For decades, physicists have struggled with the problem of how to unify the four known forces of the universe. One Harvard professor says he hopes new experimental data will lead to significant advances in solving this problem.

Professor of Physics Howard M. Georgi '68, chair of the physics department, began work in the mid-1970s on the SU(5) theory. This theory attempts to bring together and to refine two previous theories on the relationship between three of these four forces--weak force, strong force, and electromagnetic force. The fourth force, gravity, is believed to be a derivation of the weak and strong forces.

"Particle physics is, or at least was before SU(5), a very homogeneous community of interests," he says.

"The question is always, 'How small?"'

The search for smaller and smaller particles is a natural outgrowth of quantum mechanical theory, Georgi says.

Quantum mechanics is the branch of physics which attempts to explain the apparent discrepancy between light's motion as a wave and as a particle. It reconciles evidence that light consists of small bundles of energy, called photons, with the classical belief that light travels in the form of waves.

Georgi's theory unites the SU(2) and SU(3) theories, both proposed by Higgins Professor of Physics and Mellon Professor of the Sciences Sheldon L. Glasgow. Glashow was awarded the 1972 Nobel Prize in Physics for his work in the field.

SU(2) theory implies a symmetry in which particles are organized into doublets, Georgi says. Strong force, associated with three equivalent particles known as quarks, is the basis of SU(3) theory, named for three-fold symmetry.

However, the two theories did not provide a complete explanation for the relationship between the weak and electromagnetic forces, says Georgi.

A particle, known as the W particle, has proven to be a key to the formulation of a more unified theory. Physicists have long searched for such particles, which are considered the fundamental causes of physical phenomena such as forces.

The W particle has assisted scientists in creating a more unified theory because it consists of a photon-like particle and of an unknown entity, known as the Goldstone Boson.

These sub-units were discovered by Glashow and Loeb Visiting Professor of Physics Steven Weinberg. Their existence was confirmed independently by researcher Abdus Salaam of the Imperial College of Science and Technology in London.

The W particle's involvement in creating forces is, therefore, similar to the involvement of photons in electromagnetic forces. Photons convey the force between charged particles, enabling the particles to act on each other at a distance.

But this discovery led only to a partial unification.

"By the middle of 1972, we had a preliminary picture of the SU(2) and SU(3) interactions," says Georgi. "Any idiot, you might think, would realize that two plus three equals five, and try to unify the weak and strong interactions into an SU(5) theory."