successfully showed that the electromagnetic and weak forces can be unified into a single electroweak force. There is actually some pretty strong evidence that the forces of the Standard Model should all unify as well. When we examine how the relative strengths of the strong force and electroweak force behave as we go to higher and higher energies, we find that they become the same at an energy of about 1016 GeV. In addition the gravitational force should become equally important at an energy of about 1019 GeV. Unification of coupling constants The goal of string theory is to explain the "?" in the above diagram. The characteristic energy scale for quantum gravity is called the Planck Mass, and is given in terms of Planck constant, the speed of light, and Newton's constant, Physics at this high energy scale describes the universe as it existed during the first moments of the Big Bang. These high energy scales are completely beyond the range which can be created in the particle accelerators we currently have (or will have in the foreseeable future.) Most of the physical theories that we use to understand the universe that we live in also break down at the Planck scale. However, string theory shows unique promise in being able to describe the physics of the Planck scale and the Big Bang. In its final form string theory should be able to provide answers to answer questions like: * Where do the four forces that we see come from? * Why do we see the various types of particles that we do? * Why do particles have the masses and charges that we see? * Why do we live in 4 spacetime dimensions? * What is the nature of spacetime and gravity? String theory is at this moment the most promising candidate theory for a unified description of the fundamental particles and forces in nature including gravity. As a theory of quantum gravity string theory is at present our best hope to give concretely computable answers to fundamental questions such as the underlying symmetries of nature, the quantum behaviour of black holes, the existence and breaking of supersymmetry, and the quantum treatment of singularities. It might also shed light upon larger issues such as the nature of quantum mechanics and space and time. In string theory all the forces and particles emerge in an elegant geometrical way, realizing Einstein's dream of building everything from the geometry of space-time. String theory is based on the (deceptively simple) premise that at Planckian scales, where the quantum effects of gravity are strong, particles are actually one-dimensional extended objects. Just as a particle that moves through spacetime sweeps out a curve (the worldline) string will sweep out a surface (the world-sheet) In contrast with particle theories, string theory is highly constrained in the choice of interactions, supersymmetries and gauge groups. In fact, all the usual particles emerge as excitations of the string and the interactions are simply given by the geometric splitting and joining of these strings: In this way the usual Feynman diagrams of quantum field theory are generalized by arbitrary Riemann surfaces Much recent interest has been focused on D-branes. A D-brane is a submanifold of space-time with the property that strings can end or begin on it. More information on the web * Een Nieuwe Revolutie in Stringtheorie, a popular article in Dutch about recent developments, Afleiding 1 (1996) 7-11 (in Dutch, ps-file) * Nonperturbative String Theory, a somewhat more technical review in English of recent developments. * Superstring Theory, by Brian Greene. * String Theory and the Unification of All Forces, by Sunil Mukhi. * The Second Superstring Revolution, by John Schwarz Some recent popular literature on string theory * G. Taubes, A Theory of Everything Takes Shape, Science 269 (1995). * P. Townsend, Unity from Duality, Physics World, Sept 1995. * E. Witten, Reflections on the Fate of Spacetime, Physics Today, April 1996. * M. Mukerjee, Explaining everything, Scientific American, January 1996. Good starting points in the scientific literature * M.B. Green, J.H. Schwarz, E. Witten, Superstring Theory, two volumes (Cambridge University Press, 1987). Still the most complete treatment of the pre-1987 material. * J. Polchinski, What is String Theory? in Les Houches 1994, `Fluctuating Geometries in Statistical Mechanics and Field Theory,' hep-th/9409168. A good review of the material that ends just before the `1994 revolution.' It also includes the matrix models and random surface ideas of 1989/90. * J. Polchinski, Tasi Lectures on D-branes, hep-th/9611050. All you want to know about D-branes, the single most important ingredient to understand non-perturbative string theory. * B. Greene, String Theory on Calabi-Yau Manifolds, hep-th/9702155. A excellent review of (super)conformal field theory, supersymmetric sigma models, mirror symmetry etc. * H. Ooguri, Z. Yin, TASI Lectures on Perturbative String Theory, hep-th/9612254. A compact lecture series that treats the essentials of perturbative string theory in an elegant way.