 
Today's physicists are struggling with a quandary. They have accepted two separate theories that explain how the universe works: Albert Einstein's general theory of relativity, which describes the universe on a very large scale, and quantum mechanics, which describes the universe on a very small scale. Both of these theories have been supported overwhelmingly by experimental evidence. Unfortunately, these theories don't complement one another. General relativity, which describes how gravity works, implies a smooth and flowing universe of warps and curves in the fabric of spacetime. Quantum mechanics—with its uncertainty principle—implies that on an infinitesimally small scale, the universe is a turbulent, chaotic place where events can only be predicted with probabilities. In two cases where the competing theories must both be applied—to describe the big bang and the depths of black holes—the equations break down. Most physicists have a hard time accepting that the universe operates according to two separate (and sometimes contradictory) theories. They think it is more likely that the universe is governed by a single theory that explains all observations and data. The Hunt for One Theory For that reason, physicists are on the hunt for a unified theory. Such a theory would bring together under one umbrella all four forces of nature: gravity, the weakest of the four, as explained by general relativity; and electromagnetism and the strong and weak forces, as explained by quantum field theory. Einstein pursued a unified theory by trying to unite electromagnetism and gravity. Superstring theory, also called string theory, is the current formulation of this ongoing quest. String theory attempts to unify all four forces, and in so doing, unify general relativity and quantum mechanics. At its core is a fairly simple idea—all particles are made of tiny vibrating strands of energy. (String theory gets its name from the stringlike appearance of these energy strands.) Unlike everyday strings, these strings have length (averaging about 10^{33} centimeters) but no thickness. String theory implies that the particles that comprise all the matter that you see in the universe—and all the forces that allow matter to interact—are made of tiny vibrating strands of energy. The currently accepted and experimentally verified theory of how the universe works on subatomic scales holds that all matter is composed of—and interacts through—point particles. Known as the Standard Model, this theory describes the elementary particles and three of the four fundamental forces that serve as the building blocks for our world (see the Elementary Particles chart and the Fundamental Force Particles chart for a listing of these particles). This theory does not include gravity. In string theory, each type of elementary matter particle—and each type of fundamental force carrier particle that mediates interactions between matter particles—corresponds to a unique string vibrational pattern, somewhat as different notes played by a violin correspond to unique string vibrations. How a string vibrates determines the properties—such as charge, mass, and spin—of the particle it is. The equations of string theory could give rise to elementary particles like those currently known (electrons, quarks, photons, etc.), but because detailed numerical predictions cannot yet be made, it is difficult to know whether the assortment of possible vibrational patterns correctly accounts for all known matter and force carrier particles. Strings can either be openended or closed to form a loop. Whether a string is open or closed determines the type of interactions it can undergo. It is the nature of strings that unifies general relativity and quantum mechanics. Under quantum field theory, particles interact over zero distance in spacetime. Under the general theory of relativity, the theorized force carrier particle for gravity, the graviton, cannot operate at zero distance. Strings help solve this dilemma. Because they are onedimensional and have length, they "smear" interactions over small distances. This smearing smooths out spacetime enough for the graviton to interact with other quantum field particles, thus unifying the two sets of laws. A Hefty Price Tag But string theory, for all its elegance, comes with a price. For the theory to be consistent, the universe must have more than three spatial dimensions. In fact, string theory predicts a universe with nine spatial and one time dimension, for a total of 10 dimensions. (The most current version of string theory predicts 11 dimensions.) The nine spatial dimensions consist of the three extended dimensions that we experience in everyday life, plus six theorized tiny, curledup dimensions that can't be seen with existing technologies. These extra six dimensions occur at every point in the familiar threedimensional world. The existence of more than three spatial dimensions is such a difficult concept to grasp that even string theorists cannot visualize it. They often use analogies to help picture these abstractions. For example, picture a piece of paper with a twodimensional, flat surface. If you roll up this surface, it will form a tube, and one dimension will become curled. Now imagine that you continue rolling the surface until it is rolled so tightly that the interior curledup dimension seems to disappear and the tube simply looks like a line. In a similar manner, the extra dimensions predicted by string theory are so tightly curled that they seem to disappear in everyday experience. These curledup dimensions may take on certain complex configurations known as CalabiYau shapes. Unfortunately, tens of thousands of variations of these shapes exist, and it is difficult to know which ones might correctly represent the extra dimensions of our universe. It is important to know which ones are correct because it is the shape of these extra dimensions that determines the patterns of the string vibrations. These patterns, in turn, represent all the components that allow the known universe to exist. These extra dimensions might be as small as 10^{35} meters or as big as a tenth of a millimeter. Alternatively, the extra dimensions could be as large or larger than our own universe. If that's the case, some physicists believe gravity might be leaking across these extra dimensions, which could help explain why gravity is so weak compared to the other three forces. It's a Match String theory also calls for every known matter particle to have an asyetundiscovered corresponding "super" force carrier particle and every known force carrier particle to have an asyetundiscovered corresponding "super" matter particle. This idea, known as supersymmetry, helps establish a relationship between matter particles and force carrier particles. Called superpartners (see "Particles and Sparticles" below), these theorized particles are thought to be more massive than their known counterparts, which may be why they have not yet been observed with current particle accelerators and detectors.
The potential for what string theory could help explain is huge. It could reveal what happened at the moment the universe began. The big bang theory only describes what happened after the first extremely small fraction of a second. Under conventional theories, prior to that the universe shrank to zero size—an impossibility. Under the auspices of string theory, the universe may never have shrunk to a point at which it disappeared but rather may have begun at a miniscule size—the size of a single string. String theory could also help reveal the nature of black holes, which, while predicted by general relativity, have never been fully explained at the quantum level. Using one type of string theory, physicists have mathematically described miniature massless black holes that—after undergoing changes in the geometry of string theory's extra dimensions—reappear as elementary particles with mass and charge. Some theorists now think that black holes and fundamental particles are identical and that their perceived differences reflect something akin to phase transitions, like liquid water transitioning into ice. String theory also opens the door to different hypotheses about the evolution and nature of space and time, such as how the universe might have looked before the big bang or the ability of space to tear and repair itself or to undergo topological changes. When It All Started String theory is not entirely new. It has been evolving since the late 1960s. At one point, there were five variations of the theory. Then, in the mid1990s a theory known as Mtheory emerged that unified the five theories. Mtheory is considered the latest step in string theory evolution (see "Mtheory, Magic, Mystery, Mother?" at right).
No part of string theory has been experimentally confirmed. This is in part because theoreticians do not yet understand the theory well enough to make definitive testable predictions. In addition, strings are thought to be so small—less than a billionth of a billionth of the size of an atom—that technologies such as current accelerators and detectors aren't powerful enough to detect them (see "Seeking the Fundamental" below). While string theory can't yet be experimentally verified, physicists hope that some of its facets can be supported by circumstantial evidence, such as demonstrating the existence of:


