A type of unified-field theory that postulates a generalization of the general theory of relativity to higher than four space–time dimensions. In five space–time dimensions this gives general relativity and electromagnetic interactions. In higher space–time dimensions Kaluza–Klein theories give general relativity and more general gauge theories. A combination of Kaluza–Klein theory and supersymmetry gives rise to supergravity, which needs eleven space–time dimensions. In these theories it is proposed that the higher dimensions are ‘rolled up’ to become microscopically small (a process known as spontaneous compactification) with four macroscopic space–time dimensions remaining. It has been suggested by Sir Roger Penrose that the higher dimensions in Kaluza–Klein theory and other theories with compact higher dimensions might be unstable, leading to singularities. It is named after the German mathematician and physicist Theodor Kaluza (1885–1954) and the Swedish physicist Oskar Klein (1894–1977).