In 1919, Theodor Kaluza posted his 5-dimensional extension of general relativity, to Albert Einstein, who was impressed with how the equations of electromagnetism emerged from Kaluza's theory. In 1926, Oskar Klein suggested that Kaluza's extra dimension might be " curled up " into an extremely small circle, as if a circular topology is hidden within every point in space. Instead of being another spatial dimension, the extra dimension could be thought of as an angle, which created a hyper-dimension as it spun through 360°. This 5d theory is named Kaluza–Klein theory.