During the last part of the 20th century, physicists have constructed a theory of subatomic particles and their interactions known as the Standard Model of particle physics. Three forces operate in this microscopic world: electromagnetism, which encompasses the electric and magnetic forces; the strong nuclear force, which holds the protons and neutrons together in the nucleus, or heavy central core of an atom; and the weak subnuclear force, which causes some nuclei to decay in radioactive processes and helps to generate energy at the center of the Sun. The weak subnuclear force and electromagnetism have been unified into a single structure known as the electroweak theory.
is that all forces are created through the exchange
of a special set of particles called gauge bosons.
There is a reason why electromagnetism, which manifests itself at macroscopic distances, is so different from the weak subnuclear force, which only operates inside a nucleus. The unified electroweak theory has undergone what-is-called "breaking." This breaking destroys the apparent unification of electromagnetism with the weak subnuclear force. In particular, it creates masses for two subatomic particles, the W and Z, while leaving the photon without mass. One of the great principles of particle physics is that all forces are created through the exchange of a special set of particles called gauge bosons. Electromagnetism is generated through the exchange of photons, while the weak subnuclear forces are created through the exchange of W's and Z's. It is difficult to exchange particles of mass over significant distances. Since the W and Z are about 90 times heavier than a proton, they are very heavy and can only be exchanged over distances much smaller than a nucleus. This thus explains why the weak subnuclear force and the electromagnetic force are so different.
It turns out that in the Standard Model, the "breaking" mechanism also produces the masses of the electron, the proton and the neutron. It would seem that scientists have discovered the mechanism that makes all mass. However, there are several theoretically attractive ways to implement the breaking, and physicists do not know which is correct. It is also possible that nature has found a way to break the electroweak theory in a way that theorists have not imagined. One important purpose of the Superconducting Super Collider that was going to be built in Texas was to determine the breaking mechanism. The cancellation of that project shattered the dreams of many scientists, who had hoped to discover one of nature's most fundamental secrets. Physicists have turned to the Large Hadronic Collider (LHC), now being built near Geneva, Switzerland, hoping that it will have sufficient power to discover how "breaking" works. The LHC particle accelerator will be completed around 2005 and results should be available by the end of the next decade.