# Kaluza-Klein reduction The KK reduction is a way to obtain a (more complicated) theory at lower dimensions by dimensionally reducing a simple theory such as pure gravity. Some metric components of the higher dimensional theory becomes matter fields such as the $U(1)$ gauge field and the dilation field. The KK vacuum has a semi-classical instability. It can tunnel to a [[0168 Bubble of nothing|bubble of nothing]], aka Witten's bubble of nothing.