Editing Compactification (physics) (section)

{{short description|Technique in theoretical physics}}

In [[theoretical physics]], '''compactification''' means changing a theory with respect to one of its [[Spacetime|space-time]] [[dimension]]s. Instead of having a theory with this dimension being infinite, one changes the theory so that this dimension has a finite length, and may also be [[Periodic function|periodic]].

Compactification plays an important part in [[Thermal quantum field theory|thermal field theory]] where one compactifies time, in [[string theory]] where one compactifies the [[String theory#Extra dimensions|extra dimensions]] of the theory, and in two- or one-dimensional [[Solid-state physics|solid state physics]], where one considers a system which is limited in one of the three usual spatial dimensions.

At the limit where the size of the compact dimension goes to zero, no fields depend on this extra dimension, and the theory is [[Dimensional reduction|dimensionally reduced]].

[[Image:Kaluza Klein compactification.svg|frame|The space {{math|''M'' × ''C''}} is compactified over the compact {{mvar|C}} and after Kaluza–Klein decomposition, we have an [[effective field theory]] over {{mvar|M}}.]]