Science: Hanging the Universe on Strings

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In their search for a unifying theory, researchers found that they could make headway using quantum theory, in which the basic forces are transmitted through quanta, tiny packets of energy. The quanta, tossed like softballs between particles of matter, such as protons or electrons, account for the interaction between the particles. Electromagnetism, for example, had long been conceived as traveling in bundles of light known as photons. (In fact, Einstein had elaborated this concept in explaining the photoelectric effect, a feat that later won him the Nobel Prize in 1921.) More recently physicists conjured up hypothetical bits, called W and Z particles, to carry the weak force; gluons to transmit the strong force; and gravitons, which would transmit the force of gravity.

In the late '60s Weinberg and two other physicists, Sheldon Glashow of Harvard and Abdus Salam of the International Center for Theoretical Physics in Trieste, Italy, devised a model that integrated the weak and electromagnetic forces into a so-called electroweak force and predicted the characteristics of the W and Z particles. Their theory was experimentally confirmed when a team led by Carlo Rubbia discovered the W and Z particles at the CERN accelerator near Geneva. In 1979 physicists working with an accelerator in West Germany found experimental evidence for the existence of the gluon, the strong-force carrier. Most physicists believed that a theory called quantum chromodynamics, which explains the strong force, would eventually be encompassed with the electroweak theory under one grand unified theory.

Gravity, however, would still remain the odd force out. No experimental evidence has emerged to confirm the existence of its transmitting agent, the graviton. And though this hypothetical particle has been accommodated mathematically in unified theories, such models have been fatally flawed by anomalies that leave the theories meaningless. The crux of the problem: the electroweak and strong forces are quantum forces, whereas gravity is still defined only as a consequence of the curvature of space and time and thus cannot yet be explained in terms of quantum physics.

To the rescue come superstrings. One primitive version of the theory was proposed in 1971 by Schwarz and France's Andre Neveu to explain the workings of the strong force. Schwarz later refined the theory with another Frenchman, Joël Scherk, recognizing that it was potentially the ultimate Theory of Everything. But the enhanced theory initially failed to cause a stir. "No one ever accused us being crackpots," says Schwarz, "but our work was ignored." In 1979 Schwarz began working with Michael Green, and by 1984 the two were able to demonstrate on paper that their string theory was free of anomalies besetting other unified theories that included gravity. That proof finally caught the attention of other physicists. Until then, says Witten, "it was still plausible that this was just a beautiful mathematical construction with nothing to do with the real world."