NBIA Colloquium: Paolo di Vecchia

Speaker: Paolo Di Vecchia (Nordita & NBI)

Title: The Birth of String Theory

Abstract: I will start by describing how the Veneziano 4-point amplitude and its N-point generalization, the Dual Resonance Model, were constructed using basic properties of S-matrix theory. This procedure is opposite to the usual one where one constructs the S-matrix starting from a Lagrangian. In this case the S-matrix was constructed without knowing the corresponding Lagrangian. Only later it was shown that the S-matrix followed from the string Lagrangian. I will then discuss how the scattering amplitude was rewritten in terms of an infinite set of harmonic oscillators that allowed to determine the spectrum of physical states and their scattering amplitudes. Already in 1970 Nambu, Nielsen and Susskind proposed that the underlying theory was a string theory, but it took few more years, by quantizing the Nambu-Goto action, to show that the spectrum of physical states coincided with the spectrum extracted from the Dual Resonance Model. Later it was shown that also the scattering amplitudes at tree and multiloop level were the same. I will conclude by discussing the attempt at writing the scattering amplitude for pions that led to superstring theory. A string theory for pions is still missing. In modern language this will require construction of a string theory extension of the non-linear sigma-model that is the low-energy effective Lagrangian of QCD, but all attempts in this direction have generated scattering amplitudes with negative norm states, thus violating unitarity. The only theories that allow for a stringy extension are gauge theories and gravity and no string theory exists with one of the two without the other.

Brief Bio-sketch: After his PhD from Rome University, Paolo Di Vecchia held post-doctoral positions at Frascati, MIT, and CERN, before taking up a junior faculty position at Nordita in 1974. He was appointed Professor at University of Wuppertal in 1980, and 1986 he moved back to Copenhagen as Professor at Nordita. Paolo Di Vecchia was heavily involved in the early developments of string theory in the 1970's and has been engaged in the topic since, in addition to having made fundamental contributions to particle physics. He now divides his time between NBI and Nordita, working on the computation of observables of gravitational wave physics from general relativity.