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Science: Hanging the Universe on Strings

7 minute read
Natalie Angier

Like members of a family, the four basic forces of nature are all distinct personalities, with their separate quirks, abilities and housekeeping chores. Electromagnetism makes it possible for elevators to rise, light bulbs to glow and lightning to snake across the sky. Gravity holds chairs to the floor and planets in their orbital paths. The strong force binds together the protons and neutrons in an atomic nucleus. The weak force causes subatomic particles to shoot out of the nuclei of atoms during the radioactive decay of such unstable elements as uranium.

Despite these apparent differences, physicists have long believed that a sort of common blood unites the four forces. They are convinced that at the moment of the Big Bang, the violent birth of the universe, only a single, all-powerful force existed, and that not until a fraction of a second afterward did this force split into four. Like knights in pursuit of a visionary grail, scientists for decades have sought what they call–with a bit of tongue in cheek–a Theory of Everything (TOE), a single mathematical model that would describe the fundamental unity of the forces. So far, however, they have met with only some partial successes and many failures.

That is, perhaps, until now. These days physicists are astir with a concept that just may be their ultimate TOE. The theory, developed by Physicists John Schwarz of Caltech and Michael Green of Queen Mary College in London, is known by the unlikely name of superstrings. It explains the forces not as interacting pointlike particles–the conventional approach–but as infinitesimally small, winding, curling, one-dimensional strings. By manipulating the highly intricate mathematics of the string theory, physicists believe they can avoid many of the troubling discrepancies that have dogged all other TOEs. Some scientists are already comparing the idea of superstrings with the genesis of quantum physics, or even with the revolutionary work of Albert Einstein. Says Princeton Physicist Edward Witten: “It’s probably going to lead to a new understanding of what space and time really are, the most dramatic [understanding] since general relativity.”

Judging by the flurry of activity in the field, others apparently agree. Since the fall of 1984, scientific papers about superstrings have been streaming forth at an ever increasing rate that now averages 100 per month, and conferences centered around strings are becoming commonplace. Upon hearing of Schwarz and Green’s latest breakthrough in string theory, says Steven Weinberg, a physicist at the University of Texas, “I dropped everything I was doing, including several books I was working on, and started learning everything I could about string theory.” That task is far from trivial. “The mathematics,” he concedes, “is very difficult.”

The modern quest for a Theory of Everything began not long after Einstein published his theory of general relativity in 1915. Eager to continue breaking new ground, the great scientist next attempted to link his pet force, gravity, to electromagnetism. He pursued this quest without success until his death in 1955.

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.”

Although even physicists still have difficulty understanding the theory, superstrings may be thought of as one-dimensional bits of energy measuring a billionth of a trillionth of a trillionth of a centimeter in length. Depending on different versions of the theories, these strings may be either open, or closed into a loop, and they interact in two ways: either two strings coalesce into one, or one string splits into two. Depending on how the strings are vibrating and rotating, they can represent any of the known particles of matter, from quarks to electrons. The nature of the interacting particles, in turn, determines which of the four forces is manifested.

The problems have not been entirely ironed out. For one thing, superstrings require ten dimensions in order to work, although scientists know of only four in the real world: three dimensions of space, and one of time. Admits Schwarz: “We don’t live in ten dimensions.” He and his colleagues offer an explanation for the discrepancy by assuming that after the Big Bang, four dimensions were liberated onto the large scale of the universe, while the remaining six remained rolled up into a little ball at every point in space-time. “What’s pretty sure,” says Schwarz, “is that today we have to have six of the dimensions disappear. How things got to be this way is a little less clear.”

There is an even bigger stumbling block: a complete lack of experimental evidence. No particle accelerator has ever detected anything that suggests the existence of strings. Still, string theorists believe that the immediate goal is not necessarily to search for new particles but simply to reconcile the mathematics of the theory. Says Schwarz: “Experimentalists would love for me to say such and such is an unambiguous consequence of string theory, and if you find it, it’s right, and if not, it’s dead. But I can’t say that yet. They’ll just have to be patient.”

Witten is optimistic that superstrings hold the key to the long-sought TOE, though he and other theorists hesitate to predict whether the remaining problems of the new theory will be solved in five years or 50. “String theory has a very rich and complicated structure that we don’t understand much about,” says Witten. “But enough beautiful things have been discovered that we’re pretty sure we’ve just found the tip of the iceberg.” –By Natalie Angier.

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