Origins of String Theory

Quick Timeline 5
1921 - Kaluza-Klein Theory
1970 - Birth of string theory
1971 - Supersymmetry
1973 - Quantum Chromodynamics
1974 - Gravitons; advent of string theory as a unified theory
1976 - Supergravity; supersymmetry applied to gravity
1980 - Superstrings; supersymmetry applied to strings
1984 - First Revolution; anomalies are cancelled and string theory goes mainstream
1987 - E8xE8 Heterotic superstring theory that may explain dark matter
1994 - Second Revolution; understanding of how strings are related with dualities
1996 - Black Hole Entropy; microscopic origin for black hole thermodynamics achieved in string theory

String theory was originally used to attempt to explain the strong nuclear force. It had an irritating side effect, however. It required the existence of a particle without mass and two units of spin which didn't fit anywhere into the strong force. This idea was placed on the back-burner, and in 1971 supersymmetry was applied to strings which incorporated fermions as well as bosons, thus replacing bosonic string theory . In 1973, quantum chromodynamics (QCD) became the dominant theory for the strong force, nearly ending string theory's short career. The following year (1974), however, it was proposed that strings be used for a "unified theory". The unwanted particle that was problematic when strings were applied to the strong force? That became an ideal candidate for the graviton--a particle missing from physics prior to this realization 4.

The first string revolution occurred in 1984. In this year, Greene and Schwarz discovered that anomalies in the math behind strings conveniently cancelled. The immediate result of this was the advancement of five consistent superstring theories that exist in ten-dimensional spacetime. This is the year when string theory went from backyard science to mainstream 6.

In 1987, Gross, Harvey, Martinec, and Rohm discovered the E8xE8 Heterotic superstring theory. The SU(3)xSU(2)xU(1) gauge group of the standard model fits under this theory in one of the E8 groups. It would interact with the other E8 group only through gravity, which could provide an explanation for dark matter 3.

In 1994, Seiberg and Witten found supersymmetric gauge theories while Hull and Townsend found string dualities (to be discussed in the next section). The second revolution resulted in the realization that the five superstring theories, plus a sixth found later that exists in eleven dimensions, are manifestations of an underlying M-theory that ties them all together 6.