In April 1919 Kaluza noticed that when he solved Albert Einstein's equations for general relativity using five dimensions, then James Clark Maxwell's equations for electromagnetism emerged spontaneously. Kaluza wrote to Einstein who, in turn, encouraged him to publish. Kaluza's theory was published in 1921 in a paper, "Zum Unitätsproblem der Physik" with Einstein's support in Sitzungsberichte Preußische Akademie der Wissenschaften 96, 69. (1921). Kaluza's insight is remembered as the Kaluza-Klein theory (also named after physicist Oskar Klein). However, the work was neglected for many years as attention was directed towards quantum mechanics.