String Theory

String theory arose in the late 1960s in an attempt to understand the strong nuclear force. This is the force that holds protons and neutrons together inside the nucleus of an atom. The strong nuclear force also hold quarks and gluons together inside the neutron and proton. Strongly interacting particles, such as the proton and neutron, are called hadrons. 3 4 A correct theory must incorporate the well-established principles of special relativity and quantum mechanics. A theory of this type, based on point-like particles, is called a quantum field theory. In addition to the strong nuclear force, the other known fundamental forces are electromagnetism, the weak nuclear force, and gravity. 5 Strings are physical objects that are curves in space. They have energy density or tension, just as point particles have mass. A theory based on strings can account for various features of the strong nuclear force such as the rich spectrum of hadrons and the high-energy behavior of interactions. The first string theory was developed in the period 1968-70 by Gabriele Veneziano and hundreds of others. Open strings have two ends: Closed strings are loops without any ends: 6 The Basic Idea Different modes of oscillation (or quantum states) of the string behave as different particles. So, in this sense, there is a unique object (namely, the string). The dream was that this could give a unified understanding of the complicated spectrum of hadrons that was discovered in the 1960s. • Quantum mechanical consistency of the original string theory requires 26 dimensions (25 are spatial and 1 is time). This was completely unexpected – and certainly not what we wanted. • This theory only describes bosons, whereas realistic physical theories always involve fermions as well. 8 Superstrings A second string theory, containing fermions as well as bosons, was constructed in 1971 by Pierre Ramond, André Neveu, and me. It requires 10 dimensions (9 space and 1 time). The development of this string theory led to the discovery of supersymmetry, a new type of symmetry that relates bosons and fermions. Strings in theories that have supersymmetry are usually called superstrings. 9 Both string theories – the 26d bosonic string theory and the 10d superstring theory-have oscillation modes that correspond to massless particles. This was another disturbing fact, since it was known that every hadron has a positive mass. We tried for years to modify the string theories to describe only …