The first time strings were used to model particles, it was as a convenient way to look at data. In 1968 Gabriele Veneziano, a young researcher at CERN, was trying to describe the strong force. He realised that an equation, written by Leonard Euler several centuries earlier, seemed to do the job.
But while this approach worked well, no one really understood why. Several people began to work on an interpretation for Veneziano’s idea. By 1970, Yoichiro Nambu, Holger Nielsen and Leonard Susskind had all independently come to the same conclusion: the formula made sense if you thought of particles as tiny vibrating strings.
For several years, these ideas – then called dual resonance models - were very popular as a proposed model of the strong force. However this came to an end in 1973 with the discovery of quantum chromodynamics as the correct quantum field theory description of the strong force. Dual resonance models and string theory entered the scientific wilderness.
Nonetheless a few researchers persevered, motivated no longer by the strong force but by a deeper and more ambitious problem.
Their original string model of bosons (force particles) was consistent with special relativity (Einstein’s description of objects moving at very high speeds) and quantum mechanics, but required twenty-six dimensions! Their theory also predicted the existence of particles called tachyons, which had negative mass and could move faster than the speed of light. These properties seemed nonsensical to most people.
But in 1971 Pierre Ramond had modified the theory to include fermions (matter particles), and in doing so discovered supersymmetry. His superstring theory had no tachyons and reduced the required number of dimensions to ten.
Different string vibrations gave rise to different particles. One vibration was particularly interesting: it was a massless particle of spin two, precisely the properties of the hypothesised graviton.
So gravity seemed to emerge naturally from string theory. When John Schwarz and Joel Scherk discovered this in 1974, they suggested that string theory reinvent itself as a quantum theory of gravity.
With the discovery of the correct quantum description of the strong force hogging the limelight, their ideas were largely ignored by the physics community. String theory entered a fallow period, with few people continuing to work on it. To those in the know, there also appeared to be technical problems preventing any quantum description of both matter and gravity. This situation remained until the First Superstring Revolution in 1984.