Nearly 2,500 years ago, the ancient Greeks asked a seemingly simple question that has wended its way through the ages and is still very much with us: What is the universe made of? That is, what are the fundamental ingredients out of which everything in the heavens and on Earth is composed? Or, to put the question another way, if you take any object whatsoever—a block of wood, a chunk of iron—and you cut it in half and then cut that half in half again, and keep cutting on and on, what is the most basic constituent you will ultimately come upon?

According to string theory, the irreducible constituent of the universe is a vibrating thread, not a pointlike particle.

Democritus proclaimed that you would come upon what he called atoms, from the Greek for “uncuttable.” By the late 1800s, scientists had realized that substances such as oxygen and carbon did in fact have a smallest recognizable constituent, which (taking their cue from Democritus) they christened atoms. Yet over the next few decades, experiments revealed that atoms, contrary to the ancient Greek conception, surely must be cuttable, since they were an agglomeration of smaller particles: a swarm of electrons orbiting a central nucleus containing protons and neutrons. Moreover, in the early part of the twentieth century, physicists showed that understanding the behavior of these constituents meant replacing nineteenth-century ideas about matter and energy with the strange new laws of quantum mechanics. And by the early 1930s, physicists studying quantum mechanics realized that it required one of the most dramatic upheavals science has ever experienced: Science could no longer be expected to predict with certainty the outcome of experiments. In the microscopic realm, quantum mechanics showed that science could only predict the probability that a particular outcome might occur.

Although Albert Einstein contributed significantly to the early development of quantum mechanics, he focused much of his attention on gravity, a force that has its greatest relevance in the vastly larger realm of stars and galaxies. His general theory of relativity, proposed in 1916, correctly predicted the bending of starlight by the Sun and explained Edwin Hubble’s 1929 measurements indicating that the universe is expanding. But Einstein had even bigger plans. Perhaps, he mused, the universe could be explained by a “unified theory”—a single master framework that would describe physics out to the farthest reaches of the cosmos and down to the smallest speck of matter. Einstein relentlessly pursued a unified theory, but he ultimately came up empty-handed. To some extent, Einstein “failed” because many things about the workings of the universe were either still unknown or, at best, poorly understood during his lifetime.

For example, the first particle accelerators for studying the microscopic architecture of matter were built in the late 1920s. By the late 1960s, the resolving power of these machines had increased enormously, allowing physicists to reveal another layer of matter’s substructure: each proton and neutron, it was shown, is composed of three smaller particles, dubbed quarks. The proton consists of two “up” quarks and one “down” quark, while the neutron consists of two downs and one up. The detailed study of subnuclear interactions also established convincingly that two other forces besides gravity and electromagnetism are at work in nature: the weak nuclear force, which is responsible for radioactive decay, and the strong nuclear force, which is responsible for tightly binding quarks inside protons and neutrons and for cramming protons and neutrons inside the nuclei of atoms. Increasingly powerful atom smashers (today’s accelerators are more than a million times more powerful than those of 1930) have as yet found no evidence of any additional fundamental forces beyond these four. But they have revealed four more species of quarks (whimsically called charm, strange, top, and bottom) and have repeatedly confirmed the existence of a handful of other particles (known as neutrinos) and of two close cousins of the electron (called muons and taus).

If you’re having trouble keeping track of all the forces and the particles of matter, you’ll welcome the modern reformulation of Einstein’s goal of a unified theory: a theoretical framework that would show all four forces to be distinct manifestations of a single underlying force and would also establish a rationale for the presence of the particular species of apparently fundamental particles.

To fit the requirements of string theory, the universe must have more than the three spatial dimensions of common experience.

The first step toward this goal was taken by Steven Weinberg, Abdus Salam, and Sheldon Glashow, who in the 1960s proposed that the familiar electromagnetic force and the comparatively less well known weak nuclear force are intimately related. These physicists argued that although the weak and the electromagnetic forces have vastly different characteristics in the world around us, if the cosmic clock were rolled back to an early stage in the universe—less than a millionth of a millionth of a second after the big bang, when the temperature was some million billion degrees Celsius—these two forces would combine into a single force, somewhat the way a bouillon cube and water will form a homogenous broth when brought to a vigorous boil. By the mid-1980s, a central prediction of this proposed electroweak theory—the existence of certain crucial particles, known as Ws and Zs, that would perform the same force-carrying function in weak interactions that photons do in electromagnetic interactions—had been confirmed by the accelerator at the European Laboratory for Particle Physics (CERN) in Geneva, Switzerland. This represented a major step forward in the quest for unification.

The standard model of particle physics today encompasses both the electroweak theory and the theory of the strong nuclear force (known as quantum chromodynamics). Virtually all data recorded by particle accelerators the world over can be explained with this model; its creation has truly been a monumental achievement. There are, nevertheless, two main reasons physicists still aren’t satisfied.

First, the gravitational force is completely left out of the standard model. This omission creates a terribly thorny issue. The standard model, being a theory that describes microscopic processes, embraces quantum mechanics. But the problem of merging quantum mechanics with general relativity (a theory that describes macroscopic processes) has stumped physicists for more than half a century. Second, the standard model utilizes twenty or so numbers that have been established through decades of fastidious research—numbers such as the comparative strengths of the strong, weak, and electromagnetic forces, as well as the masses of the fundamental particles—but as a theory it offers no insight whatsoever into why these key parameters take the values they do. This marks a profound gap in our understanding, for if the value of some of these parameters had been even slightly different, the nuclear processes that power stars would likely have been disrupted, and without stars the universe would be a very different place.

These objections to the standard model are hard to counter, and many physicists believe that progress requires a radically new approach. During the past decade, the most promising possibility has been based on the notion of "superstrings." Superstring theory abandons the previous conception of particles as being pointlike—that is, having no spatial extent. Instead the theory envisages the elementary constituents to be tiny, one-dimensional threadlike loops or snippets, which for want of a more clever name are called strings. According to the theory, every elementary particle, if examined with a precision many orders of magnitude greater than what we are able to muster today, would be seen to contain one of these dancing, vibrating strings. And just as a violin’s string can vibrate in different patterns, thus producing different musical notes, string theory’s fundamental strings can also vibrate in different patterns. But instead of producing various tones, these patterns give rise to the distinct elementary particles. An electron is a string vibrating in one pattern, a quark is a string vibrating in another, and so on for all the other particles. The vibrational pattern of a string encodes the properties of the corresponding particle (its mass, its electric charge, its spin) and so may be thought of as the particle’s "fingerprint."

Thus the universe, rather than being built from a long list of different particles, according to string theory has one fundamental ingredient strings—and the rich variety of observed particles reflects nothing more than the various vibrational patterns that strings can execute. Moreover, even the four forces of nature, including gravity, are associated with strings vibrating in yet other patterns, and hence everything—all the particles of matter and all the forces by which they interact—is unified under the same rubric: vibrating strings.

However compelling a framework for a unified theory, superstring theory at the turn of the millennium is still very much a work in progress. For example, the theory’s equations are so involved that physicists have as yet been unable to determine whether the repertoire of vibrational patterns precisely accounts for the known particles and forces. The inability to clear this hurdle is due in part to another strange feature of the theory: it requires the universe to have more than the three spatial dimensions of common experience (left/right, back/forth, up/down). Since we don’t see the others, they must be hidden away.

One approach pictures these other dimensions as being curled up like a piece of paper that has been rolled into a thin tube. The more tightly the tube is rolled up, the harder it becomes to see that it has a circular cross section, since this circular dimension gets smaller and smaller. Physicists imagine that the extra space dimensions required by superstring theory are so tightly curled up that equipment powerful enough to detect them has not yet been built.

Although these extra dimensions are minuscule, they have a profound effect on the physics of string theory. The strings themselves are so small that they are able to vibrate in both the familiar “big” dimensions and the tiny, curled-up dimensions. The precise size and shape of these dimensions affect the ways a string can vibrate, much as the twists and turns of a French horn affect the ways that forced air streams can vibrate through its interior. And since the string’s vibrational patterns determine such things as particle masses and force strengths, the detailed geometry of the extra dimensions may one day explain why the aforementioned twenty numbers that animate the standard model of particle physics would have the values they do—in essence, why the universe is as it is.

The experimental verification of superstring theory in the near future poses quite a challenge. Since strings are thought to be less than a billionth of a billionth the size of an atom, we can’t use current technology to detect them directly. An indirect test, however, will be carried out within the next decade or so by a huge atom smasher called the Large Hadron Collider, which is now being built by CERN. Through enormously powerful collisions, physicists hope to produce a number of particle species (collectively called sparticles) that have never before been seen but are believed to be an essential part of the superstring framework. Another indirect test is now being carried out at Stanford University and the University of Colorado at Boulder, where researchers are looking for evidence of the required extra dimensions by trying to ascertain their influence on specific properties of the gravitational force.

Evidence for assessing this theory may also one day be found through increasingly refined astronomical observations, since superstring theory shows its true colors in extreme environments such as those associated with black holes and the big bang. It would surely be a wonderfully poetic emblem of unification if the theory describing the most microscopic properties of matter—a theory answering that prescient question raised by the ancient Greeks—were one day confirmed by turning powerful telescopes toward the sky and examining the grand expanse of the cosmos.

Brian Greene, a professor of both physics and mathematics at Columbia University, has been working on superstring theory for more than a decade. Greene’s research focuses on the new features of space and time that emerge from unifying the laws of physics. He is the codiscoverer of mirror symmetry (the recognition that distinct geometrical forms of space can yield physically identical universes) and of smooth topology change (which shows that the fabric of space can not only stretch but also tear). Greene “enjoys finding entertaining ways to communicate cutting-edge physics to those without technical training.” His book on the subject—The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory (W. W. Norton, 1999)—explains to the layperson the behavior of the universe on both cosmological and subatomic scales.

Copyright © 2000 American Museum of Natural History

Return to Web Site Archive