The history of string theory is not one of continuous progress along a clear direction. Indeed, much crucial work was done before physicists even realised the importance of strings. The story of progress can be divided into several eras.
We first uncover some prehistory: ideas that were developed before string theory, but find a natural home alongside strings. A superb example of this is extra dimensions, a concept which predates string theory by half a century.
The next age covers the late 1960s and early 1970s. At this time string theory was studied as a candidate model of the strong force. Although the approach failed, there were tantalising hints that string theory could become a quantum theory of gravity.
Almost no work was done on the subject for ten years. But in 1984 a revolutionary discovery by Michael Green and John Schwarz sent string theory mainstream. Since 1995, a slew of further results have deepened our understanding, providing a toolbox of techniques that many physicists use for research today.
The journey is far from complete. We don’t yet fully grasp the governing equations for M-theory. Much work has still to be done, in what promises to be an epic voyage of discovery. Here you can find out how it all began.