String Theory

String theory (also termed "superstring" theory) is a mathematical attempt to describe all fundamental forces and particles as manifestations of a single, underlying entity, the "string."

String theory's predictions are consistent with all known experimental data, and it is felt by some physicists to be a candidate for the long-sought "theory of every thing" (i.e., a single theory describing all fundamental physical phenomena); however, string theory has proved difficult to subject to definitive experimental tests, and therefore actually remains a speculative hypothesis.

Physics as defined before the twentieth century, "classical" physics, envisioned the fundamental particles of matter as tiny, solid spheres. Quantum physics, which originated during the first 40 years of the twentieth century, envisioned the fundamental particles simultaneously as particles and as waves; both mental pictures were necessary to make sense out of certain experimental results.

String theory, the first forms of which were developed in the late 1960s, proposes that the fundamental unit of everything is the "string," pictured as a bit of taut wire or string on the order of 10^-33 cm in length (a factor of 10^-20 smaller than a proton). These strings may be "open," like a guitar string, or form loops like rubber bands; also, they may merge with other strings or divide into substrings.

Beginning a few years after its initial formulation, string theory stagnated for a decade because of mathematical difficulties, but exploded in the 1980s with the discovery that the theory actually possesses a highly desirable mathematical feature termed E(8)×E(8) symmetry. Several major theoretical victories were won by string theorists in the 1990s, and intense efforts to extend string theory continue today. String theory is not the product of a single mind, like the theory of relativity, but has been produced by scores of physicists refining each other's ideas in stages.

Like the "waves" or "particles" of traditional quantum mechanics, of which string theory is an extension or refinement, "strings" are not objects like those found in the everyday world. A string-theory string is not made of any substance in the way that a guitar string, say, may be made of steel; nor is it stretched between anchor-points. If string theory is right, a fundamental string simply is.

Not only does string theory propose that the string is the fundamental building block of all physical reality, it makes this proposition work, mathematically, by asserting that the Universe works not merely in the four dimensions of traditional physics—three spatial dimensions plus time—but in 10 or 11 dimensions, 6 or 7 of which are "hidden" from our senses because they "curled up" to subatomic size. Experimental proof of the existence of these extra dimensions has not yet been produced.

Although the "strings" of string theory are not actual strings or wires, the "string" concept is nevertheless a useful mental picture. Just as a taut string in the everyday world is capable of vibrating in many modes and thus of producing a number of distinct notes (harmonics), the vibrations of an elementary string manifest, the theory proposes, as different particles: photon, electron, quark, and so forth.

The string concept also resolves the problem of the "point particle" in traditional quantum physics. This arises during the mathematical description of collisions between particles, during which particles are treated as mathematical points having zero diameter. Because the fields of force associated with particles, such as the electric field that produces repulsion or attraction of charges, go by 1/r, where r is the distance to the particle, the force associated with a zero-diameter particle goes to infinity during a collision as r 0. The infinities in the point-particle theory have troubled quantum physicists' efforts to describe particle interactions for decades, but in the mathematics of string theory they do not occur at all.

In the Standard Model, quantum physicists' systematic list of all the fundamental particles and their properties, the graviton (the particle that mediates the gravitational force) is tacked on as an afterthought because it is hypothesized to exist, not because the equations of the Standard Model explicitly predict its existence; in string theory, however, a particle having all the properties required of the graviton is predicted as a natural consequence of the mathematical system.

In fact, when the existence of this particle was calculated by early string-theory workers, they did not recognize that it might be the graviton, for it had not occurred to them that their new theory might be powerful enough to resolve the biggest problem of modern fundamental physics, the split between general relativity (the large-scale theory of space, time, and gravity) and quantum mechanics (the small-scale theory of particles and of all forces except gravity).

String theory—or, rather, the string theories, as a variety of different versions of string theory have been put forward—thus not only predict all the particles and forces catalogued by the Standard Model, but may offer a theory of "quantum gravity," a long-sought goal of physics.

Doubt lingers, however, as to whether string theory may be too flexible to fulfill its promise. If it cannot be cast into a form specific enough to be tested against actual data, then its mathematical beauty may be a valuable tool for exploring new ideas, but it will fail to constitute an all-embracing theory of the real world, a "theory of everything." Excitement and skepticism about string theory both, therefore, continue to run high in the world of professional physics.