The birth of string theory took place in the years from 1968 (Veneziano formula) to 1973–1974. At the time I was a young physicist interested in the dynamics of strongly interacting particles. This problem was clearly far from being solved but, due to the everyday increase of ‘elementary particles’ and their ‘strong’ interaction, one thing seemed sure to me: the answer was not to be found in quantum field theory!

From the Goldberger–Treiman relation, mNgA = fπGπNN, one obtained GπNN ∼ 10 for the pion–nucleon coupling constant. So a perturbative expansion for the strong interaction was clearly out of the question. But for me it was more than that.With all its ‘infinities’ popping up everywhere, quantum field theory (QFT) appeared to me not only inadequate but just conceptually ‘wrong’ on the whole, because of the large number of ‘unobservable’ quantities it introduced.

In the spirit of Heisenberg the description of the physical world was to be based only on ‘observables’! Since the only really ‘measurable’ quantities were the cross-sections in scattering processes, Chew's ‘bootstrap’ programme seemed the right conceptual framework and his book S-Matrix Theory of Strong Interactions [Che62] was (not only for me, I believe) the reference text.

Of course the calculation of the full S-matrix appeared too ambitious and the immediate objective was to obtain relations among a set of measurable parameters. In this context, a relevant method to obtain physical information was based on the idea, emphasized especially by Sergio Fubini and others, to use dispersion relations together with current algebra to obtain sum rules involving cross-sections.