“Superstring theory has been studied intensively since 1984, when the discovery of Green and Schwarz of anomaly cancellation convinced many physicists that it provides a consistent theory of perturbative quantum gravity, gauge interactions and chiral matter. The basic difficulties in quantizing general relativity and supergravity (non-renormalizability or at least strong coupling at the Planck scale) are visible at low order in the loop expansion, while superstring theory was shown to be well-defined and finite to all orders.”