory of force unification was confirmed in 1983 when the predicted carriers were discovered by scientists at CERN. These weak force carriers are known as W+, W- and Z0. The charge carriers have a mass of 80.4 GeV/c2 while the neutral carrier has a mass of 91.2 GeV/c2. Nevertheless, it is convenient to retain separate identities among the carriers of the weak and the electromagnetic forces just as we
refer separately the electric and magnetic fields.