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Revolutions never go backwards.
Wendell Phillips

For about a decade, string theory was a totally marginal subject in theoretical physics. Very few people worked on it and those who did found it very hard to get jobs. It had failed as a theory of the strong force. Although touted as a possible theory of quantum gravity, this seemed implausible to the few experts. Quantum theories of both gravity and matter particles were known to suffer from quantum anomalies that made them inconsistent, and there seemed no reason to think string theory was any different.

But in 1984 a landmark paper by Michael Green and John Schwarz changed the mood completely. They discovered an extra contribution to anomalies – now called the Green-Schwarz term - and showed that this term arose in string theory. Suddenly, the anomalies vanished due to a slew of cancellations.  The theory became objectively more promising, and attracted the attention of some influential theorists, including Edward Witten.

Researchers flocked to the subject during the so-called first superstring revolution, and strings rapidly became fashionable. Very soon physicists noted that string theory could easily give models with the main features of the Standard Model plus gravity. For a brief intoxicating moment, it seemed that only one small push would be needed to obtain the final unified quantum theory of all the forces.

In retrospect, these hopes were naive. The more physicists studied string theory, the more they realised the depth of the underlying structure. There emerged not one, but five different consistent superstring theories. There were intrinsic connections – mirror symmetry was discovered during this period – but they were not fully understood.

The ten years from 1984 led to many discoveries about weakly interacting strings, where perturbation theory is a good approximation. The physics of string theory for strong interactions essentially remained a mystery. This all changed in the mid 1990s when physicists realised the importance of D-branes in describing non-peturbative dynamics.

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