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Perturbation Theory

. Perturbation theory is the power series expansion of physical quantities in terms of a small dimensionless parameter. For Quantum Electrodynamics this parameter is the fine structure constant
Fine structure constant
Perturbation expansion
. In Quantum Field Theory, these are always divergent expansions, because T(a) is not analytic at a = 0. For example, quantum tunneling effects give non-perturbative corrections of the type
Non-perturbative effects
. The five superstring theories are defined by their perturbative expansions. Until recently, we didn't know how to study non-perturbative phenomena in string theory.

The Second Superstring Revolution // String Theory: a multihistory

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