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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
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
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.
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