= Compactification in string theory
Compactification makes some target spatial dimensions compact and sufficiently small that lower-energy observations see an effective lower-dimensional spacetime. A simple geometry is a product of noncompact spacetime with a compact internal manifold; its <momentum> and winding sectors influence the effective spectrum. Reaching a four-dimensional model from a critical ten-dimensional superstring requires such additional geometric and field-theoretic data. <Worldsheet supersymmetry> alone does not choose the compactification or establish a realistic particle spectrum.
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