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Ivancevic_Applied-Diff-Geom

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<strong>Geom</strong>etrical Path Integrals and Their Applications 1189notion of an orbifold usually describes an object that can be globally writtenas a coset M/G where M is a manifold (or a theory) and G is a group of itsisometries (or symmetries). In string theory, these symmetries do not needto have a geometric interpretation. The so–called orbifolding is a generalprocedure of string theory to derive a new string theory from an old stringtheory in which the elements of the group G have been identified with theidentity. Such a procedure reduces the number of string states because thestates must be invariant under G, but it also increases the number of statesbecause of the extra twisted sectors. The result is usually a new, perfectlysmooth string theory.Mirror SymmetryThe so–called mirror symmetry is a surprising relation that can exist betweentwo Calabi–Yau manifolds. It happens, usually for two such 6Dmanifolds, that the shapes may look very different geometrically, but neverthelessthey are equivalent if they are employed as hidden dimensions of a(super)string theory. More specifically, mirror symmetry relates two manifoldsM and W whose Hodge numbers h 1,1 = dim H 1,1 and h 1,2 = dim H 1,2are swapped; string theory compactified on these two manifolds leads toidentical physical phenomena (see [Greene (2000)]).Strominger showed in [Strominger (1990)] that mirror symmetry is aspecial example of the so–called T −duality: the Calabi–Yau manifold maybe written as a fiber bundle whose fiber is a 3D torus T 3 = S 1 × S 1 × S 1 .The simultaneous action of T −duality on all three dimensions of this torusis equivalent to mirror symmetry.Mirror symmetry allowed the physicists to calculate many quantitiesthat seemed virtually incalculable before, by invoking the ‘mirror’ descriptionof a given physical situation, which can be often much easier. Mirrorsymmetry has also become a very powerful tool in mathematics, and althoughmathematicians have proved many rigorous theorems based on thephysicists’ intuition, a full mathematical understanding of the phenomenonof mirror symmetry is still lacking.6.7.4 More on Topological Field TheoriesUnfortunately, there is no such thing as a crash course in string theory, butthe necessary background can be found in the classic two–volume monographs[Green et. al. (1987)] and [Polchinski (1998)]. A good introduction

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