Instead of the point-like particles of the Standard Model, which interact to form branching "world lines," the fundamental entities of superstring theory are tiny vibrating strings which interact to sweep out multidimensional "world sheets."

by Paul Preuss | Many beautiful theories have been proposed before there was a shred of physical evidence to support them—only to hibernate like Sleeping Beauty awaiting the kiss of experiment. Now the world's most powerful particle accelerators are poised to awaken a new, all-embracing physical theory. The most attractive candidate to succeed today's standard model of fundamental particles and interactions is some version of supersymmetry—SUSY for short. Even SUSY will not be able to answer the most fundamental questions about the origin and plan of the universe, however. Beyond SUSY lie superstring theories, which once giddily promised to be a "theory of everything."

Although superstrings have yet to demonstrate that they apply to any aspect of the real world, there are good reasons why theorists like Mary K. Gaillard of Berkeley Lab's Physics Division maintain their faith in them. For one thing, superstrings are tied to supergravity, a way in which some versions of SUSY unify the force of gravity with the strong, weak, and electromagnetic forces included in the standard model. "Supergravity, particularly when regarded as the 'low-energy' limit of superstring theory, provides the best hope at present for unification of all forces," says Gaillard, who is also a professor of physics at the University of California, Berkeley. Through supergravity, Gaillard's current research "attempts to tie superstrings to observable physics."

Bringing it all together
Both the allure and the frustration of supersymmentry, supergravity, and superstrings can be suggested by a single word: unification.

In the 1860s James Clerk Maxwell showed that electricity and magnetism are two aspects of the same force, electromagnetism. More recently, electromagnetism was shown to be united with the weak nuclear force, which governs radioactive decay. There are good prospects for a grand unified theory which would include this "electroweak" force with the strong nuclear force, which holds atomic nuclei and their constituents together. "All these forces are included in the standard model," says Gaillard. "But one force, gravity, is not." The standard model is a quantum theory, which looks at the world as a collection of pointlike objects such as quarks and photons, possessing quantum properties such as charge and spin. By contrast, general relativity treats gravity as arising from geometric properties—from the curvature of a continuous space-time manifold. Despite much effort, there is no successful quantum theory of gravity. Along comes SUSY —specifically, "local" theories of supersymmetry in which symmetry can vary over spacetime, and which include quantum particles that carry gravity the way photons carry electromagnetism. As a group, the various versions of supersymmetry have a wide focus and seek to explain many of the standard model's other unanswered questions, among them: why do particles come in two varieties, fermions and bosons, and why does matter have mass?

Fermions are objects which cannot occupy the same quantum state at the same time. All particles of matter—quarks, electrons, and neutrinos—are fermions. Bosons, such as photons, carry forces; any number of bosons can occupy the same quantum state. Laser light, for example, consists of a great many energetically identical photons.

At first glance the dot made by this pencil on a piece of paper may appear to be two-dimensional, but a closer look would reveal a third dimension. In an analogous way, the extra dimensions required by string theory are just too small to see.

"For every fermion in the standard model there's a boson in supersymmetry," Gaillard explains, "and for every boson a fermion." There are many theoretical and mathematical benefits to this pairing, including cancellation of infinities—a sort of balancing of energies between the standard particles and their super mates.

(A distinctly nonmathematical benefit of SUSY is humor. The supersymmetric partners of fermions are named by putting an "s" in front of the standard model name, which yields sleptons, sneutrinos, squarks, and other sparticles.)

To explain the phenomenon of mass, theorists posit a field known as the Higgs field, which interacts with other particles through one or more particles called Higgs bosons. Higgs bosons must be massive, but may be just light enough to be produced in today's most powerful particle accelerators. If not, the Large Hadron Collider at CERN, now under construction, will almost certainly snare them. All versions of SUSY describe superparticles more massive than almost all the heaviest particles yet observed. The heaviest particle of matter found so far, the top quark, weighs at least 175 billion electron volts (divided by the speed of light)—175 GeV for short, almost as heavy as an atom of gold. The lightest superparticles are estimated to weigh hundreds of GeV at least, energies that are barely within the range of today's accelerators.

Next: What SUSY Can't Answer

— Desperately Seeking SUSY —
What SUSY Can't Answer | Why 3-D? | Where Large Hadrons Collide

Research Review Fall '98 Index | Berkeley Lab