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| {{dablink|This article is on dimensional reduction in physics. For the statistics concept, see [[Dimensionality reduction]].}}
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| In [[physics]], a theory in ''D'' [[spacetime]] [[dimension]]s can be redefined in a lower number of dimensions ''d'', by taking all the fields to be independent of the location in the extra ''D'' − ''d'' dimensions.
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| Dimensional reduction is the limit of a [[Compactification (physics)|compactified theory]] where the size of the compact dimension goes to zero. For example, consider a periodic compact dimension with period ''L''. Let ''x'' be the coordinate along this dimension. Any field <math>\phi</math> can be described as a sum of the following terms:
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| :<math>\phi_n = A_n \cos \left( \frac{2\pi n x}{L}\right) </math>
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| with ''A''<sub>''n''</sub> a constant. According to [[quantum mechanics]], such a term has [[momentum]] ''nh''/''L'' along ''x'', where ''h'' is [[Planck's constant]]. Therefore as L goes to zero, the momentum goes to infinity, and so does the [[energy]], unless ''n'' = 0. However ''n'' = 0 gives a field which is constant with respect to ''x''. So at this limit, and at finite energy, <math>\phi</math> will not depend on ''x''.
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| Let us generalize this argument. The compact dimension imposes specific [[boundary conditions]] on all fields, for example periodic boundary conditions in the case of a periodic dimension, and typically [[Neumann boundary condition|Neumann]] or [[Dirichlet boundary condition]]s in other cases. Now suppose the size of the compact dimension is ''L''; Then the possible [[eigenvalue]]s under [[gradient]] along this dimension are integer or half-integer multiples of 1/''L'' (depending on the precise boundary conditions). In quantum mechanics this eigenvalue is the momentum of the field, and is therefore related to its energy. As ''L'' → 0 all eigenvalues except zero go to infinity, and so does the energy. Therefore at this limit, with finite energy, zero is the only possible eigenvalue under gradient along the compact dimension, meaning that nothing depends on this dimension.
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| ==See also==
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| * [[Compactification (physics)]]
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| * [[Kaluza–Klein theory]]
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| * [[String theory#Extra dimensions]]
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| * [[Supergravity]]
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| {{DEFAULTSORT:Dimensional Reduction}}
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| {{Physics-stub}}
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| [[Category:Dimension reduction]]
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