
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 nonperturbative corrections of the type
The
five superstring theories are defined by their perturbative
expansions. Until recently, we didn't know
how to study nonperturbative phenomena in string theory.

