The subject of string theory arose in the late 1960's in an attempt to describe strong nuclear force
s. This approach to the description of strong nuclear forces was a quite active subject for about
five years, until it was abandoned because it ran into various theoretical difficulties and because
a better theory came along.
There were two main problems that stymied us in our attempts to use string theory to describe
the strong nuclear forces. One was that the theory required the existence of a kind of particle
that we didn't want -- namely a particle with no mass and two units of spin. Its existence was
very generic and very frustrating. The second problem was that the theory required that
space-time
have ten dimensions (nine space and one time), whereas the correct answer is clearly four
(three
space and one time).
As if all that weren't bad enough, around 1973 quantum chromodynamics (QCD) -- the SU(3) part of
the standard model -- merged as a convincing theory of the strong nuclear force.
One curious fact about string theory in the 1968--1973 period is that it took two years studying
various complicated mathematical formulas before several people realized that these
were formulas describing the interactions of extended one-dimensional objects, which
were named ``strings.'' Once this was realized, it became clear that this type of
theory was outside the framework of conventional quantum field theory, which as we
have emphasized, is based on point-like elementary particles.
Another important development during this period (in 1971) was the discovery
that to incorporate a class of elementary particles called fermions (electrons
and quarks are examples) string theory requires a two-dimensional version of
supersymmetry.[2] This led to the development of space-time supersymmetry, which was
eventually recognized to be a generic feature of all consistent string theories
(hence the name superstrings").