In many respects, the history of physics is the history of our growing comprehension of nature’s simplicity—the astonishing fact that the entire universe is governed by just a handful of rules, and that the boiling, bewildering diversity of the world around us conceals a profound underlying unity.

Indeed, the quest to understand that unity has now become one of the defining challenges of physics. By the late 20 th century, that quest had led researchers to the realization that everything in the visible cosmos is built from just two kinds of matter: leptons, a family of lightweight particles that includes the familiar electron; and quarks, a family of somewhat heavier particles that are the building blocks of protons and neutrons. Furthermore, the scientists had described the particles that carry three of the four fundamental forces, namely: electromagnetism, the strong force that holds the components of nuclei together, and the weak force involved in radioactivity and nuclear fission. (The carrier of the fourth force, gravity, is presumed to be a particle called the graviton.) Additionally, the researchers had defined the relationships among those particles and forces in a mathematical framework called the “standard model,” which now stands as the most accurately confirmed theory in the annals of science.

Yet the quest continues—because the standard model is almost certainly incomplete. In particular, it says nothing about how leptons relate to quarks, or how either one of them relates to the fundamental forces.

Gaps like these have given physicists every reason to suspect that there are even deeper unities – that each apparently unique type of matter is actually a different manifestation of some irreducibly basic entity, and that each force is really just a different form of a single “super force.” If that is the case, it could also help explain puzzling disparities such as symmetry violation, a puzzling difference in the behaviors of closely related particles.

NSF supports a wide range of research related to such “grand unified theories.” One is the hunt for effects predicted by theory, such as the elusive Higgs field that is thought to give the known particles their perplexing variety of masses. Another is the search for evidence of a supersymmetry, the idea that even particles as different as photons, electrons and quarks are actually closely related siblings in a heretofore undetected “superfamily” that comprises all matter.

Part of that effort involves the experimental study of utterly new phenomena and physics revealed in particle accelerators as matter and antimatter collide at ever-greater energies. At the same time, theorists are working to reconcile two of the 20 th century’s epoch-making ideas: quantum mechanics, which describes the behavior of matter and energy at the smallest scales; and Einstein’s general relativity, which explains gravity as curvature of space-time. Both theories have been verified by experiments to exquisite accuracy. But so far, neither is compatible with the other. Bridging that gap will require bold new concepts, and may reveal a universe with seven or more dimensions beyond the four – three of space and one of time – that are familiar in our daily lives.

Two research areas supported by NSF hold particular promise for unification: string theory and loop quantum gravity. (See also Scientific American, Jan. 2004.) Each offers a theoretical structure for connecting quantum mechanics and gravity. And other explanations may arise as physicists draw closer to the fundamental basis of matter and energy.

The Physics of the Universe [Next]