The incomplete revolutions of String Theory

vol35 / no3-4 / anno2019 > 47 percorsi The Incomplete Revolutions of String Theory Augusto SAGNOTT I Scuola Normale Superiore, Pisa, Italy INFN, Sezione di Pisa, Pisa, Italy DESY, Hamburg, Germany 1 Quantum fields and particles Our current understanding of the Fundamental Interactions rests on Quantum Field Theory. This setting affords a novel incarnation of the point particle idea, which has nurtured Physics since the time of Galileo Galilei and the subsequent birth of Newtonian Mechanics. Rigid bodies, strings, membranes and fluids are but variations on this theme, and point particles have also provided important clues on light and waves. When Quantum Mechanics found its place in a relativistic picture, in the 1930’s, particle-wave duality found a concrete realization in Quantum Field Theory, where masses and momenta qualify field quanta together with a novel attribute, spin. Atomic Physics had indeed provided, since the early 1920’s, compelling If one tried to get a palatable picture of Electromagnetism, it would be natural to hear from an expert about charges, flux lines, potentials and waves. These are subtle concepts, and yet they can convey some valuable intuition on these phenomena, despite the intricacies of the underlying Mathematics. Similar questions about General Relativity would probably bring up falling bodies, the Equivalence Principle and deformations of the fabric of spacetime where planets slide along their orbits. String Theory, however, is still a very different matter, and experts have a hard time defining it. One could say that strings replace particles, as we shall try to explain, but in our current view particles emerge from fields, whose geometry underlies their interactions. As we shall see, appealing to such geometrical principles within the low-energy Supergravity has provided unprecedented glimpses of a unified view of Nature that transcends not only strings but our very picture of spacetime. Yet, we lack somehow satisfactory answers to some basic questions, which ought to have preceded all this. What replaces the principles of General Relativity in String Theory? What do strings tell us about spacetime at short distances? Why is our Universe the way it seems? String Theory is today an awesome unfinished monument, whose roots remain elusive despite decades of intense effort. While this brings about some distress, it also makes the subject mysterious, challenging and highly fascinating. In the following, I shall describe the origin of this unusual situation, while also trying to address some future prospects.