© Left: University of Göttingen, Right: Stanley Deser.

Theodor Kaluza (left) and Oskar Klein (right) conceived some of the most pervasive ideas in string theory today. Kaluza, a German mathematician and physicist, realized in 1919 that Maxwell's equations of electricity and magnetism arise naturally from Einstein's equations of general relativity if he added an extra dimension. Klein, a theoretical physicist in Sweden, realized in the 1920s that extra dimensions can be so small that we cannot observe them directly. While the theory known today as "Kaluza-Klein" gravity turned out not to describe the universe we live in, the novel ideas it contained are basic ingredients in nearly every modern string theory. (Unit: 4)