In the early 1970s physicists formulated the ‘standard model’ of particle physics, which unified three of the four known forces of nature – the strong force which holds the nucleus together, the weak force responsible for radioactivity, and the electromagnetic force. But including the fourth, gravitational force proved intractable, because the mathematical formalism for how gravity interacts with the other forces produced numbers which were infinitely huge, an anathema in physics. 

String theory entered the picture in the 1980s, representing fundamental particles as specific modes of string vibrations. It seemed, at first, to be a promising route to the complete unification of all four forces. Under certain conditions both the standard model and Einstein's theory of gravity could be derived from