Abstract:
This is a set of lectures given at the 99' Cargese Summer School: Particle Physics : Ideas and Recent Developments. They contain a pedestrian exposition of recent theoretical progress in non-perturbative field theory and string theory based on ideas of duality.

Abstract:
It has been often conjectured that the correct theory of quantum gravity will act as a UV regulator in the low energy limit of quantum field theory. Earlier work has shown that if the path integral defining the quantum field theory propagator is modified, so that the amplitude is invariant under the duality transformation l--> 1/l where l is the length of the path, then the propagator is UV-finite and exhibits a ``zero-point length'' of the spacetime. Since string theory uses extended structures and has a T-duality, these results should also emerge directly from string theory. We show, by explicit path integral computation, that this is indeed the case. The lowest order string theory correction to the propagator is the same as that obtained by the hypothesis of path integral duality.

Abstract:
We extend strong/weak coupling duality to string theories without spacetime supersymmetry, and focus on the case of the unique ten-dimensional, nonsupersymmetric, tachyon-free $SO(16)\times SO(16)$ heterotic string. We construct a tachyon-free heterotic string model that interpolates smoothly between this string and the ten-dimensional supersymmetric $SO(32)$ heterotic string, and we construct a dual for this interpolating model. We find that the perturbative massless states of our dual theories precisely match within a certain range of the interpolation. Further evidence for this proposed duality comes from a calculation of the one-loop cosmological constant in both theories, as well as the presence of a soliton in the dual theory. This is therefore the first known duality relation between nonsupersymmetric tachyon-free string theories. Using this duality, we then investigate the perturbative and nonperturbative stability of the $SO(16)\times SO(16)$ string, and present a conjecture concerning its ultimate fate.

Abstract:
We discuss duality between Type IIA string theory, eleven-dimensional supergravity, and heterotic string theory in four spacetime dimensions with $N=1$ supersymmetry. We find theories whose infrared limit is trivial at enhanced symmetry points as well as theories with $N=1$ supersymmetry but the field content of $N=4$ theories which flow to the $N=4$ fixed line in the infrared.

Abstract:
We analyze in detail the possible breaking of spacetime supersymmetry under T-duality transformations. We find that when appropiate world-sheet effects are taken into account apparent puzzles concerning supersymmetry in spacetime are solved. We study T-duality in general heterotic $\sigma$-models analyzing possible anomalies, and we find some modifications of Buscher's rules. We then work out a simple but representative example which contains most of the difficulties. We also consider the issue of supersymmetry versus duality for marginal deformations of WZW models and present a mechanism that restores supersymmetry dynamically in the effective theory.

Abstract:
We present new classes of string-like soliton solutions in ($N=1$; $D=10$), ($N=2$; $D=6$) and ($N=4$; $D=4$) heterotic string theory. Connections are made between the solution-generating subgroup of the $T$-duality group of the compactification and the number of spacetime supersymmetries broken. Analogous solutions are also noted in ($N=1,2$; $D=4$) compactifications, where a different form of supersymmetry breaking arises.

Abstract:
We study non-local realizations of extended worldsheet supersymmetries and the associated space-time supersymmetries which arise under a T-duality transformation. These non-local effects appear when the supersymmetries do not commute with the isometry with respect to which T-duality is performed.

Abstract:
The string theory introduced in early 1971 by Ramond, Neveu, and myself has two-dimensional world-sheet supersymmetry. This theory, developed at about the same time that Golfand and Likhtman constructed the four-dimensional super-Poincar\'e algebra, motivated Wess and Zumino to construct supersymmetric field theories in four dimensions. Gliozzi, Scherk, and Olive conjectured the spacetime supersymmetry of the string theory in 1976, a fact that was proved five years later by Green and myself.

Abstract:
In these lecture notes duality tests and instanton effects in supersymmetric vacua of string theory are discussed. A broad overview of BPS-saturated terms in the effective actions is first given. Their role in testing the consistency of duality conjectures as well as discovering the rules of instanton calculus in string theory is discussed. The example of heterotic/type-I duality is treated in detail. Thresholds of F^4 and R^4 terms are used to test the duality as well as to derive rules for calculated with D1-brane instantons. We further consider the case of R^2 couplings in N=4 ground-states. Heterotic/type II duality is invoked to predict the heterotic NS5-brane instanton corrections to the R^2 threshold. The R^4 couplings of type-II string theory with maximal supersymmetry are also analysed and the D-instanton contributions are described Other applications and open problems are sketched.

Abstract:
World-sheet and spacetime supersymmetries that are manifest in some string backgrounds may not be so in their T-duals. Nevertheless, they always remain symmetries of the underlying conformal field theory. In previous work the mechanism by which T-duality destroys manifest supersymmetry and gives rise to non-local realizations was found. We give the general conditions for a 2-dim N=1 supersymmetric sigma-model to have non-local and hence non-manifest extended supersymmetry. We then examine T-duality as a mechanism of restoring manifest supersymmetry. This happens whenever appropriate combinations of non-local parafermions of the underlying conformal field theory become local due to non-trivial world-sheet effects. We present, in detail, an example arising from the model SU(2)/U(1) X SL(2,R)/U(1) and obtain a new exact 4-dim axionic instanton, that generalizes the SU(2) X U(1) semi-wormhole, and has manifest spacetime as well as N=4 world-sheet supersymmetry. In addition, general necessary conditions for abelian T-duality to preserve manifest N=4 world-sheet supersymmetry are derived and applied to WZW models based on quaternionic groups. We also prove some theorems for sigma-models with non-local N=4 world-sheet supersymmetry.