It’s the birthday of Oskar Klein, who was born in 1894 in Danderyd, Sweden. In 1921 mathematician Theodor Kaluza cast Albert Einstein’s theory of general relativity into five dimensions (four space; one time). Kaluza’s extension had the advantage that four-dimensional general relativity and four-dimensional electromagnetism emerged neatly from theory. But what was the meaning of the extra dimension? In 1926 Oskar Klein proposed that the fourth spatial dimension is curled up in a circle of such tiny radius that a particle moving a short distance along the dimension’s axis would return to its origin. The extra dimension would be effectively invisible. Klein’s imaginative idea of compactifying spatial dimensions formed the basis of Kaluza–Klein theory, which unified gravity and electromagnetism, and of later theories, such as M-theory, which requires 11 dimensions.
from Physics Today