Theorists have long sought a "theory of everything" that would unify all the principles of physics in one aesthetically pleasing mathematical formalism. The main goal of those searching for this theory, or TOE, has in the past been to unify the force of gravity with electromagnetism and the strong and weak nuclear forces. Phrased differently, they sought to unify relativity and quantum mechanics into one framework. (If you are confused, see our pages on relativity and the four fundamental forces.) The closest physicists had come before string theory was the Standard Model, a conglomeration of equations that did not include gravity and was, according to many, unacceptably ugly. Moreover, its parameters required delicate adjustments in certain experimentally measured values in order to make successful predictions. Many physicists were fundamentally opposed to the ideas that the final theory of science would not only be so delicate, but would also require that one law - relativity - be used for the very large, and another - quantum mechanics - be used for the very small. However, physicists soon resurrected an idea that had long been rejected by the mainstream community - string theory. It postulated that particles were not zero-dimensional points, as the Standard Model supposed, but were instead tiny strings whose vibrations are reflected in the observed properties of the fundamental particles. Although string theory's full power is still being studied, many physicists believe that string theory or its derivatives will ultimately prove to be the final "theory of everything" long sought by scientists.
The incompatibility of relativity and quantum mechanics has been called the "central conflict" of modern theoretical physics. The conflict arises from the quantum foam generated by the properties of the uncertainty principle. In sub-Planck-length regions of space, the quantum foam comes into evidence, destroying the smooth geometric "fabric" of spacetime that is central to the functioning of general relativity. Typically, calculations that attempt to merge quantum mechanics and general relativity yield infinite answers, obvious impossibilities that alert physicists to the breakdown of their equations and the need for a new theory. Although a few physicists were willing to accept the fundamental division and simply choose the theory that best fit their purposes - relativity for cosmological applications and quantum mechanics for particle physics - there are a few cases where the two theories must be merged in order to accurately describe conditions. Examples include the cores of black holes and the original singularity at the beginning of the universe. Therefore, the conflict impedes our fundamental understanding of the beginning of the world we now inhabit, a phenomenon unacceptable to many researchers. Despite the valiant efforts of these physicists, all attempts to merge the two theories have failed - until string theory arrived in earnest.
Gabriele Veneziano, a research fellow at CERN (a European particle accelerator lab) in 1968, observed a strange coincidence - many properties of the strong nuclear force are perfectly described by the Euler beta-function, an obscure formula devised for purely mathematical reasons two hundred years earlier by Leonhard Euler. In the flurry of research that followed, Yoichiro Nambu of the University of Chicago, Holger Nielsen of the Niels Bohr Institute, and Leonard Susskind of Stanford University revealed that the nuclear interactions of elementary particles modeled as one-dimensional strings instead of zero-dimensional particles were described exactly by the Euler beta-function. This was, in effect, the birth of string theory. However, later experiments in the early '70s revealed that many of the theory's predictions were at odds with experimental data. As point-particle theory met success after success, string theory was left by the wayside by all but a few dedicated physicists.
Most people saw one major problem with string theory. String vibrations produce observable properties that we see in fundamental particles. For example, string theory seemed to provide vibrational configurations that corresponded to the properties of gluons. However, the theory also provided other vibrational patterns that seemed to have little bearing on reality. These "extra" patterns, however, were soon shown to correspond exactly with the postulated properties of the graviton, a particle that has not been found experimentally, but can be predicted by scientists. The vibrations were found to exactly relate to theorized properties of gravitons. This discovery was not received well by the scientific community, subtle conflicts between it and point-particle physics were again found, and the theory was once again abandoned by all but a few.
In 1984, a paper by Michael Green, then of Queen Mary College, and John Schwarz of the California Institute of Technology revealed the end product of over a dozen years of research often belittled by "mainstream" physicists. The paper not only resolved the conflict between string theory and quantum mechanics, but also showed that string theory could encompass the four fundamental forces and all the matter in existence. The result was the first superstring revolution, during which physicists around the world rushed to join the research on the same theory they had "snubbed" in the past.
The years from 1984-86 saw more than a thousand papers published on string theory, showing that the features of the Standard Model could be logically and naturally derived from the new string theory. However, the equations of the theory proved difficult - so difficult, in fact, that their exact form could not be determined and approximations had to be used to replace their correct, impossibly complex form. After years of using these approximate methods, they were found inadequate for the types of research being performed. Frustrated scientists, lacking a plan of attack on the dizzyingly complex theoretical calculations, once again abandoned strings and returned to previous projects.
At a conference called Strings 1995, held at the University of Southern California, Edward Witten dropped the bombshell that ignited the second superstring revolution. He announced a cohesive plan for moving past the approximations used during the first superstring revolution and thus into even deeper areas of this vast and complex theory. In fact, the implications of his idea are still being analyzed by string theorists seeking the ultimate answers from what they believe will prove to be the "theory of everything."