NBIA Colloquium: Paolo Di Vecchia

Speaker: Paolo Di Vecchia

Title: "The birth of string theory, or how to build a theory without knowing the Lagrangian"

Abstract: In my talk I will start by describing how the Veneziano amplitude and its 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 much later it was shown that the S-matrix followed from a string theory Lagrangian. Already in 1970 Nambu, Nielsen and Susskind proposed that the underlying theory was a string theory, but it took few more years 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 multi-loop level were the same. I will conclude by discussing the attempt to construct a string theory that is a low-energy effective Lagrangian of strong interactions, but all attempts in this direction have generated scattering amplitudes with negative norm states that violate unitarity.The only theories that allow for a string extension are gauge theories and gravity, and no string theory exists with one of the two without the other. In conclusion, one started with the idea of writing an S-matrix for mesons and ended up in writing instead a theory that unifies gauge theories with gravity.    

Brief bio-sketch: Paolo Di Vecchia graduated from Rome University and subsequently held post-doctoral positions at Frascati, MIT, and CERN. He was Assistant Professor at Nordita in Copenhagen before becoming Professor at University of Wuppertal 1980-85 before returning to Copenhagen as Professor at Nordita, later moving with Nordita to Stockholm. Paolo Di Vecchia has played a pivotal role in the development of string theory, both in the early years, and later its renaissance in the 1980's as superstring theory. Today, Paolo Di Vecchia works mainly on the development of new field theoretic methods for the computation of observable signals in gravitational wave physics.

Refreshments in the NBIA Lounge after the talk!